Evolusi adalah suatu proses perubahan makhluk hidup secara bertahap dan membutuhkan waktu yang lama dari bentuk yang sederhana, menjadi bentuk yang lebih kompleks. Diperlukan waktu jutaan tahun agar perubahan tersebut nampak lebih jelas. 

Terdapat dua macam evolusi : Evolusi Progresif : Evolusi yang menuju pada kemungkinan dapat bertahan hidup.

Evolusi Regresif (retrogresif) : Evolusi yang menuju pada kemungkinan menjadi punah.

Teori evolusi itu sendiri adalah perpaduan antara ide (gagasan) dan fakta. Berikut teori dari para ilmuan mengenai Evolusi makhluk hidup:

1) Jean Baptise Lamarck

Idenya mengenai evolusi, di tuangkan di dalam buku berjudul "Philosophic zoologique". Dalam buku tersebut, Lamarck mengungkapkan :

Alam sekitar/lingkungan mempunyai pengaruh pada ciri-ciri atau sifat yang diwariskan

Ciri-ciri/sifat tersebut akan diwariskan kepada keturunannya

organ yang sering digunakan akan berkembang, sedangkan apabila tidak digunakan akan mengalami kemunduran bahkan hilang

Contoh : Lamacrk berpendapat bahwa dahulu, jerapah memiliki leher yang pendek. Bagi keturunan jerapah yang dapat beradaptasi baik dengan lingkungan (dapat mengambil makanan di pohon yang tinggi),  leher jerapah akan berkembang menjadi lebih panjang. Jerapah yang telah beradaptasi menjadi leher panjang tersebut, akan mewariskan sifat-sifat kepada keturunannya. Namun sebaliknya, bagi keturunan jerapah yang tidak dapat beradaptasi baik dengan lingkungan, maka ia akan mengalami kemunduran.  

Teori Lamarck

2) Charles Darwin

Charles Darwin juga menerbitkan buku mengenai asal mula spesies pada tahun 1859, dengan judul "on the ofiginof species by means of natural selection" atau "the preservation of favored races in the struggle for life". Mengenai Evolusi, Darwin berpendapat :

Yang menjadi dasar evolusi organik bukan dari adaptasi lingkungan, melainkan karena seleksi alam dan seksual

Seleksi alam berupa "pertarungan" dalam kehidupan, yang kuat akan terus hidup

Setiap populasi berkecenderungan untuk tumbuh banyak karena proses bereproduksi

Untuk berkembang biak, diperlukan adanya makanan dan ruang yang cukup

Bertambahnya suatu populasi tidak berjalan terus-menerus

Teori Darwin

3) Alfred WallaceMemiliki pendapat yang sama dengan Charles Darwin, bahwa spesies yang ada sekarang, berasal dari spesies masa silam yang mampu bertahan hidup.

4) Count De Buffen Menyatakan bahwa variasi-variasi kecil yang terjadi karena pengaruh alam sekitar yang diwariskan. Dengan demikian, kemampuan beradaptasi dengan lingkungan akan menyebabkan terjadinya variasi yang mengarah pada terbentuknya spesies baru. 

5) Sir Charles LyeelMenyatakan bahwa batuan, pulau-pulau, dan benua mengalami perubahan.

6) Thomas Robert Malthus Menyatakan bahwa pertambahan jumlah penduduk naik seperti deret ukur, sedangkan bahan makanan yang tersedia, naik seperti nilai hitung. 

7) AnaximenderBumi pada awalnya merupakan lautan, lalu berkembang menjadi daratan. Para makhluk hidup aquatik pun termodifikasi sehingga dapat hidup di darat. Pada manusia, terdapat masa"part fish" dan "part human" yang disebut merman dan mermaid. penampilan seperti ikan ini ada pada masa dalam kandungan bayi selama proses perkembangan. Kemudian, penampilan tersebut akan hilang pada manusia dewasa.   

8) AristotelesBenda-benda hidup berkembang makin sempurna karena pengaruh kekuatan tertentu, yaknientelecy, dan makhluk hidup di daratan berasal dari makhluk hidup di lautan.

9) EpicurusEpicurus sependapat dengan Aristoteles bahwa organisme berubah dan berkembang makin kompleks dan makin maju. Namun bukan karena faktor entelecy yang mempengaruhinya, melainkan karena faktor "natural law".

10) August Weismann

Ilmuan ini melengkapi teori evolusi Darwin dengan teori genetika modern. Menurutnya, evolusi adalah masalah genetika, yakni soal keturunan bagaimana mewariskan gen-gen melalui sel kelamin.

Pro dan Kontra tentang berbagai pendapat masalah Evolusi

Lamarck vs WeismannWeismann menentang pendapat Lamarck, mengenai sifat-sifat makhluk hidup yang diperoleh melalui proses keturunan dari induknya. Maka, weismann di sini melakukan percobaan dengan memotong ekor tikus yang panjang, namun pada hasilnya, sampai kepada generasi-20, keturunan induk tikus yang ekornya di potong tadi, tetap berukuran panjang.

Lamack vs DarwinMereka berpendapat mengenai jerapah berleher panjangLamarck : Semula, jerapah berleher pendek, namun karena beradaptasi dengan lingkungan (selalu memakan dedaunan di pohon yang tinggi), maka lehernyapun menjadi panjangDarwin : Dalam populasi, ada jerapah berleher pendek, dan ada yang berleher panjang. Dalam kompetisi untuk mendapatkan makanan, jerapah berleher panjang lebih unggul daripada jerapah berleher pendek, sehingga populasi yang sampai sekarang masih dapat kita lihat adalah jerapah berleher panjang. 

Teori Lamarck dan DarwinBukti-Bukti Adanya Evolusi

a) Fosil Merupakan sisa-sisa, cetakan ataupun berkas dari hewan maupun tumbuhan yang telah membatu. Fosil ini sebagai bukti adanya peristiwa evolusi yang dapat menentukan umur dengan cara menghitung laju erosi, sedimentasi, kandungan garam, dan kadar radio aktif.

b) HomologiDua organ tubuh dikatakan homolog, apabila mempunyai asal sama (secara embrilogik), tetapi fungsi dan bentuknya berbeda. Contoh : alat gerak manusia dan sirip ikan. (memiliki bentuk rangka yang sama, namun berbeda fungsinya).n

Homologi

c) AnalogiMerupakan kebalikan dari homologi, yaitu mempunyai asal yang berbeda, namun memiliki fungsi yang sama. Contoh : sayap kupu-kupu dengan sayap kelelawar (memiliki bentuk rangka yang berbeda, namun memiliki satu fungsi yang sama, yaitu untuk terbang).

Analogi

d) Embriologi PerbandinganMerupakan hewan-ewan kelas vertebrata, seperti ikan, reptil, aves dan mamalia, meskipun tubuh individu dewasanya berbeda, namun fase perkembangan embrionya sangat mirip (lihat pada gambar, urutan paling atas horizontal, embrio dari manusia, hewan memiliki kemiripan). Embriologi perbandingan ini dibagi menjadi dua :

Ontogeni : perkembangan individu mulai dari sel telur hingga individu tersebut mati

Filogeni : Sejarah perkembangan organisme dari filum yang paling sederhana hingga yang paling sempurna

Perkembangan Embrio

e) Adanya variasi dalam satu spesiesIndividu yang termasuk dalam suatu spesies tidak pernah bersifat identik (sama persis). variasi ini terjadi karena faktor genetis dan lingkungan.

f) Petunjuk secara biokimiaDigunakan uji presipitin (endapan) yang pada dasarnya adanya reaksi antara antigen-antibodi. semakin banyak endapan yang terbentuk maka semakin jauh hubungan kekerabatannya.

g) Adanya Organ-Organ tubuh yang terseisaOrgan-organ tubuh yang tersisa nerupakan bukti adanya proses evolusi. organ-oran ini sudah tidak berguna, namun masih dijumpai pada manusia, antara lain :

umbai cacing (apendiks)

tulang ekor

gigi taring yang runcing

rambut pada dada

h) Petunjuk Peristiwa DomestikasiPembudidayaan makhluk hidup (domestikasi) dapat mengakibatkan terjadinya perubahan fenotipe sesuai dengan keinginan manusia. cara ini, dapat mengevolusikan makhluk hidup, artinya, dapat menghasilkan suatu varietas baru yang dikehendaki manusia berdasarkan sifat-sifat yang berbeda. 

Mekanisme Evolusi

Evolusi dapat berlangsung karena :

1) Mutasi   Mutasi adalah perubahan materi genetik (gen/kromosom) yang dapat diwariskan secara genetik pada keturunannya. Mutasi ini akan menghasilkan alel baru, kemudian melalui proses perkawinan (kombinasi) akan menghasilkan variasi baru.  

2) Seleksi Alam   Seleksi terhadap anggota populasi sehingga anggota yang kuat dan sehat yang dapat bertahan hidup. (teori Darwin : "survival of the fittest")

Pendapat Teilhard de Chardin mengenai proses evolusi 

Menurut Teilhard, proses evolusi dibagi menjadi 3 tahap :

1) Tahap GeosferMeripakan tahap pra-hidup (sebelum adanya kehidupan). tahap perubahan ini terutama menyangkut pada perubahan tata surya.

2) Tahap BiosferPermasalahan pada tahap ini adalah "loncatan" munculnya manusia.

3) Tahap NesosferMenurut Teilhard, yang penting pada makhluk hidup, dalam hal ini, manusia adalah terjadinya evolusi melalui kesadaran batinnya yang semakin mantap. http://www.pusatbiologi.com/2013/02/teori-teori-evolusi.html

Mekanisme Evolusi

Mekanisme Evolusi adalah perubahan sedikit

demi sedikit dari makhluk hidup yang sederhana ke makhluk yang lebih

kompleks. Mekanisme utama untuk menghasilkan perubahan evolusioner

adalah seleksi alam dan hanyutan genetika. Seleksi alam memfavoritkan gen

yang meningkatkan kapasitas keberlangsungan dan reproduksi. Hanyutan

genetika merupakan perubahan acak pada frekuensi alel, disebabkan oleh

percontohan acak (random sampling) gen generasi selama reproduksi. Aliran

gen merupakan transfer gen dalam dan antar populasi. Kepentingan relatif

seleksi alam dan hanyutan genetika dalam sebuah populasi bervariasi,

tergantung pada kuatnya seleksi dan ukuran populasi efektif, yang

merupakan jumlah individu yang berkemampuan untuk berkembang biak.

Seleksi alam biasanya mendominasi pada populasi yang besar, sedangkan

hanyutan genetika mendominasi pada populasi yang kecil. Dominansi

hanyutan genetika pada populasi yang kecil bahkan dapat menyebabkan

fiksasi mutasi yang sedikit merugikan. Karenanya, dengan mengubah ukuran

populasi dapat secara dramatis mempengaruhi arah evolusi. Leher botol

populasi, di mana populasi mengecil untuk sementara waktu dan kehilangan

variasi genetika, menyebabkan populasi yang lebih seragam. Leher botol

disebabkan oleh perubahan pada aliran gen, seperti migrasi yang menurun,

ekspansi ke habitat yang baru, ataupun subdivisi populasi.

1.     Variasi yang dapat diwariskan (bahan baku evolusi)Variasi yang diwariskan  merupakan bahan baku evolusi karena pada

tahun 1859 Charles Darwin seorang naturalis dari Inggris menerbitkan buku

The Origin Of Species, buku tersebut menyatakan bahwa semua makhluk

hidup yang ada di bumi ini merupakan hasil keturunan dari nenek moyang

yang sama yang mengalami modifikasi inilah yang disebut teori evolusi.

Dengan kata lain, teori evolusi menyatakan bahwa species bukanlah

merupakan suatu yang kekal dan tidak mengalami perubahan, melainkan

berevolusi melalui proses perubahan terhadap dari spesies yang tlah ada,

semua spesies mempunyai hubungan darah, sampai demikian setiap dua

spesies yang ada di bumi ini pada suatu saat dalam sejarah nya mempunyai

leluhur yang sama, jadi pada intinya variasi itu berupa keanekaragaman

spesies  yang ada di dunia ini

Ia menemukan bahwa peternak dapat mengembangkan jenis baru hanya

dari varian yang timbul secara spontan pada ternaknya. Yaitu, hanya jika

beberapa hewan dilahirkan lebih berat atau lebih besar atau warna bulunya

lebih gelap dibandingkan dengan yang lain, maka peternak dapat

mengembangkan jenis yang lebih berat atau lebih tinggi atau berbulu lebih

gelap. Darwin juga menyadari bahwa varian ini tidak dapat timbul hanya

sekedar karena di tempatkan di lingkungan lain. Individu-individu yang yang

lebih besar hanya karena makanan yang lebih baik tidak akan dapat

menyediakan bahan baku untuk jenis yang lebih besar. Hanyalah variasi

terwariskan yang dapat memenuhi tujuan ini. Ada dua bentuk variasi yaitu:

a. Variasi tak terputusBanyak sifat yang ditemukan dalam suatu populasi tumbuhan atau hewan bervariasi dalam suatu cara yang tak terputus dan halus dari satu ekstrem ke ekstrem yang lain. Berat tubuh, panjang tubuh, dan warna bulu, baru tiga sifat yang dari padanya kita harapkan dapat ditemukan variasi yang sangat banyak.b. Variasi terputusUntuk sifat-sifat tertentu, individu-individu dari beberapa populasi dapat dibagi dalam dua kelompok yang jelas tanpa adanya kelompok antara. Fakta bahwa semua manusia mempunyai satu dari empat golongan darah ABO merupakan contoh variasi yang terputus-putus. Variasi seperti ini disebut polimorfisme.

Baik variasi yang terputus maupun yang tak terputus dapat menjadi bahan baku evolusi hanya saja jika timbulnya disebabkan oleh factor-faktor menurun dan bukan karena factor lingkungan. Sifat ini dikatakan mempunyai daya temurun (heretabilitas) yang tinggi. Sebaliknya jika

keturunannya mampunyai kisaran yang sama dengan rata-rata populasi, maka hanya factor lingkungan yang bekerja. Sifat seperti ini dikatakan mampunyai heritabilitas nol.

Fenotipe suatu individu organisme dihasilkan dari genotipe dan pengaruh

lingkungan organisme tersebut. Variasi fenotipe yang substansial pada

sebuah populasi diakibatkan oleh perbedaan genotipenya. Frekuensi alel

tertentu akan berfluktuasi, menjadi lebih umum atau kurang umum relatif

terhadap bentuk lain gen itu. Gaya dorong evolusioner bekerja dengan

mendorong perubahan pada frekuensi alel ini ke satu arah atau lainnya.

Variasi menghilang ketika sebuah alel mencapai titik fiksasi, yakni ketika ia

menghilang dari suatu populasi ataupun ia telah menggantikan keseluruhan

alel leluhur.

Variasi berasal dari mutasi bahan genetika, migrasi antar populasi (aliran

gen), dan perubahan susunan gen melalui reproduksi seksual. Variasi juga

datang dari tukar ganti gen antara spesies yang berbeda; contohnya melalui

transfer gen horizontal pada bakteria dan hibridisasi pada tanaman.

Walaupun terdapat variasi yang terjadi secara terus menerus melalui proses-

proses ini, kebanyakan genom spesies adalah identik pada seluruh individu

spesies tersebut. Namun, bahkan perubahan kecil pada genotipe dapat

mengakibatkan perubahan yang dramatis pada fenotipenya. Misalnya

simpanse dan manusia hanya berbeda pada 5% genomnya.

Tidak ada spesies hidup yang tidak dapat mengisi penuh habitat dengan jenisnya jika tersedia makanan yang yang tak terbatas dan factor lain dalam lingkungannya itu menguntungkan. Beberapa akan melaksanakannya lebih cepat dari yang lain. Nyatanya setiap spesies yang dalam masa hidupnya menghasilkan lebih dari dua keturunan yang hidup, akan bertambah jumlahnya kecuali jika kelebihan itu mati sebelum memberi keturunan. Skala waktu berbeda untuk berbagai spesies, tetapi potensi untuk pertumbuhan eksponensial ada pada semua spesies. Ketentuan yang mengendalikan pertumbuhan populasi yang eksponensial adalah berupa sejumlah factor yang bekerja dan bersamaan yang oleh Darwin disebut seleksi ilmiah. Teori seleksi alamiah Darwin merupakan kesimpulan berdasarkan tiga factor alam yang dapat diamati dan dari suatu kesimpulan pendahuluan.

Fakta 1 : Semua spesies mempunyai potensi reproduksi yang tinggi. Dari bakteri sampai gajah, mempunyai kemampuan untuk memenuhi dunia ini dengan jenisnya.

Fakta 2 : Kecuali fluktuasi yang kecil, populasi spesies apapun dari tahun ke tahun agak konstan.Kesimpulan 1 : Karena itu, kita harus menyimpulkan bahwa semua mahluk menghadapi perjuangan untuk eksistensi yang terus menerus, yaitu perjuangan yang banyak diantara mereka akan mati muda.Fakta 3 : Terdapat variasi yang diturunkan diantara individu tiap spesies.Kesimpulan 2 : Karena itu, kita dapat menyimpulkan bahwa individu yang variasinya paling cocok untuk lingkungannya mempunyai kemungkinan besar untuk bertahan hidup.

Variasi individu dalam populasi dapat terjadi melalui :-          Persilangan antar individu dalam populasi tersebut-          Perubahan lingkungan yang menyebabkan mutasi bagi individu yang hidup

di dalamnya

Variasi genetikRekombinasi gen-gen yang dapat terjadi melalui perkawinan antara individu-individu yang fertil sebab dalam perkawinan terjadi penyatuan antara gamet janan dan betina yang masing-masing memiliki susunan genetik yang berbeda.

2.     Seleksi AlamiahSeleksi alam adalah proses di mana mutasi genetika yang meningkatkan

keberlangsungan dan reproduksi suatu organisme menjadi (dan tetap) lebih

umum dari generasi yang satu ke generasi yang lain pada sebuah populasi.

Ia sering disebut sebagai mekanisme yang "terbukti sendiri" karena:

Variasi terwariskan terdapat dalam populasi organisme.

Organisme menghasilkan keturunan lebih dari yang dapat bertahan

hidup

Keturunan-keturunan ini bervariasi dalam kemampuannya bertahan

hidup dan bereproduksi.

Salah satu kecaman terhadap seleksi alamiah Darwin adalah bahwa teorinya tidak dapat menjelaskan bagaimana sifat-sifat yang diinginkan diwariskan dari generasi-kegenerasi yang berikutnya. Dengan kata lain, Darwin tidak dapat menjelaskan mekanisme pewarisan sifat.

Seleksi alamiah dapat dipengaruhi penyebaran fenotipe dalam satu populasi dengan 3 cara :

1.      Seleksi pemantap ( stabilizing selection ) merupakan seleksi terhadap nilai-nilai ektrem, menyebabkan penurunan

variasi di sekitar nilai rata-rata. Hal ini dapat menyebabkan organisme

secara pelahan memiliki tinggi badan yang sama. Disini individu-individu

pada setiap ekstrem akan disingkirkan, disamping meningkatkan

keberhasilan reproduksi fenotif yang mendekati nilai rata-rata.

2. Seleksi berarah ( directional selection )

merupakan geseran nilai rata-rata sifat dalam selang waktu tertentu, misalnya organisme cenderung menjadi lebih tinggi. Suatu populasi mungkin dapat berada dalam keadaan dimana individu-individu yang menempati suatu ekstrem dari kisaran fenotif lebih disukai dari pada yang lain-lain. Hal ini dapat terjadi sebagai akibat perubahan pada lingkungan fisiknya.

3. Seleksi pemutus ( disruptive selection )

merupakan seleksi nilai ekstrem, suatu keadaan tertentu dimana individu pada kedua ekstrem dari kisaran fenotifnya lebih sesuai daripada yang terdapat di tengah-tengah, dan sering mengakibatkan dua nilai yang berbeda menjadi lebih umum (dengan menyeleksi keluar nilai rata-rata). Hal ini terjadi apabila baik organisme yang pendek ataupun panjang menguntungkan, sedangkan organisme dengan tinggi menengah tidak. Seleksi diskruptif dapat menghasilkan terpecahnya lungkang (pool) gen tunggal menjadi dua lungkang gen yang yang berbeda. Hal ini dapat merupakan satu cara pembentukan spesies baru.

Spesiasi adalah pembentukan satu atau lebih spesies turunan dari satu spesies moyang. Pada umumnya spesiasi mensyaratkan bahwa spesies moyang harus terpisah menjadi 2 atau lebih sub populasi yang terpisah secara geografik, padanya seleksi alamiah dan penyimpangan bekerja secara berbeda untuk membentuk lingkang gen yang berbeda. Kalaupun dua spesies yang masih tingkat permulaan ini bersatu kembali, mereka akan saling berkawin lagi dan membentuk lagi satu gen. ada kemungkinan satu atau lebih mekanisme isolasi akan mencegah berhasilnya saling berkawinan. Dalam hal demikian, persaingan yang hebat anatara mereka mungkin akan menimbulkan seleksi direksional yang akan mengurangi kehebatan persaingan tersebut. Akhirnya, akan tiba suatu saat ketika kedua spesies baru tersebut secara genetic tak mampu lagi melaksanakan kawin silang dengan sukses. Spesiasipun telah selesai.

Kasus khusus seleksi alam adalah seleksi seksual, yang merupakan

seleksi untuk sifat-sifat yang meningkatkan keberhasilan perkawinan dengan

meningkatkan daya tarik suatu organisme. Sifat-sifat yang berevolusi melalui

seleksi seksual utamanya terdapat pada pejantan beberapa spesies hewan.

Walaupun sifat ini dapat menurunkan keberlangsungan hidup individu jantan

tersebut (misalnya pada tanduk rusa yang besar dan warna yang cerah

dapat menarik predator), Ketidakuntungan keberlangsungan hidup ini

diseimbangkan oleh keberhasilan reproduksi yang lebih tinggi pada

penjantan.

3.     Ukuran “Kemampuan”

Ukuran “kemampuan” adalah adaptasi yang dilakukan untuk bertahan hidup dengan mengubah morfologi tubuh dan juga kemampuan dalam menghasilkan keturunan dewasa. Individu dalam suatu populasi yang dapat menghasilkan keturunan dewasa dalam jumlah yang paling besar, itulah yang disebut paling mampu.

Dalam hal ini beberapa mekanisme memegang peranan:1. Kelangsungan HidupJika tiap sifat yang dapat meningkatkan kesempatan organisme untuk

tetap hidup pada dan melalui periode reproduksi ini membuat organisme itu lebih mampu dari pada individu lain dalam spesies itu. Kita namakan sifat-sifat itu adaptasi. Sifat ini dapat menyangkut perubahan dalam struktur tubuh, perubahan dalam fisiologi, perubahan dalam prilaku, atau perubahan semua sifat tersebut.

2. Seleksi SeksualTiap sifat yang diwariskan, yang membuat beberapa individu lebih

menarik bagi lawan jenis dari pada individu lain, akan menyebabkan mereka lebih berhasil dalam perkawinan. Akibatnya dalam generasi berikutnya proporsi sifat yang diturunkan akan lebih besar. Penelitian mengenai beberapa sifat yang menyangkut seleksi seksual mengungkapkan bahwa dampak evolusi seleksi seksual tidak selamanya searah dengan seleksi ketahanan hidup individu. Sifat-sifat yang terseleksi secara seksual, dan akan terus meningkat pada generasi-generasi berikutnya, mungkin dapat menyebabkan hewan itu kurang mampu teradaptasi dengan lingkungannya.

3. Ukuran KeluargaSetiap sifat yang menyebabkan hewan mempunyai keturunan dewasa

dalam jumlah besar juga merupakan ukuran kemampuan. Perlu ada

penekanan pada keturunan dewasa karena hanya mereka yang dapat mewariskan sifat-sifat itu pada generasi berikutnya.

4.     Genetika sumber variabilitasVariabilitas adalah keanekaragaman secara genetis sebenarnya dua

tanaman tidak akan persis sama, meskipun bisa digolongkan dalam satu golongan sampai batas tertentu misalnya kultivar, varietas , spesies, general, family dst.

Variabilitas suatu populasi bisa di bagi atas variabilitas fenotopik dan variabilitas genotipik. Variabilitas fenotopik dapat diukur atau dilihat langsung untuk keanekaragaman tertentu sedangkan variabilitas genotipik tidak dapat dilihat atau diukur secara langsung sehingga harus diduga melalui analisis melalui statisktik.

Sumber-sumber variabilitas genetik-      

Plasma nutfah : Merupakan sumber genetik dalam pemuliaan tanaman untuk mengembangkan suatu kultivar baru Introduksi : Kegiatan memasukan genotip tanaman dari tempat lain yang dapat digunakan untuk program pemuliaan Mutan: Mutan hasil mutasi yang didefinisikan sebagai perubahan yang mendadak pada bahan genetik dari sel

Poliploid : Tanaman hasil penggandaan yang mempunyai lebih dari set genomHibridisasi: Hibridisasi iini sangat efektif untuk menggabungkan karakter yang baik dari dua atau lebih tanaman yang bebeda genotipnya,

Evolusi organisme terjadi melalui perubahan pada sifat-sifat yang

terwariskan. Warna mata pada manusia, sebagai contohnya, merupakan

sifat-sifat yang terwariskan ini. Sifat terwariskan dikontrol oleh gen dan

keseluruhan gen dalam suatu genom organisme disebut sebagai genotipe.

Keseluruhan sifat-sifat yang terpantau pada perilaku dan struktur

organisme disebut sebagai fenotipe. Sifat-sifat ini berasal dari interaksi

genotipe dengan lingkungan. Oleh karena itu, tidak setiap aspek fenotipe

organisme diwariskan. Kulit berwarna gelap yang dihasilkan dari penjemuran

matahari berasal dari interaksi antara genotipe seseorang dengan cahaya

matahari; sehingga warna kulit gelap ini tidak akan diwarisi ke keturunan

orang tersebut. Walaupun begitu, manusia memiliki respon yang berbeda

terhadap cahaya matahari, dan ini diakibatkan oleh perbedaan pada

genotipenya. Contohnya adalah individu dengan sifat albino yang kulitnya

tidak akan menggelap dan sangat sensitif terhadap sengatan matahari.

Sifat-sifat terwariskan diwariskan antar generasi via DNA, sebuah molekul

yang dapat menyimpan informasi genetika. DNA merupakan sebuah polimer

yang terdiri dari empat jenis basa nukleotida. Urutan basa pada molekul DNA

tertentu menentukan informasi genetika. Bagian molekul DNA yang

menentukan sebuah satuan fungsional disebut gen; gen yang berbeda

mempunyai urutan basa yang berbeda. Dalam sel, unting DNA yang panjang

berasosiasi dengan protein, membentuk struktur padat yang disebut

kromosom. Lokasi spesifik pada sebuah kromosom dikenal sebagai lokus.

Jika urutan DNA pada sebuah lokus bervariasi antar individu, bentuk berbeda

pada urutan ini disebut sebagai alel. Urutan DNA dapat berubah melalui

mutasi, menghasilkan alel yang baru. Jika mutasi terjadi pada gen, alel yang

baru dapat mempengaruhi sifat individu yang dikontrol oleh gen,

menyebabkan perubahan fenotipe organisme. Walaupun demikian,

manakala contoh ini menunjukkan bagaimana alel dan sifat bekerja pada

beberapa kasus, kebanyakan sifat lebih kompleks dan dikontrol oleh

interaksi banyak gen.

Mutasi

Variasi genetika berasal dari mutasi acak yang terjadi pada genom

organisme. Mutasi merupakan perubahan pada urutan DNA sel genom dan

diakibatkan oleh radiasi, virus, transposon, bahan kimia mutagenik, serta

kesalahan selama proses meiosis ataupun replikasi DNA. Mutagen-mutagen

ini menghasilkan beberapa jenis perubahan pada urutan DNA. Hal ini dapat

mengakibatkan perubahan produk gen, mencegah gen berfungsi, ataupun

tidak menghasilkan efek sama sekali.

Gen dihasilkan oleh beberapa metode, umumnya melalui duplikasi dan

mutasi gen leluhur ataupun dengan merekombinasi bagian gen yang

berbeda, membentuk kombinasi baru dengan fungsi yang baru. Keuntungan

duplikasi gen (atau bahkan keseluruhan genom) adalah bahwa tumpang

tindih atau fungsi berlebih pada gen ganda mengijinkan alel-alel

dipertahankan (jika tidak akan membahayakan), sehingga meningkatkan

keanekaragaman genetika.

Perubahan pada bilangan kromosom dapat melibatkan mutasi yang

bahkan lebih besar, dengan segmen DNA dalam kromosom terputus

kemudian tersusun kembali. Sebagai contoh, dua kromosom pada genus

Homo bersatu membentuk kromosom 2 manusia; pernyatuan ini tidak terjadi

pada garis keturunan kera lainnya, dan tetap dipertahankan sebagai dua

kromosom terpisah. Peran paling penting penataan ulang kromosom ini pada

evolusi kemungkinan adalah untuk mempercepat divergensi populasi

menjadi spesies baru dengan membuat populasi tidak saling berkembang

biak, sehingga mempertahankan perbedaan genetika antara populasi ini.

Urutan DNA yang dapat berpindah pada genom, seperti transposon,

merupakan bagian utama pada bahan genetika tanaman dan hewan, dan

dapat memiliki peran penting pada evolusi genom. Efek lain dari urutan DNA

yang bergerak ini adalah ketika ia berpindah dalam suatu genom, ia dapat

memutasikan atau mendelesi gen yang telah ada, sehingga menghasilkan

keanekaragaman genetika.

5.     Hukum Hardy-Weinberg

Asas Hardy-Weinberg untuk dua alel: sumbu horizontal menunjukkan

frekuensi alel p dan q, sedangkan sumbu vertikal menunjukkan frekuensi

genotipe. Tiap-tiap kurva menampilkan satu dari tiga genotipe yang

memungkinkan.

G.H. Hardy seorang ahli matematik inggris dan W. Weinberg seorang dokter

Jerman yang merumuskan hukumnya sebagai berikut :  Frekuensi alel dari

satu gen akan tetap dari generasi ke generasi bila memenuhi syarat (Di

dalam suatu kondisi tertentu frekuensi gen dan frekuensi genotip akan tetap

sama atau konstan dari satu generasi kegenerasi lain dalam populasi yang

berbiak secara seksual) 

Syarat       :-          Tidak ada mutasi (mutasi ke dua arah adalah seimbang)-          Tidak ada seleksi natural (kemampuan yang sama dalam reproduksi)-          Populasi Besar (sehingga suatu perubahan yang tidak mengubah frekuensi

gen secara berarti)-          Perkawinan acak-          Tidak terjadi migrasi ke atau dari populasi.

Rumus       :-          Alel tunggal

p²+2pq+q²=1-          Alel ganda

(p+q+r)² =1

Teori           :Frekuensi genotip AA, Aa, dan aa, serta perbandingan gen A dan a selalu sama dari generasi ke generasi

1.      AA, Aa, dan aa mempunyai viabilitas dan fertilitas yang sama2.      Perkawinan antara genotip satu dengan genotip lainnya berlangsung secara

acak3.      Kemungkinan terjadinya mutasi A ke a dan sebaliknya sama besar

Jika kita meninjau apa yang dimaksud oleh Hardy dan Weinberg dengan hukumnya itu. Kita ambil contoh, misalnya suatu populasi mempunyai frekuensi gen A = p dan a = q. maka karena pada kromosom homolog hanya terdapat sepasang locus untuk A dan a, maka p + q = 1. jadi spermatozoa yang dihasilkan oleh pria dalam populasi itu sebanyak p akan mengandung A dan sebanyak q akan mengandung gen a. demikian juga perbandingan sel telur yang mengandung A = p dan yang mengandung a = q.

Peleburan antara sel telur dan sperma dalam gen pool populasi itu adalah sebagai berikut:

Untuk membuktikan bahwa perbandingan ratio p² (AA) : 2 pq (Aa) : q²(aa) benar-benar dalam keseimbangan, maka kita lihat bagaimana perbandingan

baru gamet-gamet A dan a yang akan dihasilkan oleh generasi ini. Individu-individu AA akan menghasilkan gamet-gamet A saja. Individu-individu Aa akan menghasilkan ½ gamet A dan ½ gamet a. individu –individu aa akan menghasilkan gamet-gamet a saja.Jadi perbandingan gamet-gamet setelah satu generasi lewat menjadi :Gamet A ialah p² + ½ (2pq) = p (p+q) = pGamet a ialah q² + ½ (2pq) = q (p+q) = q

6.     Seleksi KerabatSeleksi Kerabat adalah seleksi yang terjadi pada satu spesies dengan

fenotip yang berbeda yang menyebabkan salah satunya bertahan dan yang lainnya akan punah.

Kondisi-kondisi ini menghasilkan kompetisi antar organisme untuk

bertahan hidup dan bereproduksi. Oleh sebab itu, organisme dengan sifat-

sifat yang lebih menguntungkan akan lebih berkemungkinan mewariskan

sifatnya, sedangkan yang tidak menguntungkan cenderung tidak akan

diwariskan ke generasi selanjutnya.

Skenario "evolusi manusia" ternyata sama sekali rekaan. Agar pohon kekerabatan semacam ini ada, evolusi perlahan dan bertahap dari kera ke manusia haruslah terjadi dan catatan fosil dari proses ini haruslah pernah ada. Tetapi, terdapat jurang sangat lebar yang memisahkan kera dari manusia. Struktur rangka, volume otak, dan ciri lain seperti berjalan tegak atau membungkuk rendah ke depan adalah hal-hal yang membedakan manusia dari kera.

Penemuan penting lain yang membuktikan pohon kekerabatan seperti ini

tidaklah mungkin terjadi di antara spesies-spesies yang berbeda ini adalah

kenyataan bahwa spesies yang dinyatakan sebagai nenek moyang bagi

spesies yang lain ternyata hidup pada saat yang bersamaan. Jika,

sebagaimana pernyataan evolusionis, australopithecines berubah

menjadi Homo habilis dan bila mereka pada gilirannya berubah

menjadi Homo erectus, maka zaman di mana mereka hidup sudah

seharusnya saling berurutan. Tetapi, pada kenyataannya tidak terdapat

urutan kronologis seperti ini.

Berbagai penemuan ini menunjukkan bahwa makhluk yang dikemukakan sebagai "nenek moyang manusia" oleh teori evolusi ternyata adalah spesies-spesies punah yang sama sekali tidak berhubungan satu dengan yang lain atau ras-ras manusia yang hilang.

7.     Pengaruh seleksi pada populasiMenurut Teori Darwin      : Evolusi disebabkan terutama oleh adanya Seleksi alam yang                                                            terjadi pada  Populasi

Seleksi yaitu kondisi atau tindakan yang mengakibatkan genotipe tertentu bertahan dalam populasi sedangkan genotipe lainnya tersingkirkan.

Seleksi merupakan kekuatan utama yang dapat menimbulkan perubahan frekuensi alel dalam populasi. Pengaruh seleksi dapat diukur dengan membandingkan jumlah individu sebelum dan sesudah seleksi. Jadi pengaruhnya pada populasi karena adanya persaingan hidup di bumi yang mana harus berjuang untuk mempertahankan hidupnya dari perubahan-perubahan yang terjadi di bumi akibat adanya perubahan struktur bumi. Jika suatu organisme atau makhluk hidup tidak dapat mempertahankan hidupnya, maka organisme tersebut akan punah. Bagi makhluk hidup yang mampu untuk beradaptasi dengan lingkungan maka makhluk hidup tersebutlah yang dapat bertahan hidup melanjutkan kelangsungan hidupnya.Pengaruhnya terhadap populasi adalah :

         Segala jenis kehidupan di dunia ini tidak ada yang sama       Dalam satu keturunan, antara individu satu dengan lainnya

menunjukkan suatu variasi. Dimana variasi ini menyebabkan setiap individu memiliki kekuatan yang berbeda-beda untuk bertahan terhadap Seleksi alam.

         Perkembangan populasi suatu organism dibatasi oleh adanya ruang dan makanan

     Populasi memiliki daya reproduksi yang tinggi, tetapi dengan adanya persaingan memperebutkan tempat tinggal dan ketersediaan makanan, menyebabkan timbulnya seleksi pada populasi. Siapa yang kuat, dia yang akan bertahan.

     Semakin banyak seleksi yang terjadi maka semakin beragam populasi yang ada.

         Semakin banyak populasi yang ada maka keanekaragaman suatu populasi akan semakin beragam.

         Selama populasi masih terus mangalami seleksi, maka evolusi akan terus terjadi.

   Punahnya berbagai macam populasi mempengaruhi keanekaragaman variasi yang ada pada suatu populasi.

Evidence of Evolution

The Nobel Prize winning scientist Linus Pauling aptly described science as the search for truth.  Science does this by continuously comparing its theories objectively with evidence in the natural world.  When theories no longer conform to the evidence, they are modified or rejected in favor of

new theories that do conform.  In other words, science constantly tries to prove its assumptions to be false and rejects implausible explanations.  In this way, scientific knowledge and understanding grow over time.  Religious explanations for the order of things are not science because they are based primarily on faith and do not subject themselves to be objectively falsified.  Because of this fundamental difference in the approach to understanding our natural world, the U.S. Supreme Court in effect decided in 1987 that the Biblically based "creation science" is not a science and cannot be taught as such in public schools as an alternative or in addition to the mainstream evolutionary theory of the biological sciences.  However, religious creation stories and the idea of "intelligent design" can be taught in philosophy, religion, or history courses.  Religion and Science provide different approaches to knowledge.  It is important to understand both.

What Is Evolution? Biological evolution is genetic change in a population from one generation

to another.  The speed and direction of change is variable with different species lines and at different times.  Continuous evolution over many generations can result in the development of new varieties and species.  Likewise, failure to evolve in response to environmental changes can, and often does, lead to extinction.

When scientists speak of evolution as a theory they do not mean that it is a mere speculation.  It is a theory in the same sense as the propositions that the earth is round rather than flat or that our bodies are made of atoms are theories.  Most people would consider such fundamental theories to be sufficiently tested by empirical evidence to conclude that they are indeed facts.  As a result of the massive amount of evidence for biological evolution accumulated over the last two centuries, we can safely conclude that evolution has occurred and continues to occur.  All life forms, including humans, evolved from earlier species, and all still living species of organisms continue to evolve today.  They are not unchanging end-products.

For those who have difficulty in accepting evolution because of what they perceive as contradictions with their fundamental religious beliefs, it may be useful to distinguish the ultimate origin of life from its later evolution.  Many, if not most, biological scientists accept that primordial life on earth began as a result of chance natural occurrences 3.5-4 billion years ago.  However, it is not necessary to believe in that view in order to accept that living creatures evolved by natural means after the origin of the first life.  Charles Darwin modified his religious beliefs, as did many others, as a result of the discovery of convincing proof of evolution.  Darwin's religious faith was also severely challenged by the death of his 10 year old daughter Annie in 1851.  Apparently, he came to believe that his God created the order of the universe including the rules of nature that result in biological evolution.  His famous book, On the Origin of Species, was not

a denial of his God's existence.  However, he did reject a literal interpretation of the Judeo-Christian Bible.  His religious beliefs were probably very similar to those who advocate "theistic evolution" today.

  Isn't Evolution Just a Theory--video clip from PBS 2001 series Evolution.    This       link takes you to an audio file at an external website.  To return here, you must click the       "back" button on your browser program.           (length = 6 mins, 15 secs)

  Darwin's Personal Struggles--an interview with Darwin's biographer, James Moore.  This       link takes you to an audio file at an external website.  To return here, you must click the       "back" button on your browser program.           (length = 7 mins, 38 secs)

  Evolution of the Eye--an explanation by zoologist Dan-Erik Nilsson.  This link takes        you to a video at an external website.  To return here, you must click the "back"        button on your browser program.                    (length = 4 mins, 8 secs) 

 

We now understand that there are a number of different natural processes that can cause evolution to occur.  These are presented in a later tutorial of this series (Modern Theories of Evolution).

How Do We Know That Evolution Has Occurred?

The evidence for evolution has primarily come from four sources:

1.  the fossil record of change in earlier species2. the chemical and anatomical similarities of

related life forms3. the geographic distribution of related species4.  the recorded genetic changes in living organisms

over many generations

The Fossil Record Remains of animals and plants found in sedimentary

rock deposits give us an indisputable record of past changes through vast periods of time.  This evidence attests to the fact that there has been a tremendous variety of living things.  Some extinct species had traits that were transitional between major groups of organisms.  Their existence confirms that species are not fixed but can evolve into other species over time.

 

Geological strata containing an evolutionary sequence of fossils

The evidence also shows that what have appeared to be gaps in the fossil record are due to incomplete data collection.  The more that we learn about the evolution of specific species lines, the more that these so-called gaps or "missing links in the chain of evolution" are filled with transitional fossil specimens.  One of the first of these gaps to be filled was between small bipedal dinosaurs and birds.  Just two years after Darwin published On the

Origin of Species, a 150-145 million year old fossil of Archaeopteryx was found in southern Germany.  It had jaws with teeth and a long bony tail like dinosaurs, broad wings and feathers like birds, and skeletal features of both.  This discovery verified the assumption that birds had reptilian ancestors. 

   

Archaeopteryx fossil   Archaeopteryx recreation   Archaeopteryx tail feathers

Since the discovery of Archaeopteryx, there have been many other crucial evolutionary gaps filled in the fossil record.  Perhaps, the most important one, from our human perspective, was that between apes and our own species.  Since the 1920's, there have been literally hundreds of well-dated intermediate fossils found in Africa that were transitional species leading from apes to humans over the last 6-7 million years.  This evidence is presented in the last 3 tutorials of this series. 

  Transitional Tetrapod Fossil--another example of filling what had been an evolutionary gap.        This link takes you to a QuickTime video.  To return here, you must click the "back" button        on your browser program.                     (length = 3 mins, 53 secs)

 

The fossil record also provides abundant evidence that the complex animals and plants of today were preceded by earlier simple ones.  In addition, it shows that multicelled organisms evolved only after the first single-celled ones.  This fits the predictions of evolutionary theory.

Chemical and Anatomical Similarities Living things on earth are fundamentally similar in the way that their basic

anatomical structures develop and in their chemical compositions.  No

matter whether they are simple single-celled protozoa or highly complex organisms with billions of cells, they all begin as single cells that

reproduce themselves by similar division processes.  After a limited life span, they also all grow old and die. 

All living things on earth share the ability to create complex molecules out of carbon and a few other elements.  In fact, 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements.  This is not a mere coincidence.

All plants and animals receive their specific characteristics from their parents by inheriting particular combinations of genes.  Molecular

biologists have discovered that genes are, in fact, segments of DNA molecules in our cells.

section of a DNA molecule

These segments of DNA contain chemically coded recipes for creating

proteins by linking together particular amino acids in specific sequences.

simple protein molecule

All of the tens of thousands of types of proteins in living things are mostly made of only 20 kinds of amino acids.  Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things.  This is evidence of the fundamental molecular unity of life.

In addition to molecular similarities, most living things are alike in that they either get the energy needed for

growth, repair, and reproduction directly from sunlight, by photosynthesis , or they get it indirectly by consuming green plants and other organisms that eat plants.

Many groups of species share the same types of body structures because they inherited them from a common ancestor that had them.  This is the case with the

vertebrates , which are the animals that have internal skeletons.  The arms of humans, the forelegs of dogs and cats, the wings of birds, and the flippers of whales and seals all have the same types of bones (humerus, radius, and ulna) because they have retained these traits of their shared common ancient vertebrate ancestor.

 

   Human arm bones

(typical vertebrate pattern)

All of these major chemical and anatomical similarities between living things can be most logically accounted for by assuming that they either share a common ancestry or they came into existence as a result of similar natural processes.  These facts make it difficult to accept a theory of special and independent creation of different species.

Geographic Distribution of Related Species Another clue to patterns of past evolution is found in the natural geographic

distribution of related species.  It is clear that major isolated land areas and island groups often evolved their own distinct plant and animal communities.  For instance, before humans arrived 60-40,000 years ago, Australia had more than

100 species of kangaroos, koalas, and other marsupials but none of the more

advanced terrestrial placental mammals such as dogs, cats, bears, horses.  Land mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand.  Each of these places had a great number of plant, insect, and bird species that were found nowhere else in the world.  The most likely explanation for the existence of Australia's, New Zealand's, and Hawaii's mostly unique biotic environments is that the life forms in these areas have been evolving in isolation from the rest of the world for millions of years.

Genetic Changes Over Generations The earth's environments are constantly changing, usually in subtle and complex

ways.  When the changes are so great as to go beyond what most members of a population of organisms can tolerate, widespread death occurs.  As Charles Darwin observed, however, not all individuals always perish.  Fortunately, natural populations have genetic diversity.  Those individuals whose characteristics allow them to survive an environmental crisis likely will be the only ones able to reproduce.   Subsequently, their traits will be more common in the next generation--evolution of the population will have occurred.

This process of natural selection resulting in evolution can be easily demonstrated over a 24 hour period in a laboratory Petri dish of bacteria living in a nutrient medium.  When a lethal dose of antibiotic is added, there will be a mass die-off.  However, a few of the bacteria usually are immune and survive.  The next generation is mostly immune because they have inherited immunity from the survivors.  That is the case with the purple bacteria in the Petri dishes shown below--the bacteria population has evolved. 

Evolution of antibiotic resistant bacteria   

This same phenomenon of bacteria evolution speeded up by human actions occurs in our own bodies at times when an antibiotic drug is unable to completely eliminate a bacterial infection.  That is the reason that medical doctors are sometimes hesitant to recommend an antibiotic for their patients and insist that the full dosage be used even if the symptoms of illness go away.  They do not want to allow any potentially antibiotic resistant bacteria to survive.

  Antibiotic resistance--how mutation and fast reproductive rates of        microorganisms can outpace modern medical breakthroughs.  This       link takes you to an external website.  To return here, you must click       the "back" button on your browser program. 

People have developed many new varieties of plants and animals by selective breeding.  This process is similar to the bacteria experiment described above.  Selection of specimens to breed based on particular traits is, in effect, changing the environment for the population.  Those individuals lacking the desirable characteristics are not allowed to breed.  Therefore, the following generations more commonly have the desired traits.

Species that mature and reproduce large numbers in a short amount of time have a potential for very fast evolutionary changes.   Insects and microorganisms often evolve at such rapid rates that our actions to combat them quickly lose their effectiveness.  We must constantly develop new pesticides, antibiotics, and other measures in an ever escalating biological arms race with these creatures.   Unfortunately, there are a few kinds of insects and microbes that are now significantly or completely resistant to our counter measures, and some of these species are responsible for devastating crop losses and deadly diseases.

If evolution has occurred, there should be many anatomical similarities among varieties and species that have diverged from a common ancestor.  Those species with the most recent common ancestor should share the most traits.  For instance, the many anatomical similarities of wolves, dogs, and other members of the genus Canis are due to the fact that they are descended from the same ancient canine species and still share 99.8% of their genes.  Wolves and dogs also share similarities with foxes, indicating a slightly more distant ancestor with them.

 

 

Dog variety resultingfrom selective breedingover many generations

 

  Insect with a highreproductive potential

  Genetic Tool Kit--evidence of a common set of genes for body parts shared by many, if not        most, creatures.  This link takes you to a video at an external website.  To return, here, you        must click the "back" button on your browser program.        (length = 4 mins, 47 secs) 

Given the abundant evidence supporting the theory of biological evolution, it is highly probable that evolution has occurred and is still occurring today.  However, there remains speculation in regards to the specific evolutionary path of some species lines and the relative importance of the different natural processes responsible for their evolution.

Much has been added to our understanding of the nature of evolution since the 19th century.  It is now known that there are six different processes that can operate independently or in consort to bring about evolution.  The understanding of these processes has become the basis

for an overall synthetic theory of evolution .  This theory encompasses multiple causes, including Charles Darwin's concept of natural selection, Gregor Mendel's experimental results concerning genetic inheritance, as well as a number of crucial 20th century discoveries.  The synthetic theory of evolution will be revisited with more detail in the 6th tutorial of this biological anthropology series.

The Public Perception of Evolution in the United States

Biological evolution is far from being universally accepted by Americans.  Annual national polls carried out since the mid 1980's by the Center for Biomedical Communication at Northwestern University School of Medicine indicate that the percentage of Americans who accept evolution has dropped from 45% to 40%.  Curiously, the number who reject evolution have also dropped from 48% to 39% over the same time period.  Those who are uncertain about whether evolution occurs or not have increased from 7% to 21%.  While it is encouraging that fewer people are now hostile to the idea of biological evolution, the U.S. still has a higher percentage of its population who hold this view than 33 of the 34 European nations and Japan.  This is very likely a consequence of the relative emphasis placed on teaching science in public schools in the different countries.  In

addition, anti-evolution sentiment is far stronger in American national politics.

For the vast majority of biologists, the debate over whether evolution occurs took place in the 19th century and has long been settled--evolution won.  The noted environmental biologist Theodosius Dobzhansky summed it up in 1973 by saying "nothing in biology makes sense except in the light of evolution".

  Evolving Ideas: Why Is Evolution Controversial Anyway--reconciling a belief in science and        a religion.  This link takes you to a video at an external website.  To return here, you must        click the "back" button on your browser program.          (length = 6 mins, 36 secs) 

 

NOTE:   Some critics have said that the kinds of rapid evolutionary changes in insects and bacteria referred to above are not good evidence of the process of natural evolution because they occur as a result of human interference.  However, there is abundant evidence of rapid evolution occurring today independent of people.  An example was described by Cristina Sandoval in the May 23, 2002 issue of Nature.  A species of insect called the "walking stick" (Timena cristinae) found in the Santa Ynez Mountains of California now exists in two distinct varieties or forms that are in the process of evolving into two separate species by adapting to different environments.  The insect forms differ in terms of genetically determined color patterns--one is striped and the other is not.  The striped ones hide from predators on the striped chamise plant, while the unstriped ones hide on the unstriped blue lilac plant.  Those that have inherited the appropriate camouflaging color pattern for their chosen environment survive the onslaught of lizards and birds.  In this case, the natural predators, rather than humans, are the driving forces of natural selection.  Mating experiments show that each variety of "walking stick" prefers to mate only with others having the same color pattern.  This breeding isolation is leading to the evolution of two distinct species.

NEWS:   On July 17, 2005, The Pew Research Center for the People and the Press conducted a national poll in the United States concerning the teaching of creationism and evolution.  In regards to beliefs about how life developed, 42% of the respondents said that "living things have existed in their present form since the beginning of time."  Only 26% said that they had evolved through time as a result of "natural process such as natural selection," while 18% said that evolution occurred but was guided by a supreme being.  In response to the question of whether creationism should be taught in public schools instead of evolution, 38% said yes and 49% said no.  When asked whether creationism should be taught along with evolution, 64% said yes and 26% said not.  The older the respondent, the more likely he/she was to reject evolution and its teaching

in favor of creationism.  The sample consisted of 2,000 people and the margin of error was ±3.5%.

POSTSCRIPT:   For additional information regarding "creation science" and "intelligent design", look at the 2007 PBS Nova documentary and the entertaining 2008 follow-up lecture by Dr. Kenneth Miller linked below.

  Judgment Day: Intelligent Design on Trial--analysis of a controversial 2004 court        case and its consequences.  This link takes you to a video at an external website.        (length = 1 hr 55 mins)

  God, Darwin, and Design: Lessons from the Dover Monkey Trial--a 2008 lecture at        the University of Texas by Kenneth Miller, a noted biologist.  This link takes you to        a video at an external website.  You will be asked to first download the Envivio        plugin in order to view the video.                 (length = 2 hrs 17 mins)

Evolution is a Fact and a Theoryby Laurence Moran

Copyright © 1993-2002

[Last Update: January 22, 1993]

hen non-biologists talk about biological evolution they often confuse two different aspects of the definition. On the one hand there is the question of whether or not modern organisms have evolved from older ancestral organisms or whether modern

species are continuing to change over time. On the other hand there are questions about the mechanism of the observed changes... how did evolution occur? Biologists consider the existence of biological evolution to be a fact. It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming. However, biologists readily admit that they are less certain of the exact mechanism of evolution; there are several theories of the mechanism of evolution. Stephen J. Gould has put this as well as anyone else:

In the American vernacular, "theory" often means "imperfect fact"--part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can't even make up their minds about the theory, then what confidence can we have in it? Indeed, President Reagan echoed this argument before an evangelical group in Dallas when he said (in what I devoutly hope was campaign rhetoric): "Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science--that is, not believed in the scientific community to be as infallible as it once was."

Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to

explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered.

Moreover, "fact" doesn't mean "absolute certainty"; there ain't no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are not about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us falsely for a style of argument that they themselves favor). In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent." I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.

Evolutionists have been very clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory--natural selection--to explain the mechanism of evolution.

- Stephen J. Gould, " Evolution as Fact and Theory"; Discover, May 1981

Gould is stating the prevailing view of the scientific community. In other words, the experts on evolution consider it to be a fact. This is not an idea that originated with Gould as the following quotations indicate:

Let me try to make crystal clear what is established beyond reasonable doubt, and what needs further study, about evolution. Evolution as a process that has always gone on in the history of the earth can be doubted only by those who are ignorant of the evidence or are resistant to evidence, owing to emotional blocks or to plain bigotry. By contrast, the mechanisms that bring evolution about certainly need study and clarification. There are no alternatives to evolution as history that can withstand critical examination. Yet we are constantly learning new and important facts about evolutionary mechanisms.

- Theodosius Dobzhansky "Nothing in Biology Makes Sense Except in the Light of Evolution", American Biology Teacher vol. 35 (March 1973) reprinted in Evolution versus Creationism, J. Peter Zetterberg ed., ORYX Press, Phoenix AZ 1983

Also:

It is time for students of the evolutionary process, especially those who have been misquoted and used by the creationists, to state clearly that evolution is a fact, not theory, and that what is at issue within biology are questions of details of the process and the relative importance of different mechanisms of evolution. It is a fact that the earth with liquid water, is more than 3.6 billion years old. It is a fact that cellular life has been around for at least half of that period and that organized multicellular life is at least 800 million years old. It is a fact that major life forms now on earth were not at all represented in the past. There were no birds or mammals 250 million years ago. It is a fact that major life forms of the past

are no longer living. There used to be dinosaurs and Pithecanthropus, and there are none now. It is a fact that all living forms come from previous living forms. Therefore, all present forms of life arose from ancestral forms that were different. Birds arose from nonbirds and humans from nonhumans. No person who pretends to any understanding of the natural world can deny these facts any more than she or he can deny that the earth is round, rotates on its axis, and revolves around the sun.

The controversies about evolution lie in the realm of the relative importance of various forces in molding evolution.

- R. C. Lewontin "Evolution/Creation Debate: A Time for Truth" Bioscience 31, 559 (1981) reprinted in Evolution versus Creationism, op cit.

This concept is also explained in introductory biology books that are used in colleges and universities (and in some of the better high schools). For example, in some of the best such textbooks we find:

Today, nearly all biologists acknowledge that evolution is a fact. The term theory is no longer appropriate except when referring to the various models that attempt to explain how life evolves... it is important to understand that the current questions about how life evolves in no way implies any disagreement over the fact of evolution.

- Neil A. Campbell, Biology 2nd ed., 1990, Benjamin/Cummings, p. 434

Also:

Since Darwin's time, massive additional evidence has accumulated supporting the fact of evolution--that all living organisms present on earth today have arisen from earlier forms in the course of earth's long history. Indeed, all of modern biology is an affirmation of this relatedness of the many species of living things and of their gradual divergence from one another over the course of time. Since the publication of The Origin of Species, the important question, scientifically speaking, about evolution has not been whether it has taken place. That is no longer an issue among the vast majority of modern biologists. Today, the central and still fascinating questions for biologists concern the mechanisms by which evolution occurs.

- Helena Curtis and N. Sue Barnes, Biology 5th ed. 1989, Worth Publishers, p. 972

One of the best introductory books on evolution (as opposed to introductory biology) is that by Douglas J. Futuyma, and he makes the following comment:

A few words need to be said about the "theory of evolution," which most people take to mean the proposition that organisms have evolved from common ancestors. In everyday speech, "theory" often means a hypothesis or even a mere speculation. But in science, "theory" means "a statement of what are held to be the general laws, principles, or causes of something known or observed." as the Oxford English Dictionary defines it. The theory of evolution is a body of interconnected statements about natural selection and the other processes that are thought to cause evolution, just as the atomic theory of chemistry and the Newtonian theory of mechanics are bodies of statements that describe causes of chemical and physical phenomena. In contrast, the statement that organisms have descended with

modifications from common ancestors--the historical reality of evolution--is not a theory. It is a fact, as fully as the fact of the earth's revolution about the sun. Like the heliocentric solar system, evolution began as a hypothesis, and achieved "facthood" as the evidence in its favor became so strong that no knowledgeable and unbiased person could deny its reality. No biologist today would think of submitting a paper entitled "New evidence for evolution;" it simply has not been an issue for a century.

- Douglas J. Futuyma, Evolutionary Biology, 2nd ed., 1986, Sinauer Associates, p. 15

There are readers of these newsgroups who reject evolution for religious reasons. In general these readers oppose both the fact of evolution and theories of mechanisms, although some anti-evolutionists have come to realize that there is a difference between the two concepts. That is why we see some leading anti-evolutionists admitting to the fact of "microevolution"--they know that evolution can be demonstrated. These readers will not be convinced of the "facthood" of (macro)evolution by any logical argument and it is a waste of time to make the attempt. The best that we can hope for is that they understand the argument that they oppose. Even this simple hope is rarely fulfilled.

There are some readers who are not anti-evolutionist but still claim that evolution is "only" a theory which can't be proven. This group needs to distinguish between the fact that evolution occurs and the theory of the mechanism of evolution.

We also need to distinguish between facts that are easy to demonstrate and those that are more circumstantial. Examples of evolution that are readily apparent include the fact that modern populations are evolving and the fact that two closely related species share a common ancestor. The evidence that Homo sapiens and chimpanzees share a recent common ancestor falls into this category. There is so much evidence in support of this aspect of primate evolution that it qualifies as a fact by any common definition of the word "fact."

In other cases the available evidence is less strong. For example, the relationships of some of the major phyla are still being worked out. Also, the statement that all organisms have descended from a single common ancestor is strongly supported by the available evidence, and there is no opposing evidence. However, it is not yet appropriate to call this a "fact" since there are reasonable alternatives.

Finally, there is an epistemological argument against evolution as fact. Some readers of these newsgroups point out that nothing in science can ever be "proven" and this includes evolution. According to this argument, the probability that evolution is the correct explanation of life as we know it may approach 99.9999...9% but it will never be 100%. Thus evolution cannot be a fact. This kind of argument might be appropriate in a philosophy class (it is essentially correct) but it won't do in the real world. A "fact," as Stephen J. Gould pointed out (see above), means something that is so highly probable that it would be silly not to accept it. This point has also been made by others who contest the nit-picking epistemologists.

The honest scientist, like the philosopher, will tell you that nothing whatever can be or has been proved with fully 100% certainty, not even that you or I exist, nor anyone except himself, since he might be dreaming the whole thing. Thus there is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea.

When we say a thing is a fact, then, we only mean that its probability is an extremely high one: so high that we are not bothered by doubt about it and are ready to act accordingly. Now in this use of the term fact, the only proper one, evolution is a fact. For the evidence in favor of it is as voluminous, diverse, and convincing as in the case of any other well established fact of science concerning the existence of things that cannot be directly seen, such as atoms, neutrons, or solar gravitation ....

So enormous, ramifying, and consistent has the evidence for evolution become that if anyone could now disprove it, I should have my conception of the orderliness of the universe so shaken as to lead me to doubt even my own existence. If you like, then, I will grant you that in an absolute sense evolution is not a fact, or rather, that it is no more a fact than that you are hearing or reading these words.

- H. J. Muller, "One Hundred Years Without Darwin Are Enough" School Science and Mathematics 59, 304-305. (1959) reprinted in Evolution versus Creationism op cit.

In any meaningful sense evolution is a fact, but there are various theories concerning the mechanism of evolution.

What Is Evolution?

Biological evolution is defined as any genetic change in a population that is inherited over several generations. These changes may be small or large, noticeable or not so noticeable.

In order for an event to be considered an instance of evolution, changes have to occur on the genetic level of a population and be passed on from one generation to the next.

This means that the genes , or more specifically, the alleles in the population change and are passed on. These changes are noticed in the AdsInfo Lowongan kerja 2014berniaga.com/Lowongan-KerjaRatusan Ribu Lowongan Tersedia, Temukan Posisi Impianmu Sekarang!Download Videos for Freewww.videoscavenger.comSee & Download Your Favorite Videos With Video Scavenger. It's Free!Referred Academic Journalwww.iiste.orgFast Review, Publishing & Printing International Journal(US Published)

Molecular Biology Creation VS Evolution Cell Isolation Human Biology Darwin

phenotypes (expressed physical traits that can be seen) of the population.

A change on the genetic level of a population is defined as a small-scale change and is called microevolution.

Biological evolution also includes the idea that all of life is connected and can be traced back to one common ancestor. This is called macroevolution.

What Evolution Is Not

Biological evolution is not defined as simply change over time.

Many organisms experience changes over time, such as weight loss or gain. These changes are not considered instances of evolution because they are not genetic changes that can be passed on to the next generation.

Is Evolution a Theory?

Evolution is a scientific theory that was proposed by Charles Darwin. A scientific theory gives explanations and predictions for naturally occurring phenomena based on observations and experimentations. This type of theory attempts to explain how events seen in the natural world work.

The definition of a scientific theory differs from the common meaning of theory, which is defined as a guess or a supposition about a particular process. In contrast, a good scientific theory must be testable, falsifiable, and substantiated by factual evidence. Adsgenes mutation Databasewww.biobase-international.comHuman Inherited Disease Mutations DNA and Protein Sequence DetailsHow to Convert to Islamwww.islamreligion.com/Convert-IslamWhy and how to convert to Islam with Live Help through 1-to-1 chat

When it comes to a scientific theory, there is no absolute proof. It's more a case of confirming the reasonability of accepting a theory as a viable explanation for a particular event.

What Is Natural Selection?

Natural selection is the process by which biological evolutionary changes take place. Natural selection acts on populations and not individuals. It is based on the following concepts:

Individuals in a population have different traits which can be inherited. 

These individuals produce more young than the environment can support. 

The individuals in a population that are best suited to their environment will leave more offspring, resulting in a change in the genetic makeup of a population.

The genetic variations that arise in a population happen by chance, but the process of natural selection does not. Natural selection is the result of the interactions between genetic variations in a population and the environment.

The environment determines which variations are more favorable. Individuals that possess traits that are better suited to their environment will survive to produce more offspring than other individuals. More favorable traits are thereby passed on to the population as a whole.

How Does Genetic Variation Occur in a Population?

Genetic variation occurs mainly through DNA mutation , gene flow (movement of genes from one population to another) and sexual reproduction . Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variations.

Sexual reproduction allows for genetic variations to occur through genetic recombination . Recombination occurs during meiosis and provides a way for producing new combinations of alleles on a single chromosome . Independent assortment during meiosis allows for an indefinite number of combinations of genes.

Sexual reproduction makes it possible to assemble favorable gene combinations in a population or to remove unfavorable gene combinations from a population. Populations with more favorable genetic combinations will survive in their environment and reproduce more offspring than those with less favorable genetic combinations.

Biological Evolution Versus Creation

The theory of evolution has caused controversy from the time of its introduction until today. The controversy stems from the perception that biological evolution is at odds with religion concerning the need for a divine creator. Evolutionists contend that evolution does not address the issue of whether or not God exists, but attempts to explain how natural processes work.

In doing so however, there is no escaping the fact that evolution contradicts certain aspects of some religious beliefs. For example, the evolutionary account for the existence of life and the biblical account of creation are quite different.

Evolution suggests that all life is connected and can be traced back to one common ancestor. A literal interpretation of biblical creation suggests that life was created by an all powerful, supernatural being (God).

Still others have tried to merge these two concepts by contending that evolution does not exclude the possibility of the existence of God, but merely explains the process by which God created life. This view however, still contradicts a literal interpretation of creation as presented in the bible.

In paring down the issue, a major bone of contention between the two views is the concept of macroevolution. For the most part, evolutionists and creationists agree that microevolution does occur and is visible in nature.

Macroevolution however, refers to the process of evolution that takes place on the level of species, in which one species evolves from another species. This is in stark contrast to the biblical view that God was personally involved in the formation and creation of living organisms.

For now, the evolution/creation debate continues on and it appears that the differences between these two views are not likely to be settled any time soon.

Darwin's Theory of Evolution - The PremiseDarwin's Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers -- all related. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival -- a process known as "natural selection." These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).

Darwin's Theory of Evolution - Natural SelectionWhile Darwin's Theory of Evolution is a relatively young archetype, the evolutionary worldview itself is as old as antiquity. Ancient Greek philosophers such as Anaximander postulated the development of life from non-life and the evolutionary descent of man from animal. Charles Darwin simply brought something new to the old philosophy -- a plausible mechanism called "natural selection." Natural selection acts to preserve and accumulate minor advantageous genetic mutations. Suppose a member of a species developed a functional advantage (it grew wings and learned to fly). Its offspring would inherit that advantage and pass it on to their offspring. The inferior (disadvantaged) members of the same species would gradually die out, leaving only the superior (advantaged) members of the species. Natural selection is the preservation of a functional advantage that enables a species to compete better in the wild. Natural selection is the naturalistic equivalent to domestic breeding. Over the centuries, human breeders have produced dramatic changes in domestic animal populations by selecting individuals to breed. Breeders eliminate undesirable traits gradually over time. Similarly, natural selection eliminates inferior species gradually over time.

Darwin's Theory of Evolution - Slowly But Surely...Darwin's Theory of Evolution is a slow gradual process. Darwin wrote, "…Natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps." [1] Thus, Darwin conceded that, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." [2] Such a complex organ would be known as an "irreducibly complex system". An irreducibly complex system is one composed of multiple parts, all of which are necessary for the system to function. If even one part is missing, the entire system will fail to function. Every individual part is integral. [3] Thus, such a system could not have evolved slowly, piece by piece. The common mousetrap is an everyday non-biological example of irreducible complexity. It is composed of five basic parts: a catch (to hold the bait), a powerful spring, a thin rod called "the hammer," a holding bar to secure the hammer in place, and a platform to mount the trap. If any one of these parts is missing, the mechanism will not work. Each individual part is integral. The mousetrap is irreducibly complex. [4]

Darwin's Theory of Evolution - A Theory In CrisisDarwin's Theory of Evolution is a theory in crisis in light of the tremendous advances we've made in molecular biology, biochemistry and genetics over the past fifty years. We now know that there are in fact tens of thousands of irreducibly complex systems on the cellular level. Specified complexity pervades the microscopic biological world. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10-12 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand million atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world." [5]

And we don't need a microscope to observe irreducible complexity. The eye, the ear and the heart are all examples of irreducible complexity, though they were not recognized as such in Darwin's day. Nevertheless, Darwin confessed, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree." [6]

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Footnotes:

1. Charles Darwin, "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life," 1859, p. 162.

2. Ibid. p. 158. 3. Michael Behe, "Darwin's Black Box," 1996. 4. "Unlocking the Mystery of Life," documentary by Illustra Media, 2002. 5. Michael Denton, "Evolution: A Theory in Crisis," 1986, p. 250. 6. Charles Darwin, "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured

Races in the Struggle for Life," 1859, p. 155.