Descent with Modification
Theme:• Evolutionary change is based on the
interactions between populations & their environment which results in adaptations (inherited characteristics) to increase fitness
Evolution = change over time in the genetic composition of a population
Charles Darwin (1809-1882)
•English naturalist•1831: joined the HMS Beagle
for a 5-year research voyage around the world (stopped at: Galapagos Islands)
•1859 – published Origin of Species
•Influenced by Lamarck, Hutton & Lyell, Malthus
Darwin’s Theory of Natural Selection:1. Populations produce more offspring than
can possibly survive.2. Individuals in a population vary extensively
from each other, mostly due to inheritance.3. Struggle to survive: individuals whose
inherited characteristics best fit to environment leave more offspring than less fit.
4. Unequal ability of individuals to survive and reproduce leads to gradual change in pop, with favorable characteristics accumulating over generations.
•Populations evolve, not individuals.•Fitness is determined by the environment.
In summary:Natural Selection = differential success in
reproductionProduct of natural selection = adaptations
of populations to environment
Therefore, if humans can create substantial change over short time, nature can over long time.
Natural Selection Artificial Selection
•Nature decides •“Man” decides
•Works on individual •Selective breeding
•Inbreeding occurs
•i.e. beaks •i.e. dalmations
Evidence for Evolution1. Biogeography
▫ Geographic distribution of a species▫ Geographic, reproductive isolation
2. Fossil Record – transitional forms3. Comparative Anatomy
1. Homologous structures2. Vestigial structures
4. Embryonic Development5. Molecular Biology
▫ DNA, proteins
Gill pouches
Post-anal tail
Chicken embryo
Human embryo
•Darwin’s could not explain how inherited variations are maintained in populations - not “trait blending”
•A few years after Darwin’s “Origin of Species”, Gregor Mendel proposed his hypothesis of inheritance:Parents pass on discrete heritable units
(genes) that retain their identities in offspring
•Frequencies of alleles & genotypes in a population’s gene pool remain constant from generation to generation unless acted upon by agents other than sexual recombination (gene shuffling in meiosis)
•Equilibrium = allele and genotype frequencies remain constant
Allele Frequencies:•Gene with 2 alleles : p, q
p = frequency of allele “A ” in a population
q = frequency of allele “a” in a population Note:
1 – p = q1 – q = p
Genotype Frequencies:•3 genotypes (AA, Aa, aa)
p2 = AA2pq = Aaq2 = aa
1. Extremely large population size (no genetic drift).
2. No gene flow (isolation from other populations).
3. No mutations.4. Random mating (no sexual selection).5. No natural selection.
•If any of the Hardy-Weinberg conditions are not met microevolution occurs
•Microevolution = generation to generation change in a population’s allele frequencies
1. Mutations – changes in DNA Point mutations Gene duplication
Mutation will alter or create
new alleles in a population.
2. Sexual Recombination▫ Rearrange alleles into fresh combinations
every generation
3. Natural selection
Douglas fir trees only release their seeds during fires. Fire rarely occurs in the river bottom of this valley.
4. Genetic drift: a change in a population’s allele frequencies due to chance bottleneck and founder effect
A. Bottleneck Effect – genetic drift due to drastic reduction in population size▫ Certain alleles may be over/under
represented
Northern elephant seals hunted nearly to extinction in
California
B. Founder effect – few individuals become isolated from larger population certain alleles over/under represented
Polydactyly in Amish population
5. Gene flow – genetic exchange due to migration of fertile individuals i.e. wind storm blows pollen to another field Reduces differences between populations Gain/lose alleles
Natural selection can alter frequency distribution of heritable traits in 3 ways:
1.Directional selection2.Disruptive (diversifying) selection3.Stabilizing selection
Directional Selection: eg. beak sizes of birds during wet/dry seasons in Galapagos
Diversifying Selection: eg. small beaks for small seeds; large beaks for large seeds
Stabilizing Selection: eg. average human birth weight
Preserving Genetic Variation•Diploidy: inherit 2 alleles
Recessive alleles less favorable Heterozygote protection
•Heterozyote Advantage: People hybrid for sickle cell anemia protected
against malaria.
•Sexual selection for mating success▫Intra (within same sex) – competition for
mate▫Inter (out) – mate choice
Darwinian Fitness : ability to survive AND reproduce AND pass on alleles to offspring
Sexual selection may lead to pronounced secondary differences between the sexes
• Remember:▫Individuals are selected▫Populations evolve
• Terms:▫Population = localized group belonging to
same species▫Species = members of a population that can
interbreed and produce fertile viable offspring▫Gene pool = total combo of genes in a
population at any one time▫Fixed population = all members are
homozygous for trait (usually not the case)
Speciation = origin of species
•Microevolution: changes within a single gene pool
•Macroevolution: evolutionary change above the species level▫cumulative effects of speciation over long
periods of time
Similarity between different species.
Anagenesis Cladogenesis
Anagenesis (“new” “race”)
• Phyletic evolution• A single species
gradually changes into a different species
• No original group left• Evolution in single
direction
Cladogenesis (“branch” “race”)
• Branching evolution• One species stays same, but
small portion leaves and changes to another species
• Gene pool splits• Original + new groups• Increase in diversity/# of
species
Two Patterns of Evolutionary Change
•Proposed by Ernst Mayr (1942)•Species = population or group of
populations whose members have the potential to interbreed in nature and produce viable, fertile offspring▫Reproductively compatible
•Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids
Prezygotic Barriers:▫Impede
mating/fertilizationTypes:
▫Habitat isolation▫Temporal isolation▫Behavioral isolation▫Mechanical isolation▫Gametic isolation
Postzygotic Barriers:▫Prevent hybrid zygote
from developing into viable adult
Types:▫Reduced hybrid
viability▫Reduced hybrid
fertility▫Hybrid breakdown
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Postzygotic barriers prevent a hybrid zygote fromdeveloping into a viable, fertile adult
REDUCED HYBRIDVIABILITY
REDUCED HYBRIDFERTILITY
HYBRID BREAKDOWN
HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION
Reducedhybrid
viability
FertilizationViable,fertile
offspring
Reducedhybridfertility
Hybridbreakdown
Matingattempt
Gameticisolation
Fertilization
Mechanicalisolation
Behavioralisolation
Temporalisolation
Habitatisolation
Individualsof
differentspecies
Types of Reproductive Barriers
Other definitions of species:
•Morphological – by body shape, size, and other structural features
•Paleontological – fossil record•Ecological – niche/role in community
•Phylogenetic – unique genetic history, branch on tree of life
Allopatric speciation Sympatric speciation
Two main modes of speciation
Two main modes of speciation:Allopatric Speciation
“other” “homeland”
Geographically isolated
Evolves by natural selection & genetic drift
Eg. Galapagos finches
Sympatric Speciation
“same” “homeland”
Overlapping populations within home range
Subset of population isolated from parent pop. change due to:
• chromosomal changes• nonrandom mating• habitat differentiation
Eg. polyploidy in plants (oats, cotton, potatoes, wheat)
•Emergence of numerous species from a common ancestor introduced into new environment
•Occurs when: A few organisms make way to new, distant
areas (allopatric speciation) Environmental change extinctions
new niches for survivors•Eg. Hawaiian archepelago
Founding
Parents
When 2 splintered groups rejoin geographically:
Possibilities:1. Still one species2. Two distinct species (no
interbreeding)3. Hybrid zone
Gradualism• Darwin• Slow, constant
change• Less likely
Punctuated Equilibium• Eldridge & Gould• Long period of minor
change are interrupted by short bursts of significant change
• More likely
Tempo of Evolution
• Independent development of similar features between 2 unrelated species
•Similar environments•Analogous structures•Eg. wings on bees & wings on birds
REMEMBER!!•Dear King Philip Came Over For Good Spaghetti•Dear King Philip Crossed Over Five Great Seas•Dear King Philip Came Over From Germany Stoned•Your own???
Drosophila
Lanc
elet
Fish
Amph
ibia
nBird
Human
RatM
ouse
• Phylogeny = evolutionary history of a species or group of species
Phylogram: the length of a branch reflects the number of changes that have taken place in a particular DNA sequence in that lineage
• Cladogram: diagram of evolutionary relationship of organisms▫Shared characteristics due to common
ancestry▫Uses parsimony – simplest explanation, fewest
DNA base changes for tree (“keep it simple”)
LE 25-11b
Turtle Leopard
Hair
Amniotic egg
Four walking legs
Hinged jaws
Vertebral column
Salamander
Tuna
Lamprey
Lancelet (outgroup)
Cladogram
Comparison of Structures
Homology Analogy
• Results from:▫ Adaptive radiation▫ Common ancestor▫ Similar origin
• Different functions• Eg. wing of bat, human
arm, dolphin flipper
• Results from:▫ Convergent evolution▫ Different ancestors▫ Different origin
• Similar functions• Eg. wings of bird, wings of
insect
Remember:•Adaptive radiation – emergence of many species from common ancestor•Convergent evolution – unrelated species independently evolve similarities when adapting to similar environments
Major events during each Era• Precambrian: microscopic fossils (stromatolites)
▫Photosynthesis, atmospheric O2▫Eukaryotes (endosymbiont theory)
• Paleozoic: Cambrian Explosion▫Plants invade land, many animals appear▫Permian Extinction (-96% species)
• Mesozoic: “Age of Reptiles”, dinosaur, plants▫Formation of Pangaea supercontinent▫Cretaceous Extinction – asteroid off Mexico’s coast
• Cenozoic: primates
Note: All end with major extinction & start with adaptive radiation
Land plants
Animals
Paleozoic
Meso-
zoic
Ceno-zoic
Origin of solarsystem andEarth
41
2 3Multicellulareukaryotes
Single-celledeukaryotes
Prokaryotes
Atmosphericoxygen
ProterozoicEon
ArchaeanEon
Humans
Billions of years ago
Evolution of Plants
•Non-Vascular (liverworts, hornworts, mosses) Seedless Vascular (ferns) Seed Vascular (gymnosperms, angiosperms)
•Mosses: Gametophytes = dominant form•Ferns: 1st with vascular tissue (xylem,
phloem▫wet environment (fertilization in water)▫Sporophyte = dominant form
•Gymnosperms: “naked” seeds on cones▫Conifers
•Angiosperms: flowering plants
Evolution of Animals – Body Plan
Evolution of Animals – Body Cavities
Evolution of Animals – Development
Evolution of Animals
Evolution of Animals•Porifera (sponge)•Cnidarian (jellyfish, hydra)•Flatworms (planaria)•Mollusc
▫Gastropod (snail), bivalve (clams), cephalopod (octopus)
•Annelid (earthworm)•Arthropods (insects, crustaceans)•Echinoderms (“spiny skin” = starfish, sea
urchins)•Chordates (vertebrates)
Chordate Characteristics
Phylogeny of living chordates
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