EvolutionDescent with Modification

Evolution Overview• The framework for modern biology• Definition:

– a change over time• Occurs in populations, not individuals• How new organisms are created• A theory subject to revision and

change• A hot topic long before Darwin &

Wallace suggested a mechanism

Darwin’s Big Ideas

• Darwin, in Origin of the Species, made 2 revolutionary points:– species are not created 'as is', but

evolve from ancestral species– proposed mechanism to account for

changes in species: – Natural Selection

History – Before Darwin

• Context before Darwin:• Believed organisms were created 'as is';

immutable• Nature was like a ladder - man on the

top rung• Earth thought to be 5 - 6,000 years old.• Linneaus - 18th C.

– developed classification scheme– based on similarities, hierarchy– did not subscribe to evolutionary

relationships

Early Evolutionary Ideas• Georges Buffon - 1749

– French scientist – suggested the age of the earth was

underestimated– living things had changed

• Jean Baptiste de Lamarck - 1809– proposed a theory of evolution – called his idea "the parade of nature"– organisms were continually ascending and

acquiring more perfect characteristics

Lamarck’s Key Ideas• Use and disuse of organs varies• Shaping force was environment• Organisms respond to changes in

their environment by developing or changing their structure

• Inheritance of acquired traits• Once highly respected

– couldn't support his ideas, fell in disfavor

Lamarckian Evolution

Charles Darwin• Living things evolve to adapt to their

environment• Thinking shaped by his experiences

– Naturalist aboard H.M.S. Beagle at age22– Read Charles Lyell

• earth much older• physical forces that shaped the earth still exist

– Saw similar species on the Galapagos Islands• realized they must have come from mainland and gradually

become new species

– Collected much data, but didn't publish for 20 years– Eventually presented his work with Alfred Russell

Wallace– Published Origin of the Species

• Proposed different mechanism to account for changes: Natural Selection

Voyage of the Beagle

The Basis of Darwin’s Theory

• Variation exists within species – traits vary among individuals of the same species

• All organisms compete for limited natural resources– some will get more, some less (Malthus)

• Organisms produce more offspring than can survive– leads to competition

• The environment selects organisms with beneficial traits

• Organisms with traits well suited to the environment survive and reproduce in greater numbers than those less well suited. – They pass these traits to their offspring

Adaptation & Natural Selection

• Over time the population becomes better adapted to the environment

• The environment does the selecting– therefore natural selection, – artificial selection was well known – also survival of the fittest

Artificial Selection

Members of the cabbage family bred from wild mustard plants.

Key Points• Evolution is not the same as natural selection• Evolution occurs in populations, not

individuals!– Population = a group of interbreeding individuals

belonging to a particular species & haring a common geographical area

• Variations in the population occur randomly • Natural selection acts on this random

variation– This contrasts to Lamarck’s idea that variation

occurs because of a change in the environment

Random Variation

Natural Selection• The mechanism of evolutionary change• Definition = selection by the environment• Misleading idea of an elimination contest

favoring the strong and eliminating the weak

• Darwin did not know about genetics– Makes mechanism clear

• Sometimes misapplied or misunderstood– Herbert Spencer – Extended to human affairs, politics: social

Darwinism

The Peppered Moth Example• In England before the industrial revolution

moth’s were mostly light colored• They lived on light colored trees

– blended with bark– hard for predators to see

• The industrial revolution led to pollution– tree bark became darker– light moths were easy targets for predators – this favored dark moths

• NOTE: – had to have variation in the population in the 1st

place to produce dark moths!

• Recent pollution control– return of light moths

• The Peppered Moth• Camouflage

produces a selective advantage

• Changing environmental conditions produce direct evidence of evolution

Evidence of Evolution• Fossil Record• Comparative Anatomy

– Homologous Structures– Vestigial Structures

• Comparative Embryology• Biogeography

– Adaptive Radiation– Convergent & Divergent Evolution

• Direct observation of natural selection• Molecular Biology

The Fossil Record• Paleontology = the study of fossils

– A record of change over time

• Sedimentary impressions– layers = strata– allow dating

• Most are skeletal remains– Also bogs, amber, ice

• Two theories about how changes occurred and why there are gaps:– Catastrophism: – boundaries between strata corresponded to catastrophic

events– Gradualism:– profound change is the result of slow, continual process

Fossil Formation

Fossils (Continued)• Study the relationship of fossils to existing

organisms– Look for similarities & differences

• Study order of fossils• Chronology of appearance of different

classes of vertebrates: – fishes predate other vertebrates, then

amphibians, reptiles– The oldest fossils are prokaryotes – corroborates biochemical & molecular evidence

• The fossil record is incomplete

Rock Strata & Fossil Dating

The Grand Canyon

Comparative Anatomy• Homologous Structures

– Anatomically similar structures serve different functions in different organisms

– Forelimbs of bird, horse dog have different functions yet have common internal bone structure (bat's wing, whale's flipper, homologous)

• Vestigial organs or structures – parts that are incomplete or have no

apparent function; the remains of once functioning structures.

• Evidence for common ancestry

Homologous Structures

Vestigial Structures

Embryology

• Ontogeny recapitulates phylogeny• Developing organism = embryo• The early stage of one organism is

very similar to other organisms• Similar developmental stages may

reflect common ancestry

Comparative Embryology

Biogeography• The study of the distribution of plants and animals• Places with similar climate don't always have the same

animals• Animals on islands may be more like those on the

nearest mainland than those on islands with similar habitats elsewhere

• Convergent Evolution– Similar environmental pressure may produce very similar

organisms which are actually not closely related

• Divergent Evolution– Organisms that are similar may grow farther and farther

apart due to different selective pressures in different environments

• Adaptive Radiation – A type of divergent evolution 

Convergent Evolution• Natural selection favors similar

organism in similar environments because they are acted on by similar pressures

• Two organisms in similar environments may appear very similar but may be genetically very different– fishes & dolphins; S. American rhea &

African ostrich

Convergent Evolution Illustrated

Divergent Evolution

• The process by which related organisms become less alike

• Caused by different selective pressures in their environments

Adaptive Radiation• One type of Divergent Evolution)• Many new species can arise from a single

parent species• Darwin's finches = classic example

– On the Galapagos Islands – similar in appearance, but each species has

distinctive shaped beak – distinct feeding habits and behaviors– On the Galapagos Islands, many different food

sources,– evolved into different species to fill different niches – All are closely related– Arose from a single type of finch in S. America

Darwin’s Finches

Direct Observation

• Usually difficult because of the time involved for natural selection to take place

• Recall the peppered moth • Other examples:

– bacterial drug resistance– pesticide resistance in insects– drosophila

• Direct Observationof Natural Selection

Insecticide resistance in beetles

Molecular Biology• Includes biochemistry, molecular biology,

genetics– serum proteins in blood– amino acid sequences in proteins– proteins with similar amino acid sequence in

related organisms– DNA sequencing

• Darwin did not know about genetics– Mendel existed but there is no evidence that

Darwin had read or understood Mendel's ideas

• Modern technology gives a much deeper understanding of the basis for Darwin's theory

• In a changing environment, natural selection alters gene frequencies