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Chapter 7

Evolution and the Fossil Record

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Chapter 7 - Guiding Questions

• What lines of evidence convinced Charles Darwin that organic evolution produced the species of the modern world?

• What are the components of natural selection? • What is the source of the variability that is the basis of natural

selection? • What role does geography play in speciation? • What factors lead to evolutionary radiation?• Why is convergence one of the most convincing kinds of evidence that

evolutionary changes are adaptive? • Why do species become extinct? • What is mass extinction?• In what ways can evolutionary trends develop?

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Evolution

Evolution– changes in populations,

which consist of groups of individuals that live together and belong to the same species

– a change in gene frequencies

– populations evolve, not individuals

• YOU can’t evolve!

Extinct ground sloth (20 ft long)

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Evolution

Adaptations– specialized features of

animals and plants (or any other organism) which perform one or more useful functions

– allow that organism to excel in its environment

– YOU can only modify characteristics over which your genes have control; e.g., tanning

Cat skull Horse skull

Venus Flytrap

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Inefficient Evolution

• can only operate by changing what is already present;

• it’s the business of remodeling rather than new construction from scratch

• e.g., to make a new structure, natural selection starts by modifying an already existing one– e.g., how could ‘night vision’ evolve?

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Charles Darwin

• 1831– set sail on the Beagle– schooled in

uniformitarianism• Lyell’s Principles

of Geology– a keen observer of

natural phenomena• 1859-On the Origin of

Species by Natural Selection

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Some of Darwin’s Observations

• Rhea– large flightless birds

– found only in South America

– also found extinct, fossil forms

• Some similarities but also obvious differences from ostrich (Africa) and emu (Australia)

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• sloths and extinct armadillos– unique to the Americas

More of Darwin’s Observations

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• Oceanic islands– many barren; Hawaii

has no native snakes, frogs, FW fish, etc.

– species must have originated elsewhere

• Galápagos Islands– tortoises with shells

unique on each island– shared a common

ancestry– followed later by

differentiation

More of Darwin’s Observations

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Galapagos Islands

Range from 3 to 5 my old

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Galapagos Tortoises

• 14 subspecies, 11 extant, several with very small populations– e.g., Lonesome George

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Lonesome George

Isla Pinta

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• Finches of the Galápagos– different beak types

• slender-insectivorous

• sturdy-seed crushing

• woodpecker-like-tool user

– differentiation based on lifestyle– curiously resemble a South American

mainland finch

Darwin’s Famous Finches

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Adaptive Radiation of Darwin’s Finches

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Feeding Adaptations of Ground Finches

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Charles Darwin

• Additional observations– Anatomical relationships

• embryos of many vertebrates are quite similar, even superficially indistinguishable

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Charles Darwin

• Additional observations– Anatomical relationships

• embryos of many vertebrates quite similar

• homology– presence in two different groups of animals or plants of

organs that have the same ancestral origin but serve different functions

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Charles Darwin

• Additional observations– Anatomical relationships

• embryos of many vertebrates quite similar

• homology– presence in two different groups of animals or plants of

organs that have the same ancestral origin but serve different functions

• vestigial organs– organs that serve no apparent purpose but resemble organs

that perform functions in other creatures

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Pelvic structures in whales and snakes

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Vestigial Structures

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Theory of Evolution

• Natural Selection- ‘survival of the fittest’– the process that operates in nature but parallels

the artificial selection by which breeders develop new varieties of plants and animals

• success of an individual determined by advantages it has over others

– survives to bear offspring with same trait

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Antibiotic Resistant Bacteria

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The Basis for Natural Selection

• There is much variation between members of a species.

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The Basis for Natural Selection

1. There is much variation between members of a species

2. Reproduction is way in excess of the number that the habitat can survive

e.g., Robins

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E.g., Robins

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The Basis for Natural Selection

1. There is much variation between members of a species

2. Reproduction is way in excess of the number that the habitat can survive

e.g., Robins

3. Differential reproductive success-those best adapted survive to reproduce the most

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Will the all survive?

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Theory of Evolution

• Darwin didn’t have all the tools we have today to explain his idea.

• Genetics was in its infancy—Mendel’s ideas, though contemporary with Darin, weren’t appreciated for decades.

• Genes-hereditary factors• Particulate inheritance

– Gregor Mendel’s idea that organisms retain identities through generations

– Peas• No blending• Colors could be masked for generations

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Theory of Evolution

• Mutations– alteration of genes– provides for variability– very few are helpful

• DNA– Deoxyribonucleic Acid– transmits chemically

coded information– mostly found in

chromosomes

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Theory of Evolution• sexual recombination

– each parent contributes one half of its chromosomes to offspring via a gamete• special reproductive cell containing one of each type of chromosome

– female ovum/egg; male sperm

– yields new combinations-e.g., my kids aren’t exact copies of me; genes of their mother and me are both present.

• mutations increase variability—and we all have them!

• gene pool– sum total of genetic components of a population or group of interbreeding

individuals– you have only a small subset of the human gene pool

• reproductive barriers limit the pool and keep species separate

• speciation– origin of a new species from two or more individuals of a preexisting species

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Reproductive Isolation

Spring breeder Fall breeder

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Origination

• Evolutionary radiations– pattern of expansion from some ancestral adaptive

condition represented by descendant taxa

• adaptive breakthrough– appearance of key features that allow radiation to occur

• fossil record documents patterns– E.g., Jurassic corals

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Jurassic Corals

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Origination• Rates of speciation

– Galápagos Islands-formed millions of years ago– Lake Victoria

• 13,000 years old• 497 unique species of cichlid fish, many with specialized

adaptations

• Molecular clock– assume average rate of mutation– determine pace of change– extrapolate timing of change

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Convergence

• Evolution of similar forms in two or more different biological groups

• Marsupials and placental mammals– similar form

– isolated, adaptive convergent evolution after initial divergence

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Extinction• caused by extreme impacts of limiting factors– predation– disease– competition

• pseudoextinction– species evolutionary line

of descent continues but members are given a new name

• high rates of extinction make useful index fossil– ammonoids

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Extinction

• rates– average rate has

declined through time

• mass extinctions– many extinctions

within a brief interval of time

– largest events peak at extinction of >40% genera

– rapid increase (radiation) follows

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Modern Mass Extinctions

• fossil patterns reflected in modern– tropical species

– large animals

• loss of habitat• direct exploitation• likely replacement by

opportunistic species

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Evolutionary Trends

• Cope’s rule– body size increases

during evolution of a group of animals

– structural limitations on size

• specialized adaptations limit evolution– elephants– manatees

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Evolutionary Trends

Whales• terrestrial origin

– 50 Ma– small (2 m)

mammals with feet

• marine adaptation– 40 Ma– lost hind limbs– no pelvic bones– up to 20 m

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Phylogeny

• Phylogeny– complex, large-scale

trend within a branching tree of life

– gradual large-scale change from one species to another is rare

• e.g., Jurassic coiled oysters

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Phylogeny

• Axolotl– example of rapid speciation from

parent species• parent is amphibious• offspring is aquatic throughout

life after one simple genetic change

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Phylogeny

• rates • gradualistic model

– very slow rates

• punctuational model– rapid evolution with

little change between steps

– bowfin fish• little change in 60 m.y.

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Phylogeny

• Horses– increase in body size– evolved tall complex

molars and single-hoofed toe

– change driven by climate• expansion of grasslands

• Dollo’s law– evolutionary transition from

at least several genetic changes is highly unlikely to be reversed by subsequent evolution

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