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The Evidence for Evolution

Chapter 21

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Evidence of Natural SelectionDarwin collected a closely related group of 14

finch species in the Galápagos Islands– All similar except for beak characteristics– Darwin hypothesized that different beak

shapes were related to food gathering– Darwin wrote “…one might really fancy

that…one species has been taken and modified for different ends.”

3Darwin’s finches

Evidence of Natural Selection

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• Modern research verified Darwin’s selection hypothesis

• 3 conditions of natural selection– Variation must exist in population– Variation must lead to differences among

individuals in reproductive success– Variation among individuals must be

genetically transmitted to the next generation

Evidence of Natural Selection

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• Peter & Rosemary Grant studied medium ground finch• beak depth variation among members of population• Average beak depth changed from year to the next in

a predictable fashion- Droughts: birds with deeper, more powerful beaks survived better - Normal rains: average beak depth decreased to its original size

Evidence of Natural Selection

6Evidence that natural selection alters beak shape

Evidence of Natural Selection

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• When environment changes, natural selection often favors different traits in a species

• Biston betularia: peppered moth– Light gray with black specks to jet

black coloration– Black individuals have dominant allele– Dominant allele rare in population

until 1850s

Evidence of Natural Selection

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• J.W. Tutt hypothesized that light-colored moths declined because of predation

• Light moths were easily seen by birds on darkened (sooty) trees

Evidence of Natural Selection

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Evidence of Natural Selection

• Bernard Kettlewell tested hypothesis– Dark tree trunks = more dark-colored

moths survived– Light tree trunks = more light-colored

moths survived• When environmental conditions reverse,

so does selection pressure

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• Industrial melanism: phenomenon in which darker individuals come to predominate over lighter ones

• Pollution control resulted in lichen growing on trees and bark color being lighter again

• Light-colored peppered moths now are dominant in population

Evidence of Natural Selection

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• Agent of selection may be difficult to pin down• Could poisoning by pollution be agent of natural

selection?

Selection against melanism

Evidence of Natural Selection

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Laboratory Experiments• Drosophila melanogaster (fruit fly)

– Selected fruit flies with many bristles on abdomen– Chose only those with most bristles to reproduce– 86 generations later: average # of bristles à 4X

Artificial Selection

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Artificial selection in the laboratory

Artificial Selection

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Agriculture

Corn looks very different from its ancestor

Artificial Selection

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Domestication of silver foxes result of artificial selection

Artificial Selection

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Breeds of dogs: Differences among dog breeds are greater than differences displayed among wild species of canids.

Can selection produce major evolutionary changes?

Artificial Selection

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Fossil Evidence of Evolution

• Fossils: preserved remains of once-living organisms

• Rock fossils: created when three events occur– organism buried in sediment– Ca++ in bone/hard tissue mineralizes– surrounding sediment hardens to form rock

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• Absolute dating: age of fossils estimated by rates of radioactive decay

• Relative dating: fossil’s position in sediment • Isotopes, like U238, transform at precisely

known rates into nonradioactive forms

• Rate of decay à isotope’s half-lifeC14 half-life: 5,280 yrsK40 half-life: 1.28 million yrs

Fossil Evidence of Evolution

19Radioactive Decay

Fossil Evidence of Evolution

5 Major Extinctions

• Filling in ecological barrel: many habitats/niches to exploit

• Unrestricted genetic design:freedom to try new/different morphological forms;some successful/others do not workà extinct

• Stealth predator: Anomalocaris canadensisnatural selection for 20 million yrs pressuring prey to adapt/evolve new structures to escape predator

Hypotheses for Cambrian Explosion

Cambrian Explosion

Anomalocaris canadensis

Burgess Shale

Environmental Changes

Geological Studies

I. Continental Drift

II. Plate Tectonics- 6 Major Plates1) North American2) South American3) Eurasian4) African5) Antarctican6) Pacific

A. Subduction Zones: plates collide

B. Transform Faults: plates slide past

C. Oceanic Ridges: plates pull apart

formation of islands, mountains, volcanoes

earthquakes

deep sea vents

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Fossil records document the course of life throughtime

Fossil Evidence of Evolution

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• Oldest known bird fossil: Archaeopteryxfound in 1859intermediate between bird & dinosaurpossesses some ancestral traits & some traits of present day birds

Fossil Evidence of EvolutionFossils document evolutionary transition

Archaeopteryx

birdreptilian

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Recent discoveries– Four-legged aquatic mammal

• Important link in evolution of whales & dolphins from land-dwelling, hoofed ancestors

– Fossil snake with legs– Tiktaalik: a species that bridged gap

between fish & first amphibian– Oysters: small curved shells to large flat

shells

Fossil Evidence of Evolution

Tiktaalik

375 million yr old fossil

head of a crocodile & gills of a fish

Fish

30Whale “missing links”

Fossil Evidence of Evolution

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Evolutionary change: horse body size & toe reduction

Fossil Evidence of Evolution

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Anatomical Evidence for Evolution

• Homologous structures: structures with different appearances & functions that all derived from same body part in a common ancestor

• Bones in forelimb of mammals: homologous structures

• Different functions, same ancestor structure

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I. Anatomical Homology: mammalian forelimb bones

Anatomical Evidence for Evolution

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• Strongest anatomical evidence supporting evolution comes from comparisons of how organisms develop.

• Early vertebrate embryos possess pharyngeal pouches that develop into:– In humans: glands & ducts– In fish: gill slits

Anatomical Evidence for Evolution

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II. Embryological homology: à descent from common ancestor

Anatomical Evidence for Evolution

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• Neck vertebrae– Geese: 25– Plesiosaurs: 76– Mammals: 7

• Giraffe: 7 vertebrae, very large in size, to make up for length of neck

Anatomical Evidence for Evolution

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• Eyes– Molluscs:

photoreceptors face forwardfocusing lens move forward-backward

– Vertebrate: photoreceptors face backwardfocusing—lens scrunched by muscles

Anatomical Evidence for Evolution

38Eyes of vertebrates

Anatomical Evidence for Evolution

39Eyes of Mollusks

Anatomical Evidence for Evolution

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• Vestigial structures: no apparent function but resemble structures their ancestors possessed

Anatomical Evidence for Evolution

Vestigial structures of a whale

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• Humans– Muscles for wiggling ears

• Boa constrictors– Hip bones & rudimentary hind legs

• Manatees– Fingernails on their fins

• Blind cave fish– Nonfunctional eyes

Anatomical Evidence for Evolution

III. Biochemical Homologybacteria/yeast to humans

same codons & amino acids1953 Miller & Urey

7 days4 amino acidsureafatty acids

Chemical Evolution

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Convergent Evolution

• Biogeography: study of the geographic distribution of species– Some plants & animals have similar

appearance but are only distantly related• Convergent evolution: independent

development of similar structures in organisms that are not directly related

• Convergent evolution: usually seen in animals & plants that live in similar environments

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• Marsupials & placentals– Marsupials: young born in an immature

condition & held in a pouch until they develop– Placentals: young are not born until they can

safely survive in the external environment

Convergent Evolution

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Convergent Evolution

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Convergence among fast-swimming predators

Convergent Evolution

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Biogeographical Record

• Darwin noted on his voyage that– Islands: often missing plants & animals

common on continents– Species present on islands often diverged

from continental relatives– Island species usually more closely related to

species on nearby continents

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• Darwin concluded:– Species arrive on islands by dispersing

across water– Dispersal from nearby areas more likely than

distant sources– Species that can fly, float or swim can inhabit

islands– Colonizers often evolve into many species

Biogeographical Record