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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.”
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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
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19Radioactive Decay
Fossil Evidence of Evolution
5 Major Extinctions
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• 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
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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
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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
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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
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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