The Evidence for Evolution

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Chapter 21

• The Evidence for Evolution

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OverviewEvidence of Natural Selection

• The beaks of Darwins finches
• Peppered moths and industrial melanism
• Fossils
• Anatomical
• Convergent evolution and the biogeographical
  record

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EVIDENCE OF EVOLUTION

• STRUCTURAL ADAPTATIONS
• MIMICRY
• CAMOUFLAGE

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The beaks of Darwins Finches

• Darwin collected 31 species of finches from the
  Galapagos Islands
• Probably wrens, grosbeaks, and blackbirds
• John Gould-ornithologist identified the birds
  were related species, differences in their bills

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Darwins selection hypothesis

• The differences among species in beak size and
  shape have evolved as the species adapted to use
  different food resources
• Theory of evolution by natural selection requires
  3 criteria
• Variation must exist in the population
• The variation must lead to differences among
  individuals in lifetime reproductive success
• Variation among individuals must be genetically
  transmitted to the next generation

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Testing Darwins proposal of evolution by natural
selection

• More than 130 years later
• 1973- Peter and Rosemary Grant students
  (Princeton University) begin studying the medium
  ground finch (Geospiza fortis)
• Daphne Major-Galapagos Islands
• Discovered the bird feeds on
• Small tender seeds that are produced in abundance
  during the rainy season
• Feeds on larger, drier seeds during droughts
• Measured beak depth every year
• Beak depth changed every year
• Had caught evolution in action

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Grant Grant Work

• To ensure that the changes in beak depth was not
  reflecting a response to diet
• Measured the relation of parent beak size to
  offspring beak size
• Discovered depth of beak size was same for
  parent/offspring
• Natural selection selects for beak size in
  response to the available food supply

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Peppered moths and industrial melanism

• Variety of adult shades from light gray with
  black peppered spots to jet black (melanistic)
• Different alleles of a single gene code for color
• Black is dominant, but rare before 1850
• Increase in frequency of black moths near
  industrialized cities.
• Became close to 100 of population
• Tree trunks in this area, black by soot of
  pollution and light-colored lichens were killed

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Distinct coloration of moths
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Peppered moths

• Hobbyist declared peppered moths were more
  visible to predators on sooty trees that have
  lost their lichens
• Birds ate peppered moths
• Black forms are at an advantage-camoflagued

• Tested Tutts predictions
• Step 1 released a of dark and light moths
  into 2 sets of woods
• Site A Birmingham- heavily polluted (19 of
  light moths survived 40 of dark)
• Site B Dorset- unpolluted (12.5 of light
  survived 6 of dark)
• Step 2 placed moths on trees and videotaped
  birds feeding
• Black moths blended in

• Tutt

• Kettlewell

The Agent of selection may be difficult to pin
down
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Artificial Selection

• Fruit fly lab experiments
• Selected fruit flies with lots of bristles on
  abdomen
• Chose only those with the most bristles to
  reproduce
• 86 generations later average number of bristles
  had quadrupled
• Body size, eye color, growth rate, life span,
  exploratory behavior

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Artificial Selection-Example
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Artificial Selection Important in Agriculture

• Modification of crops and livestock
• Oil contents of corn from1896 (4.5) to 1986
  (450)
• Very different now compared to ancestor

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Domesticated breeds from artificial selection

• Breeds of dogs
• Cats, pigeons, silver
• fox

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Fossil evidence of evolution

• Fossils- the preserved remains of once-living
  organisms
• Amber, Siberian permafrost, dry caves, rocks
• Rock fossils are created when three events occur
• organism buried in sediment
• calcium in bone or other hard tissue mineralizes
• surrounding sediment hardens to form rock

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The age of fossils is estimated by rates of
radioactive decay

• Absolute dating age of fossils is estimated by
  rates of radioactive decay
• Relative dating position of the fossil in the
  sediment
• Isotopes, like U238, transform at precisely known
  rates into nonradioactive forms.
• The rate of decay is known as an isotopes
  half-life

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Radioactive decay
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Fossils document evolutionary transition
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Fossil Evidence

• Fossils document evolutionary transition
• The oldest known bird fossil is the Archaeopteryx
• It is intermediate between bird and dinosaur
• Possesses some ancestral traits and some traits
  of present day birds
• Archaeopteryx was first found in 1859

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Recent Fossil Discoveries

• Four-legged aquatic mammal
• Important link in the evolution of whales and
  dolphins from land-dwelling, hoofed ancestors
• Fossil snake with legs
• Tiktaalik a species that bridged the gap
  between fish and the first amphibian
• Oysters small curved shells to large flat
  shells

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Evolutionary change in body size of horses
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Anatomical Evidence for Evolution

• Homology of the bones of the forelimb of mammals
• Homologous have same evolutionary origin, but
  have different structure and function
• Analogous have similar structure and function,
  but different evolutionary origin

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Early embryonic development shows similarities in
some groups

• Many vertebrates display similarities early on,
  but become different as they develop
• Early vertebrate embryos possess pharyngeal
  pouches that develop into
• In humans glands and ducts
• In fish gill slits

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Some structures are imperfectly suited to their
use

• Neck vertebrae
• Most organisms with long necks have increased
  neck vertebrae for flexibility
• Geese- 25
• Plesioasaurs- 76
• Most mammals- only 7
• Even giraffes
• Because of the absence of variation in vertebrae
  
• Selection led to an evolutionary increase in
  vertebra size to produce the long neck of the
  giraffe

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Comparing the eyes of vertebrates to mollusks

• Vertebrate Eye

• Mollusk Eye

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Vestigial structures- holdovers from the past

• Vestigial structures have no apparent function,
  resemble structures their ancestors possessed
• Examples

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Convergent evolution and the biogeographical
record

• Biogeography is the study of the geographic
  distribution of species
• Convergent evolution is the process that
  organisms that are not closely related
  indepedently evolve similar traits as a result of
  having to adapt to similar environments.
• Example
• Marsupials and placentals
• Hummingbirds and sunbirds

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Convergent Evolution
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Convergence evolution is a widespread phenomenon

• Fast moving marine predators
• Stream-line body to minimize friction
• Island trees
• Elsewhere are shrubs or small bushes in the
  sunflower family
• Why? Seeds from trees rarely make it to islands
  and the species that make it to islands fill the
  empty niche

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Darwins conclusions

• Species arrive on islands by dispersing across
  the water
• Dispersal from nearby areas is more likely than
  distant sources
• Species that can fly, float or swim can inhabit
  islands
• Colonizers often evolve into many species

• Islands are often missing plants and animals
  common on continents
• Species present on islands often diverged from
  continental relatives
• Island species usually are more closely related
  to species on nearby continents

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Darwins Critics

• Evolution is not solidly demonstrated
• There are no fossil intermediates
• The intelligent design argument
• Evolution violates the 2nd Law of Thermodynamics
• Proteins are too improbable
• Natural selection does not imply evolution
• The irreducible complexity argument