some evidence of evolutiondgray/evol322/chapter2.pdfthat the earth was 5668 years old –26th of...
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Some Evidence of Evolution
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Pre-Darwin westernChristian thought
• Life arose by special creation• Organisms were formed as we find them
today– That is species do not change
• Believed to be recent– In 1664 Archbishop James Ushher calculated
that the earth was 5668 years old– 26th of October, 4004 B.C., 9 AM
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Facts and Inferences
• Facts are observable things
• Inferences are extensions to what is notdirectly observed
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Claims of special creation
• Species created independently
• Species do not change through time
• Created recently
• Process inferred from those claims is thatsome God did it
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Claims of Darwin
• Species change through time
• Species related to each other as ancestor-descendant
• Life on earth is ancient
• Process inferred: ‘descent with modification’
• Process of adaptation: ‘natural selection’
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Any evidence?
• Are species independent?
• Are they unchanging?
• Is life recent?
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Are species independent, or dothey show shared ancestry?
• Shared ancestry proposed before Darwin– Comte de Buffon, Erasmus Darwin, Jean-
Baptiste Lamarck
– What Darwin provided was the mechanism ofadaptive evolutionary change• Natural selection
• Comparative anatomy was basis
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Homology and Analogy
• Homology (modern sense): similar bydescent or ancestry
• Analogy: similar not by descent– In conversation: ‘let me compare thee to a
summers day’– In biology: similar selection pressures produce
similar adaptations• E.g. desert plants in N. America share many water
saving features with other desert plants
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Vertebrate forelimbs
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Making senseof similarity
• If you built a flying limb from scratch, youwould design it differently than you woulddesign a swimming or grasping or runninglimb
• The similarity of structure despite verydifferent function suggests similarity oforigin
• Called structural homology
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Other homology examples
• Embryology– Louis Agassiz (among others) showed that
vertebrate embryos in early development arevery similar
– Early human embryos have structures like gills;also have a tail
• Called Developmental Homology
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Human embryo with tail
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Genetic Homology
• Structural homologies are similar becausethey share developmental homology
• Developmental homology is the result ofgenetic homology
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‘Universal’ genetic code
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Genetic homology inhomeodomain eye genes
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Thought question
• Biomedical researchers often seektreatments for human disease by doingexperiment in other species (e.g. mice)
• How successful would they be if mice andhumans did not share numeroushomologies?
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All this homology suggestsshared ancestry
• Shared ancestry….what does that mean?
• Often said that “humans evolved from apes”
• People think that means that modernhumans descended from modern apes
• Thoughts?
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Phylogenetic tree: tips, branches, nodes
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Parsimony
• Important principle of science
• Simply explanations are favored overcomplicated ones (Ockham’s razor)
• In general English usage: less, or sparing
• In biology usage: requiring lessevolutionary change
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So a horse, a zebra and a chickengo into a bar……..
• Which is more parsimonious?
• The horse and the zebra are more closelyrelated than either is to the chicken, or
• The chicken and the horse are more closelyrelated than either is to the zebra
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Do species change over time?
• Evidence from living species– Vestigial traits
– Direct observation
– Fossil record
• Evidence from fossils– Extinction
– Transitional forms
– Environmental change
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Vestigial structures
• Vestigial structure: functionless orrudimentary homolog of a structure that hasan important function in closely relatedspecies
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Examples of vestigial traits
Also:
Your appendix
Wisdom teeth
Goose bumps
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Vestigial traits can be:
• Structural– Tail bone in adult humans
• Developmental– Embryonic tail
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Does it make sense to have a tailbone and an embryonic tail UNLESS
your ancestors had tails?
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Vestigial Genes
• Genetic– Pseudogenes: nearly normal genes that are not
transcribed to protein
– E.g., psi-alpha locus of hemoglobin
– Resembles that alpha locus but has a mutationthat stops transcription
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Direct observation of changeover time
• Change from AZT susceptible to AZTresistant HIV strains repeatedly foundwithin patients?
• 100’s of other examples, finch beaks beingthe most famous
• Soapberry bugs another good one
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Soapberrybug beak
lengthevolution
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What about fossils?
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Fossils provide three types of evidencefor evolution
• Evidence of extinction
• Evidence of change of species through time
• Evidence of age of life
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Extinction
• Was actually controversial in early 1800’s
• By 1812 Cuvier published Irish Elk analysis
But how does the fact ofextinction help the case forevolution?
• Species do not persistindefinitely; that suggests lifeon earth changes
•Fossils prove that life isn’ t setin stone
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Change through time:Transitional fossils
• Fossils species intermediate betweenmodern living forms
• Fossil species intermediate between olderand younger fossils, or older fossils andmodern forms
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Ambulocetus natans, ca. 50 mya, described 1994, from Pakistan
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Basilosaurus isis, ca. 38 mya, described 1990, from Egypt
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Modern baleen whale
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Whale fossils from Egypt andPakistan?
• Marine fossils found high and dry– Clearly, the earth changes– But does it change fast or slow?
• Before the late 1700’s, change thought to berapid, e.g. Biblical floods, other‘catastrophies’– Catastrophism: hypothesis of catastrophic rapid
change
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Geological Evidence
• James Hutton and Charles Lyell
• British geologists, actually set out tomeasure rates of geologic change (wow,science!)
• Determined that the rates of geologicchange are very very slow
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Why is that important?
• If rates of change are very very slow (whichmostly they are)
• AND
• If the rates of change in the past were similar tothe rates observed today– Why would they be different?
• Then the earth must be very old, much much olderthan Archbishop James Ussher’s 6000 years.
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How Old?
Current estimate’s
Earth ~4.6 bya
Life ~3.7 bya
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How do you know that?
• Relative age dating techniques– Deeper layers are older– Layers are originally flat– Rocks within other rocks are older– But rocks that fill cracks in other rocks are
younger
• Those are good, but you’d like actual ages
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Actual ages: Radiometric dating• Isotopes decay at characteristic rates, called half-
lives• A half-life is the amount of time it takes for 1/2 of
the isotope to decay
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Summary of Facts
• Species are not independent– Homology: structural, developmental, genetic
• Species change– Observable in populations today
– Observable through time in fossils
• Life on earth is ancient– Earth 4.6 Billion years; Life 3.7 Billion years
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So not special creation, then how?
• Process inferred:– ‘descent with modification’
• Process of adaptation:– ‘natural selection’
• Natural selection next lecture