evidence for evolution graduate seminar introduction and overview
TRANSCRIPT
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Evidence for Evolution Graduate Seminar
Introduction and overview
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The three main claims of Darwinian evolution
• Living species are related by common ancestry
• Change through time occurs at the population not the organism level
• The main cause of adaptive evolution is natural selection (and related mechanisms)
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The three main claims of Darwinian evolution
• Living species are related by common ancestry
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Common ancestor
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The importance of common ancestry
• If two different species descended from a single ancestor then change (=evolution) is implied
All differences evolved along the lineages
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What did people believe before Darwin?
• Separate ancestry (many versions)
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A special case: Jean-Baptiste Lamarck
• French Naturalist (1744-1829)
• The first scientific theory of evolution
• Struggled to reconcile evolution and the Scala naturae (great scale of being)
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A special case: Lamarck’s theory
• Life progresses upward due to an internal drive towards perfection
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A special case: Lamarck’s theory
• Life progresses upward due to an internal drive towards perfection
• Why are “primitive” organisms still around?
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A special case: Lamarck’s theory
• Life progresses upward due to an internal drive towards perfection
• Why are “primitive” organisms still around?– Spontaneous generation of new
life constantly
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Lamarck’s view
Because all species follow the same trajectory of origin: they will form a ladder of advancement
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Lamarck’s view is basically separate ancestry
past
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Lyell, C. Principles of Geology, Vol. II, Chap. 1
Common Ancestor
Species 2Species 1 Species 3
Common Ancestor
First articulation of common ancestry
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Darwin envisaged evolution as a tree
The affinities of all the beings of the same class have sometimes be represented by a great tree. I believe this simile largely speaks the truth………The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species…..….the great Tree of Life….covers the earth with ever-branching and beautiful ramifications
Charles Darwin, On the Origin of Species; pages 131-132
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The only figure in The Origin of Species
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Two claims
• That major groups of organisms share descent from common ancestry– vs. separate ancestry
• That all living organisms share descent from common ancestry– vs. several origins
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Evidence for common ancestry (against separate ancestry)
• Fossil record
• Homology
• Vestigial Structures
• Classification
• Hierarchical distribution of traits
• Agreement between gene trees
• Evolution during domestication
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Evidence for common ancestry• Fossil record
– Transitional fossils (sometimes in temporal sequence): consistent with the existence of real common ancestors
Living sister group
Major clade
Traits
Transitionalfossils
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• Distantly related organisms share structural similarities
• Function varies
• Explicable by common ancestry
grasping
leaping
flying
swimming
running
Deep Homology
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“fish” Amphibia Reptilia
Pentadactyl limb
human whale bat
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Vestigial structures
• Structures that are non-functional (but functional in related species). For example:– Human appendix, tail bones,
– Gill slits in mammal embryos
– Hip bones of whales and snakes
– Eyes in cave fish
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Trees explain patterns in trait distribution
Fur; milk
Amnion
Four legs; lungs
Vertebral column
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Trees explain patterns in trait distribution
Fur; milk
Amnion
Four legs; lungs
Vertebral column
Applies a forteriori to molecular data
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Molecular phylogeny of Hawaiian and other Campanulaceae
(Givnish et al.)
Hawaii
Biogeography: closely related species live near each other
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Correlation among gene trees(Penny et al. 1982. Testing the theory of evolution by comparing phylogenetic trees constructed from five different protein sequences. Nature 297: 197-200.)
• When we estimate the phylogeny from different genes, we get trees that are much more similar than could happen by chance during separate ancestry
• Amenable to statistical analysis
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We see diverse forms that are descended from single ancestor
Brassica oleracea
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What about the claim of a single ancestor of all living organisms?• Shared biochemistry (e.g., same 4 nitrogenous bases, same
20 L-amino acids, ATP)– There are many possible nitrogenous bases and amino acids; Many
sugars could have form the NA backbone
– No chemical reason for L- vs. D-amino acids
• Shared structures (ribosomes, lipid bilayer membranes)
• Shared metabolic pathways (e.g., glycolysis)
• Share information processing (genetic code)– The code is a “frozen accident”
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The three main claims of Darwinian evolution
• Living species are related by common ancestry
• Change through time occurs at the population not the organism level– No organism level mechanism is currently
plausible
• The main cause of adaptive evolution is natural selection (and related mechanisms)
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The main cause of adaptive evolution is natural selection
• Claim 1: Natural selection happens– vs. Natural selection does not/cannot happen
• Claim 2: Natural selection is sufficient to explain even the most complex traits of living organisms– vs. natural selection is not sufficient
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The main cause of adaptive evolution is natural selection
• Claim 1: Natural selection happens
• Evidence: – Artificial selection and rapid natural selection– Theoretically must occur if only minimal
assumptions are met• Genetic variation
• Limited resources
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The main cause of adaptive evolution is natural selection
• Claim 2: Natural selection is sufficient to explain even the most complex traits of living organisms
• “Evidence”– On short time scales it is very effective– Time is long– No other natural mechanisms are known