Evidence for Evolution Graduate Seminar

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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

future
past
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The importance of common ancestry

• If two different species descended from a single
  ancestor then change (evolution) is implied

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What did people believe before Darwin?

• Separate ancestry (many versions)

future
past
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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 Lamarcks theory

• Life progresses upward due to an internal drive
  towards perfection

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A special case Lamarcks theory

• Life progresses upward due to an internal drive
  towards perfection
• Why are primitive organisms still around?

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A special case Lamarcks 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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Lamarcks view
Because all species follow the same trajectory of
origin they will form a ladder of advancement
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Lamarcks view is basically separate ancestry
past
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First articulation of common ancestry
Lyell, C. Principles of Geology, Vol. II, Chap. 1
Species 3
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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

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Deep Homology

• Distantly related organisms share structural
  similarities
• Function varies
• Explicable by common ancestry

grasping
leaping
flying
swimming
running
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human
whale
bat
fish
Amphibia
Reptilia
Pentadactyl limb
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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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Biogeography closely related species live near
each other
Molecular phylogeny of Hawaiian and other
Campanulaceae (Givnish et al.)
Hawaii
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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