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Measuring Evolution
• Evidence for evolution
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Estimate natural selection in the wild
Trace fossils
Infer phylogeny from behavior
Infer behavioral evolution from phylogeny
• The comparative method
– Test evolutionary hypotheses by correlating
behavior with ecological factors across
• Species
• Phylogenetically independent contrasts
Selection and Galapagos finches
14 species evolved in
about 500,000 years
Selection on beak depth in G. fortis
4% change in two years, due to change in seed sizes
Selection on G. magnirostris
Trait
Bill length
Bill depth
Tarsus length
Selection
0.45 s.d.u.
0.58 s.d.u.
0.27 s.d.u.
h2*
0.66
0.69
0.82
*h2 estimated from midparent-offspring
regression
Measuring natural selection
• When trait is linearly related to LRS
– Directional selection: S = cov (trait,fitness)
– Cov(x,w) = S(xi-X)(wi-W)/n
• When trait is unimodally related to LRS
– Stabilizing selection: S = cov (trait2,fitness)
• When trait is bimodally related to LRS
– Disruptive selection: S = cov (trait2,fitness)
Stabilizing selection on song repertoires
Sample problem
• A population of guppies has an average swimming speed of 80
mm/s, individuals having a mean of 110 survived a flood to be
parents of the next generation; their offspring had a mean of 90
mm/s. Calculate the realized heritability.
• Recall that R = h2S. Therefore, h2 = R/S where S (selection) =
mean after selection - mean before selection, R (response) =
mean of offspring generation - mean of previous generation
• S = 110 mm/s - 80 mm/s = 30 mm/s
• R = 90 mm/s - 80 mm/s = 10 mm/s
• Therefore, h2 = 10 / 30 = 0.33
Footprint fossils
Can infer:
Size
Weight
Group size
Parental care
Homo
erectus
footprints
3.8 MYA
Laetoli,
Tanzania
Marine invertebrate burrow fossils
Phylogeny from behavior: duck courtship
Ruddy duck
Asian white-headed duck
Courtship
similarity
between ducks
From Konrad Lorenz
What if behavioral phylogeny ≠ DNA phylogeny?
Infer trait evolution
• Derive phylogenetic tree from independent
data
• Assign trait values to ancestral nodes by
minimizing the number of possible changes,
i.e. use parsimony
• Use trait value at next lower node to decide
ambiguous nodes
• Note: need outgroup to infer ancestral trait
value
Inferring discrete trait evolution
a
a
Inferring trait evolution
Note: it is not always possible to obtain
unique ancestral trait values
Phylogenies can be used to test
adaptationist hypotheses
Sexual selection example
– Sensory exploitation hyothesis
• Female preference evolves prior to male trait
– Coevolution hypothesis
• Female preference evolves with male trait
Female preferences evolved before male
swordtails
Female X.m. prefer
males with swords
Phylogeny based on morphological data
or do they?
Phylogeny based on mtDNA sequence data
Black = swordtails
White = no sword
Hatched = ambiguous
Phylogeny based on mtDNA sequence data
The comparative method and
continuous characters
• Predict that some trait correlates with a
particular ecological or social variable due
to selection
• Remove effects of body size, if necessary
• Look for correlation between traits across
species
• Assumes species are independent, unlikely!
Sperm competition in primates
Since species that
live in multi-male
groups exhibit larger
testes, infer sperm
competition.
But, species are not
independent!
Infer continuous trait evolution
• Derive phylogenetic tree from independent
data
• Estimate phylogenetically independent
contrasts for each trait
– Assign trait values to ancestral nodes by using
the difference in trait values of derived taxa
– Weight change by branch length
• Regress contrasts through the origin
Phylogenetically independent contrasts
Testes size vs mating contrasts