Download Lecture 2: Analysis of Adaptation

Lecture 2: Analysis of Adaptation
• Adaptation = a feature that, because it
increases fitness, has been shaped by NS
• In other words:
NS + genetic variation = adaptation
• Adaptations are not always obvious
e.g. Eyesight vs. Giraffe’s neck
Adaptations
When analyzing adaptation we need to
remember:
• Not all features of a population are adaptive
• Not all adaptations are perfect
Analysis of Adaptation
We need to:
• Show that a trait has been shaped by NS
• Determine the agent of selection
4 Ways to Identify an Adaptation
1) COMPLEXITY
• Complex structures are usually adaptive
e.g. ampullae of Lorenzini
• Variants of complex structures may not be
adaptive (e.g. Hb)
2) Engineering
Does the trait fit efficient model predicted by
engineering?
e.g. Fish shapes
• Fits aerodynamic prediction
• Form fits function
3) Convergence
Correlational Evidence: Convergent Evolution
• Patterns of convergence are studied using
the COMPARATIVE METHOD
• Variation in character should correlate with
selective pressures of ecological context
• Problem: similarity can mean similar
adaptive response or close relationship
• Need: traits that arise independently in
different phylogenies
• Eliminate the effect of common ancestry;
therefore ecology is the determining factor
• Thus need correct phylogeny
= Biparental care
= Nest parasitism
Conclusion: biparental care = adaptive response
Experiments
4) Experimental manipulation
• Manipulate a trait and see if affects fitness
• e.g. Swallow’s tails
• e.g. Bower birds
• e.g. Zonosemata flies
Zonosemata
• Dark banded wings, waving behaviour
• Main predator: jumping spiders
• Does wing colouration or waving reduce
predation? (mimicry?)
• 5 test flies:
• Untreated Zonosemata, sham surgery, housefly
wings, housefly with Zonosemata wings, housefly
• Against jumping spider and other predator
• Needed to have both markings & waving to
repel jumping spider (no surgery effect)
• No effect on any other predators
• Mimic jumping spiders to avoid jumping
spider predation
Cepea nemoralis
• Snails vary in colour &
# of bands (polymorphism)
• Morphotype varies with habitat
• Why?
– Engineering: thermoreg’n depends on darkness
– Experimental: camouflage – thrush predation
Examples
1. Evolution of sex
2. Sexual selection
3. Evolution of sex ratio
Evolution of Sex
• Sex is costly so why is it so common?
• Asexual reproduction is only found in
patches on the phylogenetic tree
• Asexual species have higher rates of
extinction than sexual species
Model: Asexual variant
• e.g. Given each female has 2 offspring, no
difference in survival
Asexual
Sexual
Frequency
100 females 100 females (100 males) p(female) = 0.33
200 females 100 females (100 males) p(female) = 0.5
400 females 100 females (100 males) p(female) = 0.67
Sexual vs. Asexual
• Sexual females lose ½ genes in each generation
– to survive to repro females must be fit but
their mate may be less fit
• Sexual female has ½ fitness of asexual
• Plus, costs of finding a mate, STDs etc.
• Given this disadvantage, there must be a
benefit in sexual reproduction
Model’s Assumptions Violated
1. Reproductive mode does not affect number
of offspring
Parental care/Nuptial gifts (fairly rare)
2. Reproductive mode does not affect survival
of offspring
Group Selectionist Argument:
sex accelerates rate of evolution
• Increases a group’s
ability to respond to
changing environment
• Asexual populations
have a higher
extinction rate
Given 2 loci with 2
alleles (Aa Bb):
p(A) >>> p(a)
p(B) >>> p(b)
(A & B are “fixed”)
a & b interact to increase
fitness
How get aabb in one individual?
1) Asexual: AABB  aabb only by mutation
get AaBB and AABb but:
p(AABB  aabb)  0
1) Sexual: recombination
AaBB x AABb
Gives: AABB; AaBB; AABb; AaBb
AaBb x AaBb = aabb
Mutant genotype can arise quickly and prevent
extinction
Mutation rate is important
Mutation rate slow
Sexual  Asexual
No advantage to sex
Mutation rate fast
Sexual > Asexual
Thus, sexual pop’ns can outcompete asexual pop’ns
Sex is still disadvantageous to the individual