Lecture 2: Analysis of Adaptation

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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. Giraffes neck

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Adaptations

• When analyzing adaptation we need to remember
• Not all features of a population are adaptive
• Not all adaptations are perfect

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Analysis of Adaptation

• We need to
• Show that a trait has been shaped by NS
• Determine the agent of selection

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4 Ways to Identify an Adaptation

• COMPLEXITY
• Complex structures are usually adaptive
• e.g. ampullae of Lorenzini
• Variants of complex structures may not be
  adaptive (e.g. Hb)

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2) Engineering

• Does the trait fit efficient model predicted by
  engineering?
• e.g. Fish shapes
• Fits aerodynamic prediction
• Form fits function

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3) Convergence

• Correlational Evidence Convergent Evolution

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• 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

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• Eliminate the effect of common ancestry
  therefore ecology is the determining factor
• Thus need correct phylogeny

Conclusion biparental care adaptive response
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Experiments

• 4) Experimental manipulation
• Manipulate a trait and see if affects fitness
• e.g. Swallows tails
• e.g. Bower birds
• e.g. Zonosemata flies

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Zonosemata

• Dark banded wings, waving behaviour
• Main predator jumping spiders
• Does wing colouration or waving reduce predation?
  (mimicry?)

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• 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

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Cepea nemoralis

• Snails vary in colour
• of bands (polymorphism)
• Morphotype varies with habitat
• Why?
• Engineering thermoregn depends on darkness
• Experimental camouflage thrush predation

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Examples

1. Evolution of sex
2. Sexual selection
3. Evolution of sex ratio

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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

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Model Asexual variant

• e.g. Given each female has 2 offspring, no
  difference in survival

Asexual 100 females Sexual 100 females (100 males) Frequency p(female) 0.33
200 females 100 females (100 males) p(female) 0.5
400 females 100 females (100 males) p(female) 0.67
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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

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Models 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

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Group Selectionist Argumentsex accelerates rate
of evolution

• Increases a groups ability to respond to
  changing environment
• Asexual populations have a higher extinction rate

• Given 2 loci with 2 alleles (Aa Bb)
• p(A) gtgtgt p(a)
• p(B) gtgtgt p(b)
• (A B are fixed)
• a b interact to increase fitness

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How get aabb in one individual?

• Asexual AABB ? aabb only by mutation
• get AaBB and AABb but
• p(AABB ? aabb) ? 0
• Sexual recombination
• AaBB x AABb
• Gives AABB AaBB AABb AaBb
• AaBb x AaBb aabb
• Mutant genotype can arise quickly and prevent
  extinction

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Mutation rate is important

• Mutation rate slow
• Sexual ? Asexual
• No advantage to sex

• Mutation rate fast
• Sexual gt Asexual

Thus, sexual popns can outcompete asexual
popns Sex is still disadvantageous to the
individual