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 100 females Sexual 100 females (100 males) Frequency p(female) = females100 females (100 males)p(female) = females100 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