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) = 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 mutationget AaBB and AABb but: p(AABB aabb) 0

1) Sexual: recombinationAaBB x AABb

Gives: AABB; AaBB; AABb; AaBbAaBb 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