How are adaptationist hypotheses built and tested? Examples

Why do we observe sexual dimorphism in humans? H1: Because males and females consume different food resources as a consequence of division of labor. H2: Because males compete for females. There is sexual selection for bigger males.

Sexual Dimorphism – Comparative Method

Why do we observe sexual dimorphism in humans? There are many dots on the graph which significantly deviate from the regression line. Results neither confirm nor falsify adaptationism. They only show that natural selection is relevant in explaining dimorphism in some species. Adaptationism is tested in the long run.

Copulation time in dung flies: Mathematical model Given that: – Search and guard time is minutes, – First male fertilizes 20% and second male fertilizes 80% of eggs, – Fertilization rate increases with with time spent in copulation with diminishing returns, – Optimal copulation time is the one which maximizes # Fertilized eggs/unit time, What is the optimum time of copulation?

Copulation Time – Mathematical Model

Giraffe’s Neck: Why is it so long? H1: Giraffe necks have evolved to be long because long necks allowed giraffes to reach higher branches in feeding. H2: Giraffe necks evolved as a fighting tool for mating.

Giraffe Necks Evidence against H1: – Giraffes usually don’t feed on tall trees. They feed on short bushes. – They usually use their necks as a weapon in competiton for females. – There should be a correlation between giraffe neck length and mating success.

Giraffe Neck

Flies vs. Jumping Spiders: Mimicry? Zonosemata vittigera performs a special dance whence threatened. The dance resembles the defensive dance of its hunter: jumping spider. What is the function of this behavior? – H1: Flies do not mimic jumping spiders. In many fly species, these dances are used as mating displays. – H2: Flies mimic jumping spiders not for threatening them but for threatening other predators. – H3 Flies mimic jumping spiders in order to dissuade them from hunting.

Flies vs. Jumping Spiders: Mimicry? A: Normal Zonosemata B: Wings cut and pasted. C: Wings cut and replaced with transparent wings of house fly. D: Housefly with Zonosemata wings E: Normal housefly

Predictions ABCDE H1 JSAttacks OthersAttacks H2 JSAttacks OthersRetreat Attacks H3 JSRetreat Attacks OthersAttacks

Zonosemata vs. Salticidae: Results

Why do some bats have bigger testicles? Female bats mate with multiple males. Sperms of different males compete for fertilizing the egg. Greater ejaculation means a greater chance of fertilization. Bigger testicles allow this outcome. Predictions: We should find positive correlation between group size and testicle size.

Bat Testicle Size – Comparison

Trade-off Female begonias are pollinated by bees. Male begonias offer bees lots of pollens whereas female ones don't. Female begonias look like males (same size, same color, etc). H1: Female flowers are visited by bees more if they resemble male flowers better. So, optimum phenotype for females is resembling the average male flower. H2: Pollination success (attracting the bees) depends on size, there is a linear relation. The greater the better.

Trade-off

Host change in Ophraella: Constraint Hosts produce toxins. Ophraella should detoxify it. Why has some host changes occurred and others not? H1: Chance and ecological factors. There was adequate genetic variation. H2: There wasn’t adequate genetic variation.

Constraint

Additional resources Evolutionary Analysis (Freeman and Heron) 2nd edition Pratt D., Anderson, V., “Giraffe social behaviour”, Journal of Natural History, 1985, 19: