Origin of swords in swordtail fishes: the pre-existing bias hypothesis Idea: Females evolve a sensory bias which causes them to prefer males with exaggerated features. Hence, male morphology is driven by sexual selection.
Testing this idea... Female choose males based on sword length. Experiments on X. helleri show females prefer males with longer swords. Females of swordless species (platys) prefer males with swords. Experiments on X. maculatus confirm this. Absence of swords must be primitive, so that female's bias for swords predates the evolution of swords. ?
Correlated evolution Have two characters evolved together?
Gregariousness in aposematic butterfly larvae Aposematic forms tend to be gregarious. R. A. Fisher suggested warning colouration evolved through kin selection. An individual may die during the lesson that teaches naïve predator not to eat brightly coloured larvae If predator leaves kin alone, inclusive fitness of dead larvae is positive Laying eggs in clutches will result in kin groups on same plant Prediction: aggregation evolves before colouration
Correlated character change AB ab A B AB Ab ab A B Does character A always arise with character B? Does character A always precede character B?
Prediction based on kin selection unpalatable gregarious unpalatable gregarious
Results: gregariousness evolves after unpalatability unpalatable gregarious unpalatable gregarious
Comparative method Phylogeny makes cross-species comparisons non independent
Species arent independent A C B D E F ABCDEF ABCDEF
Summary Given a phylogeny we can infer ancestral states These states can be used to test hypotheses Comparisons between species are not independent The comparative method provides tools to extract independent comparisons from a phylolgeny