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Testing adaptive hypotheses 1 What is (an) adaptation? What is (an) adaptation? Generally speaking, adaptations are traits or characters that appear to be too well-fitted to their environment to have arisen by chance. That is, they must be the result of selection. Adaptations may involve morphological, physiological or behavioural traits. They arise through the accumulation of a series of small improvements over time. A trait, or integrated set of traits, that increases the fitness of an organism. The process of improving the fit of phenotype to environment through natural selection Testing adaptive hypotheses What is (an) adaptation? "If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous successive slight modifications, my theory would absolutely break down." — Darwin Testing adaptive hypotheses 2 3 Testing adaptive hypotheses Examples of adaptations The eye Bird wings The human brain Homeothermic temperature regulation Human language 4 Testing adaptive hypotheses 5 Identifying adaptations Testing adaptive hypotheses 6 Identifying adaptations – an incomplete example Why do polar bears have white coats? • Adaptive hypothesis: In order to identify a trait as an adaptation, we must first hypothesize its use or function, and then test that hypothesis. As we saw in the Gould & Lewontin article, it is important to test the hypothesis of adaptation against a variety of null and alternative hypotheses. camouflage white coat is an adaptation for • Test: observe hunting behaviour and assay use of camouflage • Result: camouflage not usually important in hunting • New adaptive hypothesis: trapping solar heat white coat is an adaptation for • Test: hairs are actually clear and translucent, and trap 16% of incident light energy – better than most hair types. Results don’t support our first adaptive hypothesis. Testing adaptive hypotheses 7 Testing adaptive hypotheses Identifying adaptations – an incomplete example Identifying adaptations – an incomplete example Why do polar bears have white coats? Why do polar bears have white coats? • Adaptive hypothesis: camouflage white coat is an adaptation for • Test: observe hunting behaviour and assay use of camouflage • Result: camouflage not usually important in hunting • New adaptive hypothesis: trapping solar heat white coat is an adaptation for • Adaptive hypothesis: camouflage white coat is an adaptation for • Test: observe hunting behaviour and assay use of camouflage • Result: camouflage not usually important in hunting • New adaptive hypothesis: trapping solar heat white coat is an adaptation for • Test: hairs are actually clear and translucent, and trap 16% • Test: hairs are actually clear and translucent, and trap 16% Results are consistent with our new adaptive hypothesis. What’s missing? of incident light energy – better than most hair types. of incident light energy – better than most hair types. 8 9 Testing adaptive hypotheses Identifying adaptations Testing adaptive hypotheses 10 Approaches to testing adaptive hypotheses The polar bear example shows that ecological and physiological patterns are consistent with one particular adaptive hypothesis – but it doesn’t show that fur color evolved via a process of adaptation. We’ve compared alternative hypotheses of adaptation – but we haven’t tested the biological null hypothesis: that no adaptation has occurred. The primary null hypothesis is that traits have evolved due to drift (according to the neutral model). Ex per im ent al st udies There are various experimental and theoretical ways to Ex . test Wing marks &ofwing waving inhas Tephritid flies. hypotheses adaptation. Each its benefits and drawbacks: • Observational studies (e.g., the polar bear study) Tephritid flies have dark bands on their wings and • Experiments wave their wings when disturbed in a manner that is Theoretical models • reminiscent of their major predator's territorial Comparative method display• -e.g., jumping spiders' leg waving. • Molecular evidence Do flies mimic their predators? Testing adaptive hypotheses Experimental approaches: example Tephritid flies have dark bands on their wings. When disturbed, they wave their wings in a manner reminiscent of the territorial behavior of their predator (leg waving in jumping spiders). Do the flies mimic their predators? Does this mimicry deter predators? Does it deter jumping spiders in particular? 11 If so, is this mimicry to deter any predator, or is it specifically to deter jumping spiders? 12 Testing adaptive hypotheses Experimental approaches: example Testing Adaptive Hypotheses 13 Testing adaptive hypotheses 14 Experimental approaches: example The evidence suggests that the morphology and behavior of the flies increases their fitness in the face of their primary predator. Text Conclusion: Unlike the observational approach, this gives direct evidence of a fitness advantage. However, experiments like this still don’t directly test the hypothesis that the traits evolved due to the process of adaptation. Testing adaptive hypotheses Theoretical approaches Two classes of models predict how a trait should evolve under a specific set of environmental circumstances (usually ignoring genetics altogether). • Optimality models assume that a trait will evolve to impart the highest possible fitness. • Evolutionarily Stable Strategy (ESS) models assume that the fitness of a phenotype depends on what other phenotypes are present. This doesn’t always lead to maximal fitness. 15 Testing adaptive hypotheses Theoretical approaches – example We might do an ESS model to explore the possibility that lekking behavior in birds is an adaptation to predation risk (because the risk of predation gets spread out among large numbers of males). The model would predict the number of males we should see on a lek as a function of the risk of predation and the likelihood of mating. We could then measure whether the number of males we observe in nature is consistent with the predictions of the model, given our observations of predation and mating probabilities. 16 Testing adaptive hypotheses 17 Theoretical approaches – example Testing adaptive hypotheses 18 The comparative method These models offer quantitative predictions for observational or experimental studies. If we have an adaptive hypothesis for a trait, then we might expect to see a correlation between the explanatory variable (X) and the trait itself (Y). However, they seldom compare predictions to alternative hypotheses (either adaptive or non-adaptive) Also, the quantitative predictions might not take into account constraints on adaptation – any failure of observations to match predictions might be due to such constraints. However, another possible explanation for such a correlation is the process of evolution itself: organisms whose common ancestors had both X and Y are also Thelikely Comparative Method to have X and Y. Ex . Testis size in bat species. Male bats vary in the size of testes. One hypothesis for this variation is that large testes produce more sperm, an advantage in sperm competition. Testing adaptive hypotheses The comparative method – Example Testis size in bats Male bats vary from species to species in the size of their testes. Adaptive hypothesis: Larger testes produce more sperm, which provides an advantage if sperm from multiple males competes for fertilization in a female. Prediction: Species with larger social groups should have males with larger testes (because more males are competing for reproductive access to females). 19 Sperm competition might be more intense in larger social groups, where more males compete for reproductive access to females. Testing adaptive hypotheses The comparative method – Example Prediction: Species groups Hypot hesis: Testis with size islarger largersocial in species withshould larger social groups. have males with larger testes (because more males are competing for reproductive access to females). 20 1. Need phylogeny 2. Calculate contrasts between sister taxa Closely related3.species Evaluate relationship with phylogeneticallymight have similarcorrected group values. size and testis size NOT But not all data points are because of sperm independent. The comparative method – Example The comparative method – Example competition, but because they share an ancestor Null hypothesis: Closely related species might have who had large testes and similar group size and testis size simply because they Closely related species lived in large groups. share a common ancestor.might have similar group size and testis size NOT But not all data pointsbecause are of sperm independent. competition, but because How do we control for they share an ancestor the effects of shared who had large testes and history? lived in large groups. Closely related species might have similar group size and testis size NOT How do we control for because of sperm the effects of shared competition, but because history? they share an ancestor who had large testes and lived in large groups. Testing adaptive hypotheses Testing adaptive hypotheses The comparative method – Example How do we control for the effects of shared Controlling for common ancestry, males in species history? larger groups sizes still have larger testes. with 21 23 Testing adaptive hypotheses Testing adaptive hypotheses 22 24 The comparative method Evidence that phylogenetically independent contrasts of a trait (e.g., testis size) are correlated with a hypothesized explanatory variable (e.g., group size) suggests that: • The trait has evolved in the absence of (or despite) phylogenetic constraint • Not all evolution of the trait has been neutral (because there is evidence of directionality) The comparative method explicitly tests adaptive hypotheses against a null hypothesis. Yes, males in species with larger group sizes also Testing adaptive hypotheses The comparative method – caveats “Correlation is not causation.” Still not a direct rejection of the neutral model. 25