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Download The nature of adaptations
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The nature of adaptations Adaptive trait • A trait that enhances fitness compared with an alternative trait • Including historical perspective – Character state may be a consequence of evolutionary history and not an adaptation Preadaptation Exaptation Not all traits are adaptations • Necessary consequence of chemistry or physics • Evolution by genetic drift • Hitchhiking and pleiotropy • Phylogenetic history How do you recognize adaptations? • Complexity • Design • Prediction and observation • Experiments • Comparative methods Complexity • Complex traits probably have an adaptive function Design • Function is inferred by design or matches predictions – Functional morphology – Ecophysiology Prediction and observation • Identify or postulate genetic variants of trait • Develop hypothesis of function of trait; develop alternative hypotheses and predictions • Test predictions Observation • Example: Cepaea (land snails) color pattern Observation • Hypothesis: Banding patterns important for camouflage • Prediction: Habitat segregation – Banded snails in complex background – Unbanded snails in uniform background Observation • Test Observation • Hypothesis: Banding pattern important for camouflage • Prediction: Higher mortality for banded snails than unbanded snails in woods due to predation Observation • Test: predation rates by song thrush • Frequency of banded in one site: 47.1% • Frequency of banded snails eaten by thrushes: 56.3%. Experiment • Manipulate trait to test hypothesis • Could paint bands on snail shells and examine whether predation rate increases • More powerful than observation (why?) Experiment • Example: touch-me-not petals Experiment • Hypothesis: Petals important in attracting pollinators • Prediction: Flowers with smaller petals will have fewer bee visits than those with big petals Experiment • Test: Compare normal flowers with surgically altered flowers Experimental control • An important control: ‘whole’ flowers were lightly trimmed around edges • Tests for unexpected effects of treatment • Another example: Should apply paint to all snail shells, in case paint itself affects predation Experiment • Example: tephritid flies and jumping spiders Why to flies wave wings? • Scare away other predators • Scare away jumping spiders What makes a good experiment? • Consideration of alternative hypotheses • Clear predictions that distinguish among hypotheses • Good controls, randomization, replication Why don’t we always do experiments? Comparative method • Compare trait among related species • A good method when interesting variation is among, rather than within, species Comparative method • Example: Testis size, body mass, and mating system in primates Comparative method • Example: Testis size and social group size in bats Comparative method • Important issue: must consider relationships among species. Are points on graph independent? Phylogenetically independent contrasts Phylogenetically independent contrasts Comparative method: our example revisited Constraints to adaptation – Genetic – Physiological and developmental – Phyletic Constraints to adaptation • Genetic – Lack of genetic variation – Genetic correlations among traits Example: genetic correlations • Sizes of different flower parts are correlated in radish • Very strong selection required to ‘break’ correlations and change shape of flowers Physiological constraint • Fuchsia flowers retained after pollination because growth of pollen tubes takes time Developmental constraint • Larval insects never have compound eyes of adults Developmental constraint • Limitations to animal body plans Constraints to adaptation • Phyletic – Everything evolves from something else – Basic idea: you’re stuck with your history Example of phyletic constraint • The panda’s thumb Example of phyletic constraint Constraints: summary • Slow or prevent adaptation • Relative importance varies among populations or species
