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