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Historical Constraints and Non-perfect Adaptations

Explore the influence of historical constraints on natural selection and the occurrence of non-perfect adaptations. Learn about the imperfections and nonadaptive differences in species, trade-offs in adaptation, and how to determine if something is adaptative.

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Historical Constraints and Non-perfect Adaptations

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  1. Lecture 8: Adaptive ExplanationNon-perfect Adaptations Continued 6) Historical constraints • Natural selection acts at each gen’n •  must be advantageous in short term • NS does not know the future – can’t plan

  2. Historical Constraint • May lead species in what will become the wrong direction & trap them there time 3 time 1 time 2 selection on character selection on character fitness global optimum local optimum character state

  3. Historical Constraint Can (& does) lead to: 1) Imperfections • b/c of “baby steps” • Local optimum  global optimum b/c small s cannot overcome “fitness valley”

  4. First evolved in fish-like ancestors Direct route from gills to brain Mammals – from brain around dorsal aorta & back to larynx Long detour in giraffes (15+ feet!) Pharyngeal Nerve

  5. Historical Constraints 2) Nonadaptive differences: • Many adaptive peaks at equal fitness • Pop’n start out close to diff’t peaks • Diff’t starting cond’ns not diff’t env’t adapt’ns

  6. time 1 time 3 time 2 Non-adaptive Differences Time 1: Character state with greatest fitness differs b/w pop’ns Time 3: Either state equally fit but pop’ns constrained by history fitness Character state (popn 1/popn2)

  7. Grouse chicks Differ in colouration Not diff’t env’tl cond’ns Diff’t genetic composition Cryptic colouration

  8. 8) Trade-offs More than one function: • can’t be optimal for both • Behavioural trade-offs • Life-history trade-offs • Can’t look at adaptations in isolation

  9. Example: Mouth • Feeding vs. Breathing • 2 palate in mammals • Boas stop breathing

  10. Summary • Adaptations do occur • They are characters appropriate to env’t • Adaptations only result from NS • Adaptations may not be recognized • Adaptations may be imperfect

  11. How to tell if something is adaptative: • Adaptive prediction • Test observed against model • Beneficial, purposive • Measure selection • Usually experimental manipulation • Fitness of character variants • Problems: selection coefficients can be small, hard to correlate with repro success, time-consuming, characters hard to measure

  12. More… 3. Heritability • NS can only work on heritable traits • Low variability = low heritability 4. Cross-species comparisons • use comparative method • Adaptations should correlate to envtl diffns Problem: Adaptation is hard to study!!

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