Lecture 8: Adaptive Explanation Non-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

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

Pharyngeal Nerve 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!)

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

Non-adaptive Differences time 1 time 2 time 3 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)

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

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

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

Summary 1.Adaptations do occur They are characters appropriate to env’t 2.Adaptations only result from NS 3.Adaptations may not be recognized 4.Adaptations may be imperfect

How to tell if something is adaptative: 1. Adaptive prediction Test observed against model Beneficial, purposive 2. 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

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!!