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

Historical Constraint

• May lead species in what will become the wrong direction & trap them there

local optimum

globaloptimum

selectionon character

selectionon character

fitne

ss

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 historyfi

tnes

s

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!!