Evolution and Adaptation

- mutation- migration- genetic drift- Natural Selection

Darwin’s Postulates:

(1) There is heritable variation

(2) There is a struggle for existence

(3) Variation influences the struggle

and Natural Selection follows ...

(1) There is heritable variation

Heritability in selected human traits:

Handedness 30%Diastolic blood pressure 45%Twinning 50%Systolic blood pressure 55%Body weight 65%Stature and tooth size 85%

Fertility 10-20%IQ 60-80%

(2) There is a struggle for existence

Resources are limitingThere is competition for resources, including mates

(3) Variation influences the struggle

dicots

monocots

vs..

Columbian ground squirrel

min energyconstraint

timeconstraint

Digestiveconstraint

Optimal diet

Dicots consumed

Mon

ocot

s co

nsum

ed

Mixture of monocots and dicots- Monocots limited by handling time- Dicots limited by digestion

Mark Ritchie compare the predicted “optimal” diet and theactual diet for 109 individuals squirrels

Dicots consumed

Mon

ocot

s co

nsum

ed

r2 = 0.94

(a) variation in the ability to forage optimally

Mark Ritchie compare the predicted “optimal” diet and theactual diet for 109 individuals squirrels

Dicots consumed

Mon

ocot

s co

nsum

ed

r2 = 0.94

(a) variation in the ability to forage optimally

Deviators

Optimal

Mother’s deviation

Off

spri

ng’s

dev

iati

on

(b) optimal foraging is a heritable trait

Mothers raise offspring

Offspring on their own

(c) There is a struggle for existence

Relative to optimal foragers, deviators have lower surplus energy intake and ....

(c) Variation influences the struggle

... as a consequence, deviators havei) lower somatic growth ii) lower survivaliii) smaller litter sizes

Mark Ritchie’s study beautifully illustrates Darwin's Postulates in action:

Heritable VariationStruggle for ExistenceVariation influences the Struggle

but...falls short of documenting Natural Selection

Evolution by Natural Selection – Guppies on the island of Trinidad

Evolution by Natural Selection – Guppies on the island of Trinidad

Life history traits

Schooling behavior- dilutes individual risk- greater vigilance- group confusion

Predator-inspection behavior- method to ascertain the identity and intentions of the assailant

lowrisk low

risk

(Magurran et al. 1996)

Male coloration and female choice

Predation risk(cichlids)

Predation risk(prawns)

Low risk

mean # spots

mean size

(Endler 1980)

Correlations vs. experimental testsHaskin’s 1957 transplant experiment

black = % females schooling

10mm

(Magurran et al. 1996)

Transplant experiment results:

Summary:

1) “Natural experiment” – Guppy populations that have experienced different regimes of predation risk show different levels of anti-predator behavior

2) Transplanted (1957) high-risk guppies behave like native low-risk guppies (evolution in 34 years or ~100 generation)

3) Changes in color-patterns that function in mate choice were apparent after

13 months!!

At what level does Natural Selection operate?

The Individual or the Group?

“lions rarely fight to the death because if they did it would endanger the survival of the species”

“salmon migrate thousands of miles from the ocean to their inland spawning grounds killing themselves in the process with exhaustion to ensure the survival of the species”

Wynne-Edwards proposed that organisms have adaptations to ensure its population or species controls its rate of consumption Likewise, individuals should restrict their birth

rate to prevent over-population

Are these accurate statements??

“lions rarely fight to the death because if they did it would endanger the survival of the species”

“salmon migrate thousands of miles from the ocean to their inland spawning grounds killing themselves in the process with exhaustion to ensure the survival of the species”

Wynne-Edwards proposed that organisms have adaptations to ensure its population or species controls its rate of consumption Likewise, individuals should restrict their birth

rate to prevent over-population

Are these accurate statements??

NO!

Natural selection acts at the level of the individual, not the group

CS

C

C

C

C

C

C

C

CS

S

S

S

S

S S

S

S

C

X

X

CS

C

C

CC C

CS

C

C

CC C

Group Selection – differential survival/reproduction of groups

But why won’t this work?

1) Groups would have to die out faster than individuals, which rarely happens

2) Groups would have to be isolated

3) “Cooperative” groups are always vulnerable to invasion of selfish individuals

This does not mean cooperation or behaviors that serve the “good of the group” cannot evolve (reality tells us differently), butrather that most of these behaviors are inherently selfish

Aka. Trajedy of the Commons

The “selfish” individual reaps the rewards in a world of self-restraintIt receives a private benefit while everyone shares the public cost

Introduction to:

alternative mating strategies sexual selection and mate choicereproductive behavior and the roles of males and femalesforaging behavioranti-predator behaviorliving in groupscooperationsocial contracts

Formulating and testing hypotheses about the evolution of behavior:

(1) Experimental approaches – particularly those that make quantitative rather than qualitative predictions

(2) The comparative approach – when experiments fail...