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 ...
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...
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