Evolution  Lecture 21

Chapter 14

Topics for today:

•         What tips the balance between in favor of cooperative or competitive behavior?

•         Manipulation

•         Individual advantage

•         Group selection

•         Kin selection

•         Evolutionary stable strategies

EvoBeaker: Snails

Exercise 1

 

Thomas Malthus’ ideas strongly influenced Darwin

  • Populations grow when they meet their needs for subsistence
  • Larger populations stimulate greater production of resources which increase arithmetically
  • Populations grow exponentially until they hit the point of crisis
  • Creates the competition that underlies natural selection

How can cooperative relationships evolve?

  • Among cells
  • Among individuals of the same species
  • Among individuals of different species

Example

Acacia trees and ants

Maybe it is just manipulation not cooperation!

§         Other birds raise cowbird chicks

§         But is this really cooperation?

Birds have learned to reject parsitic eggs

o       Female coots dump eggs in other female’s nests

o       Sometimes recognize different color and reject

o       “Acceptors” lay fewer of their own eggs

o       Lyon 2003 – paper on the web

Sometimes cooperative behavior has a direct benefit to the individual

§         Safety in numbers

§         Cooperative behavior ultimately pays off

o       Unmated males help unrelated families

o       Increase’s reproductive success at the nest

o       Unmated males best chance to obtain a territory is to replace the father at the nest

o       Fig. 14.15

Some behavior appears to be truly altruistic and/or cooperative

Altruistic trait

§         feature that reduces the fitness of the individual but benefits others

§         Ground squirrel that trills when a mammalian predator is near is twice as likely to be killed

§         What factors need to be considered?

Both cooperative and competitive interactions are involved

Fig. 14.1

 

What allows cooperative relationships?

  1. Group selection (Wynne-Edwards 1962)

§         Selfish behavior leads to higher individual fitness

§         But it also leads to rapid exhaustion of resources & population extinction

§         Group selection fails because individuals turn over more rapidly than populations

Fig. 11-13

  1. Kin selection (Hamilton 1964)

§         Focus on fitness as measured by the contribution of genes to the next generation

§         Inclusive fitness of an allele has two components

1.      Direct fitness – benefit to the individual that carries the allele

2.      Indirect fitness – benefit to other individuals that also carry the allele

Hamilton’s rule determines whether altruistic trait evolves

rb > c

§         Benefit received by the donor’s relatives, b

§         Weighted by their degree of relationship, r

§         Must outweigh the cost to the donor’s fitness, c

Example

1.      Maternal care results in self sacrifice

r = ½

c = 1

b > 2

·        Ultimate case of ultruism, eusocial animals

o       Sterile workers raise offspring of reproductive individuals

o       Many eusocial insects have haploid-diploid sex determination

§         ♀ from fertilized eggs & are diploid

§         ♂ from unfertilized eggs & are haploid

§         Coefficients of relatedness are altered, especially with single queen

o       In a single-queen colony,♀ more closely related to her sisters (r = 0.75) than to her daughter (r = 0.5) or brother (r = 0.25)

o       Inclusive fitness of ♀ > if she helps to rear reproductive sisters who may be future queens

Fig. 14.16

o       Worker ants manipulate sex ratio to optimize fitness

Ø      Single queen

§         Optimal fitness for queen is ♀1:1♂ reproductive individuals

§         Optimal fitness for workers is ♀3:1♂ reproductive individuals

Ø      Multiple queens

§         Optimal fitness for workers closer to ♀1:1♂

Fig. 14.17