Biol 4802 Evolution
Lecture 21, Chapter 16
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
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 old only
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 old only
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. 11.15 new 11.13 old
2. 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. 16.17 new 14.16 old
o Worker ants manipulate sex ratio to optimize fitness
 Single queen
 Optimal fitness for queen is ♀1:1♂ reproductive individuals
 Optimal fitness for workers is ♀3:1♂ reproductive individuals
 Multiple queens
 Optimal fitness for workers closer to ♀1:1♂
Fig. 16.19 new Fig. 14.17 old