Chapter 12

Principles of Social Evolution

1. A great puzzle identified by Darwin was the evolution of altruism. Because self-sacrificing altruists help other individuals reproduce, one would think that this behavior should be naturally selected against and so should disappear over time. After all, a behavior that favors other individuals reproducing at the expensive of one’s self seems to defy the theory of evolution by natural selection.

2. Extreme altruism is common in eusocial (caste-forming) insect colonies where workers rarely, if ever, reproduce and instead help their colony mates survive to reproduce. Many solutions to this puzzle have been offered, including the claim that group selection favors colonies that contain altruists because these colonies produce more new colonies than do groups without altruists.

3. Most students of animal social behavior employ inclusive fitness theory and the concept of kin selection, both of which operate at the level of the individual, because they have repeatedly proven to be useful for scientific investigation of sociality. Specifically, inclusive fitness theory has been used to show that altruism can spread through a population if the cost to the altruist in terms of a reduction in the number of offspring produced (multiplied by the coefficient of relatedness between the altruist and those offspring) is less than the increase in the number of related individuals helped by the altruist (multiplied by the average coefficient of relatedness between the altruist and the helped relatives).

4. Kin selection can result in an increase in the number of genes transmitted indirectly by an individual to the next generation in the bodies of relatives that exist because of the altruist’s help. An individual’s total genetic contribution to the next generation is called its inclusive fitness.

5. Inclusive fitness theory underlies modern “gene-centered thinking,” with researchers now aware that individuals should behave in ways that boost their inclusive fitness, whether this is achieved through self-sacrificing cooperation or through self-serving conflict with others, even close relatives.

6. Sociogenomics, the study of the genes that influence social behavior and social diversity, allows researchers to probe the genetic architecture of sociality and caste differentiation. Although many socially relevant genes have been co-opted from those underlying solitary behaviors, others are rapidly evolving and appear to play a key role in social evolution.

7. Although cooperation broadly and altruism specifically form the basis of all eusocial insect societies, social conflict in the form of opposition over who breeds (reproductive conflict) and occasionally over who mates with whom (sexual conflict) is an inherent part of social living. In many eusocial societies, social conflict occurs not just among queens and/or kings, but also among workers, which in some Hymenoptera can produce unfertilized eggs that become males.