Social evolution theory is the bedrock of modern evolutionary biology. Think of it as Darwin+.


Evolution By Natural Selection

Evolution occurs in many ways. However, only by natural selection are complex traits—adaptations—built and refined. Natural selection is the differential reproduction of "types" as a consequence of their differences. These "types" can be genotypes or they can be phenotypes, a gene's effects on a trait. 

Consider a trait that, across the population,

  1. Is variable. The trait in question differs among individuals.
  2. Is heritable. Differences are passed on from parents to offspring, at least in part.
  3. Affects fitness. Some individuals out-reproduce others because they bear different versions of the trait. 

Under these conditions, natural selection inevitably occurs. Versions of the trait that produce more offspring will become more common in the population, a simple fact that repeats itself over and again so long as these three conditions are met. This process of competition is how chance variation gives rise to organismal sophistication. 

Charles Darwin's theory of natural selection gives us not only an explanation of the exquisite complexity of species, but also of what that complexity was designed for: the maximization of an individual's fitness. 

Of course, organisms don't "know" how to maximize their fitness. Nor can they know this. Their legacy is of measured improvement, of better fit with their environment, one generation at a time. Adaptations are methods that enable organisms to optimize things—foraging, communication, mate selection, and more—that led to fitness maximization in the past.

This is why we marry psychology to evolutionary theory: to discover and characterize the adaptations that shape our behavior. 


Social Evolution Theory

Yet, Darwin's account was incomplete. In the 1960s, WD Hamilton pointed out that many behaviours violated the principle of individual fitness maximization. In eusocial species, for instance, such as ants and termites, most individuals give up their reproduction to help others. Rather than produce offspring of their own, they raise someone else's, collect food for the colony, or defend the colony from intruders. 

Indeed, such altruism is widespread. Further, there is growing evidence that spiteful behavior (harming oneself to harm others) is somewhat common. Both of these kinds of behavior pose a problem for classical Darwinism: one would expect that genes causing altruism and spite—those that reduce their bearers' fitness—would be driven out of the population by alternative genes causing selfish or mutually beneficial behavior—those that increase fitness or at least keep it constant. (For those seeking precise definitions of these concepts, click here; for a video that explains them, click here.)

Hamilton's solution to this problem was to consider the effects of a gene on copies of itself in other bodies. Offspring carry identical copies of their parents' genes and, by some probability, so do siblings, cousins, and so on. Thus, genes can affect their own frequency in the population by helping or harming others who—like descendant and collateral kin—may bear copies of themselves. With this, Hamilton extended Darwin's concept of fitness to include the effects of a gene on the fitness of the individual bearing it (the actorand on the fitness other individuals (the recipients), weighted by the probability that they also bear copies of the gene. He called this concept inclusive fitness, and it is this that individuals can be said to have evolved to maximize. 

Watch this video to see Hamilton's idea, known as Inclusive Fitness Theory, in action.