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

 

Evolution By Natural Selection

Evolution occurs in a number of 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 described as genotypes or as phenotypes, a gene’s effects on a trait. 

Consider a trait that, across the population,

  1. Is variable. The trait differs among individuals.

  2. Is heritable. Differences are passed on from parents to offspring, at least in part, via genes.

  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 increase reproductive success or fitness 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 blind process takes chance variation in genes and turns it into organismal sophistication. 

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

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

Social Evolution Theory

Yet, Darwin’s account was incomplete. In the 1960s and 1970s, William Donald 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 either partly or completely to help others. Rather than make 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) occurs as well. 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 more precise definitions of these concepts, 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, with 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.