Sociobiology

In recent decades, another branch of evolutionary theory has appeared, as researchers have explored the possibility that not only physical traits, but also behaviour itself, might be inherited. Behavioural geneticists have studied how genes influence behaviour, and more recently, the role of biology in social behaviour has been explored. This field of investigation, known as sociobiology, was inaugurated in 1975 with the publication of the book "Sociobiology: The New Synthesis" by American evolutionary biologist Edward O. Wilson. In this book, Wilson proposed that genes influence much on animal and human behaviour, and that these characteristics are also subject to natural selection.

Sociobiologists examine animal behaviours that are called altruistic – that is, unselfish, or demonstrating concern for the welfare of others. When birds feed on the ground, for example, one individual may notice a predator and sound an alarm. In so doing, the bird also calls the predator's attention to itself. What can account for the behaviour of such a sentry, who would seem to derive no evolutionary benefit from its unselfish behaviour and so seem to defy the laws of natural selection?

Darwin was aware of altruistic social behaviour in animals, and of how this phenomenon challenged his theory of natural selection. Among the different types of bees in a colony, for example, worker bees are responsible for collecting food, defending the colony, and caring for the nest and the young, but they are sterile and create no offspring. Only the queen bees reproduce. If natural selection rewards those who have the highest reproductive success, how could sterile worker bees come about by natural selection when worker bees devote themselves to others and do not reproduce?

Scientists now recognize that among social insects, such as bees, wasps, and ants, the sterile workers are actually more closely related genetically to one another and to their fertile sisters, the queens, than brothers and sisters are among other organisms. By helping to protect or nurture their sisters, the sterile worker bees preserve their own genes – more so than if they actually reproduced themselves. Thus, the altruistic behaviour evolved by natural selection.

Punctuated Equilibria

Evolutionary theory has undergone many further refinements in recent years. One such theory challenges the central idea that evolution proceeds by gradual change. In 1972 the American paleontologists Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual transformation within a lineage, but rather result from quick bursts of rapid evolutionary change. In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of many small genetic changes over long periods of geologic time. In the fossil record, however, new species generally appear suddenly after long periods of stasis – that is, no change. Gould and Eldredge recognized that speciation more likely occurs in small, isolated, peripheral populations than in the main population of the species, and that the unchanging nature of large populations contributes to the stasis of most fossil species over millions of years. Occasionally, when conditions are right, the equilibrium state becomes "punctuated" by one or more speciation events. While these events probably require thousands or tens of thousands of years to establish effective reproductive isolation and distinctive characteristics, this is but an instant in geologic time compared with an average life span of more than ten million years for most fossil species. Proponents of this theory envision a trend in evolutionary development to be more like climbing a flight of stairs (punctuations followed by stasis) than rolling up an inclined plane (Darwinian gradualism).