Selection, Levels of

In the Origin of Species (1859), Charles Darwin introduced his theory of natural selection, the generally accepted mechanism for evolutionary change. More organisms are born than can survive and reproduce; there will consequently be a struggle for existence. Given naturally occurring variation, the struggle will bring on a process equivalent to a breeder's artificial selection: a differential reproduction leading to evolutionary change of a kind that centers on adaptation, producing contrivances like the hand and the eye. A matter of immediate interest was the level at which natural selection was supposed to operate. Does the struggle occur between individuals or between groups like species? If the latter, can adaptations benefit the group at the expense of the individual? Could one have "altruistic" adaptations where, instead of an organism selfishly serving its own ends, it sacrifices its well-being and possibilities for reproduction to the common good? Darwin himself was inclined to think not, although he did equivocate with regard to human beings. A contrary tradition was initiated by the co-discoverer of natural selection, Alfred Russel Wallace, who, as a good socialist, was convinced that selection can work for the good of the group, even if the individual suffers thereby.

Matters went essentially unresolved until the 1960s. Although some (notably R. A. Fisher) stuck to the Darwinian line, a position like Wallace's, endorsing what came to be known as group selection, was assumed implicitly by most evolutionists. Then a strong reaction set in, and thinking swung to a Darwinian mode. Biologists realized that the trouble with group-directed altruistic adaptations is that they are open to cheating. While the altruist is working for the good of the group at its own expense, the selfish individual is benefiting thereby, and at the same time serving itself by refusing to direct any effort to others. Selfishness will therefore win out in the struggle for existence and altruism will go extinct.

At about the same time, a number of new models based on selection for self (individual selection ) were devised. Notable was the idea of kin selection, introduced by British evolutionist William Hamilton, which showed how close relatives help each other for shared biological ends. Particularly impressive was the way in which Hamilton demonstrated how his new mechanism could account for the sterility of worker ants, bees, and wasps. These groups are exceptional in that only females have both mothers and fathers, males being born of unfertilized eggs. This leads to nonstandard genetic relationships where females are more closely related to sisters than they are to daughters. Hence, selection favors adaptations (including sterility) that motivate females to raise fertile sisters rather than fertile daughters. In so doing, one is accomplishing more to increase one's genetic representation in future generations than one would if one followed more traditional patterns of reproduction.

The Hamiltonian-type approach is often referred to as genic selection because ultimately it sees evolution as a matter of the sorting of the genes, the units of inheritance, and evolutionary change as a simple function of change of gene ratios. While this is true, it does not mean that the individual organism drops out of sight, for it is organisms that package genes and it is organisms that compete in the struggle for existence. For this reason, it is helpful to distinguish between genes as replicators, the markers of evolutionary change, and organisms as vehicles or interactors, the carriers of genes and the units that struggle for supremacy. At both levels and in both appropriate senses, one has units of evolution.

Other models similar to kin selection were devised showing how "selfish genes" can nevertheless lead to cooperative behavior between organisms. The best known is perhaps reciprocal altruism (something of which Darwin had an inkling) where organisms cooperate because benefits given are linked to the expectation of benefits to be received. At the same time, students of the evolution of social behavior turned to game theory to work out how organisms, mainly animals but some plants, adopt different strategies to maximize their evolutionary success. This activity is all a thriving part of the evolutionary enterprise, both theoretically and empirically. It is true that in the past a number of evolutionists have produced theories and experiments showing that, under certain circumstances, group effects within a species can swamp individual interests, but this is in no sense a return to old-fashioned group selection. It is also true that some paleontologists think that in the course of history one sees some species succeeding in systematic ways, while others do not. But such species selection is compatible with an individualist approach at the level of the organism. There is a richness to the evolutionary process, something that can work in many ways and at many levels.

See also Adaptation; Altruism; Evolution; Fitness; Selfish Gene

Bibliography

ruse, michael. darwin and design: science, philosophy, and religion. cambridge, mass.: harvard university press, 2003.

sober, elliot, and wilson, david sloan. unto others: the evolution and psychology of unselfish behavior. cambridge, mass.: harvard university press, 1998.

sterelny, kim, and, griffiths, paul e. sex and death: an introduction to philosophy of biology. chicago: university of chicago press. 1999.

michael ruse