The units of selection

TLDR: The principle of natural selection as the motive force for evolution was framed by Darwin in terms of a "struggle for existence" on the part of organisms living in a finite and risky environment, but the logical skeleton of his argument turns out to be a powerful predictive system for changes at all levels of biological organization.

Abstract: The principle of natural selection as the motive force for evolution was framed by Darwin in terms of a "struggle for existence" on the part of organisms living in a finite and risky environment. The logical skeleton of his argument, however, turns out to be a powerful predictive system for changes at all levels of biological organization. As seen by present-day evo­ lutionists, Darwin's scheme embodies three principles (Lewontin 1) : 1. Different individuals in a popUlation have different morphologies, physiologies, and behaviors (phenotypic variation). 2. Different phenotypes have different rates of survival and reproduc­ tion in different environments (differential fitness). 3. There is a correlation between parents and offspring in the contribu­ tion of each to future generations (fitness is heritable). These three principles embody the principle of evolution by natural selec­ tion. While they hold, a population will undergo evolutionary change. It is important to note a certain generality in the principles. No particular mechanism of inheritance is specifi'ed, but only a correlation in fitness between parent and offspring. The population would evolve whether the correlation between parent and offspring arose from Mendelian, cytoplasmic, or cultural inheritance. Conversely, when a population is at equilibrium under selection (for example, a stable polymorphism due to heterozygous superiority), there is no correlation in fitness between parent and offspring, no matter what the mechanism of inheritance. Nor does Principle 2 specify the reason for the differential rate of contribution to future generations of the different phenotypes. It is not necessary, for example, that resources be in short supply for organisms to struggle for existence. Darwin himself pointed out that "a plant at the edge of a desert is said to struggle for life against the drought." Thus, although Darwin came to the idea of natural selection from consideration of Malthus', essay on overpopulation, the element of competition between organisms for a resource in short supply is not integral to the argument. Natural selection occurs even when two bacterial strains are growing logarithmically in an excess of nutrient broth if they have different division times. The generality of the principles of natural selection means that any enti­ ties in nature that have variation, reproduction, and heritability may evolve. For example, if we replace the term individual with the term population and