Discussion: Ghiselin and Mayr on species

Abstract

think." But, as she says, things do not come to us with their label on them. "The argument that for some purposes Homo sapiens ... is a class, for some purposes an individual, depending on one's point of view, does not mean that entities in the world change as we move between individual and class. Rather the argument means that Homo sapiens has many real properties" (Heise, 1981). If we look at some properties we think of it as an individual. But if we look at others we think of it rather as a class. "In each case the judgment is based on what we take to be real properties, but they do not all work to give us a single concept by means of which we unite the plural entities" (Heise, 1981). As a matter of fact, such an instrumental approach is often adopted also by Ghiselin. He prefers, for example, the pragmatic ecological criterion of the species definition, "'a reproductive community of populations ... that occupies a specific niche in nature", over Simpson's theoretical definition of a species as an evolutionary lineage (Ghiselin, 1987, p. 139). The reason seems to be that this latter, more "objective" definition, leads at once to difficulties: A lineage is said to evolve "with its own unitary role and tendencies." But what are these "roles"? And, do asexual organisms form a lineage? To summarize, in the framework of the theory of evolution through natural selection species are individuals, and individuals are contextdependent regulative entities.