Department of Genetics
604 Allison Rd
Piscataway, NJ 08854-8082
Abstract - The species problem is the persistent biological and philosophical debate on the meaning of the word "species" and the methods of species identification. With a meaning of "species" that follows from a simple model of DNA replication, species are shown to be real and non-arbitrary groups of organisms, under some circumstances. However, it also follows that many organisms do not belong to species. The criteria by which a group of organisms can be considered a species is whether they share in a process of genetic drift. This simplification is a negative resolution to the problem cases of species identification; it permits a concise listing of the causes of diversity and of the reasons why species can be very difficult to identify, but it does not simplify the process of species identification. For population biologists, a reduced species concept reveals a research plan for the study of organismic diversity that focuses on the determinants of structure in patterns of genetic drift. The finding that species exist, but that some organisms do not occur in species, reveals the central difficulty of systematic theories that assume the existence of species.
The diversity of life seems to have a pattern whereby organisms fall into a limited number of types. Although the existence of these types, or species, has long been recognized (Mayr, 1982a) , the definition of the word "species" and the identification of species remain problematic. One advance is the understanding that the word "species", as often used by biologists, signifies a distinct kind of biological individuality (akin to "organism" or "cell"), and does not simply denote a group of similar organisms (Ghiselin, 1966, 1974; Hull, 1976, 1978). However, some biologists reject species as a distinct kind of individual (Nelson, 1989) , and among those who do not, there persists a lack of consensus on the defining properties of this kind of individual (Endler, 1989). Two questions remain much discussed: is it useful to consider species as individuals?; and if so, what is the defining attribute of this individuality - the meaning of the word "species" - that applies to the different species of the world?
The approach taken in this paper does not begin with an assumption that species either are or are not individuals. Rather the approach has been to assume the existence of some simple natural phenomena concerning the nature of DNA replication, and then to consider whether these things will cause species. It is shown that these relatively modest phenomena will create a kind of individual that has a close correspondence with other concepts of the meaning of species.
The concept that is developed (the genetic species) is similar to some elements of the cohesion species concept (Templeton, 1989, 1994). In particular, both species concepts rely extensively on the idea of shared genetic drift. However, the two concepts differ in their motivation and their purpose. Templeton begins his discussion with the question "What is a species?" and the implicit assumption that species as individuals exist in nature. The genetic species concept arises from the basic question of whether organisms actually occur in groups that are individuals (i.e. species).
© 1997 Jody Hey