What are species?
Species pluralism
The definition of a species has been a historical problem since even before the days of Buffon and Linnaeus. Some believe that there is no one objective definition waiting to be found and so we should be species pluralists. Adopting a pluralistic approach means accepting that there are a number of equally legitimate definitions of 'species'. This plurality can refer to either a different definition for different types of animals, but with only one definition for each species (Kitcher 1984 and Dupré 1993), or it may apply different definitions to the same group of organisms. I believe that this choice should be based upon which is the most theoretically useful for biologists.

Pluralists differ in motivation, epistemological pluralists claim pluralism is legitimate on the basis of our cognitive limitations. There may be a single correct taxonomy in nature but we cannot uncover it and so must rely on a plurality of inaccurate descriptions. Ontological pluralism on the other hand states that there is no one natural definition, there is naturally a number of ways of defining species. This could be because, for example, the phylogenetic tree naturally diverges in numerous ways for numerous reasons. Some ontological pluralists such as Ereshefsky think that we should doubt the existence of a species category in nature at all (Ereshefsky, 1998).

I will defend a type of ontological pluralism by arguing that there are numerous correct ways to define species. These are correct not because they are objective categories, but for their instrumental capabilities. I will defend this view on the basis that species do not have essences.

Essentialism was the traditional way to define species before Darwin's theory of evolution. Essentialism is the view that species are natural kinds with essences, natural kinds are a spatiotemporally unrestricted group whose members are subject to universal laws. This can be seen in objects such as atomic elements which, by definition, must all have the same nuclear make up and do not vary by degree but have sharp distinctions. Essentialism would be the optimal method of species categorisation because there would be no anomalies, however evolutionary theory has undermined the view that species have essences.  

Without essentialism, there are no objectively defined criteria, only criteria defined by people out of instrumental necessity. If there are no objective ways to classify species then the form of classification should be judged on its instrumental advantages. In the case of species there is no single optimum way to categorise and so a number of approaches are equally legitimate. This does not mean doubting the existence of what we call species, but doubting the idea that species are an objective component of nature, an idea which has been attributed to Darwin himself (Beatty, 1985).

An instrumental view of species would claims that the basal unit, the smallest unit used to distinguish organisms as species, should be a balance between what conveys the most data and how much data it is to be theoretically useful to convey if. The anatomical distinctions between observed organisms of different species, notably sexual species, are a good distinction. They convey all the data associated with a species but without the unnecessary information of subcategorising.

The basal unit of evolution however, does not necessary correspond with these anatomical distinctions. I propose that evolution disproves the existence of a species category of natural kinds with essences. Evolution is not a monistic process, it is not limited to either reproduction, isolation, or a shared environment, all ways to define species. Evolution occurs over huge periods of time, distinct species rise from one evolutionary chain and so there must be a point when a parent becomes a different species to its offspring. Some argue that this leads to the idea that there is no real basal unit of evolution (Mishler and Donoghue, 1982, pp.12). The basal unit used depends on the biological paradigm you are working within.