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Species: Members of a Species
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HPC theory provides a more promising account of species as natural kinds than essentialism. HPC kinds need not have a common essential property, so the criticisms of species essentialism are avoided. Furthermore, HPC theory allows that external relations play a significant role in inducing similarity among the members of a kind. Traditional essentialism assumes that the essence of a kind is an internal or intrinsic property of a kind's members, such as the atomic structure of gold or the DNA of tigers. Such intrinsic essences are ultimately responsible for a kind's similarities. HPC theory is more inclusive because it recognizes that both the internal properties of organisms and the external relations of organisms are important causes of species-wide similarities.
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Although the current scientific understanding of species suggests that there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but it remains imperfect. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this definition of a species.
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The existence of various evolutionary forces does not rule out the possibility of a trait occurring in all and only the members of a species. But consider the conditions such a trait must satisfy. A species' essential trait must occur in all the members of a species for the entire life of that species. Moreover, if that trait is to be unique to that species, it cannot occur in any other species for the entire existence of life on this planet. The temporal parameters that species essentialism must satisfy are quite broad. The occurrence of a biological trait in all and only the members of a species is an empirical possibility.
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Members of a Species