The species-as-individuals thesis poses a dilemma for the way categories of individuals and species are typically understood. A close examination of meaning and reference, and the origins of the species concept, reveals a hidden dilemma.
The general category of ‘species’ is in discordance with the notion of biological species, i.e. ‘species’ is both a category and a biological entity. The two meanings are typically thought to overlap and yet a species need not be biological (see Richards, 2010). For example, Aristotle deploys the terms to explain that particular instances of circles in the world are part of the “species” of the form “circle”. The species-as-individuals thesis denies this sort of overlap. To see this, it is necessary to return to the initial work the species category was meant to achieve. In the Metaphysics, the essential “being” of an individual is what can be said about it at the species category. Because properties of an individual thing can change, what remains essential are those things that are in the species category, e.g. I am most essentially human. An organism within the species Homo sapiens is the most essential “being” of such an individual. But this use of the species category extends beyond biology. On this account, what might be most essential about an individual car is its “species”, e.g. Ford. The species-as-individuals thesis rejects this use of “species” not only biologically, but more broadly, as a category. An individual organism can, in theory, come to participate as a part (i.e. member) of a new species in virtue of performing an act, e.g. reproduction (see Hull 1978) So, if species are individuals, the long held way of determining what an individual is no longer works. We can now only trivially conclude that individuals are composed of individuals (i.e. individual parts), which is to say we are not dealing with distinct categories.
What then is an “individual”? The obvious answer provided by the species-as-individuals thesis is that it is a spatiotemporally isolated interbreeding group of organisms. But offering this answer leads us astray, for the question is not one of biology. If biological species are individuals the notion of an individual, itself, has changed. Generally accepted theories of meaning and reference (e.g. Kripke and Putnam’s origin essentialism) fall short of answering this question as they don’t make clear how to meaningfully refer to these new individuals, i.e. biological species. Biological species cannot be picked out by providing any of the traditional markers of necessary conditions (see Pedrosa, 2015), ostensive, property clusters or clear indexicals. It shouldn’t be surprising that such individuals are difficult to identify, for it appears as though we have failed to identify these individuals even as individuals. Picking a thing out in virtue of its spatiotemporal isolation might avoid confusing biological species with natural kinds or sets, but it falls short of telling us what counts as an individual - biological or otherwise. There seems to be no way to meaningfully refer to such individuals, as the concept of an individual is circularly employed to tell us what individuals are. Thus, we are faced with the very problem that initially led to Aristotle’s positing of the species category, i.e. that reference to “species” provides some immutable markers of an individual.
Because the species-as-individuals thesis fundamentally alters the very notion of individuality, there should be little surprise that analyzing different types of DNA lead to conflicting trees of decent. As long as the species-as-individuals thesis continues to be upheld as the best candidate for presenting the reality of biological species, the concept of an individual is far more permissive than ever previously thought. Until this philosophical dilemma is resolved, biology will struggle to uniformly agree on the boundary markers between one species and another.