Evolution and Philosophy: Why are natural kinds supposed to stay fixed?

efore Lamarck, species were thought to be eternal kinds, and any single organism to have all the necessary and sufficient conditions of being a member of that species. Think of it like this: To be a member of the supporters of a football team, you must have certain characteristics. For argument's sake, suppose they are:

Anyone who has one, or even two, of these criteria filled may still not be a supporter. You might get your membership from a corporate sponsorship deal in which you have no interest. You might be obsessively fixated due to a pyschological disorder. You might collect things in the hope they become valuable. Each condition is necessary , but only all conditions are sufficient for you to qualify. An organism was thought to need identifying characters - all of them - in order to be a member of the species. And these conditions never changed. 'Football supporter' was an idea that would remain the same even if nobody filled the conditions, or even played football. If something was a species, it could not change, and if it changed, it could not be a species.[note 5]

Figure: The difference between essentialist and populational notions of species. Not all members of a species may have all the diagnostic traits that tells it apart from similar species. Species which all share all the same diagnostic traits (like Species X) are called 'monotypic' species and are rare. Species usually share only some diagnostic traits among all members (Species Y, a 'polytypic' species).

This is the kind of view expressed implicitly when a creationist says that such and such a change represents "devolution": a movement away from "pure type". The great evolutionary theorist Ernst Mayr has, following the philosopher Karl Popper, called this "typological essentialism", the opinion that species have essences in some Aristotelian fashion [Mayr 1988]. While the "kinds" mentioned in the Bible ( Genesis 1:21-23) are merely observations that progeny resemble parents, that is, that some principle of heredity is active in reproduction, Aristotle held rather that living things are generated in an approximation to a "form" of that species. There is something that represents the perfect dog, for example. [note 6] This view found its way into Christian theology through the rediscovery of Aristotle from the Islamic tradition in the middle ages, primarily through Thomas Aquinas, and was enshrined in biology by Carl von Linne in the 18th century in what is now called the Linnean system of classification.

After the work of the mid-nineteenth century explorers and naturalists, scientists were no longer able to view species in this way. They were much more diverse than that. Not only were species sometimes more different internally than some members were to other species, but it became clear that what was actually common between members of a species was the ability to interbreed (at least, in sexual species).

Actually, this view (now called the biological species concept ) predated evolution by some fifty years, deriving from Buffon, who attacked the Linnean system. It meant that seeing species as morphological kinds (that is, as groups of characters of organisms) was no longer scientifically possible. Some, including Darwin, thought on occasions that this meant that species were conventional names given to record observations, but nothing more, and that 'species' were artificial constructions. Others held to the older view that there was something in virtue of which things were members of a species, but that this had nothing to do with their morphology, but with their relations of descent. Of course, if this is all that makes an organism a member of a species, and the variation that is observed is real, then there is nothing in being a species that can prevent a species - or at any rate a part of a species - becoming something different and new. Nothing else makes scientific sense.

In this century, the systematist Ernst Mayr (eg, [1970]) has championed the view that what he calls 'typological thinking' has been abandoned by modern biologists in favour of what he calls 'population thinking'. Typology is the view that there are 'types' - unchanging forms that are what makes a species what it is. It derives from the philosophy of Plato, who claimed that true knowledge is knowledge of the Idea (Greek eidos ). Population thinking is a recent development in Western thought - it is the view that aggregates of individuals, groups, have a profile that shows a distribution of characteristics. The well-known 'bell curve' of statistics illustrates this - for almost any trait of a population you will find a bell curve distribution. Some organisms will be longer or shorter, heavier or lighter, and there will be a mean around which most individuals cluster. Variation is a universal fact about all species. Some parts are located in different environments, and natural selection, genetic drift and happenstance all work to make them different if they are isolated for long enough. Thus are new species created.

Enter Michael Ghiselin [1975] and David Hull [1976, 1988]: a biologist and philosopher respectively. They proposed that species are not universal types, or classes, but are historical individuals (which is what 'species' meant to Aristotle anyway). The name of a species, according to Ghiselin and Hull, is a proper noun, the name of a single and unique individual that has a beginning, a history, and an extinction, and which also has a distribution in space. Homo sapiens is not, on this view, the name of a 'type' of rational animal as Aristotle had it, but the name of a particular lineage of hominoids that happened to develop language and ratiocination. If all humans were extinguished next year, they could never arise again. This view is also hotly debated by philosophers and biologists (cf Gayon [1996]). Mayr [1970] for example thinks that some taxa (eg, families or even orders) are 'grades' which can be arrived at more than once, which the individuality thesis rules out.

This is related to the complex and difficult area of the taxonomic methods collectively called cladistics (from the Greek word klados , meaning branch). Cladistics attempts to 'reconstruct the past' [Sober 1988] - recreate phylogeny - using as few theoretical assumptions as possible, on the basis of the present distributions of organismic traits [Panchen 1992]. This deserves an essay on its own, but not by me.

Whatever the triumphant view in philosophy, evolutionary notions of species do preclude eternal types, in favour of what the philosophers Hilary Putnam [1975] and Saul Kripke [1972], following the great American philosopher WVO Quine [1969], call 'natural kinds' - things that exist naturally at certain times and places. Like Hobbes's example of the ship of Theseus, which over the course of a voyage was completely rebuilt, species can be changed so much that they are not the same individuals they once were, but this change can happen imperceptibly (at varying rates), as Darwin expected it would. Species are biological entities that change.