Species: A History of the Idea (review)

A century and a half after the publication of Darwin's On the Origin of Species by Means of Natural Selection, it seems we are more sure of how species originate than what they are. As time passes, the less we remember what previous generations of naturalists and philosophers considered them to be, and the more we invent novel definitions to try to pin down that elusive concept.

So holds John S. Wilkins, Research Fellow in Philosophy at the University of Sydney and member of the editorial board of the Species and Systematics series of the University of California Press. In his Species: A History of the Idea (2009), Wilkins performs a masterful and sorely needed job of reviewing, criticizing, and synthesizing the concept of species from the ancient Greeks to the present, while gently correcting false impressions and false attributions of previous commentators along the way.

Wilkins emphasizes a number of points that should be engraved in every high school and college biology textbook that currently teaches a whiggish view of the history of the species concept (if they teach anything at all on the topic). First, not until medieval times did many scholars separate the notion of a biological species from that of species as a general logical concept, i.e., that a smaller category (say, chairs) could be subsumed by a larger one (e.g., furniture). Many ancients were intent to develop the logical concept per se, without concerning themselves much with whether the categories were living versus non-living, or even natural versus man-made. Second, for those early naturalists who did concern themselves with living categories, a "generative" conception of species was developed as early as the fourth-century BC, and remained and remains a major idea to the present. The modern attribution of an ancient obsession with "essences" of species that could not vary does not stand up to critical examination. Aristotle himself was not a species "fixist" (21), and ancient naturalists were well aware that variation occurred from what were thought to be the standard attributes of a particular species. Hybridization was also thought to contribute to variety in nature. Aristotle observed, "animals of diverse species meet … at watering places, and there pair together; and such pairs will breed if they be nearly of the same size …" (21). He recounted travelers' stories that the Indian dog was actually a dog–tiger hybrid.

As biology progressed in the early modern period by use of the microscope and systematic exploration of the planet, new conundrums were encountered; life, it seemed, was more complex than the naked eye had judged or more than could readily be seen in any observer's immediate neighborhood. With the publication of Darwin's theory in the mid-nineteenth century, evolutionary concepts began to be tied to the idea of species, yet they did little to tidy up the definition of species.

In our modern era, the most popular definition is the "biological species concept." Roughly, that phrase is meant to convey that a "species" is a closed group of organisms that interbreed in the wild. Although that does cover much of life, and seems commonsensical to many modern scientists, it leaves many issues unresolved. For example, bacteria do not interbreed at all (but can exchange some genetic material indiscriminately), so the definition does not apply to the greater number of organisms on the planet. Worse, history is left out of the definition. For example, how can we classify fossil organisms, since of course they no longer interbreed with anything? Are horses of today a different species than horses of a millennium ago because they cannot interbreed?

Wilkins notes the remaining difficulties, and suggests that we may simply be demanding too much of life to be able to apply a single concept across all organisms. Other broad concepts in biology are notoriously difficult to define, starting with "life". As shown by a recent article in Nature with the remarkable title "What Is a...