Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Sunday, April 02, 2006

The essence of a species

I've been rereading some of Hugh Paterson's writings on species lately, as well as revisiting Alan Templeton's more recent cluster concept. Each of these use what might be termed a "positive" conception of (sexual) species, rather than the "negative" ones of Mayr and Dobzhansky that preceded them.

In Paterson's view, a (sexual) species is a group of organisms that share a common reproductive system. In Templeton's view, a species is a group of organisms whose genetic structure is maintained by selective pressures. The older "reproductive isolation" views, in contrast, are based on what species can't do - that is, interbreed.

The difference is interesting. If a species is something that happens to occur through incidental processes, then there is no essence of a species. But if it happens through processes that actually make it a species, such as coadaptation of signalling systems (Paterson) or of gene complexes (Templeton), then species do have an essence. That is they have a "what-it-is-to-be". And what that is, I think, is the intrinsic selection for reproductive compatibility. A species is formed by the fitness of those who can reproduce. That is their essence.

Taking this tack removes some of the confusions found in the species concept literature. While the genetic composition of the population may change for reason that have nothing to do with reproductive isolation or the lack of it with the parental species, it does have something to do with the compatibility of each organism to reproduce with the other organisms in its poopulation. Moreover, if it were the case that ecological adaptations led to a diverging phenotype in the population, and hybrids were less fit, it is still selection that causes the new species that form to be species.

The allopatric consensus of the older evolutionary views was based on solid evidence and good theory, but the definition of a species as being reproductively isolated is what is called a "privative" definition: defining something in terms of what it is not. But rocks cannot interbreed with birds - that doesn't make rocks a species. To work, the isolationist definition has to assume a lot, and some of what it assumes is that species are fit to interbreed. Simply calling a population a species because it can't interbreed with other populations is to overlook that fact that organisms within the population can. And it is that which makes the species. Put another way, if all possible mating groups for a species went extinct today, the species would still be a species tomorrow.

This leads me to my next claim: asexual populations, which do not interbreed because they can't, are still species too. Consider the favourite example of this kind of thing - the whiptail lizards. Most of them are sexual, but a number of groups appear to have evolved asexuality (possibly via hybridisation). If the isolationist view were correct, then there would be nonspecies of these lizards in a clade of species. This seems odd. But if a species is a selectively maintained group, the asexuals will be species because those that vary by mutation too far from their adaptive peak (which includes developmental mechanisms) will die out. Selection maintains them at the phenotype that constitutes them. In single celled asexuals, and viruses, this is called a "quasispecies", a cluster of genotypes about a fitness peak. If the fitness peak splits, then the species will split as some of the popuation adapts to the new exigencies (for example, if the virus infects a new host successfully). But what constitutes that species remains that which is preventing the smearing out of the genotype and the phenotype.

So we might say that the notion of a quasispecies is basic to being a species. The main difference is that with an asexual species the adaptation is both intrinsic (between mating partners) and extrinsic (to ecological conditions), while for asexuals it is only extrinsic. And this means that a species does have an essence - whatever it is the population is adapting to that keeps it cohesive.

But this is not the essence feared and hated by Mayr and those others who argued against essenetialism in the Received View. It is not a definitional, a priori, essence we have here. It is entirely contingent and a posteriori. How might we accommodate this philosophically? There are two candidate views. One is by Paul Griffiths, whose essay "Squaring the Circle" argues that species have a historical essence. A species' essence is a kind of developmental essence, inherited and modified from the speciation event. It is the essence of having had a history. This is compatible with the view I'm espousing here, but it doesn't account for being a species the way this view does.

The other is more directly relevant. Richard Boyd has argued for a "homeostatic property cluster" view of natural kinds, and species are an example of this. In Boyd's view, a natural kind is maintained in the ensemble of properties it has by mechanisms that keep it coherent. This is exactly the view I propose. Selection maintains species' properties. A third view, that of Kevin de Queiroz, is that species are metapopulations (collections of connected populations) that form lineages. This is also consistent with my view, but not sufficient to account for species. It is insufficient because not all lineages form species even at the population level, and because there are subordinate metapopulations in many species that are not the whole species.

This view necessitates the conclusion that "species" is not an absolute rank. I think this is true of all taxonomic objects. An objection to asexual species often relies on the fact that there is no distinct organisational level in those organisms that could be called species. But I argue tht there is none in sexual organisms either. Reproductive compatibility is a post hoc property. And even that is insufficient to identify species, since there is introgressive breeding across species lines, for example, in many plants, corals, and even animals. The thing that makes it a species is the fitness of those who breed, due to selection against deviation from extrinsic or intrinsic fitness peaks.

So let's see if we can define species in a general way. A species is whatever group of organisms are selected for a given genetic or phenotypic fitness peak, through ecological or sexual adaptation. They form lineages, which are homeostatic, but that is not enough. They have historical essences, but that is not enough, either. Let's see how that flies...

Boyd, R. 1999. Homeostasis, species, and higher taxa. In Species, New interdisciplinary essays, edited by R. Wilson. Cambridge, MA: Bradford/MIT Press.

de Queiroz, Kevin. 2005. Ernst Mayr and the modern concept of species. PNAS 102 (Supp. 1):6600-6607.
———. 1999. The general lineage concept of species and the defining properties of the species category. In Species, New interdisciplinary essays, edited by R. A. Wilson. Cambridge, MA: Bradford/MIT Press.
———. 1998. The general lineage concept of species, species criteria, and the process of speciation. In Endless forms: species and speciation, edited by D. J. Howard and S. H. Berlocher. New York: Oxford University Press.

Griffiths, Paul E. 1999. Squaring the circle: Natural kinds with historical essences. In Species, New interdisciplinary essays, edited by R. A. Wilson. Cambridge, MA: Bradford/MIT Press.