Theories of speciation

Continuing on from my last post, let's consider the modes of speciation that are called into account for the existence of species.

Here is a list taken from Sergey Gavrilets, which I put in my most recent paper in Biology and Philosophy (2007).

Vicariant – divergent selection and stochastic factors like drift after division of a population by extrinsic factors such as geographical changes;

Peripatric – a small subpopulation, mostly isolated, at the extreme of the parent range. The idea is that it will have both a non-standard sampling of alleles, and also be subjected to divergent selection pressures in extreme environments (for that species);

Centrifugal – central populations that carry a sample of many alleles that become isolated through, say, “island” formation (such as the mountain “islands” in the Amazon);

Punctuated equilibrium – the appearance of relatively rapid speciation and subsequent stasis as the population reaches equilibria of alleles. In my opinion, this is inappropriately included here, for it is a “pattern” rather than a “process” (or “event”) of speciation, and as such can be caused by any of the other scenarios/modes;

Chromosomal speciation – the rearrangement of chromosomes, either by duplication or insertion, fission, fusion or inversion;

Hybridization – the fusion of two genetic lineages, usually from distinct species, including allopolyploidy. In allopolyploidy the genetic complement of two species is paired up by a loss of secondary division, giving a symmetrical set of chromosomes;

Reinforcement – once hybrids are of lowered fitness for whatever reason, selection will tend to reinforce separation of the gene pools (for example, a hybrid rock and grass dwelling lizard might be less able to survive in either environment as well as the “pure” lines);

Competitive – this is Darwin’s scenario. Members of a species that are in strong selection for a limited resource may result in specialized forms that are thus in less competition with the ancestral forms that make use of some other resource;

Clinal/ecotonal – Gavrilets calls it “speciation along environmental gradients”, where limited migration and selection leads to aggregation of forms at the terminal ends of the cline;

Host shift – this is the case of the Rhagoletis fruit flies mentioned above, that Stuart Berlocher (1999; 2000; 2002) and colleagues have studied. Host fidelity replaces geographic isolation;

Runaway sexual selection – this is secondary selection by mate choice of polygenic traits (Lande 1981).

In that paper, I locate each of these modes as a unique coordinate or region in a three-dimensional space, the axes of which are:

Gene flow – the rate of migration between populations, or the amount of genetic material exchanged in a mating event, from none to 50%;

Selection – the degree to which selection is endogenous to the organisms (such as climatic selection, say on the Souay sheep or the finches of the Galápagos Islands), or is intrinsic to the organisms (such as mate choice or immunological compatibility); and

Stochasticity – which is basically whether selection is directional or stabilising, or whether the rate of change of the genetic constitution of the population is due to genetic drift and other stochastic "forces".

Now, suppose we have a species of flowering plant, say. It buds off a new species due to climatic and pollinator adaptation, with no exchange of genes between populations. In binary terms, its coordinate would be 0,1,1. That is the explanation of that species in terms of how its speciation occurred. If it is in sympatry (shares a geography) with its parental species, it must have adaptations that prevent competitive exclusion, and so remaining a species, rather than merging back into the parental metapopulation, is due to selection for those niche adaptations.

This, it need not be said, will not be true of species that occupy other regions of "speciation space". The plant has its own explanation, and its own phenomenal reality. We know it is a different species simply because introgression and hybridisation are inhibited, and competitive exclusion does not occur because parent and child species occupy different points of the fitness landscape.

But is this sort of species (which may have formed in isolation from the parental species) required by any theory of speciation or evolution, or biology in general? [And generalising, is any type of species so required?] The answer is, no. Simply put, if it merged back into the parental species, it would not be identified as a species itself, but a variety (and maybe a fairly ephemeral one at that).

What we have is a post hoc phenomenon. If it is distinguished by its genetic and ecological properties, however acquired, then it is a species. If it isn't, then the issue doesn't arise. This is a bit like weak anthropomorphism - any universe that lacks humans isn't going to be discussed by them, but you cannot make the inference that there is something special about human occupied universes simply because we are here talking about them. Similarly species - if it is separate from other species (in its own unique and contingent manner) then it is a phenomenon that demands explanation, but if it doesn't, the theories of population genetics, developmental biology, ecological interactions and the like all continue on apace.

Post hoc explanation is not an illicit move in science; it can't be, because one of the major roles of a scientific theory is to explain what is observed. But that doesn't imply that the phenomena are theoretically significant. Just that they interest us enough for them to call for an explanation.

There is a school of thought in the philosophy of science that treats ontology, the set of objects that one thinks exists, as basically the bounded variables of the theory of that domain. In this case, species would be theoretical objects if they were such variables of a theory. But they aren't. So we need to establish what sort of ontology they, and other phenomenal non-theoretic objects, may have.

Consider planetary orbits. They were observed and debated for a very long time before Newton proposed that there was a general physics that accounted for them (and made predictions about them). But in so doing, Newton demoted these orbits from theoretically important objects to special cases of larger and more universal physics. "Planetary orbit" is a special kind of astrophysical dynamics, one which aperiodic comets, entire star systems, and even entire galaxies all obey. Even if no orbits actually existed (and we can perhaps envisage this in some universe) under this physics, the movements of objects would be still covered by Newtonian dynamics.

Likewise species. They obey, and when they occur are post hoc explained by, the biology of populations, interbreeding, selection, drift, and so on. But they are not themselves theoretical objects, any more than planetary orbits are in physics. [I know - the "nebular hypothesis" of planetary formation requires that bodies in the disk of accretion form more massive objects by gravatational attraction, but if there were no such disks, they needn't.] Species occur, and are explicable in a multiplicity of ways, but they do not follow formally from any theory of biology.

My general characterisation of species was that they are the nexus of coalescence of genes, haplotypes, parent-child lineages and so on. In abstract terms, species are these coalescences that are distinct from other such coalescences, something I have called the synapomorphic conception of species (in my 2003), and each and every one has a general set of properties and modes of speciation, and a unique set of these that only they have (the synapomorphies, or shared characters, which are causally active in maintaining separation).

So that's why I say species aren't theoretical objects, but are phenomenal ones. As a take-home exercise to the reader, try to imagine under which conditions organisms like ours wouldn't form species at all...

refs

Wilkins, John S. 2003. How to be a chaste species pluralist-realist: The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621-638.

———. 2007. The dimensions, modes and definitions of species and speciation. Biology and Philosophy 22 (2):247-266.