, Volume 22, Issue 2, pp 247-266
Date: 15 Mar 2007

The dimensions, modes and definitions of species and speciation

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Abstract

Speciation is an aspect of evolutionary biology that has received little philosophical attention apart from articles mainly by biologists such as Mayr (1988). The role of speciation as a terminus a quo for the individuality of species or in the context of punctuated equilibrium theory has been discussed, but not the nature of speciation events themselves. It is the task of this paper to attempt to bring speciation events into some kind of general scheme, based primarily upon the work of Sergey Gavrilets on adaptive landscapes, using migration rate, or gene flow, as the primary scale, and concluding that adaptive and drift explanations are complementary rather than competing. I propose a distinction between intrinsic and extrinsic selection, and the notion of reproductive reach and argue that speciation modes should be discriminated in terms of gene flow, the nature of selection maintaining reproductive reach, and whether the predominant cause is selective or stochastic. I also suggest that the notion of an adaptive “quasispecies” for asexual species is the primitive notion of species, and that members of reproductively coherent sexual species are additionally coadapted to their mating partners.

... evolution is a process of change or movement. Description of any movement may logically and conveniently be divided in two parts: statics, which treats of the forces producing a motion and the equilibrium of these forces, and dynamics, which deals with the motion itself and the forces producing it. Following this scheme, we shall discuss, first, the forces which may come under consideration as possible factors bringing about changes in the genetic composition of populations (evolutionary statics), and second, the interactions of these forces in race and species formation and disintegration (evolutionary dynamics).

(Dobzhansky 1937)