A collection of edifying (or, at least, interesting to me) quotations from G.L. Stebbins’ Variation and Evolution in Plants, originally posted in Sep 2006.

p. 1:
“The hierarchy of categories is a multidimensional pattern of variation in nature, and the gaps or discontinuities give reality to the various categories.”

I think (and hope) he means taxa by “categories”. This seems to be his usage elsewhere. There are several other interesting quotes in the first few dozen pages that I may put up later.

p. 34:
“All [Dobzhansky, Mayr, & Huxley] agree that species must consist of systems of populations that are separated from each other by complete or at least sharp discontinuities in the variation pattern, and that these discontinuities must have a genetic basis.”

This remains essentially the case with modern disagreements on species concepts. The disagreements are not in what species are, but in what is the best axis on which to look for discontinuities.

p. 35:
“In fact, it is likely that most families in which the genera are well-defined have suffered the extinction of many species, and further that most boundaries between neighboring genera represent gaps left by species which have perished.”

The importance of extinction in observed patterns remains often overlooked and misunderstood. In most cases monophyletic taxa, for instance, were probably previously paraphyletic groups in which sufficient lineages have subsequently become extinct.

Stebbins continues:
“If this fact is kept in mind, then the search for natural boundaries to genera has some meaning to the evolutionist and is not entirely a matter of convenience.”

pp. 189-190:

“The common ground of agreement between these definitions may be expressed as follows. In sexually reproducing organisms, a species is a system consisting of one or more genetically, morphologically, and physiological different kinds of organisms which possess an essential continuity maintained y the similarity of genes or the more or less free interchange of genes between its members. Species are separated from each other by gaps of genetic discontinuity in morphological and physiological characteristics which are maintained by the absence or rarity of gene interchange between members of different species. The above sentences are not to be construed as this authors definition of a species, since several different species definitions are possible within the framework of their meaning.”

But–isn’t it precisely the problem of existing species concepts that they try to limit us to a single axis for discerning species, rather than admitting of several different axes, as Stebbins’ sentences above do? Why not embrace such a broad and inclusive definition–merely because it could be subdivided?

p. 202:
“The second alternative [the first was multiple species concepts] would be to recognize that at any given moment in the evolutionary time sale, reproductive isolation is important in keeping distinct only those populations which are sympatric or which overlap in their distributions.”

In other words… Mayr’s Biological Species Concept is applicable only to sympatric or overlapping populations. This criticism has been hemmed and hawed over for five decades now, but has never been addressed in a coherent fashion. And it is precisely a multidimensional species concept that will allow us to overcome this problem, as well as those that plague the other species concepts. Why, after all, would we expect groups in multi-dimensional space to always be identifiable along a single axis, like that of reproductive isolation?

More bits of Stebbins; p. 262:

“Hybridization between well-established and well-adapted species in a stable environment will have no significant outcome or will be detrimental to the species populations. But if the crossing occurs under rapidly changing conditions or in a region which offers new habitats to the segregating offspring, many of these segregates may survive and contribute to a greater or lesser degree to the evolutionary progress of the group concerned.”

p. 270:

“There is little doubt, therefore, that the majority of the examples of hybridization and introgression which can be found in plant populations at the present time are associate with the disturbance of old habitats and the opening up of new ones through human activity.”