I read somewhere that there is a species of snake common throughout the entire eastern half of North America. This species is widespread, but it is constantly evolving in order to adapt to
its local environment, particularly the temperature. There are minor variations in the different
varieties as one samples the populations–the species is in the process of fractionating into a variety of subspecies, each more completely adapted to local conditions. Eventually the subspecies will become distinct species–standard Darwinian natural selection. It explains the snapshot of species differentiation Darwin noticed in the Galapagos finches.This is quite noticeable in isolated populations of organisms that have become geographically segregated. There is a species of squirrel in the southwest USA that has been trapped on mountain tops as the climate got progressively hotter and drier, each range now has distinct subspecies, and presumably they will all eventually become distinct species.
Our snake is particularly interesting because the different populations are not physically isolated, They exist as a continuum from north to south, and the evolving characteristics exhibit a gradient as one goes into more northerly ranges. But just what is a “species”, anyway? We usually define it as a population of organisms that is reproductively compatible. No two critters are identical, and it is the national selection of these variations that drives evolution. As the different subspecies become more established, eventually they become genetically isolated and reproduction becomes more difficult. Different species cannot interbreed, the same species can, but this is not a binary separation. There is a shadow zone in between. Some matings never occur, they are forbidden by separately evolving physical and behavioral differences. Other matings become less effective, or lead to sterile offspring. This is the hazy region of hybridization, which is rare in nature but which humans use extensively (with clever technology) to manage the characteristics of their domesticated plants and animals. Many orchids, for example, can only be produced by artificial pollination by botanical hobbyists. And everybody knows about mules.
Getting back to our snakes, it turns out that the Florida cohort can breed easily with the Georgia and Alabama contingents, which in turn have no problem breeding with representatives from Tennessee. These breed easily with the Kentucky and Missouri varieties, which can mate with those from Ohio and Pennsylvania, and so on into Ontario. But in turns out that snakes from the northernmost parts of the range cannot successfully reproduce with the Florida snakes. Genetic compatibility also exhibits a continuum, a gradient, of possibilities. Canadian and Florida snakes are on the boundaries because we are witnessing a instantaneous photograph of a process dynamically occurring, dispersed in both space and time.
It is useful to think of a “species” not necessarily as an isolated collection of distinct individuals, but as a hypervolume in a “species space”, where each dimension in that space is some physical characteristic of the organism. All organisms on earth share some characteristics in common, and many they do not, and the multidimensional volumes in n-space they occupy merge gradually into one another, with much overlap and separation in some dimensions but not in others. We only choose one axis in our technical definition of “species”, reproductive compatibility, but even that, as we have seen, does not exhibit sharp boundaries.
The biological example I have chosen should be pretty clear. But I have no doubt there are many other physical and human phenomena that can be considered in this way. This may be a useful concept to extend into psychological and sociological phenomena, and it certainly seems to apply in linguistics. Languages, for example, branch and grow, they hybridize and separate, and they can be merged and isolated as well, both spatially and temporally. and they evolve. Applications in anthropology, politics, economics also may come to mind.
Just thinking out loud….