A couple more ideas about speciation:
Two recent articles have examined aspects of speciation, which is of abiding tangential interest to my own subject. 

One of them (Expanding Islands of Speciation, Erin X. Kelleher and Daniel A. Barbash NATURE vol. 465 no. 7301 June 24, 2010 page 1019) is a review of a paper in Proceedings of the National Academy of Science. (Michel, A.P. et al. Proc. Natl Acad Sci USA 107, 972409728(2010))  What the authors have done is to study an incipient speciation of a kind of fruit fly that used to inhabit hawthorn trees and is now branching off into a new species that exploits apple trees.  This apparently has been going on for about 150 years.  Since the trees in question mingle in the same groves and forests, the two kinds of fly are not strictly separated.  This then is an example of sympatric speciation

To me it is strange, but there is controversy surrounding sympatric speciation.  There is a vigorous school of thought that does some arithmetic and concludes that if two populations are able to exchange genes, then gene flow between them will overwhelm any move toward their dividing into different species.  It strikes me as odd, because I am far more interested in whether something actually happens than in any theory that does not predict it.  Well there is a case, so that should end it.

But these opponents of sympatric speciation do not give up easily.  Their response is, well, yes, if it does happen, then it must happen on a very local level.  There can be a little speciation caused by a limited part of the genome and arising in a small area.  But these two strains of fruit fly are in fact very different.  They have diverged a lot over the past century and a half, which is quite fast for what is expected for evolution. 

It should not have come as a surprise.  I have not seen the calculations, but I would wager that the assumption is that gene flow across a species is quite free, that the species represents one enormous gene pool.  But that cannot be true.  A species can exist in substantial numbers only if if is strongly divided into tiny local components.  If one of these components, already mostly isolated anyway, begins to evolve into a new species there is going to be little to stop it.  So given a new niche, the introduction of apples I would suppose, rather than the introduction of hawthorn, speciation is away and running free.

The other article (Evolutionary Novelty is Concentrated at the Edge of Coral Species, Ann F. Budd and John M. Pandolfi SCIENCE vol. 328 no. 5985 June 18, 2010 page 1558) looks at corals.  It turns out that for corals, and presumably for other forms of life, evolution is more effective and the emergence and merging of species more common at the edge of the environment than in the middle.

The edge of an environment is going to be the place where survival of species or individual is least likely; the environment is not optimal as it is at the center.  That means less gene flow between adjacent populations.

Although sympatric speciation is real enough, nobody seems to doubt that isolation is a good thing for promoting speciation.  So the relative isolation at the edge of the range may foster speciation and that is the end of it.

I have a haunting feeling that there is more to it but cannot put my finger on anything.  By analogy, I suppose, I am thinking of the emergence of civilizations.  And there my thinking may be distorted because of the way Mesopotamia looms above history.  Although irrigation has long led to great agricultural wealth there, the area strikes me as rather unpromising.  It is hardly lush when unimproved.  So I have long suspected that civilizations emerge where life is marginal.  That would make sense from my own perspective.  If life is hard, then community sizes will be small, communities will survive a relatively long time and innovation becomes more likely.

If it is true of civilizations, then why would it not be true of species?  If the local population is perforce small, and therefore enduring, then that might present the possibility of genetic innovation much as marginal environments have enhanced technological and social innovation.  At least so my logic goes. 

But I am by no means sure of the initial premise, namely that civilizations tend to emerge in marginal terrain.  Civilizations have appeared at high altitudes and low, in cold climes and torrid, in wet and dry and in the context of many dangerous predators and few.  But that does not really address the point.  The question is whether civilizations are more likely to emerge under marginal circumstances.  And for now I can think of no rigorous way to test the notion.  And lacking a firm basis in anthropology, I fear that the analogy breaks down when an attempt is made to apply it to biology. 

At all events, the authors suggest that seeing as how new species seem to emerge away from the best environments for a life form, then a conservation strategy aimed at preserving diversity by creating defended areas where diversity is greatest will miss picking up the innovating areas at the stressed margins.  I am all for it, although from what I read the whole world is getting amply stressed already. 

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