Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Friday, May 12, 2006

Hybrids in the news

I have a long standing interest in hybridisation, one that appeared out of nowhere after I read Jan Sapp's Evolution by Association, which is about another intuition-breaking aspect of evolution, symbiosis.

Hybridisation as an explanation of biodiversity goes back at least to Aristotle's History of Animals (Bk VIII, ch 28).

As a general rule, wild animals are at their wildest in Asia, at their boldest in Europe, and most diverse in form in Libya; in fact, there is an old saying, ‘Always something fresh in Libya.’

It would appear that in that country animals of diverse species meet, on account of the rainless climate, at the watering-places, and there pair together; and that such pairs will often breed if they be nearly of the same size and have periods of gestation of the same length. For it is said that they are tamed down in their behaviour towards each other by extremity of thirst.
"Libya" is the name for Africa, from the Hellene perspective, of course. Aristotle had another long standing account of biodiversity too - the effects of local water, soil and climate. This view wasn't abandoned until the early years of the 20th century. But it's hybrids that fascinate me and motivate this post.

Hybrids were accepted as a way to "fill in" the empty territory of the Great Chain all through the middle ages, through the renaissance and Reformation, by the translators of the King James Bible, and even by Linnaeus, the champion of species fixity. Mendel was in part attempting to work out how hybridisation generated the material for evolution, at a suggestion of von Nägeli, who was in effect his scientific mentor.

Oddly, though, hybridisation plays a largely negative role in modern evolutionary thought - a species is defined by its inability to hybridise, according to the Mayrian hegemony, at any rate. More positive conceptions like the Recogniton Concept or the Genetic Cluster concept focus on the cohesion of species rather than the isolation between them.

So I am going to post, whenever I become aware of a case, instances of hybridisation, both as a counterexample to the Mayrian Reproductive Isolation Conception of Species, and as an example of a process that we tend to overlook and which needs more attention.

The inaugural example is a hybrid between a grizzly bear father and a polar bear mother, taken from a note on John Hawk's site. It is often the case that when a species is stressed ecologically, and members find it hard to find mates of their own species, they will attempt to mate with closely related species. This has been seen with lions and tigers, ducks, and wolves and coyotes, to mention just a few cases. I wonder if this might be due to the warming effect reducing the hunting territories of the polar bear. The polar bear is sometimes regarded as a subspecies of the Brown Bear of which the grizzly, kodiak and Mexican Brown Bear are subspecies, though, so this might be just a case of introgression between locally adapted members of the same species.