Thursday, February 09, 2006

Peripheral isolates, viruses, languages and a bird flu bleg

So I've got a question about the evolution of viruses. I myself am a mammalogist, and mammals turn out to be very different from viruses.

First, a digression to linguistics. It's generally found that languages change least in peripheral isolates, and change rapidly in their center of origin. Peripheral isolates are small populations budding off at the edges of a species' range. There are immigrant communities in Kansas (Mennonites I think) that are supposed to speak great 18th century German. This makes a certain sense; more people means more opportunities for novelties, and more connections between people means that innovations spread rapidly. Fewer people in an isolated setting leaves fewer chances for novelties and for mixing new ideas from different regions. There's strong stabilizing selection in such small, isolated populations, since any "mutation" just makes you incomprehensible. In a society with diverse accents, a little variation is normal and accepted, and language can drift.

In the evolution of sexual organisms, you tend to find peripheral isolates to be more divergent from one another and to evolve more rapidly than the central population. Small peripheral populations are subject to more rapid genetic drift (random change in allele frequency due to small population size). You're more likely to get a continuous run of heads if you only flip 10 times than if you flip a thousand times. Plus, peripheral isolates are often subject to founder effects; the genotypes of the peripheral population are a random sample of the original population, and just as you sometimes get 5 heads in 5 tosses, you can get skewed allele frequencies by chance when a population separates.

As Gould said in "The Episodic Nature of Evolution," anthologized in The Panda's Thumb: More Reflections in Natural History by Stephen Jay Gould:
Large, stable central populations exert a strong homogenizing influence. New and favorable mutations are diluted by the sheer bulk of the population through which they must spread. They may build slowly in frequency, but changing environments usually cancel their selective value long before they reach fixation. Thus, phyletic transformation in large populations should be very rare — as the fossil record proclaims. But small, peripherally isolated groups are cut off from their parental stock. They live as tiny populations in geographic corners of the ancestral range. Selective pressures are usually intense because peripheries mark the edge of ecological tolerance for ancestral forms. Favorable variations spread quickly. Small peripheral isolates are a laboratory of evolutionary change.
The H5N1 virus has been found in Africa. It's moving through Europe. It's all over Asia. Within a year it'll be in Australia, all over Africa and Europe, and I bet it's all over the Americas within a couple years.

If viruses are like the mammals that I'm used to, I would predict that the first outbreak of H5N1 in humans – the beginning of the pandemic that will sweep around the world – will be in Africa or Europe, rather than southeast Asia.

If it's more like a language, I'd look for it in China or southeast Asia.

My first inclination is obviously that it's more like the mammals I'm used to thinking about. As it hops around the world, it interacts with new host species, picks up little chunks of local viruses, interacts with different human populations, and has a better chance at hitting the right combination to make the jump to humans.

I can't put my finger on what's bothering me about that. It's something about it being haploid and the way that the viruses mix up in a host's body that reminds me of language evolution.

I first started thinking about this a month ago, when this article came out showing that Turkish avian flu infections are different from those in Asia:

In one week, Turkey announced 15 confirmed human cases of A(H5N1); Asia has seen only about 140 in the space of five years.

[T]he five cases in Ankara hospitals are different from those elsewhere in Asia. Four of the five display only mild symptoms, or no symptoms at all. Also, although all five have had some recent contact with birds, Dr. Rodier said, they are people who live on the fringes of a major city, not farmers or people who keep birds in their backyards.
Whether this represented something genuinely new, or just better detection in Turkey wasn't clear. If it was new, it would suggest that the peripheral isolates are doing their thing.

I'll make the bold prediction that the pandemic will start in Africa or Europe, and wait for people who know something about viruses to set me straight.