Wednesday, November 21, 2007

Evolution: Selection for Stability

I'll say it yet again: evolution is a very subtle and nuanced subject. An intellectual challenge with some easy misconceptions that are fostered by a careless turn of phrase.

Just last Friday, I was talking to someone who I should expect to know better. He had quite a good knowledge of paleontology, but he was caught up in the most basic Lamarckian thinking. He still misperceived that traits acquired in a lifetime (such as beefy muscles) could be passed on.

Perhaps it's too easy to think the obvious; like a lot of quantum physics, there's a converse, perverse actuality that's counterintuitive. It's plausible that a good specimen breeds good stock, but then that's only by virtue of its genetic material formed before it was born.

With this in mind, it's worth stating the obvious. Perhaps not often and loud, but with enough nuance that the thought sticks.

I've been reading a book called Plan And Purpose In Nature (George C Williams, reference below). It has its controversies, but one lucid point in particular is straightforward:

Natural selection has a core role to play in stability, not just in evolutionary change. Most of the time, selection results in a culling of mutations from a population's optimum.

Williams reports on William Bumpus, who in 1899 observed sparrows killed in a storm. Specifically, he measured their wingspans, and found that among those killed, a higher percentage (than in the general population) had wingspans that were markedly different from the norm. The suggestion is that the sparrow was sufficiently adapted to its environment that at the optimum wingspan it was better equipped to survive storms. At the margins, the wingspans were unhelpful in the environment of the time. Williams: "The process proposed by Darwin is now thought to operate mainly to prevent evolution".

This is the differential action of natural selection: optimisation within a physical range for a stable environment. Outliers are more likely to be culled.

This is the beauty of the process of random mutation coupled with natural selection for a specific environmental niche. That mechanism carries with it the ability to propel a population to adapt to a changing environment. With an important caveat: the population is carried forward (evolves) IF and WHEN environmental changes and mutational changes happen to be sufficiently optimised. As (and if) an environment changes, some outliers will be favoured, and the erstwhile optimum will be out of favour. If the environmental change is too abrupt, the outliers can't propagate quickly enough, and the population dies out.

Continued optimisation for a stable environment is undoubtedly behind the concept of punctuated equilibrium. Stable environments make for stable populations, and it's only when the environment changes that evolution is forced. Anything else is random genetic drift, which doesn't of itself have significant evolutionary outcome, and is certainly not a factor in large populations.

So is this concept of stability sound too obvious? It's still worth holding on to it. Never know when you'll need it, as Gould would undoubtedly have attested.

Williams, George C (1996): Plan And Purpose In Nature, Science Masters, Great Britain.

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