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

Wednesday, November 10, 2004

Prokaryote evolution study published

A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land has just been published in BMC Evolutionary Biology.

The conclusion is that methane was generated by early prokaryotes, causing the land surfaces to warm sufficiently for terrestrial bacteria to colonise them. In short, oxygenation of our atmosphere was caused by terrestrial adaptations.

Monday, November 08, 2004

The joys of teaching

Well, I just finished teaching my first full subject as a lecturer. I'm simultaneously impressed and aghast at the quality of my students. Some show enthusiasm and interest, and a gritty determination to delve into a subject as full of complexities as any science subject they may previously have done. Others use the grocer's apostrophe and fail to use even the simplest of grammar and spelling, let alone understand the subject. I guess I am learning my lesson as a teacher, which every other teacher already knew.

Incidentally, the subject was Philosophy of Biology at the University of Melbourne, jointly between Philosophy (the inestimable Karen Jones - thanks for your forbearance, Karen) and History and Philosophy of Science (me, standing in for Howard Sankey, who got promoted to Head and had a bad case of committees come on - I hope that's cleared up now, Howard).

As always, I owe much to my previous supervisor, Neil Thomason, who seems to think teaching really matters...

And didn't we have fun with sociobiology?

The supervenience of selection

OK, if we have readers, we must have writers...

Sober's claim is that

1. Selection is an evolutionary force, and

2. Fitness is a supervenient property.

Recall that "supervenience" is a term of philosophy of mind that means that a property may be realised physically in many ways, but that if any two physical systems are identical [in all possible worlds, *shudder*] they must perforce have that property.

I think these two claims are in contradiction to each other. If fitness, the property of increasing relative to other alleles in a population, is a supervening property over the physical substrates of the actual organisms, then I fail to see how selection can be viewed as a physical force.

But if selection is not a physical force, what kind of force can it be? It must be, like fitness which proceeds from its operations, a supervening process. In short, selection is not something that occurs, so much as something that sums up what occurs. And the summation is something that happens in the process of providing an explanation.

Sober's own view of selection, and of explanation, here, implies that selection is only a force in a metaphorical sense. I get that feeling from his original discussion, that "evolution is a theory of forces" means something more like, "in explaining an evolutionary event, we must separate and assess the actual kinds of evolutionary processes, as a prolegomenon to a full explanation". If this is how he meant it, then I think he escapes the attack of Walsh et al.; if not, he should have drawn that conclusion. It is, in any event, how investigation into selective events proceeds in science.

Consider the famous cases of selection research, such as the Kettlewell moth study or Endler's study on Jamaican guppies. They did not posit some force like "selection"; they instead posited actual physical events - like predation by birds on physical trunks with physical properties (light reflection, and visual acuity). That is the nature of selection - it is a summary of the kinematics of a population.

But, one might object, Fisherian and subsequent mathematics shows there is a proper theory, indeed a fundamental theorem, in play - surely this makes selection a force? No - a mathematical model describes the ways a system will behave assuming that it can vary only along certain defined parameters. An equation is not a theory; it is a model. The model develops into a theory only when it can provide an account of the underlying physical processes, which is why, despite its tentative nature, Newton's theory of universal gravitation was a theory and not just an equation - there was expected to be a physical underpinning, and there turned out to be one. But selection posits no such physical underpinning - in fact we have extremely good empirical reasons to think that nothing physical need be common to all selection events other than the ordinary physical properties of physics in general.

Selection, as I noted, provides a way to proceed. It organises what would otherwise be a blooming buzzing confusion of empirical data, and enables one to narrow down the sorts of variables one must look at. So, too, does a large slab of our background knowledge - that heredity is in play, and so on. But these abstractions are heuristic until the rubber meets the road in each particular case. I am saying that selection is a heuristic principle, and not a full explanation.

This may be controversial...

Sunday, November 07, 2004

*knock knock knock* Is this thing on?

I suspect this medium is ill-suited to my discursive posting style. If anyone is actually following this, please let me know by posting a comment. Otherwise, I shall stop.

This reminds me of my dearly departed mother-in-law's letter to her daughter, along the lines of "I haven't heard from you. Are you receiving my letters. Please let me know if this is so" and in pencil "or not so"...