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, January 18, 2006

Evolution, rules and laws

Evolution is a historical process, and like many such processes it is hard to make generalisations about it. But the supposed hallmark of science is that it makes generalisations, and so biologists have labored to produce these laws of evolution. As a reprobate reductionist, I think that the laws that govern evolution will be physical, in the end, but it seems that some rules have survived.

One of these is Cope's Rule, which states that body sizes tend to increase in a lineage over time. Ironically, this rule was formulated by an evolutionary biologist, Edward Drinker Cope, who rejected Darwin's mechanism of natural selection and who also thought that Lamarck was correct about inheritance. But Darwinian evolutionary biologists have tried to account for this rule without much success.

A new study has argued that there is no such tendency on the part of phylogenetic lineages, which is to say that there is no bias in speciation events to larger body forms, but that there is evidence of selection within lineages, that is, species, towards larger body size when temperatures change. I haven't seen the paper yet, but it looks like when sea temperatures dropped, the optimum body size of deep water ostracods (a kind of crustacean that looks like a clam) increased over 40 million years.

This indicates that body size is purely a microevolutionary effect of optimisation for local conditions. As such, no general rules can be formulated without knowing what conditions the organisms will encounter. And this goes also to undercut our assumptions that "evolution" is a general process of a single mechanism. It is no more so than "weather" is. We won't be able to predict more than a limited amount of change, nor develop "laws" of evolution, because the extraneous variables are so complex, contingent, and unforeseeable.

And yet evolution remains science. The atomic physicist Ernest Rutherford famously quipped that there were two kinds of science - physics and stamp collecting. Justifiably, biologists, who do a substantial amount of fact collection, protested, and before he died Ernst Mayr published a book that defended the independence of biology from physics and chemistry. But everything that we have been able to investigate substantially reduces to physics. The independence of biology is heuristic rather than ontological.

Biology needs only the laws of the physical universe - they will be economic (thermodynamic), chemical, and so forth. But for creatures of limited cognitive capacity, it makes perfect sense to break problems down into bite-sized chunks that can be addressed. If we were Laplace's Demon, able to track everything at the microstructural level, then perhaps there would be no biology. But we have only the cognitive capacities that nature has given us, and what we can generate by linking together in scientific disciplines, and so biology is a heuristic necessity.

Even Laplace himself knew this, defending his Exposition du systeme du monde with the comment that
If man were restricted to collecting facts the sciences were only a sterile nomenclature and he would never have known the great laws of nature. It is in comparing the phenomena with each other, in seeking to grasp their relationships, that he is led to discover these laws...
Biology has to make generalisations, but it doesn't have to make laws. And these generalisations are defeasible. Another "law" is Bergmann's Rule, which says that warm-blooded animals increase their body size as they move into polar, colder, climates. This makes perfect sense for endotherms, but it is a puzzle why it should also hold for exothermic animals like ostracods. I posit that the laws of physics rather than the actual temperature regulation system, are what counts here. Perhaps some metabolic reactions just need bigger bodies in colder climates, whether or not the animal regulates its temperature? Who knows?

Here's a link to the paper.

Late note: As mentioned in the Comments, coturnix blogged something else about laws that I wanted to do, but he beat me, the bastard. See Constructal Theory, which aims to show how periodicity is an expected outcome of certain systems.