Thursday, June 21, 2007

What Is Evolution?

If I'm going to reinvent evolution, I probably ought to define it. That turns out to be not very easy.

The word "evolve" comes from a Latin word meaning "to unroll", as in unrolling a scroll. As applied to living things, it's a leftover of the Great Chain of Being, the idea, dating from Aristotle, that life is arranged in a hierarchy from non-living, or barely-living, like lichens and moss, up to God, the greatest of all imaginable beings. In the original form of the Chain, each life-form was just above the one below and just below the one above; there were no gaps in the chain. There is clearly a great gap between man and God; this was filled with a hierarchy of angels. The best source of information, if you are a glutton for philosophy, is The Great Chain of Being, by Arthur O. Lovejoy.

The idea of a Chain of Being follows from the Principle of Plenitude, one variant of which holds that God, in his infinite goodness, could not have created less than everything possible. A consequence of this is that there can never be anything new. It was also used against the idea that what we now call fossils are the remains of once-living creatures.

As natural philosophers in Europe looked around, they saw gaps; for a while they believed that these would be filled by discoveries in the New World. Eventually, thugh, the New World came to be fairly well explored, with large gaps rameining in the chain. This led, in the 1700s, to what Lovejoy calls the temporalizing of the Great Chain: The plenum formarum, the fullness of forms, doesn't have to exist all at once, rather parts of the chain can come and go. Progress became philosophically possible.

The plenum has to come about eventually, though, so the scroll, on which the Book of Life is written, has to unroll. The new links in the Chain have to arrive sometime. AboutDarwin lists a number of people who expressed this idea in one way or another before Darwin.

Darwin put forth a mechanism (which I will have to wait to deal with later) by which new species can arise. He didn't believe in "progress" (at least not officially), so the idea of an unrolling didn't work for him. This may be why he didn't call his theory "evolution". In recent decades, molecular embryology (again, wait) has started to come to grips with the biochemical origin sof novelty. New forms are not part of a pre-existing pattern, but are actually new (in a sense that I will at some point develop).

John Wilkins laments that the Great Chain is not quite dead:
But the Great Chain is more than a hangover; it is a durable idea. It infects anthropology, cognitive studies, and philosophy as well as biology, and it was inserted into evolutionary traditions when Lamarck borrowed the scale from Bonnet in the early years of the 19th century and turned it on its side. No longer did the ladder range from bottom to top (approaching God in the process); now it ranged from early to late, with later being better in some way.

Darwinian evolution, however, is not a Great Chain. Later is just... later. Of course Darwin himself got a bit confused about this from time to time, but the message that Darwinian evolution was not necessarily progressive, and that humans are not the single most spectacular opus of Nature ever was clear, and offensive! Rejecting this implication of evolution, in cultural evolution as well as biological evolution, became a cottage industry, involving reinterpretations of Schopenhauer, Hegel, and Kant, as well as devising sometimes mystical evolutionary accounts (Teilhard or Bergson), or special cases for culture (Gould comes to mind, but there are many others, such as Rescher).
So if evolution is not really an unfolding, what is it? Larry Moran, a biochemist at the University of Toronto, has an extended post discussing various definitions. He notes that a definition shouldn't limit the mechanism. This would seem to eliminate definitions such as "change in gene frequency", since it assumes a particular (and somewhat obsolete) concept of genes. Susan Oyama, in The Ontogeny of Information, and Mary Ann West-Eberhard, in Developmental Plasticity and Evolution, emphasize that the development of an organism is determined (or maybe just influenced) by the entire environment, of which gene products are an important part, but not the whole.

One side note: Larry says
Note that biological evolution refers to populations and not to individuals. In other words, populations evolve but individuals do not.
The evolution of a star proceeds according to the simple (see the next post) principles of physics, so this is akin to an "unfolding" of a natural progression. So biology has left behind the original definition of "evolution", which still survives in astronomy (and maybe to a lesser extent in geology). But the development of an individual organism is also an unfolding of a sort, being the realization of a sequence of biochemical events which is, approximately and to some extent, predetermined.

So my own working definition of biological evolution is
Evolution is heritable change in the characteristics of a lineage.
I de-emphasize populations just a little, since in- and out-migration can influence what one observes in a population. I'll probably have to study some ecology to figure out how important that might be. And of course sexual reproduction mixes lineages. "Lineage" does assume common descent, although the number of original forms is an empirical question. Creationists assume a great many original forms, with limited variation in lineages. Darwin spoke of "one form or several"; evolutionary biologists have decided that there was essentially one original form (although its exact nature is sort of up for grabs). The evolutionary paradigm allows for much more variation. The causes of variation are, to me, one of the most exciting parts of current biology.

1 comment:

Rob said...

I enjoyed your opening blog John. Great stuff. I am passionately consumed studying evo devo and the great wealth of new information pouring forth from this new field, along with comparative genomics. Have you read Robert G.B. Reid’s “Biological Emergences: Evolution by Natural Experiment”?