Friday, August 21, 2009

Permanent ink

Calamari lovers, take a look at an interesting evolutionary story at the BBC web site of a fossil squid. This little gem is an estimated 150 million years old, yet seems in great detail to be similar to today's appetizers (we like them sauteed with chili peppers, olive oil, s&p, on a bed of fresh lettuce).

This particular fossil was found when a rock from a dig in Wiltshire, England, was cracked open. The ink sac still contained ink, and archeaologists were able to draw a picture of the specimen with it (drawing shown here, from the BBC web site). Many more such fossils were also found, including other squid species; researchers suggest that the squid were drawn to the area to feed, but were poisoned by algae when they arrived, and conditions were right for their preservation.

This find is one of many, long known in paleontology, that show remarkably conserved traits--again, down to the finest detail, for many different kinds of plant and animal (and bacterial) species. Indeed, stromatolites are multi-billion year-old beds of what appear to be bacterial films that look remarkably modern.

Such finds raise the obvious, but again not new, question of stasis in evolution. How can evolution, so typically portrayed as a relentless race to get ever better, be responsible for not changing at all? A common metaphor is known as the Red Queen hypothesis, from Through the Looking Glass: the Red Queen says to Alice, as she breathlessly pulls her by the hand, that they must always run as fast as they can to stay in the same place.

If you look at squid phylogeny (a fine web site for such data is maintained by Sudhir Kumar and Blair Hedges), you'll see that long-conserved creatures like flies, horseshoe crabs, many kinds of plants and much else--including squid--have DNA sequence relationships that make sense. The branches between current species roughly correspond to their ages of separation estimated from fossils and other kinds of taxonomic studies. They accumulate molecular differences in the usual way (though of course the details vary considerably and can be argued about).

But how does selection, if that's the cause, maintain such stasis? This question has been asked and answered implicitly or explicitly in various ways. Large populations essentially swallow up new variants like the sea. Selection removes the bad ones, but the good ones arise amidst such a huge population that they can basically not 'take over' the sequence for their respective locus.

Stephen J Gould made much of 'punctuated equilibrium', initially writing as if this was a dramatic new theory: change eventually occurs in a small subpopulation in some local area not documented by the fossil record. The advance then spreads throughout the species and then gives the illusion, in the fossil record, of having appeared suddenly. Sewall Wright actually developed formal theory for this as a basic means of evolution, in contrast to Fisher's ideas, way back in the 1930's. Wright envisioned major adaptive changes as being mainly possible in small isolated subpopulations, where new or helpful ('fit') variation would not just be swamped and absorbed by the species at large.

A key fact may be that, today there are many species of squid, and their traits vary both within and between species. So squid as a group clearly have evolved! But let's go back 150 million years and imagine the future course of events. If environments, like the general nature of the ocean bed, stay the same in many ways, there will be change from generation to generation, but the conservative nature of selection will usually eliminate or work against squids too different from their fellows or their habitat. Small, chance changes in DNA sequence and/or function will always occur, however, and local as well as global populations will accumulate variation. New or modified squid species will arise (and that had already happened by 150 my ago as we noted above).

In such circumstances, by chance or the gentle molding of selection, some descendants will stay pretty much the same, while others will either die out or differentiate into new species with different morphology or adaptations. So when we find a fossil and search among today's many kinds of squid, it's no surprise that we find some that didn't change.

This is a kind of problem in conditional probability, related to the degree of surprise we should feel about a given observation, or the multiple-testing phenomenon in statistics. If conditioned on searching through an open-ended array of possibilities, the 'significance' is much less (e.g., p value much greater) than if you just looked at a single instance.

We might be surprised if we held to a strictly Red Queen view that evolution always forces creatures to innovate or die. And, of course, some major or minor lineages don't stay as static, again what one would expect (dinosaurs didn't, but birds arose from among them). Otherwise, it is less clear how selection could preserve identity so precisely. These are problems associated with the kind of retrospective illusion due to the compression of the deep past that we write a lot about in our book. We think the Red Queen is misleading Alice!

Whatever the explanation, one lesson is to be careful how we interpret what is written in stone (or with permanent ink). And fried calamari are great, too!

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