Friday, April 23, 2010
By Ken Weiss
There's a new paper in PNAS that looks like one that really is interesting and relatively important. It is a story that made the popular science news, and reports paleontological findings with at least some interesting aspects.
This is the discovery of fossils ants and other insects, preserved in amber (tree resin, basically) that is estimated to be about 95 Million years ago, along with other species of plants. The insects are shown here. Apparently (we're no experts!) it has been thought from the available evidence that the ant lineages originated to the north (Asia or North America), rather than Europe. If the dating of this site is accurate, then the old idea is incorrect. At least, by this ancient time clearly recognizable ants were already in Africa.
Now this is not a story to shatter evolutionary theory, though overturning accepted views, no matter how arcane, gets played in the news headlines as being transformative. But it does seem to qualify as new and worth discovering and so on. That is, its appearance in the news doesn't seem to qualify as just more Big Discovery hype.
There may well be ecological, geological, or climatological reasons why this find is interesting. Or it could just be that, while the environments Way Back When were suitable for some of these arthropods, they just hadn't managed to get to all of them by this time, having originated elsewhere with suitable climate. So ant lovers can now start trying to explain this expansion, and EO Wilson can stop writing bad novels about ants and get back to what he does really well.
But there is something that is much more interesting to us. It's another example of a fact that is already well-known but that these specimens give us the occasion to write about. It's the very deep conservation of very modern-looking form, in this case of insects (but other groups of animals and plants show some similar characteristics).
Now, evolution is supposed to be a relentless struggle to keep ahead, to survive in changing environments (that include the ever-changing mix of other species, climate, and so on). If you look at the DNA from a group of critters like, say, ants, their sequence differences correspond to their species differences, and separation times estimated from the amount of sequence difference correspond to fossil evidence. Since Way Back When, there has been a proliferation of related species, with their own morphological differences and adaptations.
But wait! Why, then, do these very ancient fossils look just basically just like the bugs that bother you when you try to eat out on your back deck? Why can morphology stay essentially unchanged for 100 million years? Why hasn't selection or even just genetic drift (chance changes) led to changes in the form of these bugs?
The standard answer is that selection has kept it so. That means that the environments haven't changed. But why then did the species divergence also occur? Long-term stasis is not a new discovery; it was the basis of a famous, but over-hyped 'theory' called punctuated equilibrium, but that has to do with other aspects of evolution and the fossil record. For us here, what is not so clear at all is how selection keeps form so similar for so long in various lineages, yet allows divergence even in local areas as well as genomewide sequence differences to accumulate in the same way it does for less conserved branches of life. Various evolutionary ecologists, like a prominent acquaintance named Doug Futuyma, have advanced ideas about how genetic incompatibility can keep neighboring populations from interbreeding, so they retain their distinct forms and functions. But even without interbreeding as a source of variation, why doesn't form just drift--very slowly perhaps, but after 100 million years why not noticeably?
We don't have an answer, and maybe the rather superficial answer ('selection made it so') is correct. Even then, how that works with traits affected by many genes, as these insects' form certainly is, is a major question for evolutionary biology....if evolutionary biologists would think more seriously about it rather than assuming the answer is known.
We'll post on this subject again.