The definition of a species is somewhat murky, because speciation is a continuous process. Which says something about race and racism as well.
Santa Claus brought me a delightful book about bird evolution, over the holidays. The writing is workman-like, and the treatment thorough. It covers the diversity, origins, and current status of birds, with an emphasis on how they inform evolutionary theory generally. One issue that is still live in evolutionary biology is the definition of a species. It is clear enough that birds and bees are different species, even different phyla. But are the six species of herring gull separate, or just sub-species? This is not just a headache for birders filling out their lists, but very much for biologists as well, in conservation, taxonomy, and evolutionary theory. How different do species have to be? Does any interbreeding / hybridization disqualify two populations from being different species? If not, how much does? And more importantly, at which point do such populations behave like different species, avoiding mating with each other, and maintaining distinct traits? This remains a difficult question, and is generally relevant, including for our own increasingly ramified family tree of ancestors.
One point that the author (Douglas Futuyma) makes, which I had not appreciated fully before, is that in the process of speciation, genetic barriers to successful mating between two species (or nascent species) come very late. In contrast, behavioral, ecological, or geographical forms of separation come early and are the real drivers. The examples are all around us as hybrid forms that can occur at range overlaps, not only among birds, but mammals as well. Polar bears can interbreed with other bears, though they don't want to! Species are recognized as separate by traits or molecular isolation long before they differ enough that they can't make viable hybrid offspring.
"So far, there doesn't seem to be any detailed evidence about how specific genes interact to cause incompatibility in hybrids, and why the genes diverged. But genetic incompatibility is seldom the cause of speciation in birds simply because premating isolation almost always evolves long before incompatibility does. Hybrids seem to be fully fit in about half of 254 different crosses between closely related species, many of which diverged as long as 5 million years ago. We can learn more about the causes of speciation if we focus on premating isolation, especially behavior. What causes behavioral isolation to occur?" p.169
This has some significant implications. Speciation turns out to arise largely from mate choice decisions, which we know in birds are highly discriminating. Why do they have all the plumage colors, song singing, and other mating behaviors, but to carry out the most careful evaluation of mates? The ideal mate is not very closely related. There are instinctive barriers to sibling mating, for instance. On the other hand, the ideal mate is also not very distantly related- not part of a differently colored sub-population or allied species. The plumage colors of birds are usually the clearest marks of speciation, both to us, and evidently to themselves- a way to keep straight who is who. On top of all that, the ideal mate has other properties that it may display during the courtship period, like vigor, courage to sing from the top-most branches, the ability to bring gifts of food, or to to construct an intricate bower.
The typical differentiation of nascent species happens due to physical separation, such as a founder finch flying to Hawaii, and starting an amazing adaptive radiation of honeycreepers and other birds. But sometimes, (as must have happened within the Hawaiian radiation of these birds), separation can be by habit or specialization within the same location- which is called sympatric speciation, or speciation in place. While difficult to demonstrate and reconstruct, this has been documented to occur, and again must be attributed to some kind of behavioral specialization and mating preference that overrides the heretofore mixed mating system of a local population.
 |
| A happy pair of horned puffins- not to be confused with tufted puffins! |
It is evident that mate choice is a deeply significant and influential force. It causes all the decorative features typically displayed by males, but more deeply embodies and enforces the very concept of species as experienced by the species themselves. When a tufted puffin declines to mate with a horned puffin, it may not know whether offspring would have been possible or infertile, but it just knows that something isn't quite right about that prospective mate, and finds someone more suitable. And this is naturally relevant in humans as well. We are always making fine distinctions among ourselves in status and innumerable other qualities. Therefore it should come as little surprise that we have racist impulses, despite the fact that humans are, due to our rapid evolution and rampant internecine warfare, one of the least diverse species in existance, with no trace of genetic differentiation to support any kind of genetically based sub-speciation, races, etc.
No comments:
Post a Comment