Tuesday, October 6, 2009

Darwinism without Darwin: The Origin of Species without Natural Selection?

We've written here before about whether Darwin actually solved the species problem for which he named his famous book. We concluded that he did not--natural selection, geographic distance, adaptive responses to environmental changes, time since common ancestor, none of these necessarily lead to speciation, though together they constitute the processes that explain biological divergence, adaptations to environment, and ultimately mating incompatibility (the most common practical definition of species). The question of what causes speciation at the gene level is currently being addressed by molecular biologists,who have advanced various ideas, and this is what Allen Orr, from the University of Rochester, talked about at the meeting in Italy.

Allen has been publishing on this question for a long time based on evidence found in fruit flies. He crosses two particular lines of flies, one from the US, and one from Colombia, and finds that their offspring are either sterile or a process called segregation distortion or meiotic drive results in an overwhelming proportion of offspring of just one sex (male or female, depending on the particular cross), so that the F1 (offspring) generation can't reproduce. They thus cannot form their own population, which demonstrates that, in this sense, the US and Columbian populations are separate species.

As we understand his work, the effect is not 100%, so by classical concepts the speciation is not complete, but this is a detail rather than a profound aspect of the results, which indicate that genetic mechanisms--or genetic 'conflict'--can be responsible for speciation, rather than the classical Darwinian idea that it is a suite of adaptations to the environment that leads to species formation.

Five or six genes, including several found in Orr's lab, have been shown to be responsible for these processes. They have nothing in common, according to Orr, except that they are evolving very fast. They are not related to particular adaptations to climate, diet, predators, pathogens, and the like. Instead, they can be referred to as 'hybrid sterility' genes, and a number of such genes or mechanisms have been found over the years, indicating that this particular example is not unique.

Hybrid sterility was discussed at some length by Darwin in the Origin of Species, and the phenomenon was well-known in his time, though of course nothing about its genetics. The interesting fact, perhaps, is that mating between such species leads to offspring, which one would think meant the parents were not separate species after all, but the hybrid cannot reproduce. This is quite different from the inability of, say, dogs and cats to mate with each other (should they even want to!). Nobody doubts that, by any definition, dogs and cats are different species.

The interest in hybrids is that they are formed from closely related 'species', such as members of the horse family (or fruit flies, these days), so hybrid sterility seems relevant to our understanding of the process by which species differences arise.

Darwin was wrong in many respects. Indeed, one can find flaws, shallow evidence, forced reasoning, and speculation throughout his work. He was thoroughly wrong about many things, perhaps most notably the nature of inheritance, the inheritance acquired traits, and the age of the earth (and he was quite anthropologically naive in terms of his ideas about the evolution of human 'races', or those with different cultures). Thus, Darwin's work is not a 'revealed' truth the way some people take the Bible or other sacred texts to be.

Instead, Darwin's work is masterfully integrative, assembling grand ideas from a wide diversity of carefully considered data. Given the data of his time, his deep insight was that life is a history of process rather than separate creation. Once that idea dawned on him, he was incredibly capable of developing the case. The proof of that is the same fact: his overall idea withstood the many errors that he made about the details. That is a powerful test of any scientific idea.

Indeed, the 'species problem' (or 'transmutation' as it was often called then) is a rather trivial technical issue, of interest to systematists (those who build trees of biological relationships). The species relationships were key to Darwin, because they showed that life was a history and as he said extensively in the Origin, he could see no better explanation than his (and certainly not creationism) to account for the diversity of facts that he presented.

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