Wednesday, May 17, 2006

The Science of Evolution

Despite the fact that I studied physics at college, I've always had an appreciation for certain aspects of the life sciences. A few years ago I listened to an unabridged audio version of the Darwin's Origin of the Species read by the late, great David Case. I came away impressed with rigorous inductive logic of that treatise and the great achievement of the theory. Recently, the New York Review of Books noted some of the latest trends in the field of evolutionary research in a book review entitled "Evolving Evolution" by Edward Ziff and Israel Rosenfield. Among the more fascinating items was the following:
What they and others discovered were genes that regulate the development of the embryo and exert control over other genes by mechanisms analogous to that of the repressor molecule studied by Monod and Jacob. Eight of these controlling genes, called Hox genes, are found in virtually all animals —worms, mice, and human beings— and they have existed for more than half a billion years. Fruit flies and worms have only one set of eight Hox genes; fish and mammals (including mice, elephants, and humans) have four sets. Each set of Hox genes in fish and mammals is remarkably similar to the eight Hox genes found in fruit flies and worms. This discovery showed that very similar genes control both embryological and later development in virtually all insects and animals.
The diagram that goes along with this description illustrates the point made. The reviewers further suggest that
These findings strongly support the Darwinian view that animals descend from one or a few ancestors. However, contrary to the previously accepted neo-Darwinian view, the same findings showed that different animal forms are not primarily a function of distinct gene pools that have evolved over millions of years. How then do similar collections of genes create the enormous diversity of living forms? In Sean Carroll's view, what creates diversity is the patterns in which genes are turned "on" and "off." The different appendages found in centipedes, fruit flies, lobsters, and brine shrimp are created by varying combinations of Hox gene activity in the developing insect or crustacean embryo.
This kind of switching activity is key and has enormous implications:
Evolution, then, depends on new patterns of gene regulation rather than the creation of new genes. Indeed, it is not meaningful to talk about the function of a single gene in isolation. Genes only function in the context of the organism. There is no single gene for an eye, a limb, or language, much less such tendencies as homosexuality. Genes function in relation to other genes and intercellular signals, much as words vary in meaning and function depending on the way they are used in sentences and the contexts in which they are spoken. It is the combinations of gene activity, which may be different in different species, that create the form of the organism. "We can begin to think of individual groups—insects, spiders, and centipedes, or birds, mammals, and reptiles, as well as their long extinct fossil relatives—not so much in terms of their uniqueness, but as variations on a common theme," Carroll writes. And surprising, too, is the evidence that all animals, from worms to humans, probably descend from one or a few primitive bacteria. Darwin would have been pleased to discover molecular evidence for his "common descent."
The authors conclude:
We now have a far deeper understanding of evolution than even a decade ago. And although our knowledge is still incomplete, our new understanding, as the books under review admirably show, has opened the way toward a comprehensive account of evolution and has supplied solid answers to the critics of evolutionary theory.
I would have to agree. The progress of evolutionary theory is inspiring.

No comments: