What would happen if a modern biologist could get in touch with Charles Darwin and enter into a lively and stimulating discussion about recent developments in evolutionary theory? Here, we have the answer. Molecular biologist Gabriel Dover uses imaginary correspondence as a literary device for explaining how our ideas about evolution have evolved since Darwin's day.
Dover argues that evolution involves more than just natural selection and cites such phenomena of sampling error as genetic drift. The basic idea behind sampling error is easily recalled. Gene frequencies inevitably fluctuate at random. Some alleles within a population may happen not to be present in any of the zygotes that ultimately become the next generation of adult organisms. This is more apt to happen when the alleles are rare and in small populations. So pure accident may result in the elimination or fixation of an allele. The result is change in gene frequencies, but not adaptation.
According to Darwin's theory of natural selection, adaptation results when one organism has properties that allow it to out-reproduce another of the same species. Dover does not mention another of Darwin's mechanisms -- correlated variability, or pleiotropy. Variation sometimes involves traits that always go together, so that they both increase in frequency even if only one of them is selectively advantageous. Dover's additional mechanisms are something different. Because of the way in which chromosomes behave in the course of reproduction, the genome is constantly being reorganized. Sexual reproduction generates change, and Dover sees in it a cause of evolution (a "molecular drive") that interacts with selection and sampling error.
Although Dover explains all this very well, his real goal is to rebut the metaphysics of Richard Dawkins, an Oxford behaviorist who studied chickens before branching out and writing popular books. Dover laments the influence of Dawkins's reasoning on persons who are not equipped to see through it, especially textbook writers and social scientists. Let me clarify what Dover is complaining about. Dawkins decided to call genes, and other things of which copies are made, "replicators." The problem with that term is that in ordinary English, the suffix "-or" refers to the doer of the action. Thus, a replicator should be that which does the replication, not that which is replicated. (Likewise, a photocopier is not the copy that is produced by the machine.) Dawkins's term is apt to dupe the unwary into thinking that a passive participant is an active agent. This is what Dover has to grapple with: the metaphysics of agency. He makes it abundantly clear that genes are not replicators, in the sense of things that carry out replication. Rather, they are replicated by the cells that contain them. More important, he argues that organisms are active agents in evolution by virtue of their roles in restructuring the genome in producing compatibility among genes, chromosomes, and other components of organisms and species.
This suggests an important role for sex. Dover maintains that molecular drive produces compatibility within reproductive populations. The compatibility within species, together with the lack of it between them, is fundamental to the modern "biological species concept." The point that species are not just abstractions but, rather, higher-level units that play an important part in evolution is very much in line with Dover's antireductionist metaphysics. However true it may be that species and other populations are not likely to have adaptations over and above those of their component organisms, there is no legitimate reason to extrapolate and treat species and organisms as mere epiphenomena of molecules.
Species are important because they are historical units -- things that evolve and give rise to the branches of the phylogenetic tree. Dover is off the mark when he suggests that biology is history, pure and simple, and that we seek in vain for its laws of nature. Rather, biologists have been seeking laws of nature in the wrong place. Although it is true that there are no laws of nature for organisms and species, this is because organisms and species are concrete, particular things, or "individuals" in the broad, metaphysical sense. All laws of nature are about kinds, or things in general, and not about instances of such kinds. In evolutionary biology, laws apply to kinds of populations, such as large populations and small populations. For example, as the effective population size goes down, the frequency with which alleles are fixed by sampling error goes up. This is a perfectly legitimate law of nature: it is necessarily true of everything to which it applies, irrespective of time and place. However, it is a statistical law, since it does not predict which particular version of a gene will be eliminated from the population. The fact that so much of the lawfulness (such as it is) of biology must be conceptualized in such statistical terms gives scant comfort to those who would have us treat human behavior in Laplacian, deterministic style.
One might wonder whether debunking the metaphysical pretensions of one's colleagues is perhaps beneath the dignity of a good scientist like Dover. Isn't it enough to joke about selfish chromosomal deletions and then get on with one's research? The trouble is that selfish genes are becoming part of popular culture. The medical community should brace itself for an onslaught of belief systems, alternative therapies, and nostrums -- all justified on the basis of what purports to be legitimate science.
Michael T. Ghiselin, Ph.D.
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