| Epilogue
WHERE IS DARWINISM going? What are the prospects for its second century? I claim no clairvoyance, only some knowledge of the past. But I do believe that an assessment of future direction must be tied to an understanding of what has been—particularly to the three central ingredients of Darwin’s own world view: his focus on individuals as primary evolutionary agents, his identification of natural selection as the mechanism of adaptation, and his belief in the gradual nature of evolutionary change.
Did Darwin hold that natural selection acts as an exclusive agent of evolutionary change? Did he believe that all products of evolution are adaptive? During the late nineteenth century, a debate arose in biological circles over who rightly wore the title of “Darwinian.” August Weismann, a rigid selectionist who granted almost no role to any other mechanism, claimed the mantle as Darwin’s true descendant. G.J. Romanes, who gave Lamarck and a host of other aspirants equal billing with natural selection, demanded the title for himself. Both and neither were right. Darwin’s view was pluralistic and accommodating—the only reasonable stance before such a complex world. He certainly granted overwhelming importance to natural selection (Weismann), but he did not reject an influence for other factors (Romanes).
The Weismann-Romanes debate is playing itself out again, as the two most widely discussed movements of recent years line up behind the old advocates. I suspect that Darwin’s middle position will prevail again, as extreme formulations on either side retreat before the multifariousness of nature. On the one side, human “sociobiologists” are presenting a series of elaborate speculations rooted in the premise that all major patterns of behavior must be adaptive as the products of natural selection. I have heard adaptive (and even genetic) arguments for such phenomena as the inheritance of wealth and property through male lines and the higher incidence of fellatio and cunnilingus among the upper classes.
With supreme confidence in universal adaptation, sociobiologists are advocating the ultimate atomism—reduction to a level even below the apparently indivisible individual of Darwin’s formulation. Samuel Butler, in a famous remark, once stated that a chicken is merely the egg’s way of making another egg. Some sociobiologists take this epigram literally and argue that individuals are no more than instruments that genes use to make more genes like themselves. Individuals become temporary receptacles for the “real” units of evolution. In Darwin’s world, individuals struggle to perpetuate their kind. Here genes themselves are generals in the battle for survival. In such intense combat, only the fittest win; all change must be adaptive.
Wolfgang Wickler remarks: “It follows from evolutionary theory that the genes run the individual in their own interest.” I confess that I cannot regard such a statement as much more than metaphorical nonsense. I am not bothered by the false attribution of conscious purpose; this is a literary convention, and I am guilty of it myself. I am disturbed by the erroneous idea that genes are discrete and divisible particles, using the traits that they build in organisms as weapons for their personal propagation. An individual is not decomposable into independent bits of genetic coding. The bits have no meaning outside the milieu of their body, and they do not directly code for any bounded piece of morphology or any specific behavior. Morphology and behavior are not rigidly built by battling genes; they need not be adaptive in all cases.
While the sociobiologists try to out-Weismann Weismann, many molecular evolutionists take the opposite view that much evolutionary change is not only uninfluenced by selection, but truly random in direction. (In Darwin’s formulation, the raw material of variation may be random, but evolutionary change is deterministic and directed by natural selection). The genetic code, for example, is redundant. More than one sequence of DNA yields the same amino acid. It is hard to imagine how a genetic change from one redundant sequence to another can be controlled by natural selection (since selection will “see” the same amino acid in both cases).
We may choose to regard such “invisible” genetic change as irrelevant, for if variation is not expressed in an organism’s morphology or physiology, natural selection cannot act upon it. Still, if most evolutionary change were neutral in this sense (I don’t believe that it is), then we would need a new metaphor for Darwinian influence. We might have to view natural selection as an epiphenomenon, touching only the few genetic variations that translate into adaptively meaningful parts of organisms—a mere surface skin on a vast sea of hidden variability.
But the challenge of molecular evolutionists is more serious than this—for they have detected more variability in proteins (i.e. in visible genetic products) than models based on natural selection should permit a population to maintain. In addition, they have inferred a strikingly regular, almost clocklike, rate for evolutionary changes in proteins over long periods of time. How can evolution work like a clock if it is directed by a deterministic process like natural selection—for intensity of selection maps rates of environmental change, and climate does not tick like a metronome. Perhaps these genetic changes are truly neutral, accumulating at random and at constant rate. The issue is not settled; copious variability and clocklike rates can arise by natural selection with the aid of some ad hoc hypotheses that may not turn out to be absurd. I only wish to argue that we have no final answers.
I predict the triumph of Darwinian pluralism. Natural selection will turn out to be far more important than some molecular evolutionists imagine, but it will not be omnipotent, as some sociobiologists seem to maintain. In fact, I suspect that Darwinian natural selection based on genetic variation has rather little to do with the very behaviors now so ardently cited in its support.
I hope that the pluralistic spirit of Darwin’s own work will permeate more areas of evolutionary thought, where rigid dogmas still reign as a consequence of unquestioned preference, old habits, or social prejudice. My own favorite target is the belief in slow and steady evolutionary change preached by most paleontologists (and encouraged, admittedly, by Darwin’s own preferences). The fossil record does not support it; mass extinction and abrupt origination reign. We cannot demonstrate evolution by recording gradual change in some brachiopod as we climb a hillslope. To sidestep this unpleasant truth, paleontologists have relied on the extreme inadequacy of the fossil record—all the intermediate stages are missing in a record that preserves only a few words of the few lines of the few pages left in our geological book. They have purchased their gradualistic orthodoxy at the exorbitant price of admitting that the fossil record almost never displays the very phenomenon they wish to study. But I believe that gradualism is not exclusively valid (in fact, I regard it as rather rare). Natural selection contains no statement about rates. It can encompass rapid (geologically instantaneous) change by speciation in small populations as well as the conventional and immeasurably slow transformation of entire lineages.
Aristotle argued that most great controversies are resolved at the aurea mediocritas—the golden mean. Nature is so wondrously complex and varied that almost anything possible does happen. Captain Corcoran’s “hardly ever” is the strongest statement that a natural historian can make. A person who wants clean, definitive, global answers to the problems of life must search elsewhere, not in nature. In fact, I rather doubt that an honest search will reveal such answers anywhere. We can resolve small questions definitely (I know why the world will never see an ant 25 feet long). We do reasonably well with middle-sized questions (I doubt that Lamarckism will ever enjoy a resurgence as a viable theory of evolution). Really big questions succumb to the richness of nature—change can be directed or aimless, gradual or cataclysmic, selective or neutral. I will rejoice in the multifariousness of nature and leave the chimera of certainty to politicians and preachers.
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| Index
Achatinella apexfulva, 233
Adams, John Quincy, 28
Adaptation by evolution, 91–96, 103–4
aphids, 95–96
Cecidomyian gall midges, 91–92
, 95
comparative method and, 92–93, 95–96
Micromalthus debilis, 92, 96
Mycophila speyeri, 93–94
r- and K-selection theory, 94–95
African Genesis (Ardrey), 240
After Many a Summer Dies the Swan (Huxley), 63–64
Agassiz, Louis, 92, 148–49, 151, 152, 243
Agate Springs Quarry, 156
Alexander’s Feast (Dryden), 49
Algae, 114, 121
eukaryotic, 115
Allen, Mel, 70
Alleröd interstadial phase, 90
Allopatric theory, 61–62
Allosaurus, 190
American Journal of Science, 130
American Museum of Natural History, 207
American Scientist, 180
Anaximander, 202
Andrews, Roy Chapman, 207
Annual Review of Ecology and Systematics, 98
Anthropocentrism, 114
Anti-Duhring (Engels), 210
Antioch College, 160
Aphids, 95–96, 98
Aphis fabae, 96
Apollo missions, 194
Archaeopteryx, 189
Ardrey, Robert, 237, 239–40, 242
Aristotle, 205, 271
Army Mental Tests (World War I), 244
Arness, James, 113
Astronomy, 147
Australian Aborigines, 236
Australopithecus, 57, 60, 182, 183–85, 207–8, 212
Australopithecus africanus, 57, 58–59, 60, 182, 183, 184, 241–42
Australopithecus boisei, 56–57, 58
Australopithecus robustus, 58, 60, 241
Babies as embryos, 70–75
brain size, 73, 74, 75
gestation period and, 72–73
Portmann’s position on, 72
postnatal development, 73
Baer, Karl Ernst von, 208, 209
Bamboo (Phyllostachys bambusoides), 97–102
flowering of, 97–98
Barrett, P. H., 23
Beagle, 21, 23, 28–33
Beauvoir, Simone de, 259
Ever Since Darwin: Reflections in Natural History Page 24