Genetics of Original Sin (13 page)

Particularly impressive in this respect are the many instances of mimicry, the phenomenon by which some animals have acquired appearances that make them almost indistinguishable from their environment, for example, insects that look like leaves or branches, fish that look like the bottom of the sea, and so on. Obviously useful as a protection against predators, these phenomena are astonishing, as they imply a multistage evolutionary process—a beetle cannot suddenly come to look like a leaf—in which, at every stage, the organism underwent a small change that made it look a little more like its environment, enough to give it a selective edge over its unmutated congeners. What is remarkable is that a favorable mutation took place at each of those stages.

Also remarkable are the many instances of animals that have separately developed the same specializations in the face of the same challenges. Anteaters, moles, felines dependent on hunting, and herbivores built for speed look very much the same in widely different parts of the world, where they evolved in completely independent fashion. This phenomenon, called “evolutionary convergence” or “convergent evolution,” has attracted considerable interest in recent years. It is cited in recent books by Simon Conway Morris (
Life's Solution: Inevitable Humans in a Lonely Universe,
2003) and Richard Dawkins (
The Ancestor's Tale: A Pilgrimage to the Dawn of Life,
2004), two British evolutionists of the younger school, as convincing proof of the inevitability and repeatability of many evolutionary happenings. Dawkins even goes so far as directly to counter Gould by stating that if the tape were replayed, essentially the same story would unfold. Interestingly, Conway Morris defends a religious vision, whereas Dawkins is a militant atheist. So, it is not ideological bias that prompts either of these scientists to assert this view.

One last feature of natural selection that deserves emphasis is its dependence on environmental conditions. Whatever genetically determined attribute is naturally selected and therefore survives is directly related to the kind of challenge the evolving organism is exposed to. Thus, if increasing temperature is the challenge, the most heat-tolerant form present among the offered varieties will be selected. In the face of increasing cold, on the other hand, the form best adapted to a low temperature will emerge. And so on. In all the examples of optimization given above, whether biodiversity, mimicry, or convergence, the common determinant has been adaptation to a given environment. If evolving life had encountered different environmental conditions, it no doubt would have generated a different biosphere. Here, indeed, the word “contingency” finds its deserved place. The history of life on Earth is undoubtedly unique, even if there should be billions of life-bearing planets, as no planet can go through precisely the same kind of history as another one, given all the erratic cosmic, geological, climatic, and biological upheavals it would undergo.

Such being the case, while the history of each planet is certainly unique, the events that compose it are less so. Tectonic movements, volcanic eruptions, dryness, floods, glaciary periods, tropical episodes, even the fall of meteorites are events
that have a considerable likelihood of happening at one or another moment in the history of a planet situated, like Earth, in the habitable zone surrounding its sun.

Within such a context, one may wonder whether certain key events of the history of life had not become almost necessary at the stage reached by evolution and waited, so to speak, for the environment to provide them with an opportunity to take place. As an example, if the fall of a meteorite had not precipitated the disappearance of dinosaurs sixty-five million years ago, is it not conceivable that those monstrous animals were in any case slated to disappear, together with the luxuriant vegetations from which they drew their subsistance, and that, if not the fall of a meteorite, some other accident would at some time have triggered their eradication. As we shall see, the same kind of question arises with respect to the advent of humanity.

Thus, the notion of pure contingency as the driving force of evolution should perhaps be replaced by that, more subtle, of coincidence between an evolutionary stage potentially capable of leading to a critical step in the history of life and the environmental conditions needed for this step to be accomplished. In such a case, the main lines of the evolutionary course would be more or less probable depending on the likelihood of such coincidences.

Perhaps the picture, long taken for granted by a majority of evolutionists, of an evolutionary process largely dominated by the vagaries of the environment should be replaced by that of a process dominated, at least in its main lines, by its internal dynamics but dependent on the environment for the actualization of its potentialities. We have seen that several modern evolutionists lean in favor of such a conception.

D
arwinian selection constitutes the main mechanism of biological evolution. As we saw in the preceding chapter, the evidence supporting this statement is overwhelming. But natural selection is not solely in charge. Several other mechanisms have been proposed that, while not replacing natural selection, may play a significant additional role.

The first evolutionary theory to be conceived antedates natural selection by half a century. It was elaborated, together with evolution itself, by the Frenchman Lamarck, in his 1809 opus
Philosophie zoologique.
This theory postulated that useful traits acquired during life could be transmitted to progeny. A favorite example cited by Lamarck is the giraffe, which he saw as having acquired its long neck as a result of efforts by generations of giraffes to reach the highest branches of trees. A
Darwinian explanation of the same fact would be, for example, that giraffes that happened by chance to be born with hereditarily transmissible longer necks had a better chance of surviving and transferring this property to their offspring because they alone could still feed when the lower branches, which were the only ones that giraffes with shorter necks could reach, had no leaves left.

Lamarck's theory of inheritance of acquired characters lost a lot of ground—though not without a struggle—after natural selection was proposed by Darwin. In the beginning of the twentieth century, the Austrian biologist Paul Kammerer announced that he had succeeded in causing toads that copulate on land to acquire the “nuptial pads” whereby the males that copulate in water grip the slippery body of the female. He claimed to have accomplished this transformation simply by forcing the animals to copulate in water, generation after generation. Accused of falsifying his results, Kammerer even-tually committed suicide. This did not prevent the British writer of Hungarian origin Arthur Koestler from defending Kammerer's memory and presenting him as a victim, in
The Midwife Toad,
published as late as 1971, well after the advances of molecular biology had invalidated the foundations of Lamarckism.

A much more dramatic instance of Lamarckian fraud was perpetrated under Stalin by the Soviet agricultural biologist Trofim Lyssenko, who claimed to have converted winter wheat into the more productive and faster-growing spring wheat by the simple device of “vernalization,” which depends on soaking and chilling the grains. When this claim was contested by geneticists, Lyssenko used the apparent agreement between his theory and Marxism as an argument to have the dissenters condemned, with catastrophic consequences for the
future of genetics and of agriculture in his country. This scandalous affair caused a number of prominent leftist scientists in the West to cut ties with Soviet Russia and resign from the Communist Party.

The inheritance of characters acquired by the parents' experience was ruled out as a possible evolutionary explanation by the findings of modern molecular biology, at least as concerns transmission via DNA. There is no way whereby such an acquired trait could travel up to DNA and become hereditarily encoded in it. The pathway from gene to trait is strictly one-way. Crick has gone so far as to call this rule the “Central Dogma.” This choice was unfortunate, since science knows no dogmas, but it underlines the strict incontrovertibility of the rule. The Darwinian mechanism, on the other hand, is perfectly compatible with modern biology and supported by it.

Lamarck still has a few rearguard defenders, especially in France, where he has become something of a national symbol in the old struggle against Anglo-Saxon cultural hegemony. Not so long ago, after giving a lecture in a French university town, I was treated to a lengthy harangue by an elderly biology professor, extolling the qualities of Lamarck and lamenting his abandonment, which the professor saw as a betrayal in favor of the Englishman Darwin.

Note, however, that the ban against Lamarckism concerns only DNA-mediated heredity. Recent years have seen discovery
of several other forms of heredity susceptible to a Lamarckian explanation. Biological membranes offer a revealing example. Cellular membranes grow by accretion, that is, incorporation of new components into preexisting membranes. In this process, the pattern of the recipient membrane influences the “choice” of the new constituent to be inserted. A membrane modified by usage could thus induce the insertion of a different constituent and change the pattern of the new material added to it. This material would, by the same mechanism, transmit this modification to progeny, with as consequence the inheritance of an acquired character, in Lamarckian fashion. Another, similar, example concerns the pattern of implantation of swimming appendages, called cilia, on the surface of some protists named paramecia. Let this pattern be changed by environmental influences, and later progeny will display the new pattern.

The discovery of prions reveals a particularly dramatic case of shape transmission. Prions are infectious agents of protein nature that owe their pathogenicity to their abnormal shape. Their molecular sequence is normal, but the manner in which the chains are folded is abnormal and—accounting for the infectious nature of the molecules—can be transmitted to normal molecules by contact. Mad cow disease and its human version, Creutzfeldt-Jakob disease, which made headlines a few years ago, are typical instances of prion diseases. The importance of this kind of transmission in normal heredity is not known but could be significant. Future work will tell.

Yet another instance of possible Lamarckian inheritance is represented by a set of phenomena recently grouped under the term “epigenetics,” a term long used in an entirely different meaning by developmental biologists and neurobiologists (see
chapter 16
). In its new meaning, epigenetics refers to a
number of inheritable traits that are not written into the DNA sequences but accompany the DNA in germ cells and influence subsequent events in the fertilized egg. Such traits include the blockage of certain bases by chemical groups (for example, methyl groups) or the manner in which DNA is combined with the local proteins, or histones, in the chromosomes. Some of the most exciting new findings are being made in this area.

Another non-Darwinian form of inheritance concerns traits that are transmitted without being selected. Many genetic mutations seem close to neutral, with little selective value, whether positive or negative, and just accompany the others by inertia, so to speak, because of the simple fact that their elimination is not sufficiently advantageous. The Japanese theoretician Motoo Kimura has developed a mathematical theory, under the name of “genetic drift,” that explains how mutated genes are inherited in this way. Many drifting genes exist. The best proof of this is given by the genetic diversity exploited by molecular phylogenetics. We have seen (
chapter 1
) that this technology uses the sequence differences among genes that play the same role in different organisms. The very fact that such differences exist, and actually do so on a large scale, is proof that many different versions of the same gene can exist and, apparently, perform satisfactorily. Whether these mutations are truly neutral is, however, debatable. For example, modern medicine has identified a number of human genetic variants that affect the probability of falling victim to a disease, such as diabetes or breast cancer. Such “risk genes” are not neutral; they may significantly
influence the life span of the individuals concerned. On the other hand, to the extent that the genes do not affect fertility—the diseases they influence often break out late, after the individual has ceased to be reproductively active—the genes are indeed neutral with respect to natural selection.

An evolutionary factor, unrelated to heredity and believed by some of its proponents to be as important as natural selection, lies in the ability, attributed to certain living systems, to spontaneously settle from an initially unstable situation into a stable, organized pattern. Variously referred to as “self-organization,” “autopoiesis,” “order out of disorder,” and the like, the phenomena underlying this ability have been the object of many ingenious theoretical studies but little experimental work so far.

One last alternative to natural selection has been proposed under the name of
intelligent design.
It warrants attention, not because it offers a valid scientific explanation of evolution, which it does not, but because of the political and educational issues it has generated, especially in the United States but also, in recent years, in France and other countries. Intelligent design should not be confused with creationism. Theoretically, intelligent design presumes no biblical roots. Many of its defenders accept evolution. Some even accept natural selection. All they claim is that natural selection does not account
for everything and that certain evolutionary steps cannot be explained naturally and must have required supernatural intervention.