Back To Darwin: A Richer Account of Evolution

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Marx was very clear as to the importance of accidents. It would, on the other hand, be of a very mystical nature if 'accidents' played no part. These accidents themselves fall naturally into the general course of development and are compensated again by other accidents. Evolution is not "of a very mystical nature.

In numerous species these accidents happen often enough to give rise to statistical certainty.

If the gene for haemophilia arises afresh on an average once in 50, generations, it is very nearly certain that it will arise between and 11, times in the next million people born. On the other hand, with a rare species such as the Indian elephant, comprising perhaps only 20, individuals, chance assumes a great importance. The accidental character of mutation is clear in many other ways. Almost, though not quite, all mutations lower the fitness of an organism in its natural state.

This is equivalent to saying that organisms are pretty well fitted to their environment fitness is defined later and any change due to chance is likely to be for the worse.

If mutation were an adaptive phenomenon like the growth of a muscle when exercised, as Lamarck believed, this would not be so. Most mutations would be useful. The same would I suppose be true if mutation were a manifestation of the Life Force whatever he, she or it may be. Naturally enough, biologists to whom dialectical materialism means nothing, or means a weapon of the abominable Marx, cannot understand how harmful mutations can be a condition of evolutionary progress.

They therefore deny them any importance. We have now taken our first step. The self-repairing, self-reproducing organism is negated by accidents of a certain type. It can no longer reproduce itself unchanged. But since it does reproduce itself in the changed form say as a white mouse in place of a brown, or a beardless wheat in place of a bearded the negation is negated. This dialectical process is called "mutation," and leads to inheritable variations within a species. If we do not look at it dialectically, we are apt to label it either as pathological or progressive. In fact it constitutes a union of these opposites.

Taking mutation as a given process, what do we expect from it? Certainly not evolution.

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Different mutations will affect different organs in different directions. The average man can get his best idea of the effects of mutation by going to a show of fancy poultry breeds.

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As a result of mutation the feathers can be any color of a large range from black to white. They can be longer or shorter than in the wild form.

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  • The comb may be doubled, as in a Sicilian Buttercup, or reduced to a mere button, as in a Malay. The number of toes may be increased, the rooster may have feathers like a hen, and so on. These changes are due to new genes which have arisen by mutation. Other genes produce still fiercer effects, for example young chickens with mere knobs for limbs, which die long before hatching.

    Mutation alone, then, would cause every species to break down into a collection of freaks, some of which could only be preserved alive by a miracle. We have every reason to ask whether it is really of evolutionary importance. The answer is decisively "yes. In such a case, it is generally found that a good deal of the difference between the two species is due to one or two genes, and that much of the remaining difference behaves statistically, as if it were due to a number of genes each having a small effect by itself.

    John B. Cobb Jr., ed., "Back to Darwin: A Richer Account of Evolution." Reviewed by

    In some cases, and particularly where the species are so far apart that they cannot be crossed, or yield a sterile hybrid, we find differences of a higher order than genes — that is to say, differences in the arrangement of the chromosomes. As genes of new kinds and chromosomal rearrangements only arise by mutation, we cannot avoid attributing to it a certain evolutionary importance.

    The only question is whether it accounts for all or only some of the differences between species. The antithesis to mutation, which nearly negates its effects, is natural selection. We can best understand it by studying the effects of artificial selection. Suppose that we start with a group of wheat plants or flies, and in each generation breed two hundred individuals and select the ten best, in some respect, as parents of the next generation.

    For the first five or ten generations we shall make dramatic progress. We may increase the yield of our wheat plants, on the average, by 30 per cent. We may nearly double the number of bristles on our flies. Darwin knew this. He did not know that after ten or twenty generations the process comes to an end.

    We started with a population containing several different genotypes — that is to say, sets of genes. We end by eliminating all but the one which best satisfies our criterion. We have got to a pure line, and further selection is useless. When this fact was discovered it was at once stated that Darwinism was dead. But that cat has at least nine lives. The critics forgot the transformation of quantity into quality.

    In a large enough population, selection never gives us a pure line, because new genes are always turning up by mutation before selection has eliminated all the original heterogeneity. Hence an experiment on a small population is misleading as a model for the natural process. In artificial selection we can select for anything we please — for example, for innate capacity to sing, to produce many eggs, to grow long hair, or to develop cancer, though unless the suitable genes are available selection is fruitless.

    But natural selection selects for one character only, which Darwin called fitness, but never defined rigorously. It can be defined only in mathematical terms, and R. Fisher and the writer were the first to do so.

    JBS Haldane-A Dialectical Account of Evolution

    The full definition is complicated, but a simple example will serve. Consider a population of self-fertilized plants, say wild peas, which die down every winter and reproduce themselves annually from seed. Suppose several different genotypes, say purple, red and white, with and without tendrils. Now take one hundred plants of each sort in a particular environment, counted at a particular point in the life cycle, say the opening of the first flower, and see how many progeny they leave, on the average, on the same day or at the same point in their life cycle of next year.

    This average number is the fitness. In the long run the average fitness of any species is quite close to unity, except during rare periods of rapid spread or extinction. A change in any quality may affect the fitness.

    Back To Darwin (A Richer Account of Evolution)

    Extra fertility will do so, other things being equal. But other things rarely are.

    Charles Darwin

    A wild hen which produced two hundred eggs per year would certainly not be able to bring up two hundred chickens. She would be worse off than the mother of a mere ten, because she would be turning an immense amount of food into eggs which would never be hatched. Darwin laid a considerable stress on survival, rather than fertility, but a change in either will affect the fitness.