Harmony and Conflict in the Living World - Alexander F. Skutch [72]
The prominence given to individual selection in contemporary discussions of evolution, the widespread rejection of group selection, ignore the significance of sexual reproduction. The genes of an individual lack evolutionary importance unless they are contributed to the gene pool of its species; and the first step in this incorporation is their mingling with the genes of a second individual. The pair rather than the individual appears to be the primary unit of selection that does not lead to extinction, but this is only a step toward the wider diffusion of its genes through a larger group of interbreeding organisms. The sharp distinction between individual selection and group selection erects an artificial boundary in a continuum. Natural selection has both a negative and a positive aspect. It acts like a sieve, which holds back, for rejection, coarser particles, while permitting the finer grains to passfiner, in the present context, meaning fitter or more adequate to confront their
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environment. And these finer individuals gain their evolutionary significance by mingling their genetic endowments with those of their contemporaries.
For examples of pure individual selection, we must turn to organisms that reproduce with never the intervention of sex. Their progeny form clones, all of whose members bear precisely the same constellation of genes, so that differences in their survival must be attributed to external factors instead of intrinsic differences. When feasible, horticulturists and agriculturists frequently prefer vegetative propagation, which is often quicker and more efficient than reproduction by seeds, and has the great advantage that plants so multiplied nearly always "breed true." Many of the most valuable agricultural plants, including potatoes, cassava, sugarcane, and bananas, are regularly propagated by vegetative parts; some of their varieties never set seed. Occasionally by a "sport," or bud variation, one of these cultivars produces a new and valuable variety, which must be propagated as a clone by vegetative means.
Despite the vigor of many plants that can reproduce only asexually, they rarely spread widely without our help; most would probably become extinct without human care. Among vertebrate animals, parthenogenesis is rare. Why has sexual reproduction, indirect and frequently wasteful, become so much more widespread among all the more highly evolved plants and animals than the more direct and efficient methods of asexual multiplication? The reason appears to be that strict individual selection does not produce the genetic diversity that promotes adaptability to confront changing conditions and the capacity for continuing evolution. Sexual reproduction and group selection have evolved because they promote evolution.
Darwin was aware of the inadequacy of individual selection. In The Origin of Species he wrote: "I did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetuated." Farther along he elaborated: "But in the great majority of cases, namely, with all organisms which habitually unite for each birth, or which occasionally intercross, the individuals of the same species inhabiting the same area will be kept nearly uniform
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by intercrossing; so that many individuals will go on simultaneously changing, and the whole amount of modification at each stage will not be due to descent from a single parent." And again: "Hence in order that a new species should suddenly appear. . ., it is almost necessary to believe, in opposition to all analogy, that several wonderfully changed individuals appeared simultaneously within the same district. This difficulty . . . is avoided on the theory of gradual evolution, through the preservation of a large number of individuals, which varied more or less in any favourable direction,