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Volume 86, 1959
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Struggle for Existence

The third chapter in the “Origin” is devoted to the “Struggle for Existence”, the general principles of which, as set forth by Darwin and Wallace in 1858, need no explanation. Darwin explained it in a nutshell as “one general law, leading to the advancement of all organic beings—namely, multiply, vary, let the strongest live and the weakest die”45. Such epigrammatic brevity, out of context, has led to misunderstanding, and, indeed, to the unjustified restriction of the term “Darwinian selection” to differential survival or mortality, which is only one aspect of selection.

So-called Darwinian selection, which operates in the life-time of the individual, has recently been neatly demonstrated by analysis of the dimensions of the molar teeth in a fossil population of the extinct cave bear of Europe46. This bear hibernated in caves, and suffered a mortality peak near the end of winter, so that the fossil samples from a cave near Odessa (Ukraine) show an age grouping with modes at annual intervals. Frequency plots of “paracone index” of the second molar in successive ages show a continuous reduction in mean values with increasing age, indicating a strong linear selection in favour of lower paracone index, due to the death of young bears suffering from quite slight malocclusion between the molars. This example graphically demonstrates how effectively, in a natural population, selection can operate on minor differences; but as the differences were not genetically controlled no evolution took place.

The classic case of Natural Selection operating on wild populations is the spread of “industrial melanism” during the past century among moths living near the industrial cities of Great Britain47. Smoke and smog killed the lichens of tree trunks and replaced them with a black sooty covering. Change of background colour reversed the normal selection pressure in favour of “normal” coloration, so that whole populations changed colour in the course of about fifty years.

But this crude form of selection involving the death of the unfit was far less important in Darwin's mind than many of his critics have claimed. It is important to emphasize that Darwin himself used the term Struggle for Existence “in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny48 (my Italics). During the past two decades the use of mathe-

[Footnote] 45 Origin: 209.

[Footnote] 46 Kurten, B., 1957. Evolution, 11 (4): 412–416.

[Footnote] 47 Ford, E. B., Polymorphism and Taxonomy, in The New Systematics, Clarendon Press: 508.

[Footnote] 48 Origin: 54.

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matical methods has given a firm basis for selection theory, and Fisher49 has shown that even genes with small selective advantage will eventually spread over entire populations

[Footnote] 49 Fisher, R. A., 1930. The Genetical Theory of Natural Selection, Clarendon Press.