Letter | Published:

Evolution, and the Age and Area Hypothesis



DR. WILLIS'S assumption that new genera and new species may arise directly by mutation is rather startling to most students of evolution. He supports his contention, chiefly, by the observation that the frequency distribution of genera containing 1, 2, 3 species follows a regular, hollow curve, with monotypic genera the most frequent. Mr. Yule (Phil. Trans. Roy. Soc. B. 403) has shown that assuming (1) that species give new species by chance at an irregular rate, constant on the average and the same for all species, and (2) that species give new genera in the same way, by mutation, then the frequency distribution of size of genus will approximate closely to that observed in Nature; the latter being such that log. number of species plotted against log. number of genera gives practically a straight line. That all genera arise directly by mutation is implied throughout, since they are all supposed to start as monotypes. Finally, Mr. Yule concludes that viable specific mutations probably do not occur, in all the flowering plants over the whole earth, more often than about once in thirty years; hence that our failure to observe them cannot disprove their occurrence. This conclusion is disquieting; and we clearly cannot accept this mechanism if we can otherwise explain the evidence adduced for it.

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