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Notes and References
Wheldon’s polemical journal, Questions of the Day and of the Fray, did not cease publication till 1924 (?); and the anti-Mendelian influence was strong enough to ensure that Fisher’s first major genetical paper, arguing amongst other things that continuous variation is explicable on Mendelian principles, was rejected by the Royal Society of London, and published by the less prestigeful Royal Society of Edinburgh, in 1918.
Wright, S. Stochastic processes in evolution, in Stochastic Models in Biology and Medicine (ed. Garland) (Univ. Wisconsin Press 1964) p. 199.
Levins, R. Amer.Nat. 96(1962)361; Amer. Nat. 97(1963)75: J. Theoret. Biol. 7 (1964) 224: Evolution 18 (1965) 635: Genetics 52 (1965) 891.
MacArthur, H. H. Proc. Nat. Acad. Sci. Wash. 48 (1962) 1893.
Slobotkin, L. B. Am. Sci. 52 (1964) 343.
Waddington, C. H. The Nature of Life (Allen and Unwin 1961) p. 109, Prolegomena (1967) p. 22.
Several ecologists argue that there will be, for ecological reasons, a selection pressure towards the development of more complex ecosystems, involving an ever-increasing number of species. The fundamental selection is towards increasing the stability of the ecosystem; when there are only a small number of interacting species, there is a tendency for violent fluctuations in numbers, which may lead to the extinction of some species and the complete collapse of the whole system. See, for example, Hutchinson, G. E. Amer. Nat. 93 (1959) 145; MacArthur, R. H. Ecology 36 (1955) 533: Dunbar, M.J. Ecological Development in Polar Regions (Prentice Hall 1968).
Green, M. M. Proc. 11th Int. Cong. Genet. 2 (1965) 37.
For this and other examples see The Genetics of Colonising Species, IUBS Symposium (ed. Baker and Stebbins) (Acad. Press 1965).
In connection with the nature of the mutational events which may play a part in evolution I would like to advance some ideas, which are, I freely admit so speculative that I have relegated them to a note rather than the body of the text; but many biologists will consider them so outrageously heterodox that they may refuse to consider them at all—and these I should beg to think again. One of the major problems of evolution theory is to understand how the sharp discontinuities between major taxonomic groups—Phyla, Families, Species-Groups, and so on—have come into being. A simple-minded empirical inspection of the facts would suggest, as it did for instance to Goldschmidt when he wrote his The Material Basis of Evolution 1940 and Theoretical Genetics 1955, that it might be profitable to contemplate the possibility of the very occasional occurrence of what Goldschmidt called “systemic mutations,” which result in a complete restructuring of the genome, achieved either in a single step, or at least in rather few generations, When Goldschmidt wrote, no clear-cut example could be given in which the occurrence of such a process had been observed. The orderly minded orthodox biological world closed its ranks against this suggestion that revolutionary processes may happen. It became accepted that the only respectable doctrine is that evolution never involves anything but step-by-step Fabian gradualism, plodding along a weary way similar to that by which the annual milk yield of dairy cows or egg yield of hens is slowly improved—the occurrence of a little allopolypoidy or rearrangement of chromosomes by two or three breaks could be admitted, but would only push the basic philosophy from bourgeois-liberal to right-wing social democrat. It is still impossible—so far as I know—to quote a compelling instance in which a systemic mutational event has been observed in an evolving multicellular organism. But events which appear to be essential of this kind are becoming well known in the field of cell culture. It is a common experience that cells isolated from vertebrate tissues usually grow in culture for a fairly restricted number of cell generations—a hundred or two—and then die out, unless they undergo some sort of change which brings into being cells capable of forming an “established line,” which can then be sub-cultured in perpetuity. The nature of the change from a “strain” to an “established line” is highly obscure, but it often involves what looks like a complete restructuring of the genome; there may be a considerable reduction in number of chromosomes, accompanied in some cases by considerable changes in chromosome morphology. (For a recent review see The mammalian cell as a differentiated microorganism by Howard Green and George J. Todaro, Ann, Rev. Microbial. 21 (1967) pp. 574–600.) The fact that cells in culture can throw up, within at most a few cell generations, new types of cells capable of giving rise to “established lines,” and that the change may involve a very drastic reshuffling of the genome (usually, in the cases observed, with a loss rather than a gain of chromosomal material), is evidence that something like a “genetic revolution” or “systemic mutation” can occur. It is, of course, more difficult to see how such an event in an evolving population could be propagated so as to affect the future, but if such events are not ruled out of court by the nature of genetic processes it seems silly to close one’s mind to the possibility that evolution has found some way of making use of them. An example of an evolutionary phenomenon which suggests a very radical reorganization of the genome between nearly related species is the astonishing difference found by Forbes Robertson (in press, Genet. Res.) in the DNA sequences of Drosophila melanogaster and simulans, as tested by molecular hybridization. RNA manufactured in vitro on a melanogaster DNA template hybridizes only one-third as well with simulans DNA as does the RNA made on the simulans template: the results of a reciprocal experiment are very similar. Although it seems certain that the hybridizations only occur between substances related to the highly reiterated stretches of DNA, this is still strongly suggestive that the differences between these two species involve much more radical and pervasive alterations of base sequences than were contemplated a few years ago, when it seemed that all that was involved was a few inversions, and translocations of large sections of the chromosomes.
Lewontin, R. C. Bioscience 18 (1966) 25.
Barnet, S. A. Biol. Rev. 40 (1965) 5.
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This essay is dedicated, with respect and affection, to my good friend Theodosius Dobzhansky, in celebration of his seventieth birthday.
Originally published in Waddington CH, ed, Towards a Theoretical Biology, Vol. 2, Sketches, International Union of Biological Sciences & Edinburgh University Press, 1969, pp. 106–123.
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Waddington, C.H. Paradigm for an Evolutionary Process. Biol Theory 3, 258–266 (2008). https://doi.org/10.1162/biot.2008.3.3.258
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DOI: https://doi.org/10.1162/biot.2008.3.3.258