By the 1950s the “neo-Darwinian” or “modern synthesis” in evolutionary theory had discounted or even rejected the significance to evolution of characters acquired in an organism’s lifetime. It became taboo to express sympathy with Lamarck. The irony is that the rejection of Lamarck revolved around individual-to-individual transmission of characters, not the population thinking that was a hallmark of neo-Darwinism. Let me review how, historically and conceptually, the possible evolutionary significance of appropriate acquired characters has been excluded.
In the 1930s there was not yet a settled consensus about the efficacy of natural selection to produce evolutionary change that results in, to quote Darwin, “that perfection of structure and coadaptation which most justly excites our admiration.” At a 1936 Royal Society of London symposium, “A Discussion on the Present State of the Theory of Natural Selection” (Watson 1936), Fisher and Haldane expressed their confidence in the explanatory power of differential survival among different genotypes. Watson, the chairman of the discussion, was less convinced, awaiting definitive tests to discriminate between a “Darwinian explanation” or a “direct environmental influence,” and noting that “the problem of the co-ordination of the evolutionary changes which is necessary to ensure that animals remain workable wholes.” MacBride, an outspoken (neo-)Lamarckian, rejected both natural selection and direct environmental influence, proposing instead that Lamarckism, correctly interpreted, was all but proven:
Lamarck’s theory is that… what alters the animal is its own functional activity stimulated by its response to the environment. That such changes take place in the lifetime of the individual is common knowledge; what is needed for the full proof of Lamarckism is to show that some resideue of this effect is passed over to the offspring (p. 71 in Watson 1936).
Instead of proof, history, however, records disproof. The strong experimental tradition from the late 19th century into the 20th, from Weismann and Entwicklungsmechanik through into Morgan’s gene school of research, emphasized direct causal connections. The more that became known about the cell nucleus and chromosomes, the less plausible it was that change acquired by an individual during its lifetime in response to the environment could be transcribed back into the hereditary material to be transmitted from parent to offspring. Neo-Lamarckians held onto the possibility of individual-to-individual transmission; in doing so they allowed the impossibility of such transmission to reject Lamarckism. (With more recent discoveries of ways that the germline is tweaked, “impossibility” should be replaced now by “rarity.”)
This is where the irony enters: Darwinism requires population thinking, where in evolution is a change in a character’s frequency in a population. The issue is not direct transmission of hereditary elements (genes), but heritability, which (at least in its technical sense) is a population phenomenon. The impossibility (rarity) of individual-to-individual transmission does not disprove the possibility of an acquired character increasing in frequency in a population. C.H. Waddington argued that the inheritance of acquired characters was compatible with neo-Darwinian population genetics (Waddington 1942) and supported his argument with experiments in the 1950s. In these experiments, variation in certain characters, originally seen only in response to environmental stressors, e.g., enlarged anal papillae of Drosophila larvae in response to higher salt concentrations (Waddington 1959), eventually occurred in populations even when the stressor was withdrawn. A future post will explore possible mechanisms and interpretation of these results, but, for now, what is important to note is that the variation originated as an appropriate response to the stressor—enlarged papillae ameliorate the osmotic pressure from the increased salt concentration.
This post is adapted from P. J. Taylor, “Historical versus Selectionist Explanations in Evolutionary Theory” Cladistics 3: 1-13,1987. The occasion for this post was reading about epigenetics giving new credibility given to Kammerer’s mid-wife toad observations in the 1920s (New Scientist 11 Sept 2010, p. 37); noting the passing of GC Williams, neo-Darwinism’s staunchest advocate of gene-centered view of natural selection; and thinking about extensions of the family-population conflation that is addressed in recent posts of mine.
Waddington, C. H., 1959 Canalisation of development and genetic assimilation of acquired characters. Nature 150, 563-565 (14 November 1942) | doi:10.1038/150563a0
Waddington, C. H., 1959 Canalisation of development and genetic assimilation of acquired characters. Nature 183:1654-1655.
Watson, D. M. S., et al. A Discussion on the Present State of the Theory of Natural Selection. Proc. R. Soc. Lond. B August 1, 1936 121:43-73; doi:10.1098/rspb.1936.0052