1. Darwin's problem
How could heritable variation in fitness relevant traits (which is a prerequisite for natural selection) be maintained, if sexual reproduction halved it every generation because of blending inheritance (Jenkin 1867, 158)? The remedy, here, was the inheritance of acquired variation via Pangenesis, that is, a Lamarckian source of variation in fitness relevant traits.
2. Weismann's solution
[slightly extended because Weismann's is often regarded as identical with later views]
Weismann rejected the inheritance of acquired variation. He saluted Nägeli's idioplasm, a concept of the minute molecular structure of the basis of life, but rejected the internal self-changing force that Nägeli thought this idioplasm must have.
"Nägeli has very ingeniously worked out his conception of idioplasm, and this conception is certainly an important acquisition and one that will last, although without the special meaning given to it by its author. [...] The only proof that idioplasm must necessarily change, in the course of time, as the result of its own structure, is to be found in the fact that Nägeli has so constructed it; and no one will doubt that the structure of idioplasm might have been so conceived as to render any transformation from within itself entirely impossible." (Weismann 1886; translated in Weismann 1891, p. 264; see also Appendix I, pp. 306-308)Indeed, Nägeli's idioplasm minus the internal force of change is nothing but Weismann's germ-plasm.
"The germ-plasm or idioplasm of the germ-cell (if this latter term be preferred) certainly possesses an exceedingly complex minute structure, but it is nevertheless a substance of extreme stability, for it absorbs nourishment and grows enormously without the least change in its complex molecular structure." (Weismann 1891, p. 278)These rejections posed two problems for Weismann. Firstly, heritable variation could not enter the germ-plasm via the inheritance of acquired differences. Secondly, it could neither emerge from an internal self-changing force of the germ-plasm. Therefore the problem of the maintenance of heritable variation in fitness came back to him with a vengeance. Where do heritable variations come from?
"We are clearly compelled to find some other source of hereditary individual differences, or the theory of natural selection would collapse, as it would if hereditary individual variations did not exist." (Weismann 1891, p. 274)Fortunately, Weismann's non-blending conception of inheritance also pointed a way out. Without blending inheritance he could regarded sexual reproduction as the source of heritable variation in fitness.
"I believe that such a source is to be looked for in the form of reproduction by which the great majority of existing organisms are propagated: viz. in sexual, or, as Häckel calls it, amphigonic reproduction." (Weismann 1891, p. 279)Sex was no longer the problem but the solution.
3. Fisher's & Muller's theory
Fisher (1930) and Muller (1932) knew that the ultimate source of variation was mutation, and they explicitly rejected Weismann's idea of recombination as the ultimate source. In their theories, sexual recombination accelerates adaptation in an arms race between asexual and sexual populations.
4. Maynard Smith's & Williams's paradox: Fisher's and Muller's theory implies a benefit of sex to species not individuals. Their theory could not be accepted and group selection be rejected at the same time.
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Fisher, Ronald A. [1930] 1999. The genetical theory of natural selection, a complete variorum edition. Edited by Henry Bennett. Oxford: The Claredon Press.
- Jenkin, Fleeming. 1867. “Art. I.—The origin of species.” The North British Review, June, 46:149-171. [http://archive.org/details/northbritishrev24unkngoog]
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Maynard Smith, John. 1978. The evolution of sex. Cambridge: Cambridge University Press.
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Muller, Hermann J. 1932. “Some genetic aspects of sex.” The American Naturalist 66:118-138.
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Weismann, August. 1886. Die Bedeutung der sexuellen Fortfplanzung für die Selektions-Theorie. Jena: G. Fischer. In translation: Weismann A (1891) The significance of sexual reproduction in the theory of natural selection. Pp. 257-342 in: Poulton EB, Schönland S, and Shipley AE (eds)) Essays upon Heredity and kindered Biological Problems. Authorised translation, second edition. Claredon Press, Oxford.
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Williams, George C. 1975. Sex and evolution. Princeton: Princeton University Press.
