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  • 1 Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK
  • 2 Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield, S10 2TN, UK
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Abstract

The ‘generalized Trivers-Willard hypothesis’ (gTWH) proposes that heritable traits associated with reproductive success of one sex will be positively associated with a genetic tendency to produce offspring of that sex. However, unlike the original Trivers-Willard hypothesis, the predictions of gTWH are proposed to be borne out regardless of environmental conditions. This is a problem because it ignores the influence of the hypothetical genetic variance in offspring sex-ratio on population operational sex-ratio and thus offspring's likely success in finding a mate. Accordingly, there is a notable lack of evidence to support the existence of such heritable variation in offspring sex-ratio in humans or other mammals. The genetic tendency for all individuals within populations of birds and mammals to produce a male offspring with the same probability as one another is well-established. In fact it is a cornerstone of population sex-ratio theory, upon which is built hypotheses of facultative (environmental) sex-ratio adjustment, including the original Trivers-Willard hypothesis. I therefore suggest that any phenotypic correlations between offspring sex-ratio and traits that may be associated with the reproductive success of offspring of one sex are most likely to be environmental in origin.

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No Evolutionist is an Island

A review of Kevin N. Laland and Gillian R. Brown (2011) Sense and Nonsense. Evolutionary perspectives of human behaviour Oxford: Oxford University Press. 270 pages, ISBN: 978-0-19-958696-7