Chance has been a focus of attention ever since the beginning of population genetics, but neutrality has not, as natural selection once appeared to be the only worthwhile issue. Neutral change became a major source of interest during the neutralist–selectionist debate, 1970–1980. It retained interest beyond this period for two reasons that contributed to its becoming foundational for evolutionary reasoning. On the one hand, neutral evolution was the first mathematical prediction to emerge from Mendelian inheritance: until then evolution by natural (...) selection was considered the alternative to the fixity of species; now it appears to be the alternative to continuous change. Second, neutral change generated a set of clear predictions on standing variation. These could be used as a reference for detecting more elusive alternative mechanisms of evolution including natural selection. In the wake of the transition from Mendelism to genomics, the combination of coalescent theory, DNA sequence variation, and numerical analysis made it possible to integrate contingent aspects of the history of species into a new null model, thus opening a new dimension in the concept of population that the Modern Synthesis formerly considered as a mere gene pool. (shrink)

The encounter between the Darwinian theory of evolution and Mendelism could be resolved only when reductionist tools could be applied to the analysis of complex systems. The instrumental reductionist interpretation of the hereditary basis of continuously varying traits provided mathematical tools which eventually allowed the construction of the Modern Synthesis of the theory of evolution.When genotypic as well as environmental variance allow the isolation of parts of the system, it is possible to apply Mendelian reductionism, that is , to treat (...) the phenotypic trait as if ti causally determined by discrete genes for the trait. howeverm such a beanbag genetics approach obscures the system's eye-view. The concept of heritability, defined as the proportion of the total phenotypic variance due to (additive) hereditary variation, asserts that genetic elements have discrete effects; but by relating to the genotypic variance, it avoids the trap of reffering to genes for characters. (shrink)

The dichotomy between ‘typological thinking’ and ‘population thinking’ features in a range of debates in contemporary and historical biology. The origins of this dichotomy are often traced to Ernst Mayr, who is said to have coined it in the 1950s as a rhetorical device that could be used to shield the Modern Synthesis from attacks by the opponents of population biology. In this two-part essay, I argue that the origins of the typology/population dichotomy are considerably more complicated and more interesting (...) than is commonly thought. In the first part, I argued that Mayr's dichotomy was based on two distinct type/population contrasts that had been articulated much earlier by George Gaylord Simpson and Theodosius Dobzhansky. Their distinctions made eminent sense in their own, isolated contexts. In this second part, I will show how Mayr conflated these type/population distinctions and blended in some of his own, unrelated concerns with ‘types’ of a rather different sort. Although Mayr told his early critics that he was merely making “a temporary oversimplification,” he ended up burdening the history and philosophy of biology with a troubled dichotomy. (shrink)

The Human Genome Project (HGP) has been criticised from an evolutionary perspective for three reasons: completely ignoring genetic variation; improperly treating either all or some genetic variation as deviation from a norm; and mistakenly seeking to define species in terms of essential properties possessed by all and only member organisms. The first claim is unfounded; the second and third claims are more on target. Nevertheless, it is a mistake to use the typological-population distinction to oppose molecular genetics and evolutionary genetics (...) in order to characterise HGP mapping and sequencing aims, especially the production of a DNA reference sequence, as 'anti-evolutionary' and 'pre-Darwinian.' These aims are consistent with certain strands in twentieth-century evolutionary thought: Muller's classical theory, Kimura's neutral and 'effectively neutral' theories, and, to a lesser extent, Dobzhansky's balance theory of the genetic structure of natural populations. In practice, population-based approaches to human genetic variation are similarly vulnerable to charges of 'typological' and 'essentialist' thinking in their treatment of genetic variation as deviation. This means that the Human Genome Diversity Initiative will not provide a population-based panacea for an overly typological HGP, as its proponents contend. Substituting related, and less rhetorically-charged, conceptual, empirical, and metaphysical distinctions for the typological-population distinction furnishes a better way to assess the concept of the normal genome from an evolutionary perspective. (shrink)