George Romanes, William Bateson, and Darwin's ‘weak point’

Abstract

Thomas Huxley identified as Darwin's ‘weak point’ the failure of breeders when crossing varieties within a species to simulate the sterility of hybrids derived from crosses between allied species. As a result of the sterility, the parents of a hybrid were, in an evolutionary sense, reproductively isolated from each other, so they would be members of distinct species. In his theory of ‘physiological selection’, Romanes postulated germline ‘collective variations’ that accumulate in certain members of a species; these members are thus ‘physiological complements’ producing fertile offspring when mutually crossed, but sterile offspring when crossed with others. Unlike Darwin's natural selection, which secured reproductive isolation of the fit by elimination of the unfit, physiological selection postulated variations in the reproductive system that were not targets of natural selection; these sympatrically isolated the fit from the fit, leaving two species where initially there had been one. Bateson approved of physiological selection. He noted that Mendel's ‘unit characters’ were ‘sensible manifestations’ of what we now refer to as ‘genes’, but postulated a ‘residue’, distinct from genes, that might affect gene flow between organisms and so originate species. The reproductive isolation of the parents of a sterile hybrid was due to two complementing non-genic factors (the ‘residue’) separately introduced into the hybrid by each parent. Modern studies, especially of yeast hybrids, support the Romanes–Bateson viewpoint.