Species Survival and Arrival

Abstract

In principle there is no blending inheritance since characters correspond to discrete Mendelian units (genes) that are inherited quantally, in an either/or fashion, like the sex of an individual. However, the products of allelic genes (e.g. proteins) can sometimes interact to give the appearance of blending. Furthermore, many quantitative traits, such as height, depend on several non-allelic genes that can recombine in varying proportions resulting in a continuous, rather than discontinuous (quantal), distribution in a population. In this circumstance there is blending inheritance. Linear evolution proceeds cyclically through transmission of gametes, development of fertilized egg to adult, and gametogenesis. For a line of organisms to avoid blending inheritance, there must be branching into two lines (speciation) by interruption of the unitary generational cycle. Two independent cycles result. Since more genes are involved in development than in transmission or gametogenesis, if cycle interruption is of genic origin it is most likely to be first due to differences between parents in developmental genes – later trumped by differences in transmission genes. On this basis, differences between parents in genes affecting gametogenesis in their child (hybrid) are unlikely to become manifest. However, Goldschmidt proposed cycle interruption due to failed gametogenesis because of a defective chromosome pairing of non-genic origin. Below the ‘radar screen’ of natural selection, “systemic mutations” would slowly accumulate to change chromosomal “patterning,” so that hybrids, although otherwise healthy, would be sterile. Nevertheless, with fresh compatible partners, the reproductively isolated parents would remain free to follow new evolutionary paths under the influence of natural selection. New species could then emerge.

We have all grown up in the greatest confidence that we all knew … what Darwin meant. I am very tired of having some excessively loosely expressed truism, such as that ‘all defective deer must be devoured by tigers ’, put forward as ‘the ordinary Darwinian argument’.

Ronald Fisher (1930) [1]