Abstract
The origin and maintenance of discrete morphs within natural populations has long fascinated evolutionary biologists (Darwin, 1862; Fisher, 1930; Ford, 1971). Because of the discrete nature of the phenotypic differences observed, this variation was often assumed to have a genetic origin, making its study of primary interest for early geneticists (see Chapter 6 of Ford, 1971 for an overview of genetic programmes of the time). Moreover, the diverse nature of the traits that usually differ between these morphs (colour, morphology, sexual-compatibility, etc), led to the hypothesis that the genetic basis of this discrete variation is a functional unit containing multiple linked loci, termed a "Super-gene" (spelled supergene hereafter; see glossary) by Darlington and Mather (1949) (Fisher, 1930; Ford, 1965; Mather, 1955). Indeed, as early as 1930, Fisher had anticipated that genetic linkage between multiple selected loci could evolve under natural selection (Fisher, 1930) and Ford later suggested that linkage could be increased via translocation or chromosomal inversions (Ford, 1971). Chromosomal inversions can indeed strongly reduce recombination between the genomic segments they cover, a property leading to their discovery by Sturtevant (1921). Specifically,