the colour-pattern genes, which can be distinguished by comparing their effects with those found in other species. Analysis of Nabour's data by Darlington & Mather concluded that the genes responsible for the morphs of Paratettix texanus have been gradually aggregated into a group which acts as a single switch-mechanism. This explanation was accepted by E.B. Ford and incorporated into his accounts of ecological genetics. This process might involve suppression of crossing-over, translocation of chromosome fragments and possibly occasional cistron duplication. That crossing-over can be suppressed by selection has been known for many years; Detlefsen and Roberts were able to reduce recombination between the loci for white eyes (w) and miniature wings (m) in Drosophila melanogaster from the normal 36% to 6% in one line and 0.6% in another. Debate has tended to centre round the question, could the component genes in a super-gene have started off on separate chromosomes, with subsequent reorganization, or is it necessary for them to start on the same chromosome? Many scientists today believe the latter, because some linkage disequilibrium is initially needed to select for tighter linkage, and linkage disequilibrium requires both the previous existence of polymorphisms via some other process, like natural selection, favouring gene combinations. If genes are weakly linked, it is probable that the rarer advantageous haplotype dies out, leading to the loss of polymorphism at the other locus. Most people, following J.R.G. Turner, therefore argue that supergenes arose in situ due to selection for correlated and epistatic traits, which just happened to have been possible to select via the existence of suitable loci closely linked to the original variant. Turner calls this a "sieve" explanation, and the Turner explanation might be called the "Turner sieve" hypothesis. Maynard Smith agreed with this view in his authoritative textbook. Nevertheless, the question is not definitively settled. The problem is connected to an even larger question, the evolution of evolvability. Genomic structure Genomic rearrangements such as inversions can suppress recombination. Suppressed recombination leads to accumulation of repetitive elements (including to degenerative expansion in early supergene evolution [Ref Papaya, Fire ant], and to changes in gene expression [ref Fire ant, anther smut]. Gene complexes are not supergenes Gene complexes, in contrast, are simply tightly linked groups of genes, often created via gene duplication (sometimes called tandem duplication if the duplicates remain side-by-side). Here, each gene has similar though slightly diverged function. For example, the human major