Résumé
Since the publication of Dobzhansky’s Genetics and the Origin of Species, the notion of genomic coadaptation has played a prominent role in the study of the genetics of evolution. The genome is thought of as a collection of harmoniously collaborating genes, which are pooled and maintained by selective processes over the evolutionary histories of populations. Genomic coadaptation corresponds to the overall genie balance resulting from this selection. In fact, different levels of interactions can be distinguished (table 8.1): a first levelconcerns interactions among genes between nonhomologous chromosomes, a second level involves interactions within chromosomes (the internal balance), and a third level results from interactions among genes between homologous chromosomes (the relational balance). For diploid or polyploid organisms, it can then be assumed that, when favored by natural selection, any combination of alleles at a particular locus (at a heterozygous or homozygous state) can be viewed as a kind of relational coadaptation (see Woolf and Markow, chapter 7, this volume). In summary, and in more quantitative genetics terms, coadaptation refers to selected nonadditive effects between (epistasis) and within (dominance and overdominance) loci.