Signature of sexual, environmental and intrinsic selection in the genome of Xiphophorus hybrid fish

Sexual ornaments are known not to be influenced just by male-male competition and female mate choice. They have to deal with several trade offs: energetical cost, risk of predation, reduction of genetic variability… On the other hand, they can accelerate adaptation through purging of deleterious alleles or reduce genetic load of intrinsic incompatibilities in hybrids. Genomic tools can be used to assess those effects and reveal interaction with other selective pressures. We focus on the evolution of a sexually-selected ornament, the sword, in replicated experimental populations of hybrid swordtails (Xiphophorus malinche × X. birchmanni), distributed along an elevational gradient. Over approximately 10 generations, males rapidly evolved towards the phenotype of the locally adapted parental species: long sword in highland populations, like X. malinche, and no sword in lowland populations, like X. birchmanni. Allelic divergence at candidate sword loci outpaced overall local adaptation of the whole genome. The sword seems to have a relevant role in the process of ecological adaptation to local thermal regimes. This observation challenges the idea that the ornament is merely a product of a “taste for the beautiful”.