Analysis of morphological characters linked to survival and differential reproduction in hybrids between Xiphophorus birchmanni (Lechner, Radda, 1987) and Xiphophorus malinche (Rauchengerger, Kallman, Morizot, 1990)

Ecological selection, sexual selection and genetic drift play a fundamental role in shaping morphological change over time. This work attempts to highlight the strengths of the three main evolutionary forces in an experimental model. Observing the effects of microevolution is particularly easy in hybrid populations, because they demonstrate greater phenotypic variability than parental species, so, a crossbreed beetwen two mexican swordtail fish, Xiphophorus birchmanni and Xiphophorus malinche, has been investigated. Hybrids manteined into indipendent stocktank at two altidtudinal sites for several generations show differentiation in many characters, in paticular for traits linked to body size, driven by temperature; contemporarily, for some characteristics such as the presence of melanistic pigmentation, an asymmetric distribution between the tanks is observed, due to random segregation of carrier individuals. These results suggest the persistence across generations of the effects of genetic drift, most evidently for qualitative traits, and, at the same time, the emergence and gaining greater importance of ecological selection, especially for quantitative traits.