Summary of population-level analyses in cave and surface populations of Astyanax mexicanus . Over the course of several millions of years, ancestral Astyanax (surface) fish repeatedly invaded the limestone karst region of northeastern Mexico (inset). These invasions have led to 29 named populations in the El Abra region, many of which are significantly distant from one another. Several reports have added to our growing understanding of the interrelationships among cave forms. The results of these reports, based on mitochondrial and nuclear gene sequence analysis, are summarized here (colored boxes; see legend). Note that several caves have not yet been sampled (e.g., the northern Jineo and Escondido caves). Classical reports first suggested that recent cave forms may have originated from a single colonization , however contemporary reports argue for a much earlier colonization into northeastern Mexico and subsequent colonization of subterranean caves by an “old” (red dashed line) and “new” stock (green dashed line) of surface-dwelling ancestors [7–11, 21].