• bilateral asymmetry;
  • endangered species;
  • fitness;
  • Gila topminnow;
  • heterozygosity.

A well-known example of a positive association between the level of genetic variation and fitness in endangered species is the studies in Gila topminnow. The work of Vrijenhoek and his colleagues showed lower values for four fitness correlates in laboratory-raised fish from a population that was monomorphic for all 25 allozyme loci examined (Monkey Spring) than for fish from a population that was heterozygous for two of the allozyme loci (Sharp Spring). Here, bilateral asymmetry in wild-caught fish from these sites is examined to determine if the same environmental stressor (or one with similar effects) was present in natural populations of Gila topminnows. There were no differences for all three traits, lateral-line scales, pectoral-fin rays and pelvic-fin rays, previously found to be significantly different between Monkey Spring and Sharp Spring. This, coupled with our earlier finding that fish raised in our laboratory (where there is low mortality) had low bilateral asymmetry, supports the hypothesis that some unknown, and perhaps unnatural, environmental factor in the Vrijenhoek laboratory was responsible for the differences observed in bilateral asymmetry between Monkey Spring and Sharp Spring.