The llyodon of the Rio Coahuayana river system (Colima and Jalisco, Mexico) have undergone extensive chromosomal differentiation without associated morphological differentiation. Two cytotypes differing by at least 6 fixed pericentric inversions were found. Hybrids between the two cytotypes produced in the laboratory were found to be fertile, indicating there is little or no heterozygote disadvantage associated with these inversions.

Models depending primarily on drift and inbreeding to fix rearrangements were rejected because the apparent population structure makes them unlikely and the lack of a heterozygote disadvantage makes them unnecessary. Instead, it is concluded that selection differences operated to fix these inversions.