Certain populations of the goodeid fish genus Ilyodon occur as bimodal assemblages of broad and narrow-mouthed individuals which have been regarded as sympatric species. Allozyme surveys of four bimodal populations indicated no significant differences in allele frequencies between the presumed species in seven to fifteen polymorphic loci, but substantial interpopulation genic differentiation. These results suggested regular gene flow between the supposed species at each of four independent localities and conspecificity of two mouth width morphs. Progeny of field inseminated females yielded some offspring with morphs opposite to those of their mother and some progeny of laboratory crosses were of morphs different from their parents. The analysis of Ilyodon broods conclusively proved that the two presumed species were conspecific and displayed a morphological polymorphism in mouth width. Variation in the number of metacentric chromosomes has been reported for populations of Ilyodon along the length of the Rio Coahuayana. Allozyme analyses of fifteen populations including three of the known cytotypes showed that the chromosomally diverse populations of the Rio Coahuayana were half as genetically diverse as chromosomally non-divergent populations from the adjacent Rio Armeria. Evolutionary events which fixed the various cytotypes did not produce detectable differences in either levels of polymorphic loci or average heterozygosity. Genetic similarity indices produced phenograms of genetic relationships which clustered different cytotypes together; chromosomal variation and allozymic differentiation are uncoupled processes in Ilyodon. / Ph. D.

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. / Master of Science

The role of gene flow in population differentiation was examined by electrophoretic analysis of populations of two species of goodeid fishes, Goodea atripinnis and Chapalichthys encaustus, from lakes and streams on the Mesa Central of Mexico. Microgeographic differentiation was observed among continuous stream populations of Goodea. Highly significant genic heterogeneity was exhibited among continuous lacustrine populations of both species. Levels of differentiation (based on a genetic distance coefficient) among populations of G. atripinnis in Lake Chapala were similar to levels among populations from a number of isolated drainages. These results suggested that population continuity and gene flow do not necessarily imply genetic continuity and allele frequency homogeneity. Neighborhood effects (population subdivisions due to behavioral constraints such as homing or low vagility) were proposed as contributing to reductions in gene flow among populations from lakes and streams, but at least in the case of Goodea were not of major importance. Data from Lake Chapala for both species lended support to intralacustrine or sympatric models of lacustrine species flock evolution. Population comparisons of Goodea from a number of drainage systems suggested that a simple time-since-divergence model was insufficient to explain the observed patterns of genetic variation. Local effects (drift, bottlenecks, selection) were proposed as important mediators of genetic variation and population differentiation. It is suggested that levels of gene flow much greater than the “one migrant” rule would still permit differentiation of populations in the absence of selection. / Ph. D.