Spatial and temporal patterns in the covariance of genetic and morphologic characters in a pupfish hybrid swarm

Scope and method of study. Pupfish populations from the Pecos River, New Mexico-Texas, were sampled from 1984 to 1988 to evaluate geographic and temporal patterns of variation in intergrade populations that originated with the introduction of sheepshead minnow Cyprinodon variegatus into the range of an endemic species, the Pecos pupfish C. pecosensis. Allozyme surveys were used to document the geographic distribution of introduced genetic elements. A combination of allozyme and morphological analyses was used to assess the congruence in geographic and temporal variation in unrelated character sets and to study temporal patterns in covariance (correlation) among traits.; Findings and conclusions. Introduction of C. variegatus into the Pecos River, between 1980 and 1984, and subsequent hybridization with the endemic C. pecosensis have resulted in the spread of intergrade (=genetically mixed) pupfish throughout some 430 km of the Pecos River in Texas and adjacent New Mexico. Intergrades are virtually ubiquitous in permanent and semi-permanent waters throughout Texas portions of the river and their dispersal is a major threat to the genetic integrity of remaining "pure" populations of C. pecosensis. In general, geographic and temporal variation in allozyme and morphological characters was congruent in intergrade pupfish. There is evidence of a gradual increase in the frequency of introduced characters, but there is no obvious evidence of selection. Correlations among characters generally were nonsignificant. Persistent, weak correlations among some characters may be being maintained by gene flow between upstream and downstream sites.