Genetic variation in fundulus heteroclitus: Geographic distribution

Studies of 16 polymorphic loci in the fish Fundulus heteroclitus have uncovered significant directional changes in gene frequencies with latitude (i.e., clines). These spatial patterns could have arisen by primary and/or secondary intergradation. While we cannot presently distinguish between these two models, mitochondrial DNA analyses indicate that if previous isolation occurred as required for secondary intergradation, it must have been relatively recent and of short duration. Herein we discuss the roles of genetic drift, random migration, nonrandom migration, selection and others as potential driving forces for both modes of cline formation. In addition, we address the potential role of the last glacial period in (1) creating isolating barriers, (2) the opening of unoccupied habitats for range extension, and (3) as a mechanism for thermal selection. While some evidence exists that at least one of the driving forces is selection, the extent to which it and other deterministic forces participate as opposed to various stochastic processes must await further analysis