Spatio-temporal dynamics of the Allonemobius fasciatus-A. socius mosaic hybrid zone: a 14-year perspective

Long-term studies of hybrid zones can provide valuable insight into a numbe r of questions that have long attracted the attention of evolutionists. The se questions range from the stability and fate of hybrid zones to the relat ive fitness of hybrids. In this paper we report the results of a 14-year su rvey of the Allonemobius fasciatus-Allonemobius socius hybrid zone. Populat ions were collected intensively in 1986 and 1987 and then more sporadically through the end of the 1980s and throughout the 1990s. By documenting chan ges in the genetic composition of populations near and within the zone duri ng this period of time we assessed: the strength of the reproductive isolat ion between the two species; the relative growth rates (which can be consid ered a surrogate of relative fitness) of genotype classes corresponding to hybrids and to pure species individuals; and, the power of single-year and multi-year measurements of relative growth rates to predict changes in the genetic composition of mixed populations through time. In brief, we found v ery large year-to-year variation in the relative growth rates of pure speci es and hybrid individuals. This variation may reflect the fact that both sp ecies are at the edge of their range and perhaps at the limits of their abi lity to deal with environmental perturbations. As a consequence of the vari ation, even multi-year estimates of relative growth rates often provided im precise predictions regarding the future genotypic composition of mixed pop ulations. Despite our limited ability to predict the dynamics of individual populations, some trends are apparent. A. socius, the southern species, ha s clearly increased in frequency along a transect through the Appalachian M ountains, indicating that the zone is moving north in this region. In contr ast, the zone appeared to be more stable along the East Coast transect. Wit hin mixed populations, character-index profiles are often bimodal and stabl e through time, indicating relatively strong reproductive isolation between the two species that is not being reinforced, nor is it breaking down.