DYNAMICS OF A HYBRID ZONE IN PHLOX: AN EXPERIMENTAL DEMOGRAPHIC INVESTIGATION

The dynamics of a narrow hybrid zone between Phlox drummondii ssp. drummondii and ssp. mcallisteri was investigated using an experimental demographic approach. Three populations of each subspecies as well as those from the hybrid zone were involved. Experimental plots containing seeds from each of the nine populations were established in each of the eight sites. Germination, survivorship to reproduction, and fecundity were monitored at 4–6 wk intervals. Finite rates of increase were estimated from these data. There were significant site effects for each demographic variable and rate of increase as well as significant population effects for germination and finite rate of increase. Although there were population effects, no significant differences were found between plant types in sites of the subspecies or the hybrid zone. We reject the hypothesis that the narrow width of the hybrid zone is the result of hybrid disadvantage. We propose that the narrow width is a result of restricted gene flow and discuss the supporting evidence.