Estimates of Gene Flow among Populations, Geographic Races, and Species in the Ipomopsis Aggregata Complex

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Interpopulational gene flow within a species can reduce population differentiation due to genetic drift, whereas genetic exchange among taxa can impede speciation. We used allozyme data to estimate gene flow within and among geographic races and species of perennial herbs in the Ipomopsis aggregata complex (Polemoniaceae). Estimates of interpopulational gene flow within taxa from two methods (F statistics and private alleles) were correlated with one another. Gene flow among populations within each geographic race (subspecies) of I. aggregata was relatively high (Nm > ~1.0). Gene flow was also high among populations of I. arizonica and among four northern populations of I. tenuituba. However, gene flow was low (Nm < 1.0) for I. tenuituba when a population representing subsp. macrosiphon was included. This is consistent with previous findings that subsp. macrosiphon has had an independent origin and is reproductively, as well as geographically, isolated. A recently developed model, based on hierarchical F statistics, was employed to estimate genetic exchange among taxa. Gene flow estimates were generally high among races of I. aggregata (dNm(race) > 1.0) but were low among subspecies of I. tenuituba (dNm(race) < 1.0). Consistent with morphological evidence, estimates of interspecific gene flow were moderate between I. aggregata and I. tenuituba, which hybridize in several areas. However, contrary to morphological evidence, we estimated relatively high levels of interspecific gene flow involving I. arizonica. Our results suggest that I. arizonica has hybridized with other species without the transfer of morphological traits. In the I. aggregata complex, gene flow appears to be an important evolutionary force shaping geographic variation for allozymes within species, but is insufficient to prevent morphological divergence among taxa.

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