Natural hybridization is common in plants. If plant distribution is patchy across the region of hybridization, we can distinguish four kinds of gene movement in a hybrid zone. Intraspecific gene flow can occur within or between local populations. Interspecific gene flow can occur between distinct patches each occupied by a single species, or within a patch (hybrid swarm). Using two hybridizing species of Ipomopsis (Polemoniaceae) as a model system, I discuss ongoing efforts to estimate these four kinds of gene movement and the methods appropriate in each case.
The primary means of gene movement in this system is transfer of pollen by hummingbirds. Behavioral observations of hummingbirds, combined with measures of pollen carryover from flower to flower, explain the highly localized pollen movement (usually a few m) within local populations. Similar mechanistic studies predict asymmetrical gene movement between I. aggregata and I. tenuituba and generally high rates of interspecific gene flow. These predictions were tested using a single genetic marker in mixed-species experimental populations. Work in progress uses multiple RAPD markers to estimate patterns of gene movement within a naturally hybridizing population.
At the between population level, paternity analysis with multiple allozyme markers suggests moderately high rates of gene flow from outside. Minimum rates of gene flow ranged from 9-25% into experimental populations isolated by as much as 150 m. The discrepancy between these results and the short distances of pollen flow within populations suggests that gene movement between versus within populations may result from different pollinator behaviors, making it difficult to extrapolate across scales. Determining the distance of gene flow between populations presents a special challenge. These studies illustrate the value of combining genetic markers with mechanistic methods in determining patterns of gene flow.