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Frequency of Purple Morphs

Fig. 12.2. The relationship between morph frequency and relative reproductive success for two morphs of the unrewarding orchid Dactylorhiza sambucina. The dashed line indicates the line of equal morph fitness and the solid line the least- squares regression line (y = 1.177 _ 1.320 x).

Dactylorhiza sambucina, a non-model, deceptively pollinated orchid (Nilsson 1980). Widespread in mainland Europe, it has a dramatic corolla-color dimorphism, with both yellow and purple morphs present throughout its range (Tutin et al. 1980). Reproductive success through both fruit set and pollinia removal is pollinator-limited; pollinators are newly emerged queen bumble bees (Nilsson 1980). I measured morph frequencies and relative reproductive success in 17 populations of D. sambucina in southern France in May 1996. I sampled 20 individuals of each morph from each population (or all individuals in a population for rare morphs) and measured relative reproductive success as the number of pollinia removed per plant, (corrected for inflorescence size). Environmental parameters (altitude, location, substrate) affected neither mean morph frequency nor relative reproductive success, nor were there significant effects of populationsize (A. Smithson, unpublished data). Relative reproductive success and morph frequency were negatively correlated (Table 12.3, Fig. 12.2). The regression line relating relative reproductive success and morph frequency shows a reversal in fitness advantage at a frequency of 13.4% purple morphs (Fig. 12.2). This predicted equilibrium morph frequency approximates the actual mean frequency of the populations studied (18.96% ± 4.52%), but the strength of the FDS selection recorded -as indicated by the slope of the regression line - is comparatively weak (1.18). It could be argued that specific mimicry could produce the observed relationship. However, we have no evidence of specific mimics being present in the populations, and co-flowering species varied greatly among populations. To date, we have not found density to significantly interact with morph frequency or absolute reproductive success or relative reproductive success (L. Gigord & A. Smithson, unpublished data). Overall, the D. sambucina data currently support the hypothesis that there is a reproductive advantage for rare corolla color variants of unrewarding plant species; further manipulative experiments are under way to test competing hypotheses.

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