From individual behavior to selection frequencydependence in plant populations

Evidence for FDS in rewarding plant populations

Four published studies of rewarding plant populations provide data relating the relative reproductive success of alternative morphs to morph frequency (Table 12.3). I have excluded floral size traits from consideration, along with Levin's (1972) data set for reasons discussed above. The studies include those from Table 12.2, with the exception of Cirsium palustre, and include an additional study on Phloxpilosa (Levin & Kerster 1970). For each study, I calculated relative fitness for one of the morphs considered, then used rank correlations to test for a relationship between relative reproductive success and morph frequency. I predicted that, for rewarding species, there should be a positive correlation between morph frequency and relative fitness (positive FDS). Note that this method tests for FDS in a broad sense; an indication of reversal of fitness advantage at some frequency must also be observed to implicate FDS in its stricter sense.

None of the published studies indicates a significant relationship between morph frequency and relative reproductive success (Table 12.3). In all three cases where seed set is considered, there are positive trends but the correlations are not significant. Outcrossed seed set in Ipomoea purpurea suggests a tendency for a negative correlation between relative reproductive success and morph frequency, but this result differs from that of the original authors (Epperson & Clegg 1987). They analyzed the deviation from expected for each data point individually; results must therefore be regarded as tentative. The only experiment to consider out-cross seed paternity found a positive but non-significant relationship between relative reproductive success and morph frequency. Meta-analy-sis of these data was not appropriate due to the diversity of fitness indices used and the small sample sizes for some studies. Overall, results from rewarding species fail to support the hypothesis that differences in pollinator visitation rates between morphs induce positive FDS in plant populations.

Evidence for FDS in unrewarding plant populations

Only one previously published study has measured FDS in an unrewarding plant species: again, this is Tolumia variegata (Table 12.3) (Ackerman et al. 1997). For an unrewarding species, we expect a negative correlation between morph fitness and relative reproductive success (negative FDS). However, the pollinators did not discriminate strongly between different scent morphs; there were no significant effects of frequency on male or female reproductive success (Ackerman et al. 1997).

In 1996, I attempted a test of the negative FDS hypothesis using

Table 12.3. The impact ofmorphfrequency on relative reproductive success in six plant species




Fitness index'

Frequency dependence^


Clarkia gracilis

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