Family outcrossing rates and neighborhood floral density in natural populations of swamp milkweed (Asclepias incarnata):: potential statistical artifacts

To evaluate how environmental and genetic factors influence mating-system evolution, accurate estimates of outcrossing rates of individual plants (families) are required. Using isozyme markers, we observed wide variation in family outcrossing rates in three natural populations of Asclepias incarnata using three statistical methods: (1) a multilocus maximum-likelihood procedure (t(m)); (2) a multilocus method-of-moments procedure (t(a)); and (3) a direct comparison of progeny phenotypes against maternal phenotypes (t(d)). Neighborhood floral-display size was positively correlated with t(a) in one population, but showed no relationship with any of the other estimates of outcrossing for any population. Monte-Carlo simulations revealed that statistical variation associated with these estimation procedures can be large enough to explain all of the observed variation in outcrossing. We also found that significant, spurious correlations with neighborhood floral display could arise, on average, 7% of the time by chance alone. Our observations suggest that it is difficult to obtain accurate estimates of outcrossing in naturally pollinated plants using the estimation procedures currently available. Moreover, we caution that attempts to interpret observed variation in family outcrossing estimates by observing variation in ecological parameters could be misleading.