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 e volution, accurate estimates of outcrossing rates of individual plants (fam ilies) are required. Using isozyme markers, we observed wide variation in f amily outcrossing rates in three natural populations of Asclepias incarnata using three statistical methods: (1) a multilocus maximum-likelihood proce dure (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 o ne population, but showed no relationship with any of the other estimates o f outcrossing for any population. Monte-Carlo simulations revealed that sta tistical variation associated with these estimation procedures can be large enough to explain all of the observed variation in outcrossing. We also fo und that significant, spurious correlations with neighborhood floral displa y could arise, on average, 7% of the time by chance alone. Our observations suggest that it is difficult to obtain accurate estimates of outcrossing i n naturally pollinated plants using the estimation procedures currently ava ilable. Moreover, we caution that attempts to interpret observed variation in family outcrossing estimates by observing variation in ecological parame ters could be misleading.