Documentation of numerous transitions from diploidy to polyploidy helps clarify diversification, geographic distribution, and the species problem in Amelanchier. Despite the infrequent occurrence of triploids, their retention of 25% sexuality and capacity for triploid bridge may be important steps between sexual diploids and predominantly apomictic tetraploids.

Amelanchier diploids differ strikingly from polyploid apomicts, in that hybridization among them is rare, and they form taxa that would qualify as species by most species concepts. Knowledge of diploid morphology, phylogeny, and ecogeography provides a foundation for understanding the evolutionary history of polyploid apomicts, their patterns of diversification, and their species status.

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Many rare and endemic species experience increased rates of self‐fertilization and mating among close relatives as a consequence of existing in small populations within isolated habitat patches. Variability in self‐compatibility among individuals within populations may reflect adaptation to local demography and genetic architecture, inbreeding, or drift. We use experimental hand‐pollinations under natural field conditions to assess the effects of gene flow in 21 populations of the central Appalachian endemic Trifolium virginicum that varied in population size and degree of isolation. We quantified the effects of distance from pollen source on pollination success and fruit set. Rates of self‐compatibility varied dramatically among maternal plants, ranging from 0% to 100%. This variation was unrelated to population size or degree of isolation. Nearly continuous variation in the success of selfing and near‐cross‐matings via hand pollination suggests that T. virginicum expresses pseudo‐self‐fertility, whereby plants carrying the same S‐allele mate successfully by altering the self‐incompatibility reaction. However, outcrossing among populations produced significantly higher fruit set than within populations, an indication of drift load. These results are consistent with strong selection acting to break down self‐incompatibility in these small populations and/or early‐acting inbreeding depression expressed upon selfing.