Using Phylogenetic Approaches for the Analysis of Plant Breeding System Evolution

Until recently, studies of plant reproductive systems have been at the population level, using microevolutionary approaches. The development of cladistic approaches, combined with the emergence of molecular systematics, has resulted in an explosion of phylogenetic studies and an increase in interdisciplinary approaches combining ecological and systematic methodology. These new approaches offer the possibility of testing explicit hypotheses about the number of evolutionary transitions in reproductive characters and the evolutionary relationship of these characters to changes in the environment. Character mapping may be especially useful for detecting convergent evolution. In a number of cases, character mapping has provided new insights into the evolution of plant breeding systems and pollination biology, especially in suggesting the number of times evolutionary transitions have taken place, indicating where there have been reversals and suggesting when preadaptation has been important. The insights provided by character mapping are determined by a number of factors, including the degree of confidence in phylogenies underlying these studies and the identification of appropriate outgroups. Assumptions about character coding, character ordering, inclusion vs. exclusion of characters that are mapped on trees in the data matrix, and weighting of characters will have profound effects on interpretation of character evolution. Highly labile characters that evolve frequently and have the potential to undergo reversals may make it difficult to detect the pattern of character evolution. Characters that are very strongly correlated with each other or with ecological shifts may make prediction of cause and effect using phylogenetic approaches difficult because changes in characters and ecological shifts will occur, apparently simultaneously, on the same branches. Results from microevolutionary studies have been used in several cases to weight transitions, suggesting that results of phylogenetic studies may not provide fully independent assessments of character evolution. While not a simple cure to understanding problems that have been studied only in the realm of microevolutionary studies, phylogenetic approaches offer clear potential for providing new insights for evolutionary studies.