Species Selection, Evolutionary Irreversibility, and Plant Breeding Systems

Abstract.  I will discuss the intersection of two broad evolutionary questions: how evolution plays out at multiple hierarchical levels simultaneously, and the extent to which complex traits, once lost, can be regained. The ability of plants to self-fertilize is an exemplar trait for studying both. First, I consider that the commonness of a trait is influenced both by its effects on the fitness of individuals (selection within species) and by its effects on speciation and extinction (selection among species, a.k.a. species selection). I describe existing mathematical methods, statistical and conceptual difficulties, and new directions for the enduring challenge of inferring species selection. Our empirical support for species selection relies on the knowledge that evolution of self-fertilization ability has proceeded overwhelmingly in one direction. Second, then, I consider the long-standing question of whether the evolutionary loss of complex traits is irreversible. I propose that it is more productive to reframe this question as continuous rather than binary, investigating the extent to which biological complexity inhibits evolutionary reversibility.