CLASSIFICATION OF HYPOTHESES ON THE ADVANTAGE OF AMPHIMIXIS

A classification of hypotheses on the advantage of amphimixis over apo mixis is presented. According to ''Immediate Benefit'' hypotheses, amp himixis is advantageous regardless of reciprocal gene exchange, becaus e either it directly increases fitness of the progeny, reduces the del eterious mutation rate, or makes selection more efficient. In contrast , ''Variation and Selection'' hypotheses attribute the advantage of am phimixis to the reciprocal gene exchange that alters genetic variabili ty and response to selection among the progeny. Most such hypotheses a ssume that amphimixis increases variability and efficiency of selectio n, but some claim that amphimixis decreases response to selection. Var iation and Selection hypotheses require that some factor, either rando m drift or epistatic selection, makes distributions of different allel es nonindependent, while another factor, either changes of the genotyp e fitnesses or deleterious mutations, makes overrepresented genotypes non-optimal. Numerous Variation and Selection hypotheses, dealing with either unstructured or spatially structured populations, are reviewed . Two of them seem most plausible: better responsiveness of the amphim ictic population to widely fluctuating selection, and lower mutation l oad in the amphimictic population under synergistic selection against deleterious mutations. In both cases the large advantage of amphimixis requires rather stringent conditions, which could be falsified by car eful experiment. Further progress in understanding the evolution of am phimixis will depend mostly on such experimental work.