A Graphical Approach to Lineage Selection Between Clonals and Sexuals

Abstract

Theories for the evolutionary advantages and disadvantages of sex address two fundamentally different questions: (i) Why does the genome of sexual lineages not “congeal,” (i.e., move toward a lowered recombination rate)?, and (ii) When there is a mixture of reproductively isolated clonal and sexual lineages, why do the clonals not accumulate and lead to a predominance of asexual reproduction within a clade? Here, we focus on the latter question. The relevant theory in this case is necessarily based on a special form of “lineage” selection between sexuals and clonals that do not share a common gene pool. We first briefly review the major genetic costs and benefits of clonal reproduction and conclude that the extant assemblage of theories provides an essentially complete description of the phenomenon. We next set out to combine and simplify these seemingly disparate theories by graphically representing the frameworks previously developed by Felsenstein (Genetics 78: 737–756, 1974) and Kimura and Maruyama (Genetics 54: 1337–1351, 1966) to show that the all of the proposed disadvantages to clonal reproduction can be expressed by a single factor: a decreased efficiency of natural selection in non-recombining lineages. This reduced efficiency derives from two distinct processes that only operate in clonal lineages: (i) background-trapping and (ii) the compensatory linkage disequilibrium that accrues in response to epistatic selection.