Structured populations and the maintenance of sex Peter A. Whigham 1 , Grant Dick 1 , Alden Wright 2 , and Hamish G. Spencer 1 1 Otago University, Dunedin, New Zealand, peter.whigham@otago.ac.nz, 2 University of Montana, Montana, USA alden.wright@umontana.edu Abstract. The maintenance of sexual populations has been an ongo- ing issue for evolutionary biologists, largely due to the two-fold cost of sexual versus asexual reproduction. Many explanations have been pro- posed to explain the benefits of sex, including the role of recombination in maintaining diversity and the elimination of detrimental mutations, the advantage of sex in rapidly changing environments, and the role of spatial structure, finite population size and drift. Many computational models have been developed to explore theories relating to sexual pop- ulations; this paper examines the role of spatial structure in supporting sexual populations, based on work originally published in 2006 [1]. We highlight flaws in the original model and develop a simpler, more plausi- ble model that demonstrates the role of mutation, local competition and dispersal in maintaining sexual populations. Keywords: sexual selection, Muller’s ratchet, spatial structure 1 Introduction The enigma of sex remains a central problem in biology [2]. Since a sexual individual transmits only half of their genes to its offspring, compared with an asexual organism, there is a two-fold cost associated with sex. Hence, unless a sexual individual can breed twice as often as an asexual individual, there is a transmission loss that infers a disadvantage to the sexual population [3]. However, given that the majority of eukaryotes are sexual some set of factors must contribute to overcoming the basic two-fold cost from reproduction. Two main issues relate to the question of sexual reproduction: how did sex- ual reproduction come into being, and what factors support the maintenance of sexual versus asexual populations. As noted by Maynard Smith [2], the question of how sexual populations arose is difficult because we cannot rerun the process. However the mechanisms involved in the maintenance of sexual populations can be modeled and therefore studied to support theories involved in their continu- ation over time. Consequently, a large number of theories have been published regarding this issue, including the role of recombination in maintaining diver- sity [4] and eliminating detrimental mutations [5], producing robust populations under environmental variation, and allowing the rapid accumulation of beneficial