One of the greatest challenges for evolutionary biology is explaining the widespread occurrence of sexual reproduction and the associated process of genetic recombination. A large number of theories have been developed that provide a sufficient short‐term advantage for sex to offset its two‐fold cost. These theories can be broadly classified into environmental (or ecological) and mutation‐based models. Traditionally, the different theories have been viewed as competing, and empirical work has attempted to distinguish between them. Here we highlight the advantages that may be gained from considering that multiple mechanisms (environmental and mutational) may be at work, and that interactions between the theories may be very important.

Host-parasite coevolution has been shown to provide an advantage to recombination, but the selective mechanism underlying this advantage is unclear. One possibility is that recombination increases the frequency of advantageous genotypes that are disproportionately rare because of fluctuating epistasis. However, for this mechanism to work, epistasis for fitness must fluctuate over a very narrow timescale: two to five generations. Alternatively, recombination may speed up the response to directional selection by breaking up linkage disequilibria that decrease additive genetic variance. Here we analyze the results of a numerical simulation of host-parasite coevolution to assess the importance of these two mechanisms. We find that linkage disequilibria may tend to increase, rather than decrease, additive genetic variance. In addition, the sign of epistasis changes every two to five generations under several of the parameter values investigated, and epistasis and linkage disequilibrium are frequently of opposite signs. These results are consistent with the idea that selection for recombination is mediated by fluctuating epistasis. Finally, we explore the conditions under which an allele causing free recombination can spread in a nonrecombining host population and find general agreement between the predictions of a population genetic model of fluctuating epistasis and our simulation model.

High risk of infection by parasites may select for early reproduction in natural host populations. In a previous study of a freshwater snail (Potamopyrgus antipodarum) we found (1) that different clones of the snail are associated with different depth‐structured vegetation zones and (2) that snails in shallow water, where the age‐specific risk of infection is highest, mature at a smaller size than snails in deeper habitats. This result suggests that there has been selection for early reproduction in these snails, and that different clonal genotypes have different life‐history strategies. Alternatively, the observed life‐history variation in the snails might be due to ecological factors that are independent of parasites, but correlated with depth. In the present study, we decoupled parasitism and depth by examining life histories and clonal population structure in a second lake (Lake Tennyson) where the mean prevalence of trematode parasites was low and unrelated to depth. Consistent with the previous results, clones were structured according to vegetation zones in Lake Tennyson. However, we found no relationship between depth and life‐history traits, which is inconsistent with the idea that depth‐associated factors other than parasites affect snail life histories. Taken together, these results suggest that life‐history variation is more likely to result from a depth‐specific risk of infection than from depth per se, and that partitioning of habitat zones by different groups of clones may be a general phenomenon in P. antipodarum populations.

Siblings identified multiple ways that providers can support and assist them in coping with the impact of schizophrenia. Education and support for siblings without schizophrenia and services for their ill siblings will become increasingly important for the well-being of siblings as they are faced with the responsibility of being the primary caregivers in the future.

No abstract