Why Sex?

There are clear benefits of asexual reproduction. The main advantage is that by reproducing asexually, there is an exponential increase in population size. There is no problem of finding a mate and the organism is perfectly adapted to environment. However, there are also detriments of exclusively asexual modes of reproduction. There is recent modeling work that postulates that rare beneficial mutations can also result in the accumulation of deleterious mutations (Hadany and Feldman 2005). The model argues that an individual that acquires a rare beneficial mutation will also have several deleterious mutations. There are two outcomes for this situation. The new beneficial mutation cannot overcome the cumulative deleterious effects of the accumulated mutations it has, and thus is unlikely to increase in frequency in the population. Alternatively, and less likely, the new beneficial mutation will increase fitness such that it overcomes the cumulative deleterious effects of the accumulated mutations it also has, and thus become fixed in the population. In either case, the rate of adaptation is slower than with a sexually reproducing population. Additionally, if the beneficial allele goes to fixation, it actually reduces the overall fitness of the population due to the hitchhiking effect of the deleterious mutations it brings along. Another problem with asexual reproduction is that there will be less variation in the population, and less for natural selection to work on. The benefit of asexual reproduction relies on a non-changing environment. The organism is perfectly adapted, and replicates rapidly, but once the environment changes, there may be little variation from which to select a new optimal genotype. Orr (2000) determined that the rate of adaptation is determined by the strength of selection against deleterious mutations. Finally, with most modes of asexual reproduction, DNA doesn’t get repaired, and there is potential for loss of genomic DNA. Fungi have circumvented this process somewhat by uncoupling meiosis from sex, and have a specific process called parasexuality. It is possible that this process underlies the origins of sexual reproduction. In other words, the mechanism of sexual reproduction, meiosis, originally developed to repair DNA, and evolution has acted on this process to evolve sex.