Two-Fold Cost - Background
All else being equal, an asexually reproducing population will grow at twice the rate of a sexually reproducing population. The reason for this is simple: members of a sexually reproducing population must produce both sexes, but effectively only half of them can "give birth" to offspring of their own. In contrast, all offspring of an individual from an asexually reproducing population can potentially "give birth". This is the two-fold cost of sexual reproduction.
Despite this cost, sexual reproduction is a prevalent mode of reproduction. Therefore, there must be some advantage to reproducing sexually, or else the sexually reproducing populations would soon become replaced by the much larger sexual populations. One advantage of sexual reproduction is that sexual reproduction maintains more genetic variation than asexual reproduction.