Many organisms reproduce asexually during part of their life-cycle while at other times they reproduce by sexual means. The advantage of asexual reproduction is that the population can be more rapidly increased to take full advantage of transiently abundant resources. The disadvantage is that the individuals in the population will have little variety in their genetic make-up since they are clones of their parents. The combination of asexual cycles and sexual cycles can lead to increased survival possibilities for some organisms. Clearly in the life cycle of an organism, the production of Diploid (2n) individuals from diploid individuals, or of haploid (n) individuals from haploid individuals will be by the process of mitosis. The production of n individuals, or gametes, from 2n individuals can only occur through meiosis. Haploid (n) individuals can never undergo meiosis, because they do not have paired chromosomes.

In this example the waterflea reproduces rapidly by asexual means during periods of plentiful food supply, but in the autumn when food supplies dwindle they revert to a sexual reproductive phase, producing fertilised eggs (zygotes) which are able to lie dormant through the winter to hatch and grow in the following spring. The use of a sexual phase enables variation to occur in the offspring and this allows for the evolution of the organism through the process of natural selection.