Asexual reproduction limits genetic diversity

A disadvantage of asexual reproduction is that the offspring are genetically identical to one another and to their parents. Thus asexual reproduction does not generate genetic diversity. Genetic diversity is the raw material that enables natural selection to shape adaptations in response to environmental change. When environmental changes occur, lack of genetic diversity can be disadvantageous to a population. Asexual reproduction is, however, a good way to preserve a successful genotype in a stable environment. Aphids are an example of a species that has it both ways (Figure 42.1). In the spring, the aphids that hatch on rose bushes from overwintering eggs are all females. Each of those aphids reproduces asexually to produce many more females—all genetically identical. This is a time when there is plenty of food and the environment is stable. When the conditions change due to drought or season, the females still reproduce asexually, but some of the offspring develop into males. The ensuing sexual reproduction introduces genetic variation into the eggs that will remain dormant until favorable conditions return.

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Figure 42.1 Aphids Are Sexual and Asexual When environmental conditions are favorable, such as in a rose garden in the summer, aphids make the most of resources by rapidly reproducing asexually. They are actually making identical copies of themselves. But when environmental conditions are unsure, such as in the fall, aphids reproduce sexually and thereby increase their genetic diversity in the face of changing, unpredictable conditions.

Three common modes of asexual reproduction are budding, regeneration, and parthenogenesis.