Parthenogenesis, or asexual reproduction, is the act of reproduction without using more than one source of genetic material. The natural world has many examples of parthenogenic species and relatively few that only reproduce sexually. The only known species that do not practice parthenogenesis occasionally, to some extent, are dragonflies and mammals. Aphids are the most well adapted species for utilizing the benefits of asexual reproduction.
The aphid life cycle accurately represents parthenogenic reproduction. A female aphid, without wings, hatches from an egg in the early spring. This aphid grows and feeds until, by parthenogenesis, gives birth to clones of herself. These genetic copies, once fed and matured, bring forth cloned offspring of themselves. When food within range of these aphids, which are all wingless, becomes scarce, they reproduce asexually female aphids with wings. These winged aphids then fly to a new feeding area and start their own parthenogenic population. This process continues until fall, when winged female aphids return to the hatching place of the original aphid progenitor. Once there, they again, without sex, give birth to a new type of female aphid. This caste can only give birth through sexual reproduction. Males produced asexually by females on other plants fly to the hatching place and mate with the sexually able females. These female lay eggs, which will hatch when conditions are once again favorable. Thus, the life cycle of the aphid is completed.
Asexual reproduction gives many advantages to its practitioners. Parthenogenesis allows for a much higher rate of increase in total population size than sexual reproduction. This ability allows species to inhabit environmental niches by having large populations only when resources are abundant. Also, if a new unusual environment is realized, then parthenogenic species can very quickly inhabit it.
There is one distinct disadvantage to the conservative system of asexual reproduction. Parthenogenic organisms will never overcome inherent flaws in their own genetics. A sudden change in the specie’s environment can reveal these flaws and leave the species unable to adapt. This can either cause the species eventual extinction or cause the species to return to sexual reproduction.
Genetic variation, or sex, is needed to some extent to allow a species to thrive, overcome environmental changes, and surpass genetic dead ends.