Parthenogenesis

Most organisms reproduce sexually because there is a competitive advantage in producing offspring with genetic contributions from two individuals rather than one. The genetic recombination which occurs during meiosis and on fertilization allows new gene combinations to come together in the next generation. Organisms with new gene combinations are more variable and offer more options for selection pressures to select the best adaptations for the environmental conditions, for example making use of different food resources or being more resistant to pathogens.

Parthenogenetic animals receive all of their genes from one parent and therefore no new gene combinations are created. It may seem that this method of reproduction would put species that use it at a competitive disadvantage to sexually reproducing animals but it may be advantageous in some cases. To reproduce, a sexually reproducing organism must first find a mate and then combine gametes with this mate. This process requires a great deal of time and energy, and it may well result in no offspring. Parthenogenic organisms do not experience this cost of reproduction and therefore usually can reproduce sooner after birth and produce more offspring. Animals which live in environments that are hospitable for only a short time period are often parthenogenic because mating would take time that these organisms do not have; these animals need to produce large numbers of offspring to compensate for the low survival rate of the offspring. Minnows found in the southwestern United States living in rivers that dry to the point where only puddles remain, demonstrate parthenogenetic reproduction so eliminating the need for a suitable mate to be present in a given puddle. Another advantage of parthenogenetic reproduction is that most offspring are unlikely to survive the dry months, regardless of whether or not sexual recombination occurs. Therefore organisms which produce a greater quantity of offspring are more likely to have one survive to the next generation.

Parthenogenesis may also be advantageous in stable environments with ample food resources. These environments favor organisms with the ability to reproduce quickly allowing their offspring to consume the food resources before others do. This is the reason why certain cyclical parthenogens are so successful. For example, aphids reproduce parthenogenetically in the summer to exploit the abundant leaves which they feed upon. In the fall aphids produce fertilized eggs which may endure fluctuating environmental conditions when dormant during the winter or limited food supplies when they hatch in the spring.