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What is an advantage of sexual reproduction?

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The primary advantage of sexual reproduction for a species is increased genetic variability. Species that reproduce asexually have the benefit of not having to locate an appropriate partner to reproduce with. However, when they reproduce, the offspring are genetically identical to the parent and to one another. If organisms and environments were consistent this would not be a problem, but in reality, the environment is in constant fluctuation and species must adapt to changes in their surroundings. In order to adapt, species need to have heritable variation in their traits.  

A famous example is the Peppered Moth, which has speckled wings that help it blend into lichens on trees. While the majority of the moths were white with black speckles, a genetic mutation somewhere along the line led to a a morph with almost completely black wings. During the rise of industrialism in the 1800's, lichens began to die off in urban areas near factories, and as a result tree trunks became blackened. The primarily white moths now stood out on the dark surface, and instead the black moths were better camouflaged. Before long, black moths became more common than white in urban areas. Having a diverse pool of genes in the population allowed the moths to quickly adapt to their changing environment. If this variation had not been present and all peppered moths were white, they may very well have become extinct. Since the 1800's, air quality improvements have allowed lichens to return to urban trees and the peppered moths are now primarily white.

Asexually reproducing organisms are not incapable of adapting. Variation still occurs as a result of random gene mutations.  However, by sexual reproduction a species is able to better mix the alleles in it's gene pool, leading to more diversity between individuals and making it easier for successful traits to spread throughout the population and persist.  

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