When a virus was allowed to spread through a group of five genetically identical mice, the virus became more effective. That is, it spread more quickly and became more severe with each mouse it infected. The same test was done, this time with the mice being genetically diverse. This time around, the virus became less virulent as it struggled to adapt to the dissimilar genomes of the mice.
The University of Utah uses this simple example to urge the owners of livestock and other bred animals to keep their genetic diversity up. A herd of genetically similar cows is the vertebrate version of a monoculture, and a single pathogen capable of killing one of them will affect all of them. Whereas a population of genetically diverse animals will cause a pathogen to kill less effectively, both on the individual level and on the level of the whole herd. The first author of the study, Jason Kubinak, believes that sexual reproduction may have evolved to keep pathogens at bay. Another hypothesis on the evolution of sexual reproduction is that it evolved to regulate the inevitable mutations that occur when DNA is replicated.
article: http://unews.utah.edu/news_releases/viruses-impaired-if-their-targets-have-diverse-genes/
Without genetic variation viruses are able to determine what they are up against and quickly modify themselves to become more dangerous. Genetic variation allows for a defense mechanism against the virus and keeps the virus on its toes in a sense. The varying genetic makeup of a population prevents the virus from knowing exactly what its attacking thus allows for us as humans to defend against it and hopefully ultimately eliminate it.
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