In a monoculture, if one plant is vulnerable to a disease or an insect, they all are. Thus diseases or pests can rip through it like nobody’s business. (As John Scalzi observes in one of his books, it’s also why clone armies, popular in science fiction, are a bad idea in reality, as they would be highly vulnerable to engineered diseases.) A uniform population is a high-value target.
This is also why nature fosters genetic diversity. Sexual reproduction is a lot of trouble compared to, say, fission or budding, or even parthenogenesis. Despite its undoubted pleasures, it’s resource-expensive, requiring a search for a mate, possibly competition to mate at all, and risks like sexually transmitted diseases and childbirth, all for a paltry 50% (average) genetic pass-on. Unlike asexual reproduction, which produces a copy of what is, by definition, a successfully reproducing individual, sexual reproduction produces a genetically unique offspring who may turn out to be worse-adapted to the environment than either parent.
But on the other hand, it produces a genetically unique offspring. This means that parasites that might be well adapted to the parents may well be less adapted to the offspring. Sexual reproduction imbues a population with genetic diversity, making it a moving target for diseases, parasites, etc.
This isn’t the first time I’ve thought of this phenomenon in a political connection.
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