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A gamete is a sex cell. For instance, the two gametes in the human species are the sperm cell and the ovum. Each of these gametes contains one complete set of chromosomes (23 in humans). When these two gametes fuse during the reproductive cycle, the zygote that is formed has two sets of chromosomes (46 in humans.)
It is found in nature that completely random reproduction - that is, reproduction that is not limited to reproduction between close relatives - leads to offspring that is more robust and healthy than would otherwise be the case. It is commonly known in biology as "hybrid vigour." Reproduction in this manner means that genetic abnormalities, diseases etc. are less likely to be passed down the generations.
Consider the example of thouroughbred horsebreeding. Close-relative reproduction is encouraged in order to propagate certain desirable traits in the horse (e.g. stamina or speed.) The payback is that the offspring is generally a less robust animal than a nag. The same is true in pedigree dog breeding.
The statement in the heading is simply a reflection that evolution has "recognised" this hybrid vigour and hence random union is preferred.
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