Generating Genetic Diversity is of a great deal of importance to the survival and continuity of Humanity. Paradoxically, the evolutionary process is both revolutionary and conservative. It is revolutionary in that the pace of evolutionary change is quickened by genetic recombination, much of which results from sexual reproduction. It is conservative in that change is not always favored by selection, which may instead preserve existing combinations of genes.
These conservative pressures appear to be greatest in some asexually reproducing organisms that do not move around freely and that live in especially demanding habitats. In vertebrates, on the other hand, the evolutionary premium appears to have been on versatility, and sexual reproduction is the predominant mode of reproduction. Whatever the forces that led to sexual reproduction, its evolutionary consequences have been profound.
No genetic process generates diversity more quickly; and, as you will see, genetic diversity is the raw material of evolution, the fuel that drives it and determines its potential directions. In many cases, the pace of evolution appears to increase as the level of genetic diversity increases. Programs for selecting larger stature in domesticated animals such as cattle and sheep, for example, proceed rapidly when new genetic combinations arise by crossing different varieties. Breeding programs can only accomplish so much, however. There are limits to what can be accomplished, because genes often affect more than one aspect of an individual, setting limits on how much a character can be altered. For example, selecting for large clutch size (more eggs) in barnyard chickens eventually leads to eggs with thinner shells that break more easily.
For this reason, we do not have chickens that lay twice as many eggs as the best layers do now, or gigantic cattle that yield twice as much meat, or corn with an ear at the base of every leaf instead of just at the base of a few leaves. Sexual reproduction increases genetic variability through independent assortment in metaphase I of meiosis, crossing over in prophase I of meiosis, and random fertilization.
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