The variation that selection requires arises from two distinct sources. The ultimate sources of variation are gene mutation, gene duplication and disruption, and chromosome rearrangements. Gene mutations are randomly occurring events that at a molecular level consist mostly of substitutions or small losses or gains of nucleotides within genes. Gene duplication makes new copies of existing genes, while gene disruptions destroy functional copies of genes, often through insertion of a mobile genetic element. Chromosome rearrangements are much larger changes in chromosome structure, in which large pieces of chromosomes break off, join up, or invert. Individually, such mutations are rare. Most small mutations are either harmful or have no effect, and they may persist in a population for dozens or hundreds of generations before their advantages or disadvantages are evident.
The second source of variation arises from the shuffling processes undergone by genes and chromosomes during reproduction. During meiosis, maternally and paternally derived chromosome pairs are separated randomly, so that each sperm or egg contains a randomly chosen member of each of the twenty-three pairs. The number of possible combinations is over eight billion. Even more variation arises when pair members exchange segments before separating, in the process known as crossing over. The extraordinary variety in form exhibited even by two siblings is due primarily to the shuffling of existing genes, rather than to new mutations.
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