A pair of parents would have to have 1,000,000,000,000,000 (a quadrillion) babies before they possibly might have a child with the same genes as any of their other children. This genetic variation between individuals is the key to how species have evolved.
Genetic variation
https://evolution.berkeley.edu/evolibrary/article/evo_17
Without genetic variation, some of the basic mechanisms of evolutionary change cannot operate.
There are three primary sources of genetic variation, which we will learn more about:
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Mutations happen for several reasons.
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Mutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural — even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutatio
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Gene flow
Gene flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another.
Sex and genetic shuffling
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Sex can introduce new gene combinations into a population and is an important source of genetic variation.
You probably know from experience that siblings are not genetically identical to their parents or to each other (except, of course, for identical twins). That's because when organisms reproduce sexually, some genetic "shuffling" occurs, bringing together new combinations of genes. For example, you might have bushy eyebrows and a big nose since your mom had genes associated with bushy eyebrows and your dad had genes associated with a big nose. These combinations can be good, bad, or neutral. If your spouse is wild about the bushy eyebrows/big nose combination, you were lucky and hit on a winning combination!