In every species, there is genetic which is produced by mutations and is what natural selection acts on (or "selects" from).

Variation

Explanation:

Population of an organism changes by time through evolution beacuase of genetic variation. The cause of genetic variation is may be gene mutation or genetic recombination.

Mutation is random constant process and the mutations that occurs only in egg or sperm cells be passed on to next generation. After the occurence of mutation the natural selection chooses, which organism will survive and which will die. If the mutation is bad there are very less chances of survival of an organism compared to an organism in which a good mutation has occured. So the surviving organism will pass on this mutation to offspring and Natural selection plays its role to incoprate only good mutations into species and extint the bad.

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