Without genetic variation, there could be no evolution. Imagine that all individuals in a species had the exact same set of genes. Their offspring would all be the same, and would all have an equal chance of survival. It's the variations in genes, which are expressed as variations in traits, that cause some individuals to thrive and others to succumb to predators and other threats in their environment.
There's an interesting apparent paradox, though. Natural selection does not work *directly* on genes. Charles Darwin worked out his theory of evolutoin without even knowing that genes existed. How can this apparent paradox be explained?
Consider the finch, the bird that famously caught Darwin's attention on his voyage to the Galapagos Islands. Finches have different kinds of beaks, each one best suited for a particular task. Some beaks are best for cracking hard nuts, others for reaching into tight spaces, still others for eating small seeds.
The type of beak that an individual finch has will affect that finch's ability to survive in a given environment. If there is a drought that kills the plants that produce small seeds, the finches with large, strong beaks that can crack hard nuts will do better than the finches whose beaks were best suited to eating the now scarce small seeds.
Natural selection acts on observable traits such as the size, strength, and shape of the finches' beaks. An individual's observable traits are called its phenotype, but it's an individual's DNA, its genotype, that determines what the phenotype will be.
If every individual had the exact same DNA, the only observable differences among individuals would be those caused by the environment alone. There wouldn't be any variation in inherited traits such as the finches' beak size. If all the finches had the same genes, they would all have the same type of beak.
Genetic variation comes from two sources. The first is mutation. Most mutations are neutral or harmful, but when a mutation is beneficial and creates a trait that helps an individual survive, then natural selection can cause that trait, and the gene that created it, to spread through a population in later generations.
The second source of genetic variation, in sexually reproducing organisms, is recombination. An individual receives half its genes from one parent and half from the other. There are so many different possible combinations of these genes that each individual's genotype will be unique, even among siblings, with the exception of identical twins who share the same set of genes.
Darwin observed individuals with different phenotypes, and he was able to reason out the relationship between natural selection and the changing distribution of traits within a population. What he didn't know was the mechanism whereby traits were inherited. Ironically, he was a contemporary of Gregor Mendel, the pea-planting monk who was the pioneer of the study of genetics, but most historians believe that Darwin was unaware of Mendel's work.
"The Beak of the Finch," by Jonathan Weiner, Knopf 1994