A gene is a section of the DNA strand that acts as a template for a new cell. Cells reproduce by making copies of genes on the DNA strand. Sometimes genes are copied incorrectly and become mutations. Many of these are beneficial to the organism, but other mutations can be harmful, causing a wide variety of genetic disorders and conditions.
Scientists disagree about how and why genes mutate. For a long time they believed that genetic mutations were unrelated to whatever consequences such mutations would have on the species. But in the last decade, researchers have found evidence that cells might be able to "choose" which mutations will occur to give them an advantage in stressful situations. For example, when starving, certain bacterial cells might generate multiple mutations. If the mutated cell survives, the mutation is passed on; cells that die take their unsuccessful mutations with them.
The gene pool increases when a mutation changes a gene and the mutation survives; it shrinks when a mutation dies out. A small gene pull reduces variation within a species, which can lead to genetic disorders, such as low fertility, deformities, and genetic diseases. This is why animal breeders will sometimes allow an animal inside the breed to mate with one outside the breed. The offspring increases the size of the gene pool and decreases the likelihood of genetic diseases being passed on to future generations.