Any given gene in a gene pool is likely to exist in the form of several duplicate copies, either all descended from the same original mutant, or descended from independent parallel mutants. Some genes, such as the albino gene, are rare in the gene pool, others are common. At the genetic level, evolution may be defined as the process by which gene-frequencies change in gene pools. Immigration, emigration, and random drift are not of much interest from the point of view of adaptation, although they may be quite important in practice. Genes in bodies exert an influence on the development of those bodies.
Some bodies are better at surviving and reproducing than others.
Not all selective deaths lead to evolutionary change. On the contrary, much natural selection is so-called stabilising selection, removing genes from the gene pool that tend to cause deviation from an already optimal form. Evolution under the influence of natural selection leads to adaptive improvement. Evolution, whether under the influence of natural selection or not, leads to divergence and diversity. From a single ultimate ancestor, many hundreds of millions of separate species have, at one time or another, evolved.