Evolution of Genes
Most changes that occur in genes are subject to natural selection, the process first outlined by Charles Darwin in 1859. In natural selection, a heritable change arises by chance. If the organism with that change is better able to survive and reproduce, it will leave more descendants in future generations. These descendants will also carry the new genetic change, and as they reproduce, the change will become more widespread in the population. On the other hand, if the change decreases an organism's survival rate, it will be lost from the population. It is also possible to have a neutral change, with no immediate effect on survival. Such "hidden" genetic variation within a population provides grist for evolution when it offers a selective advantage under new environmental conditions.
It is important to remember that a genome (all the DNA of an organism) is more than just its genes. The genome includes vast amounts of DNA outside of genes, and this too is subject to change over time. In fact, non-gene portions usually change at a faster rate than genes, because many of these changes have little or no effect on the organism's survival.
Evolution of genes and genomes includes sequence changes to existing genes, gene duplication, recombination of gene segments, and the varied actions of transposable elements as they move through the genome.