The products of natural, and human, selection are all around us. Humans have transformed wild plants into useful crops by selective breeding. Human selection has also produced pets and other domesticated animals with sizes and shapes very different from their wild ancestors. Controlled genetic crosses can be used to identify and locate the genes responsible for artificial selection in domesticated species. Genetic crosses in maize and dogs, for example, suggest that few genetic changes are needed to dramatically transform the shape and structure of plants and animals.
Natural selection in wild populations can also generate amazing diversity in a surprisingly short amount of time. Ocean stickleback fish, for example, colonized numerous freshwater streams and lakes produced by retreating glaciers after the last Ice Age. Differential survival and reproduction under natural selection have generated dramatic changes in morphology, physiology, and behavior as the fish adapted to different food sources, predators, and water conditions. Genetic studies of recently evolved freshwater fish confirm that many evolutionary traits are controlled by relatively few genes. It appears that natural populations, like domesticated populations, can evolve rapidly under the influence of a few simple genetic changes.