Animals engage in sexual selection. For example, birds may choose mates on the basis of plumage or other physical features. As a result, populations can change as those without favorable looks are denied mates, which increases the numbers with certain traits over time. Almost all animals engage in sexual selection which may or may not be based on environmentally useful traits.
The environment where a population lives also affects its development. Changes in the environment determine what type of life can flourish there. Existing organisms must therefore adapt to survive. If there is a drought, individuals in populations that cannot handle drought will die off, but the survivors will be better suited to drought in the future. Therefore, over time populations become suited for their conditions. Alternatively, a disease could affect a population and kill off large numbers, but future generations will be resistant to future outbreaks.
The most prevalent form of artificial selection, selective breeding has been used for thousands of years. By controlling the reproduction of plants and animals, people can breed out unfavorable traits. This allowed early civilizations to domesticate cattle, horses and smaller pets like dogs, each bred for a certain purpose. The most common example of selective breeding is the wide variety in appearances and demeanor in the many different breeds of dogs.
Advances in technology and the discovery of DNA now allow scientists to directly change the genes of plants and animals. New traits can be added, even from completely different species, that can create very high yield crops or cats that glow under ultraviolet light. Genetic engineering is the ultimate in artificial selection, since generations of breeding are not required and the first generation can be designed for the desired result.