When discussing biological selection it is important to consider the three possible methods involved: natural selection, sexual selection, and artificial selection. The reason we have such a diverse array of organisms on Earth today is primarily due to natural selection. By definition, natural selection can be defined as the process in which favorable traits are promoted and passed on, and unfavorable traits are "weeded" out, oftentimes due to environmental factors. An example of this is giraffes and the length of their necks. Over time, the giraffes with longer necks were able to reach higher branches and therefore have food more readily accessible; whereas the giraffes with shorter necks could not compete and would often die. The driving force behind natural selection is survival and adaptation to environmental factors. The second method that occurs naturally is sexual selection; the process in which a male or female is interested in characteristics of the opposite ***, such as color, behavior, or form. Because of these species-specific attractions, modifications of a specific nature are brought about in a species. The organisms with more interesting and appealing characteristics are likelier to reproduce and pass on the favorable trait. An example of this is the male species of the superb-bird-of-********, who flashes his display to the female in an attempt to mate. The brighter and larger his display, the higher chance he has of reproducing. The driving factor behind sexual selection is reproducing and the passing on of attractive characteristics. The final method of gene selection is one that does not occur in nature, artificial selection. This is a process in which certain genes or traits are sought after; humans will breed plants and animals that have the desired traits. An example of artificial selection would be the breeding of white tigers, which involves the inbreeding of individuals and often causes the white tigers to be born with defects. The driving factor behind this method is to appeal to aesthetics and make money. The similarities between these biological selection methods are that the desired traits are typically preferred, whether it be nature or humans who decide. It also takes time and multiple generations for these traits to become prevalent. One main difference is that in nature it normally takes about one million years for a significant evolutionary feature to arise, whereas humans are able to do it at a faster rate. Take for example dogs, who range in size from a chihuahua to a mastiff. Humans have been documented to be breeding dogs for at least 11,000 years, but possibly up to 36,000 years. Compare even 36,000 years to the time it would take in nature and the difference is clear. One very clear similarity is that natural and sexual selection both occur in nature, whereas artificial selection is different because it is performed by humans and is not natural.