Divergent evolution occurs when plant or animal characteristics that share an evolutionary origin become more distinct over evolutionary time (many thousands or perhaps millions of years). An example of divergent evolution is the human arm, the whale’s fin, and the bat’s wing. All three limbs perform different purposes, but all contain the same bones in different sizes and shapes because all three animals (human, whale, bat) evolved from a common ancestor whose legs contained those bones. For humans, upright walking on the ground required alterations in the foot for better speed and balance. The whale and bat modified the ancestral structure in response to environmental pressures of their own. These differing traits soon became characteristics that evolved to permit movement over the ground and in the water and air. Although humans and monkeys are genetically about 99% similar, their natural habitat required different physical traits to evolve for survival.
If different selective pressures are placed on a particular organism, a wide variety of adaptive traits may result. If only one structure on the organism is considered, these changes can either add to the original function of the structure, or they can change it completely. Divergent evolution leads to speciation, or the development of a new species. Divergence can occur when looking at any group of related organisms. The differences are produced from the different selective pressures. Any genus of plants or animals can show divergent evolution. An example can involve the diversity of floral types in the orchids. The greater the number of differences present, the greater the divergence. Scientists speculate the greater that two similar species diverge indicates a longer length of time that the divergence originally took place.
There are many examples of divergent evolution in nature. If a freely-interbreeding population on an island is separated by a barrier, such as the presence of a new river, then over time, the organisms may start to diverge. If the opposite ends of the island have different pressures acting upon it, this may result in divergent evolution. Or, if a certain group of birds in a population of other bird of the same species varies from their migratory track due to abnormal wind fluctuations, they may end up in new environment. If the food source is such that only birds of the population with a variant beak are able to feed, then this trait will evolve by virtue of its selective survival advantage. The same species in the original geographical location and having the original food source do not require this beak trait and will, therefore, evolve differently.
Divergent evolution has also occurred in the red fox and the kit fox. While the kit fox lives in the desert where its coat helps disguise it from its predators, the red fox lives in forests, where the red coat blends into its surroundings. In the desert, the heat makes it difficult for animals to eliminate body heat. The ears of the kit fox have evolved to have greater surface area so that the fox can more efficiently remove excess body heat. The different evolutionary fates of these two related animals are determined primarily on the different environmental conditions and adaptation requirements, not on genetic differences. If they were in the same environment, it is likely that their ears would not have diverged. Divergent evolution is confirmed by DNA analysis, by which species that diverged can be shown to be genetically similar.
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Bryan Cobb, PhD