The exact form organisms will evolve into in the future is very nearly impossible to predict. Evolution is usually described as being opportunistic, rather than deterministic, and so species' gene pools tend to adapt to the surrounding environment. Insofar as the environment of the future is difficult to predict, the forces driving natural selection also cannot be predicted.Continue Reading
In the 19th century, some evolutionary theorists experimented with the idea of orthogenesis, or straight-line evolution, which postulated that evolutionary trends took on a momentum of their own. A thorough understanding of the way selection works to shape gene pools, however, discredits this idea. In each generation, the individuals who thrive and reproduce are acted upon by environmental factors such as climate, endemic diseases and predators. A trend that has persisted for generations can be reversed very quickly if any of these environmental factors change.
Given the plasticity of gene pools to the environment around them, and given the large amount of random variation in a population, the exact evolutionary path any given species takes is almost impossible to know in advance. The embryological development of organisms does impose some constraints on the forms species can take, such as whales being unable to re-evolve gills, but within these restraints, the potential variation to evolve what Darwin called "endless forms most beautiful" is effectively limitless.Learn more about Zoology