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Modes of Speciation

Author(s): Tadzia GrandPre, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Rate of Speciation

We have considered some of the modes, mechanisms and evolutionary forces that play a role in the formation of new species, but we have not addressed how long the process of speciation takes. This question is difficult to address scientifically because, although the full process of speciation can happen almost instantaneously through polyploidy, in the vast majority of cases the evolutionary changes that lead to speciation occur over large periods of time. It is therefore difficult (if not impossible) for scientists to study speciation rates directly. In theory, this problem could be at least partially resolved by studying the fossil record, which chronicles the patterns of evolutionary change over millions of years. However, the fossil record is incomplete; only a small proportion of species that have existed have been successfully preserved as fossils, and many of those fossils have either been damaged or have simply not yet been discovered.

Due to these inherent problems, scientists who are interested in understanding the pace of speciation and the factors that influence it must consider information from a variety of sources including the fossil record, anatomical studies of living organisms, and the fields of ecology and genetics. These studies suggest that a number of factors influence speciation rates including the degree to which a population is geographically isolated, the novelty and stability of the environment, the size of the population, and the amount of genetic variability that exists within the population. As a result, the rate of speciation can vary widely, both between populations and within a single population over time. Some populations of snapping shrimp (Alpheus) that are found on opposite sides of the Isthmus of Panama have not yet achieved full reproductive incompatibility, despite the fact that they have been separated ever since the isthmus emerged, approximately three million years ago. In contrast, it is estimated that for some species of Hawaiian Drosophila, speciation occurred in less than 800,000 years.