Adaptive Radiation

Adaptive radiation is a term used to describe a period in evolutionary history when a single or a few species diversified and underwent relatively rapid speciation. The event is distinguished from the background pattern of evolution in two ways: in pace and in the extent of diversification. These deviations from the slow and gradual flow of evolution are “adaptive” in that they are driven by natural selection as new species expand into vacant ecological niches. They are “radiation” events in that they result in expansion of the taxonomic group from one or a few to several or many species.

This rapid speciation is often accompanied by an increase in extinction rates as well, because not all the initial emergent forms are successful. For example, if one were to consider an incomplete assemblage of the fossil record, a seemingly linear history could be traced to today's genus of horses (Equus) from its ancestor Hyracotherium some 55 mya (million years ago). However, the true lineage of this group of mammals actually involves several branching events with subsequent extinctions. Rather than a straight line from one species evolving into the next, its history resembles a bush with many branches cut short and a rather crooked path leading from Hyracotherium to Equus. Adaptive radiations can be considered times of evolutionary experimentation, when many new traits are tested but only a fraction survive the process of natural selection. The Cambrian explosion was a rapid diversification in animal body forms that occurred between 545 and 525 mya. Over the past 100 years, studies of the fossil record of one particular area, the Burgess Shale in the Canadian Rocky Mountains, have provided unique insights into this process. Of the 60,000 unique fossils found at the location, most represent species now extinct, with some of them not clearly related to any animals existing today.

Adaptive radiations typically follow one of three events: (1) the emergence of an evolutionary innovation, (2) colonization of a new habitat, or (3) a mass extinction. In each case, relatively rapid speciation is driven by natural selection favoring individuals that have escaped their competitors and/or reached unoccupied ecological niches. An ecological niche is defined as the totality of an individual's role in its environment, including its habitat requirements and competitive constraints.

First, an adaptive radiation can follow the evolution of an innovative characteristic that gives a taxon access to a novel niche. These radiations are typically widespread, even worldwide, rather than regional. Examples include the radiations following the colonization of land by plants, insects, and tetrapods, each involving the evolution of innovative characters facilitating success on land. A more specific case would be the evolution of flight in an ancestral dinosaur that led to the radiation of birds, which are now the most diverse terrestrial vertebrates, with approximately 10,000 species. Another example is the radiation of angiosperms (flowering plants) into more than 250,000 species after divergence from gymnosperms between 200 and 140 mya.

Second, a population may reach a novel habitat by colonizing a relatively empty system free from traditional competitors, such as an island. Adaptive radiation can be seen in the divergence following the initial colonization of these relatively isolated ecosystems. Examples of these regional events are found in archipelagoes and island-like systems around the world. The most famous example are the finches of the Galapagos, which served as a partial muse for Charles Darwin's theory of natural selection and descent with modification. Current evidence suggests that an ancestral population of approximately 30 individuals colonized the islands some 2.3 mya, having been carried on ocean wind currents, and then evolved to include [Page 15]the 14 species known today. An example of an island-like system that displays the same principle can be found in the cichlid fishes in Lakes Malawi, Victoria, and Tanganyika in East Africa. The genetic and geographic origin of these fishes appears to be Lake Tanganyika, with subsequent speciation leading to 2,500 species in East Africa in as little as 100,000 years.

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