Adaptive radiation

In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits. An example of adaptive radiation would be the avian species of the Hawaiian honeycreepers. Via natural selection, these birds adapted rapidly and converged based on the different environments of the Hawaiian islands.

Much research has been done on adaptive radiation due to its dramatic effects on the diversity of a population. However, more research is needed, especially to fully understand the many factors affecting adaptive radiation. Both empirical and theoretical approaches are helpful, though each has its disadvantages. In order to procure the largest amount of data, empirical and theoretical approaches must be united.

Four features can be used to identify an adaptive radiation:

The evolution of a novel feature may permit a clade to diversify by making new areas of morphospace accessible. A classic example is the evolution of a fourth cusp in the mammalian tooth. This trait permits a vast increase in the range of foodstuffs which can be fed on. Evolution of this character has thus increased the number of ecological niches available to mammals. The trait arose a number of times in different groups during the Cenozoic, and in each instance was immediately followed by an adaptive radiation. Birds find other ways to provide for each other, i.e. the evolution of flight opened new avenues for evolution to explore, initiating an adaptive radiation. Other examples include placental gestation (for eutherian mammals), or bipedal locomotion (in hominins).

One famous example where adaptive radiation is seen is with Darwin's finches. It has been observed by many evolutionary biologists that fragmented landscapes are often a prime location for adaptive radiation to occur. The differences in geography throughout disjointed landscapes such as islands are believed to promote such diversification. Darwin's finches occupy the fragmented landscape of the Galápagos Islands and are diversified into many different species which differ in ecology, song, and morphology, specifically the size and shapes of their beaks. The first obvious explanation for these differences is allopatric speciation, speciation that occurs when populations of the same species become isolated geographically and evolve separately. Because the finches are divided amongst the islands, the birds have been evolving separately for several million years. However, this does not account for the fact that many of the species occur in sympatry, with seven or more species inhabiting the same island. This raises the question as to why these species split when living in the same environment with all the same resources. Petren, Grant, Grant, and Keller proposed that the speciation of the finches occurred in two parts: an initial, easily observable allopatric event followed by a less clear sympatric event. This sympatric event which occurred second was adaptive radiation. This occurred largely to promote specialization upon each island. One major morphological difference among species sharing one island is beak size and shape. Adaptive radiation led to the evolution of different beaks which could access different food and resources. Those with short beaks are better adapted to eating seeds on the ground, those with thin, sharp beaks eat insects, and those with long beaks use their beaks to probe for food inside cacti. With these specializations, seven or more species of finches are able to inhabit the same environments without competition or lack of resources killing several off. In other words, these morphological differences in beak size and shape brought about by adaptive radiation allow the island diversification to persist.

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