adaptive radiation

Four of the 14 finch species found on the Galápagos Archipelago, are thought to have evolved by an adaptive radiation that diversified their beak shapes to adapt them to different food sources.

In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage.^[1] 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 with which they can exploit a range of divergent environments. ^[1]

Adaptive radiation, a characteristic example of cladogenesis, can be graphically illustrated as a "bush", or clade, of coexisting species (on the tree of life). ^[2]

Identification

Four features can be used to identify an adaptive radiation:^[1]

1. A common ancestry of component species: specifically a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.
2. A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.
3. Trait utility: the performance or fitness advantages of trait values in their corresponding environments.
4. Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.

Causes

Innovation

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.^[3] 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.^[4] Other examples include placental gestation (for eutherian mammals), or bipedal locomotion (in hominins).^[2]

Opportunity

Adaptive radiations often occur as a result of an organism arising in an environment with unoccupied niches, such as a newly formed lake or isolated island chain. The colonizing population may diversify rapidly to take advantage of all possible niches.

In Lake Victoria, an isolated lake which formed recently in the African rift valley, over 300 species of cichlid fish adaptively radiated from one parent species in just 15,000 years.

Adaptive radiations commonly follow mass extinctions: following an extinction, many niches are left vacant. A classic example of this is the replacement of the non-avian dinosaurs with mammals at the end of the Cretaceous, and of brachiopods by bivalves at the Permo-Triassic boundary.

See also

• Evolutionary radiation—a more general term to describe any radiation
• Cambrian explosion—the most famous evolutionary radiation
• List of adaptive radiated Hawaiian honeycreepers by form
• List of adaptive radiated marsupials by form

References

Further reading

• Wilson, E. et al. Life on Earth, by Wilson, E.; Eisner, T.; Briggs, W.; Dickerson, R.; Metzenberg, R.; O'brien,R.; Susman, M.; Boggs, W.; (Sinauer Associates, Inc., Publishers, Stamford, Connecticut), c 1974. Chapters: The Multiplication of Species; Biogeography, pp 824–877. 40 Graphs, w species pictures, also Tables, Photos, etc. Includes Galápagos Islands, Hawaii, and Australia subcontinent, (plus St. Helena Island, etc.).
• Leakey, Richard. The Origin of Humankind—on adaptive radiation in biology and human evolution, pp. 28–32, 1994, Orion Publishing.
• Grant, P.R. 1999. The ecology and evolution of Darwin's Finches. Princeton University Press, Princeton, NJ.
• Mayr, Ernst. 2001. What evolution is. Basic Books, New York, NY.
• Kemp, A.C. 1978. A review of the hornbills: biology and radiation. The Living Bird 17: 105–136.
• Gavrilets, S. and A. Vose. 2005. Dynamic patterns of adaptive radiation Proc. Natl. Acad. Sci. USA 102: 18040-18045.
• Gavrilets, S. and A. Vose. 2009. Dynamic patterns of adaptive radiation: evolution of mating preferences. In Butlin, RK, J Bridle, and D Schluter (eds) Speciation and Patterns of Diversity, Cambridge University Press, pp. 102-126.