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Discussion 2 - Grant and Grant 2006

Discussion 2 - Grant and Grant 2006 - REPORTS Evolution of...

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Evolution of Character Displacement in Darwin’s Finches Peter R. Grant* and B. Rosemary Grant Competitor species can have evolutionary effects on each other that result in ecological character displacement; that is, divergence in resource-exploiting traits such as jaws and beaks. Nevertheless, the process of character displacement occurring in nature, from the initial encounter of competitors to the evolutionary change in one or more of them, has not previously been investigated. Here we report that a Darwin’s finch species ( Geospiza fortis ) on an undisturbed Gala ´pagos island diverged in beak size from a competitor species ( G. magnirostris ) 22 years after the competitor’s arrival, when they jointly and severely depleted the food supply. The observed evolutionary response to natural selection was the strongest recorded in 33 years of study, and close to the value predicted from the high heritability of beak size. These findings support the role of competition in models of community assembly, speciation, and adaptive radiations. C haracter displacement ( 1 , 2 ) is an evolu- tionary divergence in resource-exploiting traits such as jaws and beaks that is caused by interspecific competition ( 3–5 ). It has the potential to explain nonrandom patterns of co-occurrence and morphological differences between coexisting species ( 6–10 ). Supporting evidence has come from phylogenetic analyses ( 11 ) and from experimental studies of stickle- backs, in which the role of directional selection in character divergence has been demonstrated ( 12 ). The process of character displacement occurring in nature, from the initial encounter of competitors to the evolutionary change in one or more of them as a result of direction- al natural selection, has not previously been investigated. The situation on the small Gal " pagos island of Daphne Major (0.34 km 2 ) has been referred to as the classical case of character release ( 1 , 2 , 13 ), which is the converse of character displacement. Here, in the virtual absence of the small ground finch ( Geospiza fuliginosa ; weighing È 12 g) and released from compe- tition, the medium ground finch ( G. fortis ; È 18 g) is unusually small in beak and body size. Lack ( 14 ) proposed that its small size reflects an evolutionary shift enabling G. fortis to take maximum advantage of small seeds made available by the absence of its competitor. Sub- sequent field studies demonstrated an associa- tion, previously only inferred, between beak sizes and seed diets ( 13 , 15 ). In 1977, a drought on Daphne revealed that small seeds are pre- ferred when they are abundant, but when they are scarce, finches turn increasingly to large and hard seeds that only the large-beaked mem- bers of the population can crack ( 13 , 15 ). Most finches died that year, and mortality was heavi- est among those with small beaks ( 13 , 16 , 17 ).
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