2004 The Society for the Study of Evolution. All rights reserved.
Evolution, 58(4), 2004, pp. 814824
TWELVE YEARS OF CONTEMPORARY ARMOR EVOLUTION IN A THREESPINE
MICHAEL A. BELL,1,2 WINDSOR E. AGUIRRE,1,3 AND NATHANIEL J. BUCK4,5
1Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794-5245
4Department of Natural Sciences, Long Island University, Southampton College, Southampton, New York 11968-4196
Abstract. Loberg Lake, Alaska was colonized by sea-run Gasterosteus aculeatus between 1983 and 1988, after the
original stickleback population was exterminated. Annual samples from 1990 to 2001 reveal substantial evolution of
lateral plate (armor) phenotypes. The 1990 sample was nearly monomorphic for the complete plate morph, which is
monomorphic in local sea-run populations; the low plate morph, which is usually monomorphic in local freshwater
populations, was absent. By 2001, the frequency of completes had declined to 11%, and lows had increased to 75%.
The partial plate morph and two unusual intermediate plate phenotypes were generally rare, but occurrence of the
intermediates was unexpected. These intermediate phenotypes rarely occur in other, presumably older, polymorphic
populations. When low morphs first appeared, they averaged 6.8 plates per side, indicating that the ancestral plate
number of low morphs is high, and their mean has subsequently declined. Contemporary evolution in this population
indicates that threespine stickleback adapt to freshwater habitats within decades after invasion from the ocean, and