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Euphytica 2006 148 35 46 DOI 10 1007 s10681 006 5939 3 C Springer 2006 Hybridization as a stimulus for the evolution of invasiveness in plants Norman C Ellstrand1 Kristina A Schierenbeck2 1 Department of Botany and Plant Sciences and Center for Conservation Biology University of California Riverside CA 92521 0124 USA 2 Department of Biology California State University Chico CA 93740 USA author for correspondence e mail ellstrand ucrac1 ucr edu Key words evolution gene flow hybrid species hybridization invasive plants weeds Summary Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change Likewise they are increasingly important economically and environmentally as pests Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena Many of those species that become successful only do so i after an unusually long lag time after initial arrival and or ii after multiple introductions We propose an evolutionary mechanism that may account for these observations Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur Our examples demonstrate that invasiveness can evolve Our model does not represent the only evolutionary pathway to invasiveness but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants Introduction Invasive species have always held a special place for ecologists and evolutionary biologists Successful invaders that have colonized new regions within historical time provide real life examples of ecological and evolutionary change The demographic change from a small number of colonists to a sweeping wave of invaders is a dramatic ecological event Likewise those demographic changes a founder event followed by a massive increase in numbers may have dramatic This paper was presented at the National Academy of Sciences colloquium Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins held January 27 29 2000 at the Arnold and Mabel Beckman Center in Irvine CA Reprinted from PNAS 97 13 7043 7050 2000 evolutionary consequences Not surprisingly whole books have been dedicated to the basic science of invasive species for example see refs Elton 1958 Mooney Drake 1986 Also the applied

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