restricted access 22. Rapid Natural and Anthropogenic Diet Evolution: Three Examples from Checkerspot Butterflies
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311 What are the causes of bouts of rapid evolution and selective sweeps in the field, and what proportion of such changes in managed or “pristine” landscapes are nowadays anthropogenic? Evolutionary events may occur independently of environmental change, as when a mutation or hybridization confers increased fitness in existing environments or when an immigrant arrives in a population suffering from inbreeding depression. Alternatively, evolution may comprise responses to environmental changes— changes that may be natural but are more and more often anthropogenic. Anthropogenic triggers of evolution may be obvious, as in the introduction of exotic species that both evolve adaptation to their novel habitats and cause evolution of native species with which they interact (Maron and Vila 2001; Carroll et al. 2005; Schlaepfer et al. 2005). However , they may be less than obvious, as in the gradual but near-universal pressure of climate change (Hill et al. 1999; Parmesan et al. 1999; Pounds et al. 1999; Huey et al. 2000; Bradshaw and Holzapfel 2001; Warren et al. 2001; Bale et al. 2002; Parmesan and Yohe 2003; Root et al. 2003, 2004; Parmesan 2005) or anthropogenic changes in the nutritional value of native plants that affect their interactions with insects (Jefferies and Maron 1997; Spiller and Agrawal 2001). Here, we describe a small but unbiased sample of evolutionary changes: three evolutionary host shifts undertaken by a single butterfly species, Edith’s checkerspot, Euphydryas editha (Nymphalidae: Melitaeinae). The first host shift is a straightforward and clearly anthropogenic event at Schneider ’s Meadow (Carson City, NV) triggered by introduction of an exotic. The second, at Rabbit Meadow (Sequoia National Forest, CA) is also anthropogenic but less obviously so, since it is caused by changes in quality and distribution of native plants. The third host shift, at Sonora Junction CA, is a natural one, in which a natural population extinction and recolonization drove an expansion of diet breadth when a previously rejected host was incorporated into the diet. These observations of host shifts were made by repeatedly censusing naturally laid eggs and larvae of E. editha at more than 50 sites, many of which are shown in (Fig. 22.1). Only sites that have shown substantial changes of diet are depicted; many sites that have been repeatedly censused and have shown relatively little change are not depicted on Fig. 22.1. Checkerspot Butterflies as a Study System Melitaeine butterflies, known as fritillaries in Europe and as checkerspots in the United States, are nonmigratory, relatively sedentary insects that often occur as discrete populations or classical metapopulations in patchily distributed habitats (Ehrlich 1965; Harrison et al. 1988; Hanski 1999; Wahlberg 2000). The relatively low dispersal rate of melitaeines has had two consequences for their scientific study. First, they are good subjects for mark-release-recapture since they normally move over distances than can be readily traveled on foot by an individual Homo sapiens. Butterflies that have been marked can be released and found again, days or even weeks later, either in the same patch where they were marked or in a nearby habitat. This may explain why melitaeines were already the focus of population dynamic studies in the 1930s (Ford 1945) and why the very first field studies of animal metapopulation dynamics were done by Paul Ehrlich with E. editha in the 1960s (Ehrlich 1965). The second consequence of low dispersal is that substantial local adaptation can be more readily studied than in species with more open population structure (Rausher 1982; Singer et al. 1992a; Singer and Parmesan 1993). We might hypothesize that a species that roams the landscape widely should be adapted in a general way to the set of habitats that it encounters. This does not seem like a very testable hypothesis, since in practice we can only ask to what extent TW E NTY-TWO Rapid Natural and Anthropogenic Diet Evolution: Three Examples From Checkerspot Butterflies MICHAEL C. SINGER, BRIAN WEE, SARA HAWKINS, AND MARIE BUTCHER a population is adapted to specific habitats and we normally cannot test adapation at a very large number of sites. The evolutionary forces that adapt such insects to their habitats operate at scales that are not readily studied. On the other hand, we have a much better chance of understanding the local adaptation of sedentary species such as checkerspots. The degree of adaptation of a checkerspot population to its habitat may be influenced by its recent dynamic history, which can be studied (Hanski 1999), or by gene flow emanating...


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