S C OT T A . PAV E Y E T A L . process can occur rather quickly (Hendry et al. 2007), but cases involving populations that were not the result of introductions or manipulations by humans are rare (Diamond et al. 1989; Carroll et al. 1997). Here we present an example of ecological divergence following colonization that is both recent (∼500 years, ∼100 generations) and occurring with migration. Sockeye salmon (Oncorhynchus nerka) reproduce in fresh- water habitats throughout much of the North Pacific region (Burgner 1991). Several adult phenotypic traits are highly cor- related with breeding environment and are believed to be the re- sult of parallel evolution (Burgner 1991; Taylor 1991; Blair et al. 1993). Recent work has shown that adult body size and shape in sockeye are strongly related to depth and water velocity of their breeding habitat (Quinn et al. 2001). In general, sockeye males breeding along lake beaches have deeper bodies than those breed- ing in riverine habitats (Blair et al. 1993; Hendry et al. 2000). This appears to be a response to natural and sexual selection in the breeding environments (Quinn et al. 2001; Hamon and Foote 2005). Sockeye egg mass is correlated with breeding substrate size (Quinn et al. 1995). Because these traits are adaptations to the ecology of the site of reproduction, they may be traits that are directly responsible for reproductive isolation (Schluter 2001; Rundle and Nosil 2005). Aniakchak National Monument and Preserve (ANMP; Fig. 1) in southwest Alaska contains the most active volcano in the Eastern Aleutian arc, having erupted more than 40 times in the last 10,000 years (Neal et al. 2001). Several of these cataclysmic geologic events are well documented and provide a framework to evaluate the timing of divergence. A massive volcanic eruption 3650 years ago formed a large caldera (Ani- akchak Caldera) that filled with water forming a lake (McGimsey et al. 1994; Pearce et al. 2004). Sometime after this but before a more recent eruption that occurred 500 (standard error [SE] 369–565) years ago, the caldera wall collapsed resulting in a large flood and the formation of the Aniakchak River, which connects the caldera lake (Surprise Lake; elevation 321 m) with the Pacific Ocean through “The Gates,” a chasm breaching the caldera wall (McGimsey et al. 1994) (see Fig. 1). Sometime after this connection, sockeye salmon colonized Surprise Lake. In addition to the well-documented eruptions mentioned above, the volcano erupted again in 1931 (McGimsey et al. 1994). These eruptions probably affected breeding, rearing, and incu- bating conditions and may have impacted or eliminated any sockeye populations present in the caldera during that time. In fact, much of the inlet waters to the lake are presently devoid of dissolved oxygen as a result of volcanic activity (Cameron and Larson 1993) and a large portion of the associated beaches are unused by breeding sockeye. Current sockeye populations in Aniakchak Caldera may have colonized after the original ocean access following the flood, after the substantial eruption Figure 1. Aniakchak National Monument and Preserve (ANMP) showing Aniakchak Caldera and the two study populations. 1 7 7 4 EVOLUTION JUNE 2010