S C OT T A . PAV E Y E T A L . Table 5. Comparison of the high point and average values of the posterior distribution of divergence and average migration dates in years before sample collection for all three runs of the IM program. Direction of migration Outlet to beach Beach to outlet Run 1 Run 2 Run 3 Mean SD Run 1 Run 2 Run 3 Mean SD High point 44 120 61 74.9 (39.7) 42 99 56 66.0 (29.7) SD of five loci (3.4) (31.2) (2.8) (2.8) (28.1) (3.5) Average 177 276 205 219.5 (51.1) 185 305 224 238.1 (60.8) SD of five loci (1.3) (2.3) (1.7) (2.8) (6.1) (3.3) Discussion We have described the most recent ecological divergence re- ported in a fish species following natural colonization. The di- vergence observed here is probably in a very early stage, but the morphological differences are consistent in direction with that documented for similar ecological differences in other sockeye populations. Our results indicate that migration occurred after di- vergence onset suggesting that this divergence is occurring despite migration. There are many examples of ecological divergence follow- ing natural colonization in fish including stickleback (Gasteros- teus aculeatus; Lavin and McPhail 1985, 1993; Schluter 1996b; Rundle et al. 2000; Reusch et al. 2001), lake whitefish (Core- gonus clupeaformis; Lu and Bernatchez 1999; Rogers et al. 2002; Derome et al. 2006), arctic char (Salvelinus alpinus; Gislason et al. 1999; Klemetsen et al. 2002; Johnston et al. 2004), and sockeye salmon (Blair et al. 1993; Wood and Foote 1996). However, all of these important examples are on a postglacial retreat timescale (∼10,000–15,000 years). The present study demonstrates a very recent ecological divergence following natural colonization. This ecological divergence is extremely recent (∼500 years, 100 gen- Table 6. Effective migrants per generation for all three runs of the IM program. We include the high point and the average of the pos- terior probability distribution for both directions of migration. Run 1 2 3 Outlet to beach Beach to outlet High point Average High point Average 0.16 9.00 3.83 16.15 16.67 18.41 1.23 4.41 1.01 12.25 12.71 8.86 1 7 8 0 EVOLUTION JUNE 2010 Figure 7. Posterior probability of effective population sizes of beach, outlet, and ancestral populations. Solid lines are the aver- age probability for all three runs of the IM program. The ﬁelds represent the standard error of the runs for each parameter. The inset shows detail. erations), between populations of close geographic proximity (∼1500 m), and occurred despite migration. It is possible that the actual mutation rates of the microsatel- lite markers used in this analysis were different than our estimate. This difference would proportionately change our converted de- mographic parameters: time since divergence onset, NE, average date of migration, as well as migration rate would all be affected by mutation rate. However, the relationship between divergence time and time of average migration would change similarly, mak- ing this relationship between them independent of mutation rate. The estimate for effective number of migrants is also independent of mutation rate, because mutation cancels out in the conversion process. MORPHOLOGY Most sockeye salmon populations were established following glacial retreat on the order of 10,000 years ago (Wood 1995). Populations that breed in deep water along lake beaches con- sistently have greater average male body depth than populations breeding in flowing water environments (Blair et al. 1993). The differentiation of populations is variable, but for some popula- tions the body depth as a function of body length is so different that there is little overlap between habitats (Hamon et al. 2000). This is particularly the case with access limiting streams or inlets and high levels of predation (Quinn et al. 2001; Hamon and Foote 2005), which is not the situation with either of the habitats in this study. Also, gill net fisheries may impose selection on body depth (Hamon et al. 2000), but the only commercial fishery on sockeye in this study is a seine fishery. Our results indicate that the male sockeye in Aniakchak Caldera are deeper bodied along the lake beaches than in the riverine outlet breeding habitat, the predicted nature of the difference based on patterns of sockeye differentiation elsewhere.