Table 7. Comparison of demographic parameters obtained with IM and MIGRATE programs, including high point, mean, and 95% credibility intervals for effective population size and migration rate for both populations. Program IM run 1 MIGRATE Parameter NE beach NE outlet m outlet to beach m beach to outlet NE beach NE outlet m outlet to beach m beach to outlet High point 794 528 0.00010 0.00024 1979 1062 0.00350 0.00622 Egg size of female sockeye salmon is also differentiated among other sockeye populations since the last glaciation, with females that breed over larger substrate generally having larger eggs (Quinn et al. 1995). The substrate size along the breeding areas in Aniakchak Caldera is quite different between the beaches and the outlet river. The egg size of females in these locations has diverged in the manner that was expected; females breeding in larger substrate had larger eggs. Both egg mass and body shape in salmon have genetic com- ponents (Gall and Huang 1988; Su et al. 1997; Kinnison et al. 2001, 2003; Gall and Neira 2004), but can also vary due to plas- tic responses. Outlet breeding salmon expend more energy af- ter migration, which could lead to smaller eggs (Kinnison et al. 2001) and shallower bodies (Kinnison et al. 2003; Crossin et al. 2004). The difference in egg mass that we document here is in the opposite direction, whereas the differences in body size are consistent with energetic trade-offs. We also note that many of the environments experienced by these populations are similar. Incubating and spawning environments differ, but both popula- tions have access to the same lake environments, migrate down the same river and the same distance to the ocean, have access to the same ocean environment, and again migrate up the same river for the same distance back to Surprise Lake. These populations have access to the same habitats, and the differences experienced are a consequence of an individual’s choice, with the exception of incubation and emergence habitats, which are a consequence of an individual’s parent’s choice. Because we did not perform common garden rearing experiments, we cannot exclude the al- ternative hypothesis that phenotypic plasticity contributed to our measured morphological differences. The observed pattern of divergence in body depth and egg size, taken together with a heritable basis for these traits estab- lished in closely related species, and the expected plastic response of egg size in the opposite direction of our measured difference, suggests some element of genetic divergence in these traits. How- ever, the relatively small degree of divergence observed in these traits relative to other studies may have a number of causes. First, R E C E N T D I V E R G E N C E D E S P I T E M I G R AT I O N Mean 1384 961 0.00583 0.00638 2208 2147 0.00335 0.00625 95 Low 390 311 0.00018 0.00045 1417 417 0.00190 0.00486 95 High 3889 3098 0.03115 0.03506 2458 1875 0.00439 0.00747 it may reflect relatively similar optimal phenotypes for the two en- vironments in question. Second, it may result from the relatively recent divergence of these populations, and reflect that they have not yet reached the phenotypes that would be optimal for their breeding habitats. Third, it may be a plastic response that has not been previously described that is in the opposite direction as found in Kinnison et al. (2001). Fourth, selection in this case for this trait may be relatively weak. Finally, it may result from migration, and resulting gene flow, between the habitats constraining greater divergence. DIVERGENCE WITH MIGRATION Our estimates obtained from the IM model indicated that diver- gence began recently (389–503 years; 78–100 generations ago) in a time frame that coincides with the 500 year old eruption event, and that migration occurred since (m = 0.003–0.006). Our IM mi- gration and NE estimates were similarly estimated in MIGRATE. The MIGRATE 95% credibility intervals for all estimated pa- rameters are within the bounds of the IM credibility intervals. The actual high point and mean parameter estimates are slightly higher than the IM estimates. Some differences are to be expected because MIGRATE does not have a time since divergence pa- rameter in the model, but the overall convergence of the estimates suggest that demographic processes are more important in shaping the genetic structure than the recent common ancestry. These results allow us to reject our first scenario of two sources that diverged long ago with an unknown geographic re- lationship. Also, our measurement of migration since divergence suggests that migration is present in this ongoing divergence. To assess the relative importance of allopatry and sympatry, we com- pared the estimates of time of divergence onset with the average time of migration (Tables 4 and 5). The time of average migra- tion was estimated approximately midway between the estimated onset of divergence and time of sample collection in 2001–2003. This occurred in two of three runs in the comparison of high point posterior probabilities and three of three runs in the com- parison of average posterior probabilities (Tables 4 and 5). These EVOLUTION JUNE 2010 1 7 8 1