500 years ago, or following the most recent eruption (77 years ago). Sockeye in Aniakchak Caldera use two breeding habitats, the outlet and the beaches, which are spatially separated by less than 1.5 km. These populations are genetically distinct (FST = 0.01; P < 0.001) and together form a clade that is distinct from other populations in the area (Pavey et al. 2007). There are several different scenarios that could have resulted in this current situa- tion. First, divergence may have occurred prior to colonization of the caldera with two different source populations; that is, a pop- ulation adapted to breeding in outlets colonized the outlet and a population adapted to breeding at beaches colonized the beaches. Subsequent recent gene flow may result in convergence at neutral loci, whereas divergent ecology maintains adaptive differences. In this “two-source” model, we would expect divergence time to be considerably earlier than caldera access, perhaps >10,000 years ago when most sockeye divergence occurred following glacial re- treat. In this case, the ecology and proximity of the current habitats are not informative about population divergence. Alternatively, these populations may represent a mono- phyletic group that colonized the caldera and subsequently di- verged in response to selection, which would yield a more recent time for divergence in comparison to the timing of colonization. This colonization prior to divergence can be considered under two scenarios that describe relatively different roles of migra- tion in divergence (Rundle and Nosil 2005; Xie et al. 2007). In the first of these two scenarios, migration was greatly restricted through a double invasion of the habitat from a common source. In this scenario, colonization and local adaptation to one habi- tat occurred first, followed by a second invasion from a com- mon source that colonized the unoccupied habitat (Schluter et al. 2001). Alternatively, in the second scenario, colonization may have occurred only once and populations diverged despite gene flow (Johannesson 2001). In this study, we first measured ecological parameters of the breeding habitats (substrate size and rate of water flow) of these recently colonized populations. Next, we measured adult body depth and egg mass, morphological characters important to reproduction and shown to be correlated in relation to these eco- logical differences in many populations across the species range. Then, through applying coalescent techniques to a microsatellite database (Hey and Nielsen 2004; Won and Hey 2005), we esti- mated the time of onset of population divergence to see if the data suggest that the divergence took place after the availability of the habitat. Finally, we determined whether migration occurred after the onset of divergence and the relative timing of any migration events. If there is no detectible migration after divergence, the hypothesis of double colonization is supported. Migration after divergence is consistent with the hypothesis of ecological diver- gence despite gene flow. 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 Material and Methods ECOLOGICAL PARAMETERS We determined average water velocity of the outlet from cross- sectional area and previously recorded discharge (Bennett 2004). Substrate composition was determined by Wolman pebble counts (Quinn et al. 1995). MORPHOLOGICAL TRAITS Adult body shape We captured 301 breeding adult males by net in 2001–2003. All measurements were to the nearest millimeter. Sampling consisted of measuring midorbital to hypural length (MOH; body length) and body depth at the anterior insertion of the dorsal fin. Body depth of breeding males was compared between habitats (outlet and beach) by analysis of covariance (ANCOVA). The model in- cluded year to account for variation in overall growth and size among the different sample years, and MOH as a covariate. In addition to the measurements, we assigned spawning condition to one of three categories for each individual. We recorded males as prespawning if the fish was bright red and in good physical con- dition, but not expressing milt under gentle abdominal pressure. Males still in good physical condition but expressing milt were judged to be spawning, and males with extensive scarring, worn away skin, and showing a lack of slime production were catego- rized as senescent. We did not sample sockeye salmon showing silver coloration, as this indicates that they are still immature and their eventual spawning location and mature body shape are not finalized at that point. Egg mass Females were captured in August 2006 during spawning activity by net in the same manner as males were captured above, and MOH was measured in the same manner as for males. About 20 eggs were taken from each of 50 females at the beach habitats and 30 females at the outlet. We selected only females that were expressing eggs upon abdominal pressure. Eggs were preserved in 10% formalin. Back in the laboratory we blotted each batch of eggs with a Kimwipe (Kimberly-Clark, Dallas, TX) to remove external formalin solution. Then we measured each group of eggs to the nearest 0.1 mg. The source population of each sample was concealed during measurement. Of the 80 females for which egg samples were obtained, we eliminated 17 samples from the beach collection and six samples from the outlet collection due to connective tissue attachment. We excluded eggs with adhesions or that did not freely separate from one another. These samples may represent incomplete development so the eggs and the females that they came from were removed from the analysis. With each sample, we divided the total mass of all the eggs by the number of eggs collected to get an average mass. Egg mass was compared between habitats by ANCOVA. The model included MOH as a EVOLUTION JUNE 2010 1 7 7 5