Examining evidence of reproductive isolation in sockeye salmon.

Reproductive Isolation in Sockeye Salmon The study of speciation has recently undergone a revival, with much controversy centering on whether new species can originate quickly and within the geographic range of their ancestor. Hendry et al. (1) described a case of reproductive isolation arising between two sockeye salmon populations in only 13 generations. If true, this finding would deserve considerable attention. However, Hendry et al. have failed to make the case that the two populations of salmon are indeed reproductively isolated. First, the evidence cited by Hendry et al. for significant genetic divergence of the beach and river populations is not convincing. Their conclusion that reproductive isolation has evolved rests on a small amount of genetic differentiation between a Cedar River population of sockeye salmon in Lake Washington and a nearby beach population, both apparently founded some time after 1937. That genetic differentiation, they claimed, provides evidence for the rapid evolution of reproductive isolation in the wild (i.e., a reduced reproductive success of fish who migrate from river to beach), because the differentiation occurred despite supposedly large amounts of migration between the populations. The observed level of differentiation between the two populations at six microsatellite loci (FST 5 0.025), however, was substantially lower than the level of differentiation found among populations within most anadromous fish species [median FST 5 0.081 (2)]. Also, although the FST of 0.025 was significantly greater than zero, it was not significantly greater than the FST between Cedar River residents and Pleasant Point Beach immigrants, groups of individuals presumed to come from the same population [see overlap in 95% confidence intervals in table 1 of (1)]. Moreover, Nei’s unbiased genetic distance (D), another measure of genetic differentiation, was 0.000 between river residents and beach residents at 20 allozyme loci (3)—an indication of no perceptible difference in the combined frequency of 20 nonmicrosatellite genes. A second problem is that the evidence for substantial migration from river to beach is weak at best. Hendry et al. estimated that 39% of adults breeding at the beach were immigrants from the river. This estimate was based on natural marks found in otoliths of adults collected from Pleasant Point Beach. Otoliths of sockeye salmon born in variable thermal regimes differ from otoliths of salmon born in isothermal regimes (4). Because Pleasant Point Beach seemed to have an isothermal regime (4), Hendry et al. assumed that adult salmon collected from this population that have “variable-regime” otoliths actually originated from Cedar River, which was characterized as having a fluctuating thermal regime (4). Otoliths of fry born at Pleasant Point Beach, however, were not examined by Hendry et al. Thus, without further study, one cannot assume that fry born at this site have otoliths characteristic of an isothermal regime. In the absence of this crucial control, we can conclude nothing about the rate of migration between beach and river populations. Emphasizing the need for caution in using otoliths to estimate migration is the observation by Hendry et al. that in 1993, 21% of the fry from the Cedar River population actually had otoliths typical of salmon that had developed in isothermal conditions. The close morphological similarity between supposed beach immigrants and beach residents [figure 2 of (1)] also advises caution. Until better estimates of migration between the beach and river populations are available, it is premature to regard the low level of genetic differentiation between these populations as indicating even a slight amount of reproductive isolation. Third, the evidence that river and beach salmon evolved different sizes and shapes— differences that the authors believe may cause reproductive isolation—is nonexistent. Hendry et al. have provided no evidence that observed phenotypic differences have a genetic basis. They did not rear fish from both populations in a constant environment, yet such “common garden” experiments are essential for demonstrating whether size and shape differences represent evolved adaptations, the plasticity of genetically similar organisms developing in different environments (5, 6), or a combination of these genetic and nongenetic factors. Fourth, Hendry et al. did not adequately consider reasonable alternative explanations for genetic differentiation in the face of gene flow. For example, habitat-specific selection may be operating on either the assayed microsatellite loci or genes closely linked to them. This possibility seems plausible in view of the lack of differentiation at allozyme loci, and is strengthened if differentiation between the two populations is attributable to only one or two microsatellite loci. Hendry et al. explored this possibility by examining the effects of removing the most divergent microsatellite locus on overall FST values. They claimed that, after deleting this locus, interpopulation divergence was still substantial, but they provided no P values or 95% confidence intervals for the revised FST values [table 1 of (1)]. The absence of these significance tests may mean that, when one excludes the most divergent locus, the FST of 0.017 between river residents and beach residents is not significantly greater than zero. Such a result would implicate selection, not reproductive isolation, as the factor responsible for genetic differentiation of these populations. One should also consider (although Hendry et al. did not mention it) that native sockeye salmon existed in Lake Washington before the introduction of sockeye salmon from Baker Lake, Washington, in 1937 (3). Differential introgression of alleles from these genetically distinct native populations, which still exist in the lake, could explain the slight genetic divergence between the river and beach populations. We have no quarrel with the idea that reproductive isolation may arise quickly in the wild; indeed, we encourage research in this area. Nevertheless, much more work must be done before the sockeye salmon in Lake Washington can be seen as a compelling example of rapidly evolving reproductive isolation. These salmon may represent only populations that have evolved some genetic differences by adapting to different habitats, a common occurrence in animal species (7). But, as with Homo sapiens, most differentiated populations do not go on to become new species or even evolve any reproductive isolation. Population differentiation is not a sufficient condition for incipient speciation.