Utter et al.: Genetic isolation of Oncorhynchus tshawytscha of Snake and Klamath Rivers 



775 



respectively. The present data, then, clearly identify 

 two genetically-distinct groups on the basis of the 30 

 polymorphic loci that were examined. 



This genetic distinction clearly rejects a hypothesis 

 of a recent common ancestry for populations of these 

 regions. The topography of the clustering within 

 Klamath and Snake River groups and the relative 

 genetic distance between them are very similar to those 

 distinguishing Klamath River populations from other 

 genetically-distinct population groups of California and 

 the Oregon Coast based on a similar set of polymor- 

 phic loci (Hartley et al. 1992). 



Comparison with previous information 



Because the clear separation of Snake and Klamath 

 River populations reported here contrasts sharply with 

 the minimal differences detected between these groups 

 by Utter et al. (1989), an examination of results from 

 that earlier study is warranted. A direct comparison 

 of the original study with the two more recent studies 

 is complicated by (1) the addition of a number of new 

 gene loci in the more recent studies, (2) the greater 

 discriminatory capabilities for some loci used in the 

 newer studies, and (3) the more extensive sampling of 

 populations in the newer studies. A comparison of the 

 15 loci common to both the original and more recent 

 studies was made for the five Snake River sampling 

 sites (81, S2, S5, S6, S8) and two Klamath River sites 

 (K6, KIO) that were sampled in both investigations. In 

 general, very similar allele frequencies were found at 

 most loci in the two sets of samples (Table 3). None of 

 the allele frequency differences between the original 

 and the more recent studies exceeded 0.06 (atPEPA* 

 in the Klamath River comparisons). Thus, the more re- 

 cent samples confirm the minimal differences between 

 the two regions reported by Utter et al. (1989) based 

 on the loci and populations originally examined. 



The improved resolution in the more recent studies, 

 therefore, can be attributed to an increase in the num- 

 ber and type of usable genetic characters. Particular- 

 ly important was the addition of 15 gene loci not in- 

 cluded in the earlier study (Table 3). Although regional 

 differences are strongest at mMDH-2* and sMEP-1*, 

 clear contrasts between the regions are also seen 

 at five other loci {mAH-Jt* . GAPDH-3*, HAGH*, 

 PEP-LT*, and TPI4*). In addition, the more recent 

 studies resolve individual loci that had previously been 

 considered isolocus pairs, which further enhanced the 

 discriminating power of two genetic systems. This ef- 

 fect was most apparent for the enzyme IDH. Utter et 

 al. (1989), as have other previous studies (e.g.. Utter 

 et al. 1987), reported variation for the isolocus pair 

 sIDHP-1,2* ; subsequently, Shaklee et al. (1990) showed 

 that it is possible to resolve the two loci individually. 



Whereas the most extreme frequency difference 

 between the two regions at sIDHP-1,2* was 0.087 

 (1.0-0.913; Table 3) in the original study, the maximum 

 difference at sIDHP-1* in the newer studies was 0.217 

 (1.0-0.783). Similarly, the protocol of Gall et al. (1989) 

 for partitioning variation at the PGM-1,2* isolocus 

 increased the discriminatory power of this genetic 

 system. 



General implications of the results 



During the 1960s, the newly found capability to resolve 

 numerous genetic systems exhibiting Mendelian in- 

 heritance led to a flood of studies that continues to this 

 day (see Lewontin 1991). Protein electrophoresis has 

 been used extensively in fishery research and manage- 

 ment (Utter 1991); such data have proven particularly 

 useful in modifying previously held assumptions about 

 the genetic structure of fish species (Allendorf et al. 

 1987). The results discussed here are instructive with 

 regard to both the power and the limitations of such 

 information. 



The power of Mendelian data lies in the identifica- 

 tion of genetic differences among individuals, popula- 

 tions and species. The regional differences among 

 populations of North American chinook salmon orig- 

 inally described by Utter et al. (1989) have also been 

 apparent in subsequent studies (Hartley and Gall 1990, 

 Waples et al. 1991, Bartley et al. 1992). These differ- 

 ences have generally been interpreted to reflect more 

 recent ancestries of populations within a particular 

 genetically-defined region than between populations of 

 different regions. 



However, in spite of the power of electrophoretic 

 data to detect genetic differences when present, there 

 are limits to the conclusions that one can draw from 

 the failure to detect such differences. That is, although 

 a finding of a statistically-significant allele frequency 

 difference may provide evidence that gene flow is 

 restricted (or that some other evolutionary force is 

 operating), the inability to identify such differences 

 does not prove that genetic differences do not exist. 

 The present example, in which genetically divergent 

 groups were not well distinguished in a previous study, 

 emphasizes the potential significance of this limitation. 

 Although Utter et al. (1989) hypothesized that the ap- 

 parent similarity between Klamath and Snake River 

 chinook salmon was a coincidence that did not reflect 

 a common ancestral origin, the distinctness of the two 

 groups could not be demonstrated until new data 

 became available. The situation is analogous to a 

 classical genetic comparison between populations of 

 Drosophila pseudoobscura from Berkeley, California 

 and Bogata, Colombia, in which an initial apparent 

 genetic similarity was puzzling in view of the exten- 



