Southern California Populations Research Articles

Effects of Temperature on Fecundity and Longevity of <I>Phoracantha recurva</I> and <I>P. semipunctata</I> (Coleoptera: Cerambycidae)

The cerambycid beetles Phoracantha recurva Newman and Phoracantha semipunctata F. were accidentally introduced into southern California where they have caused significant mortality to eucalyptus trees. In recent years, P. recurva populations have increased rapidly with a concomitant decrease in P. semipunctata numbers in their shared habitat in southern California. P. recurva has been collected in field surveys earlier in the season, suggesting that this species may be active over a broader range of temperatures and consequently have a longer seasonal activity period. We conducted lifetime fecundity studies to examine egg production, egg hatch, and adult longevity of beetles of both species subjected to temperatures typical of late fall and late winter/early spring in southern California. Overall, there were no differences in lifetime fecundity between P. recurva and P. semipunctata. P. recurva did not seem to reproduce over a larger range of temperatures than P. semipunctata, and there did not seem to be any substantial differences in their reproductive biologies. P. recurva eclosed slightly faster and lived slightly longer at reproductive temperatures than P. semipunctata, suggesting that P. recurva may have a slight reproductive advantage over P. semipunctata. However, it seems unlikely that such an advantage can account for the rapid increase in P. recurva populations and the simultaneous decrease in P. semipunctata populations in southern California.

Variation in owl limpetLottia giganteapopulation structures, growth rates, and gonadal production on southern California rocky shores

The size structures of Lottia gigantea populations were studied at 8 southern California sites, 4 of which were longstanding Marine Protected Areas (MPAs). Greater mean sizes and higher frequencies of larger individuals occurred at sites with lowest human visitation. Mean L. gigantea shell lengths (SLs) were negatively correlated with the number of visitors and collectors per 10 m of shoreline. Limpets never achieved SLs greater than 79.0 mm at any of our 8 sites, a size well below the maximum for this species. Mean SLs were comparable to sizes at other sites where L. gigantea is intensely collected. The presence of collectors, small mean SLs, and the absence of large limpets sug- gest that L. gigantea populations are affected by humans at our sites. Lower densities and higher fre- quencies of larger limpets were found in a subpopulation living on open-rock surfaces compared with a subpopulation occupying smaller, patch habitats within mussel beds. Larger limpets were mostly female and gonadal mass increased exponentially with SL in both subpopulations. Limpets 8yr. Given the age of larger limpets and the potential for collectors to remove larger individuals and shift populations towards smaller size structures, effective MPAs may take decades to reverse the effects of human impacts on L. gigantea populations in southern California.

Genetic Structure of Endangered Stephens' Kangaroo Rat Populations in Southern California

Stephens' kangaroo rat (Dipodomys stephensi) was listed as an endangered species in 1988 due to significant loss of its habitat through agricultural and urban development. We examined allozyme variation in 245 individualsfrom 10 locations in Riverside and San Diego counties, California, to assess the genetic consequences of habitat fragmentation. Populations were polymorphic at 0.050 to 0.175 of the loci surveyed (mean P = 0.125), and mean heterozygosities varied from 0.005 to 0.032 (mean H = 0.019). Despite low levels of genetic diversity within populations, significant F S T values were observed at 5 of 10 polymorphic loci (mean F S T = 0.266), indicating population differentiation. Genetic distances for individual loci and across all loci were not significantly correlated with geographic distances. The patterns of genetic diversity observed suggest that habitat fragmentation has influenced population genetic structure in this species. Genetic drift and restricted gene flow stemming from the fragmentation of once contiguous populations into isolated populations are suggested as contributing factors. Demographic data suggest that periodic bottlenecks in population size may also contribute to these patterns.

Geographic and genetic variation in the leaf surface resin components of Mimulus aurantiacus from southern California

Nearly 30% of the dry weight of leaves of the chaparral subshrub Mimulus aurantiacus comprise leaf surface resins. This resin provides some defense against the insect herbivore, Euphydryas chalcedona and may also protect plants from desiccation and UV light injury. The resin is composed of several components and the different components may contribute differentially to the resin's multiple protective roles. The quantities of each of seven resin components from field-grown plants, from six southern California populations of M. aurantiacus spanning a range of water availability and differing in attack by E. chalcedona, and clones from these plants grown in a common garden, were determined. Two predictions were tested: (1) E. chalcedona will attack plant populations with relatively low quantities of total resins and/or concentrations of the ortho dihydroxy resins, and (2) plant populations in drier environments will produce resin that is relatively richer in the methoxylated components than plant populations in more mesic environments. Although resin composition differed genetically among plant populations, the pattern of variation was not as predicted. Insect attack was not associated with lower levels of ortho dihydroxy compounds, and populations with higher proportions of methoxylated flavonoids were not found in the drier sites. The pattern of genetic variation among populations in resin composition therefore was not explained by the differences in chemical structure that were predicted to influence the role of the components as herbivore protectants or antidesiccants.

Interspecific and intraspecific differences in two Liriomyza leafminer species in California

AbstractIn recent years, the pest status of Liriomyza trifolii (Burgess) and L. huidobrensis (Blanchard) (Diptera: Agromyzidae) has changed in California, as well as other areas of the world. In California, L. huidobrensis has become the predominant Liriomyza species in valleys along the central coast, while L. trifolii remains the predominant species in southern California. To investigate possible reasons for this change in status, differences in host plant use and reproductive success of intraspecific populations were examined for Liriomyza trifolii and L. huidobrensis from both central and southern California. The southern L. trifolii fed, oviposited and reproduced successfully on all five hosts tested, but the central population fed significantly less on all hosts and was restricted to reproducing on pepper only. With the exception of pepper, southern L. trifolii had significantly greater larval survival on all hosts than central L. trifolii. In contrast, the central L. huidobrensis population had greater reproductive success than the southern population of that species on all hosts plants tested. However, pepper was not a suitable host for either L. huidobrensis population. Both species showed positive assortative mating, with homotypic mating occurring more frequently than heterotypic mating; however, the difference between L. trifolii populations was much more pronounced than between L. huidobrensis populations. These data indicate that central and southern California populations of each species are distinct biotypes. Furthermore when coupled with previous genetic data, our results suggest the possible existence of cryptic species within L. trifolii.

Phylogeography of the tidewater goby, Eucyclogobius newberryi (Teleostei, Gobiidae), in coastal California.

The tidewater goby, Eucyclogobius newberryi, inhabits discrete, seasonally closed estuaries and lagoons along approximately 1500 km of California coastline. This species is euryhaline but has no explicit marine stage, yet population extirpation and recolonization data suggest tidewater gobies disperse intermittently via the sea. Analyses of mitochondrial control region and cytochrome b sequences demonstrate a deep evolutionary bifurcation in the vicinity of Los Angeles that separates southern California populations from all more northerly populations. Shallower phylogeographic breaks, in the vicinities of Seacliff, Point Buchon, Big Sur, and Point Arena segregate the northerly populations into five groups in three geographic clusters: the Point Conception and Ventura groups between Los Angeles and Point Buchon, a lone Estero Bay group from central California, and San Francisco and Cape Mendocino groups from northern California. The phylogenetic relationships between and patterns of molecular diversity within the six groups are consistent with repeated, and sometimes rapid, northward and southward range expansions out of central California caused by Quaternary climate change. Plio-Pleistocene tectonism, Quaternary coastal geography and hydrography, and historical human activities probably also influenced the modern geographic and genetic structure of E. newberryi. The phylogeography of E. newberryi is concordant with phylogeographic patterns in several other coastal California taxa, suggesting common extrinsic factors have had similar effects on different species. However, there is no evidence of a phylogeographic break coincident with a biogeographic boundary at Point Conception.

Biogeography of two species of Symbiodinium (Freudenthal) inhabiting the intertidal sea anemone Anthopleura elegantissima (Brandt).

We have analyzed the genetic profiles of dinoflagellate populations obtained from the Pacific coast sea anemone Anthopleura elegantissima (Brandt) at collection sites from Washington to California. Genetic differences within the symbiont populations of California anemones have been uncovered by restriction length polymorphism (RFLP) analysis of the small subunit (SSU) and large subunit (LSU) ribosomal RNA genes, and by denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer region 2 (ITS 2). The existence of two Symbiodinium species is substantiated by sequence analysis of the variable regions V1, V2, and V3 of the SSUrDNA, which also establishes their phylogenetic relatedness to other members of the genus Symbiodinium. Anemones from Washington and Oregon harbor a single dinoflagellate species, for which we propose the name S. muscatinei sp. nov. At these northern locations, S. muscatinei either exists alone or co-occurs with the Chlorella-like green algal symbiont. Our results indicate that S. muscatinei co-occurs with a second dinoflagellate, S. californium, in mixed populations in central and southern California. We suggest that the geographic distribution of these dinoflagellates is related to the temperature cline created by latitude.

The resolution of Californian populations of Liriomyza huidobrensis and Liriomyza trifolii (Diptera: Agromyzidae) using PCR.

Liriomyza trifolii (Burgess) and Liriomyza huidobrensis (Blanchard) are important vegetable pests in California. Populations of each species differ in their impact in central and southern regions. This difference may be explained by geographical or host plant differences in each of the regions. We used random amplified polymorphic DNA polymerase chain reactions to assess genetic differences between two laboratory populations of each species collected from central and southern California. Individual L. trifolii from the two regions could be discriminated by the presence/absence of PCR products. No such qualitative differences were apparent in PCR products amplified from L. huidobrensis individuals, but the origins of individuals could be differentiated using a bootstrap analysis of marker frequencies. Marker primers were used to compare field and laboratory individuals. No evidence was found for the existence of further populations or of hybrid populations in central and southern California. The distribution of populations of L. huidobrensis was explained completely by geographical differences. As a consequence of the absence of leafminer infestations on the same host plant varieties in both regions, factors governing L. trifolii population distribution differences were less apparent. The presence of the same host plant varieties at both sites suggests that the two L. trifolii populations differ in host plant preference.

Burrowing owls and agricultural pesticides: Evaluation of residues and risks for three populations in California, USA

AbstractWe examined selenium, organophosphorus, and organochlorine pesticide residues in egg, footwash, and feather samples from burrowing owls in three populations in central and southern California. Eggs from all sites contained detectable levels ofp,p′‐dichlorodiphenyldichloroethylene, with the San Joaquin Valley site containing up to 33 μg/g (geometric mean x¯ = 7.52). Only low levels of polychlorinated biphenyls were detected, however (geometric mean x¯ = 1.98,n= 2). Selenuim concentrations were low in all samples (geometric mean x¯ = 0.426,n= 20). Eggshells collected in 1996 were 22% thinner than eggs collected prior to 1937. In addition, feather samples contained low levels of dichlorodiphenyldichloroethylene, and footwash samples indicated exposure to the pesticide chlorpyrifos. Pesticide‐use records indicated that one population might also be at risk from applications of aldicarb near nests during the breeding season.

Genetic monitoring of malathion-exposed agricultural workers.

The aerial application of malathion over large urban populations in Southern California during the early 1990s raised concerns about adverse health effects, including the potential to cause genetic damage. Workers in the Mediterranean fruit fly eradication program, which involved application of malathion as ground treatment, were studied to examine micronucleus formation and mutation frequencies assessed by the glycophorin A (GPA) assay. In the 1992 pilot project the mean micronuclei level appeared higher in lymphocytes of exposed workers (n = 13) compared to controls (n = 4) (20.1 +/- 7.1 vs 14.3 +/- 7.2 respectively, P = 0.09). During the 1993 season, neither of the cohorts examined showed a higher level of micronuclei in workers exposed to malathion compared to unexposed, nor did the pooled total (n = 53; means = 17.8 +/- 7.2 vs 18.5 +/- 6.3, respectively), even after adjustment by multiple regression. The GPA variant frequency was not associated with malathion exposure in any of the cohorts. These results suggest that any potential risk of genotoxic damage from exposure to malathion is relatively low, but other assays may be more sensitive, and the sample size was small.

PATTERNS OF REPRODUCTION, GENETIC DIVERSITY, AND GENETIC DIFFERENTIATION IN CALIFORNIA POPULATIONS OF THE GENICULATE CORALLINE ALGA LITHOTHRIX ASPERGILLUM (RHODOPHYTA)1

ABSTRACTThe reproductive composition and genetic diversity of populations of the red seaweed Lithothrix aspergillum Gray (O. Corallinales) were studied at three southern California sites (Shaw's Cove and Treasure Island, Laguna Beach; Indian Rock, Santa Catalina Island) and at a fourth site (Bodega Bay) located in northern California. Sexually reproducing populations were confined to southern California. Diploid individuals were numerically dominant over haploid (gametophytic) individuals at all sites. Intertidal and subtidal subpopulations from Shaw's Cove differed in their reproductive profiles. Most intertidal specimens found on emersed surfaces were densely branched, turf‐forming, and bore tetrasporangial (68.6%), carposporangial (11.4%), or spermatangial (5.7%) conceptacles, reflecting a sexual life history; none produced asexual bispores. In contrast, 74.3% of the larger, loosely branched subtidal specimens bore bisporangial conceptacles indicative of asexual reproduction. Nearly 70% of the Indian Rock thalli showed no evidence of conceptacle formation. Only asexual, diploid bispore‐producing thalli were obtained from the Bodega Bay site. Genetic diversity (mean number of alleles per locus, percent of polymorphic loci, and average expected heterozygosity) of diploid L. aspergillum populations varied with life‐history characteristics and geographic location. A total of 30 alleles was inferred from zymograms of 16 loci examined by starch‐gel electrophoresis; of these loci, 11 were polymorphic. The genetic diversity of sexual, diploid populations of L. aspergillum (alleles per locus [A/L] = 1.4‐1.5; percent polymorphic loci [%P] = 37.5‐50.0) was relatively high compared with other red seaweeds. Lowest diversity (A/L = 1.0; %P = 0.0) occurred in the exclusively asexual Bodega Bay population which consisted of genetic clones. All sexual L. aspergillum populations deviated significantly from Hardy‐Wein‐berg expectations due to lower than expected heterozygosity. Genetic differentiation (Wright's Fstatistic [FST]; Nei's Genetic Distance [D]) among sexually reproducing southern California populations was low (FST= 0.030) on a local scale (ca. 5 km), suggesting high levels of gene flow, but high genetic differention (FST= 0.390 and 0.406) occurred among southern California populations separated by ca. 70 km. Very high genetic differentiation (FST= 0.583–0.683) was obtained between northern and southern California populations separated by 700–760 km. Our genetic and reproductive data suggest that the L. aspergillum population from Bodega Bay is sustained by perennation, vegetative propagation, or asexual reproduction by bispores and may represent an isolated remnant or a population established by a founder event.

Mitochondrial DNA and Nuclear Microsatellite Diversity in Hatchery and WildOncorhynchus mykissfrom Freshwater Habitats in Southern California

We examined mitochondrial control-region haplotype diversity and allelic frequency distributions for three polymorphic microsatellite loci in 541 coastal Oncorhynchus mykiss collected from six habitats associated with different levels of human activity and ocean access in southern California. Extensive urbanization, climatic unpredictability, and the accelerated rate of decline in anadromous fish suggested a probable loss of genetic diversity in this area due to habitat fragmentation, geographic isolation, and population bottlenecks. Unexpectedly high levels of genetic diversity were found in southern California populations of O. mykiss. Haplotype diversity (HS) was highest in anadromous fish (HS = 0.74) and lowest in the Whale Rock Hatchery trout (HS = 0.32). The proportion of variation attributable to population differentiation among habitat groups (GST) was 10%. Haplotype frequencies showed a close relationship between anadromous steelhead and resident rainbow trout from closed habitats (DST = 0.03). Combined microsatellite allelic diversity (at loci Omy77, Omy207, and Ssa289) was highest in rainbow trout from closed habitats (88%), and lowest in Whale Rock Hatchery fish (29%). Greatest microsatellite distance (δμ = 17.1) was between anadromous steelhead and reservoir rainbow trout, and closest identity (δμ, = 1.8) was among rainbow trout from closed habitats, hatchery rainbow trout, and reservoir rainbow trout. Analysis of genetic distance measures for both molecular markers showed that considerations of life history patterns and freshwater habitats that retain ocean access remain important factors in the preservation of the unique genetic diversity found in southern California coastal O. mykiss.

Effects of oxidant air pollutants on western pine beetle (Coleoptera: Scolytidae) populations in southern California

The attack rates, brood survival, and emergence rates of the western pine beetle, Dendroctonus brevicomis LeConte, and incidence of entomophagus associates, were compared between photochemical oxidant damaged, and apparently healthy, ponderosa pine trees, Pinus ponderosa Dougl. ex Laws in the San Bernardino Forest in Southern California. The results from this study suggest that oxidant-damaged trees attacked by western pine beetle produced about the same total brood with lower initial attacks when compared with healthier trees, whereas the numbers of predators and parasitoids were higher in the healthier trees. This higher productivity trend for western pine beetle is most evident in trees attacked by the first beetle generation. Trees attacked by the second generation, both damaged and healthy, produced much less western pine beetle brood than generation 1 attacked trees, regardless of oxidant damage. The implication of these results is that, in stands with a higher proportion of oxidant damaged trees, a given population of western pine beetle could kill more trees, and increase at a greater rate, than in a stand with a lower proportion of damaged trees.

Genetic diversity and structure in Pelvetia fastigiata (Phaeophyta: Fucales): does a small effective neighborhood size explain fine-scale genetic structure?

We investigated the genetic diversity and genetic structure of southern California populations of the common intertidal fucoid seaweed Pelvetia fastigiata, (J. Ag.) De Toni by means of allozyme electrophoresis and estimates of genetic neighborhood area and size, which are the first for seaweeds. We predicted that P. fastigiata populations would exhibit relatively low genetic diversity and high genetic structure because the seaweed is monoecious and has limited dispersal of gametes and zygotes. This prediction was supported; genetic diversity indices were all low compared to other seaweeds studied, but high genetic structure was evident particularly within individual reefs. Geospatial statistical analyses (second-order analyses) revealed clustered distribution of glucose-6-phosphate isomerase (GPI) alleles at the scale of 1 to 6 m within three reefs. The rare alleles were distributed only at the landward third of the reefs. Genetic neighborhood area (2.3 m2) and size (133 individuals) were estimated from parent-offspring dispersal distributions of gametes and zygotes from attached thalli and also detached reproductive fragments, which contributed very little to the effective neighborhood size. The neighborhood size was in the small theoretical range in which genetic drift could be responsible for the within-reef genetic structure. This result was equivocal, because the stereotyped distribution of rare alleles on the tips of each reef was highly unlikely to be due to random events (6.9×10-24). These results emphasize (1) the importance of allele mapping in addition to spatial statistics to elucidate genetic structure, and (2) that interpretation of genetic-structure statistics as evidence for gene flow can be complicated, even when supported with independent estimates of gene flow, if data are lacking on selection and sporadic migration events. The emerging pattern of low levels of polymorphisms in brown seaweeds will limit the use of Wright's F-statistics and will require alternative, more direct techniques for the analysis of mechanisms responsible for population genetic structure.

Genetic Structure of Endangered Clapper Rail (Rallus longirostris) Populations in Southern California.

We assessed the genetic structure of two subspecies of endangered Clapper Rails (Rallus longirostris) in Southern California using DNA fingerprinting to uncover variation in minisatellite DNA. Minisatellite DNA variation in the Salton Sea population of the R. I. yumanensis subspecies was at a level typical of outbred avian species (average proportion of fragments shared, or S, was 0.33). Variation was extremely low (S from 0.63 to 0.77), however, within four coastal, salt-marsh populations of the subspecies R. I. levipes located along a transect extending about 260 km northwest from the Mexican border. Between-population similarity (Sij ) was also high for the four levipes populations, although individuals of the small, isolated population at Mugu Lagoon consistently clustered separately in phenograms constructed using neighbor-joining or other algorithms. Individuals of yumanensis always clustered as a sister group to all levipes individuals. The minisatellite data were contrasted with the extremely low mtDNA and RAPD variation we found in both subspecies. We propose that variation in these less-mutable markers was lost in a bottleneck that occurred at least 1000 years ago, thus allowing sufficient time for recovery of variation in the rapidly mutating (μ∼ 0.001/gamete/generation) minisatellites (t = 1/μ, or 1000 generations). A second, more-recent bottleneck, or series of bottlenecks within a metapopulation structure, likely resulted in the depauparate variation seen in levipes today. We suggest that translocations from large to small levipes populations could restore important genetic variation to the small populations and would not compromise genetic boundaries. Estructura genética de las poblaciones de Rallus longirostris en peligro de extinción en el sur de California.

DNA Fingerprinting Reveals a Lack of Genetic Variation in Northern Populations of the Western Pond Turtle (Clemmys marmorata).

I used DNA fingerprinting to provide the first analysis of the genetic composition of western pond turtle (Clemmys marmorata( populations in Washington, Oregon, and California. Populations of the western pond turtle in Washington and northern Oregon are rapidly approaching extinction. Genetic similarity within the largest northern populations, which are located inland, is high. An analysis of population substructure (Fst ) revealed significant genetic divergence between inland populations, indicating a lack of dispersal and gene flow between sites. In contrast, northern coastal sites are not genetically distinct, but there are few if any viable populations remaining in this region. Genetic variability within southern California populations is a great deal higher than in northern inland sites. Similarly, a low Fst value indicated a lack of genetic differentiation between southern sites. An inter-regional analysis of population substructure (Fst = 0.24) revealed a significant degree of genetic divergence between geographical regions throughout the range. In addition, an estimate of western pond turtle phylogeny showed a genetic break in the species between northern and southern populations. Both population subdivision and phylogenetic analyses suggest a lack of appreciable gene flow between geographical regions for a considerable period of time. Genetic analyses support traditional subdivision based solely on the morphological variation of Clemmys marmorata into two subspecies: northern Clemmys marmorata marmorata and southern Clemmys marmorata pallida. Recovery of dwindling northern populations must combine demographic and genetic considerations. A first step should be to preserve local gene pools while augmenting population numbers, with the goal of preventing the extinction of this genetically and morphologically distinct subspecies. La falta de variación genetica en poblaciones norteñas de la tortuga de agua dulce del oeste (Clemmys marmorata).

Effects of light, temperature and desiccation on the net emersed productivity of the intertidal macroalga Colpomenia peregrina Sauv. (Hamel)

The brown saccate macroalga Colpomenia peregrina Sauv. (Hamel) was incubated in the laboratory under 27 combinations of light, temperature and desiccation and the emersed net photosynthetic ( P net) and respiratory ( R) responses measured using a non-dissolved gaseous oxygen analyzer. Estimates of net emersed carbon gain were made based in the P net: R ratio. These were highly variable (−0.02 to 2.86) depending on the combinations of physical factors to which algae were exposed and differences in their physiological responses. Results from a three-way analysis of variance (ANOVA) suggest that C. peregrina has a threshold of water loss, beyond which interactions between temperature and desiccation have a significant effect on net carbon gain. Interactions between individual factors can substantially increase the complexity of estimating net carbon gain during emersion. During winter, C. peregrina populations in southern California are subject to reductions in primary productivity as a result of shorter daylengths, lower seawater temperatures and desiccation stress. Emersion can compensate for these conditions only if rates of photosynthesis during emersion are higher than under submersion. Otherwise, emersed photosynthesis only ameliorates deterimental effects of desiccation caused by prolonged periods of daytime emersion.

Isolation of Asexually Proliferative Tetrathyridia (Mesocestoides sp.) from the Southern Pacific Rattlesnake (Crotalus viridis helleri) with Additional Data from Two Previous Isolates from the Great Basin Fence Lizard (Sceloporus occidentalis longipes)

Isolation of Asexually Proliferative Tetrathyridia (Mesocestoides sp.) from the Southern Pacific Rattlesnake (Crotalus viridis helleri) with Additional Data from Two Previous Isolates from the Great Basin Fence Lizard (Sceloporus occidentalis longipes)

EVOLUTIONARY AND GEOGRAPHIC TRENDS IN ADAPTIVE WOOD ANATOMY IN ERIASTRUM DENSIFOLIUM (POLEMONIACEAE)

Anatomical features of Eriastrum densifolium populations in southern California show variation that correlates well with ecological setting and geographical distribution. Vessel element size and density in shoots suggest strong adaptation among different subspecies to efficient water movement in different habitats. Indices of mesomorphy, devised by Carlquist to indicate relative adaptivity of wood, likewise show considerable variation among populations, suggesting that different subspecies have anatomical components that can define them. Relationships among the five subspecies of E. densifolium may be explored through MI value relationships.

Genetic Differentiation of Disjunct Gulf of California and Pacific Outer Coast Populations of Hypsoblennius jenkinsi

jenkinsi. Specimens were collected from a single site in the northern Gulf of California and from two study sites in southern California waters. The standard genetic distance value (D) of 0.003 for the two populations in southern California waters is substantially less than the D values for either of these populations and the Gulf population (0.035, 0.038). Slight genetic differentiation of the southern California populations nonetheless is indicated by a significant (P < .001) difference between the local populations in allele frequencies at the Est-1 locus. The geographic proximity of the southern California populations coupled with the extended planktonic larval stage of H. jenkinsi suggest that the genetic differentiation of these local populations may be due to differences in local selection pressures rather than restricted gene flow. In contrast, statistically significant (P < .005 in five cases, P < .01 in one case) differences in allele frequencies were found between northern Gulf of California and southern California populations at six loci (Ada-A, Adh-A, Ck-C, Gapdh-B, Pep-2, Pgdh-A). The nature and magnitude of these differences suggest that, although no fixed allelic differences were found, there is limited gene flow at present between these disjunct populations. The low D values of the disjunct populations are compatible with the hypothesis that this disjunct distribution originated when the range of these temperate fish, continuous around Cabo San Lucas during the Pleistocene glacial periods, was disrupted in the Cape region due to post-glacial warming of surface waters. The distributions of all marine fish species that have been regarded as 'disjuncts' are discussed.