Shaping of genetic structure along Pleistocene and modern river systems in the hydrochorous riparian azalea, Rhododendron ripense (Ericaceae)

To determine the effects of hydrochory on the formation of the present range of a species and the spatial distribution of genetic variation, we assessed the rangewide genetic structure of a hydrochorous riparian Japanese species (Rhododendron ripense) using four nuclear microsatellite loci. The patterns of isolation by distance and Bayesian clustering analyses of 33 populations suggested that the present range, characterized by both localized and disjunct distributions across the sea, arose from two contrasting colonization events: (1) primary colonization along two Pleistocene rivers that have been submerged and become partly isolated by marine transgression by 6000 years ago, and (2) additional range expansions from these rivers into unconnected neighboring rivers as a result of river captures. Along the Pleistocene rivers, frequent gene flow by hydrochory resulted in the retention of considerable genetic diversity within each population and genetic homogenization among populations. Within unconnected neighboring rivers, genetic diversity was also retained by the simultaneous redistribution of many individuals as a result of river captures, whereas restricted gene flow within a river resulted in genetic divergence among the river populations. Thus, the evolutionary history of hydrochorous R. ripense appears to have been strongly shaped by both ancient and modern rivers.     

Geographic differentiation of <Emphasis Type="Italic">Cobitis shikokuensis</Emphasis> inferred from mtDNA RFLP analysis

Mitochondrial DNA divergence among populations of the Japanese spinous loach Cobitis shikokuensis, endemic to Shikoku Island, was investigated by restricted fragment length polymorphism analysis. A total of 68 restriction sites on DNA fragments from the cytochrome b to D-loop regions and from the 12S rRNA to 16S rRNA regions, amplified by PCR, were analyzed. A total of 12 haplotypes (plus 6 in outgroups) were detected in 268 specimens collected from 19 localities in seven rivers (and 41 specimens from four localities in three rivers in outgroups). Three of the seven river populations of C. shikokuensis were shown to have unique haplotypes, and four of the seven river populations were monomorphic. The nested structure of the haplotype network for populations of C. shikokuensis exhibited two large clades corresponding to (1) populations from the Shimanto River and its neighbors and (2) two genetically divergent populations in the Shigenobu and Iwamatsu Rivers. The population from the Shimanto River, the largest river inhabited by C. shikokuensis, maintains great haplotype diversity as well as the allozyme diversity previously reported. On the other hand, populations from the Hiji River, the second largest river, which exhibited the highest allozyme diversity, were monomorphic in their mtDNA. The nested clade analysis (NCA) revealed that past fragmentation between the above two clades could occur in the initial distribution process of C. shikokuensis. The large genetic divergence of two river populations from the Shigenobu and Iwamatsu Rivers was inferred to be caused by a process of long distance colonization and fragmentation. MtDNA introgression into the Hiji River population from southern river populations was suggested. Taking genetic divergence into consideration, each river population of C. shikokuensis should be conserved separately as like a distinct species, and conservation programs for the small populations showing less genetic variability should be invoked as soon as possible.     

Spatial structure and clonal distribution of genotypes in the rare Typha minima Hoppe (Typhaceae) along a river system

We studied the genetic diversity of the dwarf bulrush (Typha minima) along a 60 km section of the Isère river using AFLP markers. Total clonality was relatively low (proportion of distinguishable genotypes = 0.70) but extremely variable among populations with one monoclonal population and several populations where all sampled individuals were different. Genetic diversity was high (He = 0.129) and again variable among populations. Although no major genetic discontinuity could be detected, gene flow was found to be limited. Our results show a much higher diversity compared to Swiss populations. The high genetic diversity within most populations despite the species’ potentially important clonal growth indicates that populations are relatively young. This is in line with the fact that the species lives in open habitats created by high floods that erase river banks leading to a metapopulation dynamics. However the metapopulation dynamics is at least partly disturbed as gene flow appears to be restricted so that our populations are probably at risk if no action is taken to re-establish more natural river flow dynamics.     

Catchment-Scale Conservation Units Identified for the Threatened Yarra Pygmy Perch (Nannoperca obscura) in Highly Modified River Systems

Habitat fragmentation caused by human activities alters metapopulation dynamics and decreases biological connectivity through reduced migration and gene flow, leading to lowered levels of population genetic diversity and to local extinctions. The threatened Yarra pygmy perch, Nannoperca obscura, is a poor disperser found in small, isolated populations in wetlands and streams of southeastern Australia. Modifications to natural flow regimes in anthropogenically-impacted river systems have recently reduced the amount of habitat for this species and likely further limited its opportunity to disperse. We employed highly resolving microsatellite DNA markers to assess genetic variation, population structure and the spatial scale that dispersal takes place across the distribution of this freshwater fish and used this information to identify conservation units for management. The levels of genetic variation found for N. obscura are amongst the lowest reported for a fish species (mean heterozygosity of 0.318 and mean allelic richness of 1.92). We identified very strong population genetic structure, nil to little evidence of recent migration among demes and a minimum of 11 units for conservation management, hierarchically nested within four major genetic lineages. A combination of spatial analytical methods revealed hierarchical genetic structure corresponding with catchment boundaries and also demonstrated significant isolation by riverine distance. Our findings have implications for the national recovery plan of this species by demonstrating that N. obscura populations should be managed at a catchment level and highlighting the need to restore habitat and avoid further alteration of the natural hydrology.     

Population differentiation and gene flow within a metapopulation of a threatened tree, Magnolia stellata (Magnoliaceae)

We examined genetic differentiation among eight local populations of a metapopulation of Magnolia stellata using 10 nuclear and three chloroplast microsatellite (nSSR and cpSSR) markers and evaluated the influence of historical gene flow on population differentiation. The coefficient of genetic differentiation among populations for nSSR (F(ST) = 0.053) was less than half that for cpSSR (0.137). An isolation-by-distance pattern was detected for nSSRs, but not cpSSRs. These results suggest that pollen flow, as well as seed dispersal, has significantly reduced genetic differentiation among populations. We also examined patterns of contemporary pollen flow by paternity analysis of seeds from nine seed parents in one of the populations using the nSSR markers and found it to be greatly restricted by the distance between parents. Although most pollen flow occurred within the population, pollen flow from outside the population accounted for 2.5% of the total. When historical and contemporary pollen flows among populations were compared, the levels of pollen flow seem to have declined recently. We conclude that to conserve M. stellata, it is important to preserve the whole population by maintaining its metapopulation structure and the gene flow among its populations.     

Multiple spatial scale patterns of genetic diversity in riparian populations of Ainsliaea faurieana (Asteraceae) on Yakushima Island, Japan

Habitat and geographical features of river systems strongly influence gene flow and spatial genetic patterning in riparian plant populations. We investigated the patterns of genetic diversity within and among populations of Ainsliaea faurieana relative to different spatial conditions (along a river, among rivers, and among regions on an island), based on nuclear and chloroplast microsatellite DNA variations. Within an individual river system, we found higher haplotype diversities in downstream populations, and in a Bayesian analysis of recent migration, we detected unidirectional gene movements from upstream to downstream, indicating water-mediated dispersal along the river. Mantel tests detected no isolation-by-distance in genetic variation, suggesting the maintenance of a metapopulation with wide-range seed dispersal by water. Moreover, the observed high level of genetic differentiation, especially in the cpDNA (F(ST) = 0.539), indicated a metapopulation structure with frequent extinction and colonization. On a larger scale, we found high population differentiation and clear genetic structuring among regions, suggesting that gene flow was restricted by geographical features (mountains separating river systems) for relatively long periods. Our findings of genetic structures based on different spatial conditions elucidated patterns and ranges of historical and contemporary gene movement in a plant species that is persistent in extremely disturbed riparian environments.     

Phylogeographic structure in the threatened Yarra pygmy perch <Emphasis Type="Italic">Nannoperca obscura</Emphasis> (Teleostei: Percichthyidae) has major implications for declining populations

Molecular genetic information should be a pre-requisite when evaluating conservation priorities in highly structured species such as freshwater fishes. Nuclear (allozyme) and mitochondrial (cytochrome b) markers were used to investigate phylogeographic structure in the Yarra pygmy perch Nannoperca obscura (Klunzinger), a threatened freshwater fish endemic to mainland south-eastern Australia. Complementary patterns of strong, geographically defined sub-structure were observed including a major east–west divergence (at the Glenelg River), four diagnosable lineages, and statistically-significant differences between most populations. Accordingly, four Evolutionarily Significant Units (ESUs) are defined and multiple, drainage-scale Management Units (MUs) suggested. Since Nannoperca obscura is a relatively poor disperser with no apparent gene flow between most populations, any regional extirpation would see the irreversible loss of genetic diversity. This is problematic, as several populations, most notably a recently discovered ESU in the Murray-Darling Basin, are feared extirpated through a combination of anthropogenic threats and severe drought. The potential loss of unique evolutionarily components within N. obscura soon after their discovery highlights with some urgency, the need to define and protect conservation units in highly modified freshwater habitats.     

Low but contrasting neutral genetic differentiation shaped by winter temperature in European great tits

Gene flow is usually thought to reduce genetic divergence and impede local adaptation by homogenising gene pools between populations. However, evidence for local adaptation and phenotypic differentiation in highly mobile species, experiencing high levels of gene flow, is emerging. Assessing population genetic structure at different spatial scales is thus a crucial step towards understanding mechanisms underlying intraspecific differentiation and diversification. Here, we studied the population genetic structure of a highly mobile species – the great tit Parus major – at different spatial scales. We analysed 884 individuals from 30 sites across Europe including 10 close‐by sites (< 50 km), using 22 microsatellite markers. Overall we found a low but significant genetic differentiation among sites (F_ST = 0.008). Genetic differentiation was higher, and genetic diversity lower, in south‐western Europe. These regional differences were statistically best explained by winter temperature. Overall, our results suggest that great tits form a single patchy metapopulation across Europe, in which genetic differentiation is independent of geographical distance and gene flow may be regulated by environmental factors via movements related to winter severity. This might have important implications for the evolutionary trajectories of sub‐populations, especially in the context of climate change, and calls for future investigations of local differences in costs and benefits of philopatry at large scales.     

Genetic differentiation in red‐bellied piranha populations (Pygocentrus nattereri,Kner, 1858) from the Solimões‐Amazonas River

Red‐bellied piranhas (Pygocentrus nattereri) are widely caught with different intensities throughout the region of Solimões‐Amazonas River by local fishermen. Thus, the management of this resource is performed in the absence of any information on its genetic stock. P. nattereri is a voracious predator and widely distributed in the Neotropical region, and it is found in other regions of American continent. However, information about genetic variability and structure of wild populations of red‐bellied piranha is unavailable. Here, we describe the levels of genetic diversity and genetic structure of red‐bellied piranha populations collected at different locations of Solimões‐Amazonas River system. We collected 234 red‐bellied piranhas and analyzed throughout eight microsatellite markers. We identified high genetic diversity within populations, although the populations of lakes ANA, ARA, and MAR have shown some decrease in their genetic variability, indicating overfishing at these communities. Was identified the existence of two biological populations when the analysis was taken altogether at the lakes of Solimões‐Amazonas River system, with significant genetic differentiation between them. The red‐bellied piranha populations presented limited gene flow between two groups of populations, which were explained by geographical distance between these lakes. However, high level of gene flow was observed between the lakes within of the biological populations. We have identified high divergence between the Catalão subpopulation and all other subpopulations. We suggest the creation of sustainable reserve for lakes near the city of Manaus to better manage and protect this species, whose populations suffer from both extractive and sport fishing.     

Determinants of hierarchical genetic structure in Atlantic salmon populations: environmental factors vs. anthropogenic influences

Disentangling the effects of natural environmental features and anthropogenic factors on the genetic structure of endangered populations is an important challenge for conservation biology. Here, we investigated the combined influences of major environmental features and stocking with non‐native fish on the genetic structure and local adaptation of Atlantic salmon (Salmo salar) populations. We used 17 microsatellite loci to genotype 975 individuals originating from 34 French rivers. Bayesian analyses revealed a hierarchical genetic structure into five geographically distinct clusters. Coastal distance, geological substrate and river length were strong predictors of population structure. Gene flow was higher among rivers with similar geologies, suggesting local adaptation to geological substrate. The effect of river length was mainly owing to one highly differentiated population that has the farthest spawning grounds off the river mouth (up to 900 km) and the largest fish, suggesting local adaptation to river length. We detected high levels of admixture in stocked populations but also in neighbouring ones, implying large‐scale impacts of stocking through dispersal of non‐native individuals. However, we found relatively few admixed individuals suggesting a lower fitness of stocked fish and/or some reproductive isolation between wild and stocked individuals. When excluding stocked populations, genetic structure increased as did its correlation with environmental factors. This study overall indicates that geological substrate and river length are major environmental factors influencing gene flow and potential local adaptation among Atlantic salmon populations but that stocking with non‐native individuals may ultimately disrupt these natural patterns of gene flow among locally adapted populations.     

Patterns of genetic diversity and migration in increasingly fragmented and declining orang-utan (Pongo pygmaeus) populations from Sabah, Malaysia

We investigated the genetic structure within and among Bornean orang-utans (Pongo pygmaeus) in forest fragments of the Lower Kinabatangan flood plain in Sabah, Malaysia. DNA was extracted from hair and faecal samples for 200 wild individuals collected during boat surveys on the Kinabatangan River. Fourteen microsatellite loci were used to characterize patterns of genetic diversity. We found that genetic diversity was high in the set of samples (mean H(E) = 0.74) and that genetic differentiation was significant between the samples (average F(ST) = 0.04, P < 0.001) with F(ST) values ranging from low (0.01) to moderately large (0.12) values. Pairwise F(ST) values were significantly higher across the Kinabatangan River than between samples from the same river side, thereby confirming the role of the river as a natural barrier to gene flow. The correlation between genetic and geographical distance was tested by means of a series of Mantel tests based on different measures of geographical distance. We used a Bayesian method to estimate immigration rates. The results indicate that migration is unlikely across the river but cannot be completely ruled out because of the limited F(ST) values. Assignment tests confirm the overall picture that gene flow is limited across the river. We found that migration between samples from the same side of the river had a high probability indicating that orang-utans used to move relatively freely between neighbouring areas. This strongly suggests that there is a need to maintain migration between isolated forest fragments. This could be done by restoring forest corridors alongside the river banks and between patches.     

Genetic diversity and hierarchical population structure of a rare autotetraploid plant,Aster kantoensis (Asteraceae)

We sampled 17 populations of a rare autotetraploid Aster kantoensis (Asteraceae) from three river systems located in central Japan, and studied them for allelic variation at 22 enzyme loci. There was no significant correlation between the actual population size and three genetic diversity parameters, suggesting that the effective population size was very small even for the large populations, i.e., even large populations may still have a high probability of being of recent origin and remain influenced by the founder effect. Compared to other autotetraploid species, the total genetic variation of A. kantoensis is small. The number of alleles and gene diversity of a population were not significantly different among the river systems, although the percentage of polymorphic loci was different. Genetic differentiation among river systems was larger than between populations within the river systems, thereby indicating that gene flow between river systems is small, especially between the Kinu River system and Tama or Sagami River systems.     

High intra-population genetic variability and inter-population differentiation in a plateau specialized fish, <Emphasis Type="BoldItalic">Triplophysa orientalis</Emphasis>

Triplophysa orientalis (Herzenstein) is one of the Nemacheilinae (Cypriniformes: Balitoridae) fish species distributed in the Tibetan Plateau area. In order to understand the impact of plateau uplift on population history and the isolation effect of plateau lakes on T. orientalis, we examined its genetic structure and phylogenetic relationships. A total of 98 individuals from five wild populations, three from plateau lakes and two from branch rivers in upper reaches of the Yangtze River, in the eastern peripheral of the Tibetan Plateau were sampled. An 848 base pair fragment from the mitochondrial DNA (mtDNA) control region was sequenced for analyses. Overall, very high intra-population genetic variability was found in all populations except for one lake population (Rannicuo); nucleotide diversity ranged from 0.0025 to 0.0159 and haplotype diversity ranged from 0.641 to 0.879. Furthermore, the genetic distance between river populations (0.0326) was much higher than that among lake populations (Rannicuo and Barencuo 0.0035, Bannicuo and Yibicuo 0.0038, Rannicuo and Yibicuo 0.0049). Additionally, the analysis of molecular variance demonstrated that most of the observed genetic variability occurred among populations, accompanied with significant Fst values except for that between the Yibicuo and Barencuo populations. This evidence suggested a strong population structure of the species and a lack of inter-population connection. Lastly, the rate of migration indicated there were large historic gene flows among lake populations. Demographic analysis also indicated there were bottlenecks or expansions in three lake populations, suggesting a potential isolation effect of plateau lakes on population differentiation. Molecular dating of intra-specific divergence showed the plateau uplift has shaped the genetic structure of T. orientalis.     

Population genetic structure,differentiation, and diversity in Tetrix subulata pygmy grasshoppers: roles of population size and immigration

Genetic diversity within and among populations and species is influenced by complex demographic and evolutionary processes. Despite extensive research, there is no consensus regarding how landscape structure, spatial distribution, gene flow, and population dynamics impact genetic composition of natural populations. Here, we used amplified fragment length polymorphisms (AFLPs) to investigate effects of population size, geographic isolation, immigration, and gene flow on genetic structure, divergence, and diversity in populations of Tetrix subulata pygmy grasshoppers (Orthoptera: Tetrigidae) from 20 sampling locations in southern Sweden. Analyses of 1564 AFLP markers revealed low to moderate levels of genetic diversity (PPL = 59.5–90.1; Hj = 0.23–0.32) within and significant divergence among sampling localities. This suggests that evolution of functional traits in response to divergent selection is possible and that gene flow is restricted. Genetic diversity increased with population size and with increasing proportion of long‐winged phenotypes (a proxy of recent immigration) across populations on the island of Öland, but not on the mainland. Our data further suggested that the open water separating Öland from the mainland acts as a dispersal barrier that restricts migration and leads to genetic divergence among regions. Isolation by distance was evident for short interpopulation distances on the mainland, but gradually disappeared as populations separated by longer distances were included. Results illustrate that integrating ecological and molecular data is key to identifying drivers of population genetic structure in natural populations. Our findings also underscore the importance of landscape structure and spatial sampling scheme for conclusions regarding the role of gene flow and isolation by distance.     

Genetics and telemetry indicate unexpected movements among structured populations for Brachyplatystoma platynemum in the Amazon

The genetic analysis of Brachyplatystoma platynemum individuals sampled from the lower Madeira River reinforces the existence of two structured populations in the Amazon Basin (Madeira and Amazon populations). However, the recapture of an individual from the Amazon population in the Solimões River, which was telemetry-tagged in the Madeira River after the damming, indicates that fish from the Amazon population move between the two river systems. This has not yet been observed, however, in the Madeira River population, which is currently divided and isolated in the lower and upper Madeira River by the construction of two dams.     

Habitat islands, genetic diversity, and gene flow in a Patagonian rodent

The effects of terrestrial habitat islands on gene flow and genetic diversity in animal populations have been predicted and discussed in theoretical terms, but empirical data are needed to test these predictions and provide an understanding of the relationships of life-history characteristics to genetics of insular species. We studied saxicolous mice ( Phyllotis xanthopygus ) in Patagonia to explore genetic structure, phylogeography, and gene flow in a species inhabiting natural habitat islands. Phylogeographic analyses based on mtDNA sequences revealed two haplotype clades, which presumably reflect early Pleistocene factors that temporarily separated the mice into two geographically isolated groups. The Río Chubut, which lies within a glacial drainage basin bisecting northern Patagonia, might have affected gene flow in the species. Although we anticipated isolation by distance and founder phenomena associated with habitat islands, in some habitat patches we found evidence of high local genetic diversity. The amount of divergence in the mitochondrial cytochrome b gene (≈ 3.4%) in animals at a single locality could best be explained through a combination of historical factors and metapopulation source–sink theory. Demographic shifts, dispersal, and episodic recolonization are important in the life history and genetic population structure of P. xanthopygus .     

Multiscale analysis of Hymenocallis coronaria (Amaryllidaceae) genetic diversity, genetic structure, and gene movement under the influence of unidirectional stream flow

Understanding gene movement patterns in unidirectional flow environments and their effect on patterns of genetic diversity and genetic structure is necessary to manage these systems. Hypotheses and models to explain genetic patterns in streams are rare, and the results of macrophyte studies are inconsistent. This study addresses Ritland's (Canadian Journal of Botany 67: 2017-2024) unidirectional diversity hypothesis, the one-dimensional stepping stone model, and the metapopulation model within and among populations. Hymenocallis coronaria, an aquatic macrophyte of rocky river shoals of the SE USA, was sampled in four river basins. Within populations and among populations <16.2 km apart had significant isolation by distance. However, the rate of gene flow decay was not consistent with a one-dimensional stepping stone model, nor was evidence strong or consistent for Ritland's hypothesis. Some evidence indicates that localized metapopulation processes may be affecting genetic diversity and structure; however, gene flow patterns inconsistent with the assumptions of the linear and unidirectional models are also a possible influence. We discuss three variants on the one-dimensional stepping stone model. Future research in linear environments should examine the expectations of these models. This study is also one of the first efforts to calculate population genetic parameters using a new program, TETRASAT.     

Genetic structure of mountain lion (Puma concolor) populations in California

Analysis of 12 microsatellite loci from431 mountain lions (Puma concolor)revealed distinct genetic subdivision that wasassociated with geographic barriers andisolation by distance in California. Levels ofgenetic variation differed among geographicregions, and mountain lions that inhabitedcoastal areas exhibited less heterozygositythan those sampled inland. The San FranciscoBay and Sacramento-San Joaquin River Delta, theCentral Valley, and the Los Angeles Basinappeared to be substantial barriers to geneflow, and allele frequencies of populationsseparated by those features differedsubstantially. A partial barrier to gene flowappeared to exist along the crest of the SierraNevada. Estimated gene flow was high amongmountain lions inhabiting the Modoc Plateau,the western Sierra Nevada, and northern sectionof the eastern Sierra Nevada. SouthernCalifornia mountain lion populations mayfunction as a metapopulation; however, humandevelopments threaten to eliminate habitat andmovement corridors. While north-south geneflow along the western Sierra Nevada wasestimated to be very high, projected loss andfragmentation of foothill habitat may reducegene flow and subdivide populations. Preservation of existing movement corridorsamong regions could prevent population declinesand loss of genetic variation. This studyshows that mountain lion management andconservation efforts should be individualizedaccording to region and incorporatelandscape-level considerations to protecthabitat connectivity.     

The effects of river barriers and range expansion of the population genetic structure and stability in Greenside Darter (Etheostoma blennioides) populations

The genetic structure of a stream-dwelling fish, the Greenside Darter, Etheostoma blennioides, is described from variation at nine microsatellite loci in 26 populations in the northern-most portions of the species’ range in southern Ontario, Canada in two sampling years. We found relatively high levels of genetic structure at the among- and within-watershed scales, with some watersheds and populations exhibiting very high divergence. The Ausable River populations were especially isolated, containing distinct populations of potential conservation concern. Temporal replicates at selected localities showed evidence of substantial temporal variation in genetic structure, perhaps resulting from movement among sites. We found strong evidence for an effect of river barriers (dams and weirs) on dispersal measured by genotype assignment techniques. However, we found no bias in upstream vs downstream dispersal. Significant isolation-by-distance relationships in both sample years indicate that river distance is an important factor regulating gene flow in these watersheds. The Canadian Greenside Darter populations are expanding their range into more northerly watersheds, but also show substantial within-watershed genetic structure despite substantial dispersal.     

All grown‐up and nowhere to go: paedomorphosis and local adaptation in Ambystoma salamanders in the Cuenca Oriental of Mexico

Facultative or obligate paedomorphosis has evolved several times in Mexican populations of the salamander genus Ambystoma, leading to increased genetic divergence among populations with alternate life histories and contributing to population divergence in this species complex. In the present study, we surveyed the genetic diversity of Ambystoma populations in lakes of the Cuenca Oriental, a high elevation closed drainage basin that encompasses permanent crater lakes harbouring salamander populations. We genotyped individuals from five populations aiming to better understand population dynamics and the evolution of paedomorphosis in this system. Specifically, we tested the hypotheses that the evolution of paedomorphosis in Ambystoma taylori resulted in reduced genetic exchange with populations of Ambystoma velasci in neighbouring lakes. Second, we tested whether the populations in brackish lakes of the Cuenca Oriental, Lake Atexcac, and Lake Alchichica show restricted gene flow across the basin, possibly as a result of local adaptation to those microhabitats. Using various indices of population genetic diversity, Bayesian assignment, and approximate Bayesian computation methods, we show that genetic exchange between brackish lakes and freshwater lakes is negligible, despite continued gene flow among freshwater lakes. We show that the first divergence among populations occurred between Alchichica and the remaining populations and that the evolution of paedomorphosis in A. taylori was likely favoured by local adaptation to saline conditions, thus increasing its genetic isolation. An apparently similar process appears to be in progress independently in lake Atexcac, showing that local adaptation may play an important role in population isolation and, ultimately, in speciation.