Population genetic structure in Myrtus communis L. in a chronically fragmented landscape in the Mediterranean: can gene flow counteract habitat perturbation?

Ancient managed landscapes provide ideal opportunities to assess the consequences of habitat fragmentation on the patterns of genetic diversity and gene flow in long-lived plant species. Using amplified fragment length polymorphism (AFLP) and allozyme markers, we quantified seed-mediated gene flow and population genetic diversity and structure in 14 populations of Myrtus communis (myrtle), a common endozoochorous shrub species of forest patches in lowland agricultural Mediterranean areas. Overall, allozyme diversity for myrtle was low (P₉₅   =   25%; A   =   1.411; H_e = 0.085) compared to other known populations, and a significant portion of populations (57%) had lower levels of allelic diversity and/or heterozygosity than expected at random, as shown by simulated resampling of the whole diversity of the landscape. We found significant correlations between allozyme variability and population size and patch isolation, but no significant inbreeding in any population. Genetic differentiation among populations for both allozyme and AFLP markers was significant (Φ_ST = 0.144 and Φ_ST = 0.142, respectively) but an isolation-by-distance pattern was not detected. Assignment tests on AFLP data indicated a high immigration rate in the populations ( ca. 20–22%), likely through effective seed dispersal across the landscape by birds and mammals. Our results suggest that genetic isolation is not the automatic outcome of habitat destruction since substantial levels of seed-mediated gene flow are currently detectable. However, even moderate rates of gene flow seem insufficient in this long-lived species to counteract the genetic erosion and differentiation imposed by chronic habitat destruction.     

Population differentiation in the open sea: insights from the pelagic copepod Pleuromamma xiphias

Although a number of recent studies of marine holoplankton have reported significant genetic structure among populations, little is currently known about the biological and oceanographic processes that influence population connectivity in oceanic plankton. In order to examine how depth preferences influence dispersal in oceanic plankton, I characterized the genetic structure of a copepod with diel vertical migration (DVM) (Pleuromamma xiphias), throughout its global distribution, and compared these results to those expected given the interaction of this species' habitat depth with ocean circulation and bathymetry. Mitochondrial COI sequences from 651 individuals from 28 sites in the Indian, Pacific, and Atlantic Oceans revealed highly significant genetic differentiation both within and among ocean basins. Limited dispersal among distinct pelagic provinces seems to have played a major role in population differentiation in this species, with strong genetic breaks observed across known oceanographic fronts or current systems in all three ocean basins. The Indo-West Pacific (IWP) holds a highly distinct genetic population of this species that was sampled in both the western Pacific and eastern Indian Oceans. This suggests that the IWP does not act as a strong barrier to gene flow between basins, as expected, despite the relatively shallow water depth (<200 m) and vertically extensive (>400 m) diel migration of this species. A pattern of isolation by distance was observed in the Indian Ocean with genetic differentiation among samples down to spatial scales of ～800 km, indicating that realized dispersal in P. xiphias occurs over much smaller spatial scales than in previously reported oceanic holoplankton. Given its highly regionalized population genetic structure, P. xiphias may have some capacity to adapt to local oceanographic conditions, and it should not be assumed that populations of this species in distinct pelagic biomes will respond in the same way to shared physical or climatic forcing.     

Matching genetics with oceanography: directional gene flow in a Mediterranean fish species

Genetic connectivity and geographic fragmentation are two opposing mechanisms determining the population structure of species. While the first homogenizes the genetic background across populations the second one allows their differentiation. Therefore, knowledge of processes affecting dispersal of marine organisms is crucial to understand their genetic distribution patterns and for the effective management of their populations. In this study, we use genetic analyses of eleven microsatellites in combination with oceanographic satellite and dispersal simulation data to determine distribution patterns for Serranus cabrilla, a ubiquitous demersal broadcast spawner, in the Mediterranean Sea. Pairwise population F_ST values ranged between ?0.003 and 0.135. Two genetically distinct clusters were identified, with a clear division located between the oceanographic discontinuities at the Ibiza Channel (IC) and the Almeria‐Oran Front (AOF), revealing an admixed population in between. The Balearic Front (BF) also appeared to dictate population structure. Directional gene flow on the Spanish coast was observed as S. cabrilla dispersed from west to east over the AOF, from north to south on the IC and from south of the IC towards the Balearic Islands. Correlations between genetic and oceanographic data were highly significant. Seasonal changes in current patterns and the relationship between ocean circulation patterns and spawning season may also play an important role in population structure around oceanographic fronts.     

Habitat fragmentation and genetic diversity of an endangered, migratory songbird, the golden-cheeked warbler (Dendroica chrysoparia)

Landscape genetic approaches offer the promise of increasing our understanding of the influence of habitat features on genetic structure. We assessed the genetic diversity of the endangered golden-cheeked warbler (Dendroica chrysoparia) across their breeding range in central Texas and evaluated the role of habitat loss and fragmentation in shaping the population structure of the species. We determined genotypes across nine microsatellite loci of 109 individuals from seven sites representing the major breeding concentrations of the species. No evidence of a recent population bottleneck was found. Differences in allele frequencies were highly significant among sites. The sampled sites do not appear to represent isolated lineages requiring protection as separate management units, although the amount of current gene flow is insufficient to prevent genetic differentiation. Measures of genetic differentiation were negatively associated with habitat connectivity and the percentage of forest cover between sites, and positively associated with geographic distance and the percentage of agricultural land between sites. The northernmost site was the most genetically differentiated and was isolated from other sites by agricultural lands. Fragmentation of breeding habitat may represent barriers to dispersal of birds which would pose no barrier to movement during other activities such as migration.     

Rare long-distance dispersal of the Island Night Lizard, <Emphasis Type="Italic">Xantusia riversiana</Emphasis>, maintains high diversity in a fragmented environment

The Island Night Lizard (Xantusia riversiana) is endemic to three of the Channel Islands off the coast of California, USA. Introduced species such as goats, sheep, and cats have profoundly affected the fauna and flora of the islands for over 150 years, but most of these non-native species have been recently removed. We measured the distribution of genetic diversity in Island Night Lizards across San Nicolas Island using DNA microsatellites to assess the impacts of historical habitat change on effective population size, gene flow, and population divergence; to provide baseline data for future monitoring of genetic diversity; and to provide recommendations to inform the restoration of degraded habitat. Despite a history of profound anthropogenic habitat disturbance, genetic diversity was high within sites, and there was no evidence of population bottlenecks. Divergence between sites was extraordinarily high, as expected for this sedentary species. Landscape resistance modeling using circuit theory showed that unsuitable habitat is relatively permeable to gene flow compared to suitable habitat, and yet populations separated by very short geographic distances remain genetically distinct. We found no evidence of a need for short-term intervention such as artificial translocations to maintain genetic diversity. Instead, we suggest that management should focus on maintaining, improving, and increasing habitat, especially in creating patches of habitat to link existing sites.     

Gene flow of Acanthaster planci (L.) in relation to ocean currents revealed by microsatellite analysis

Population outbreaks of the coral-eating starfish, Acanthaster planci , are hypothesized to spread to many localities in the Indo-Pacific Ocean through dispersal of planktonic larvae. To elucidate the gene flow of A. planci across the Indo-Pacific in relation to ocean currents and to test the larval dispersal hypothesis, the genetic structure among 23 samples over the Indo-Pacific was analysed using seven highly polymorphic microsatellite loci. The F -statistics and genetic admixture analysis detected genetically distinct groups in accordance with ocean current systems, that is, the Southeast African group (Kenya and Mayotte), the Northwestern Pacific group (the Philippines and Japan), Palau, the North Central Pacific group (Majuro and Pohnpei), the Great Barrier Reef, Fiji, and French Polynesia, with a large genetic break between the Indian and Pacific Oceans. A pattern of significant isolation by distance was observed among all samples ( P =  0.001, r  = 0.88, n  = 253, Mantel test), indicating restricted gene flow among the samples in accordance with geographical distances. The data also indicated strong gene flow within the Southeast African, Northwestern Pacific, and Great Barrier Reef groups. These results suggest that the western boundary currents have strong influence on gene flow of this species and may trigger secondary outbreaks.     

Multilocus evidence for globally distributed cryptic species and distinct populations across ocean gyres in a mesopelagic copepod

Zooplanktonic taxa have a greater number of distinct populations and species than might be predicted based on their large population sizes and open‐ocean habitat, which lacks obvious physical barriers to dispersal and gene flow. To gain insight into the evolutionary mechanisms driving genetic diversification in zooplankton, we developed eight microsatellite markers to examine the population structure of an abundant, globally distributed mesopelagic copepod, Haloptilus longicornis, at 18 sample sites across the Atlantic and Pacific Oceans (n = 761). When comparing our microsatellite results with those of a prior study that used a mtDNA marker (mtCOII, n = 1059, 43 sample sites), we unexpectedly found evidence for the presence of a cryptic species pair. These species were globally distributed and apparently sympatric, and were separated by relatively weak genetic divergence (reciprocally monophyletic mtCOII lineages 1.6% divergent; microsatellite F_ST ranging from 0.28 to 0.88 across loci, P < 0.00001). Using both mtDNA and microsatellite data for the most common of the two species (n = 669 for microsatellites, n = 572 for mtDNA), we also found evidence for allopatric barriers to gene flow within species, with distinct populations separated by continental landmasses and equatorial waters in both the Atlantic and Pacific Ocean basins. Our study shows that oceanic barriers to gene flow can act as a mechanism promoting allopatric diversification in holoplanktonic taxa, despite the high potential dispersal abilities and pelagic habitat for these species.     

Genetic differentiation between populations of the European rose hip fly Rhagoletis alternata

We tested for genetic differentiation between populations of Rhagoletis alternata Fall. (Diptera: Tephritidae) on three different host species. We collected larvae from three rose species of the section Caninae ( Rosa canina L., Rosa corymbifera Borkh . , and Rosa rubiginosa L.) from 15 sites across Germany, where the three roses occurred together. Additionally, we sampled three sites in Switzerland. Roses differ in morphology (e.g. leaf glands) as well as phenology. We were able to score nine allozyme loci (five polymorphic). Populations from the three hosts did not differ in genetic variability. We found significant genetic differentiation between populations from different host species. However, the differentiation was very low (0.9%). Hence, we found no indication for host races. Furthermore, surprisingly little geographical structure of genetic differentiation was found between populations of this fruit fly across central Europe. We offer three mutually non-exclusive explanations for these findings. First, gene flow between populations of Rh. alternata is high. Second, the pattern of genetic differentiation is based on a recent expansion of the distributional range . Third, the ongoing gene flow between roses of the section Caninae acts as a hybrid bridge.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 619–625.     

Clones or clans: the genetic structure of a deep‐sea sponge,Aphrocallistes vastus,in unique sponge reefs of British Columbia,Canada

Understanding patterns of reproduction, dispersal and recruitment in deep‐sea communities is increasingly important with the need to manage resource extraction and conserve species diversity. Glass sponges are usually found in deep water (>1000 m) worldwide but form kilometre‐long reefs on the continental shelf of British Columbia and Alaska that are under threat from trawling and resource exploration. Due to their deep‐water habitat, larvae have not yet been found and the level of genetic connectivity between reefs and nonreef communities is unknown. The genetic structure of Aphrocallistes vastus, the primary reef‐building species in the Strait of Georgia (SoG) British Columbia, was studied using single nucleotide polymorphisms (SNPs). Pairwise comparisons of multilocus genotypes were used to assess whether sexual reproduction is common. Structure was examined 1) between individuals in reefs, 2) between reefs and 3) between sites in and outside the SoG. Sixty‐seven SNPs were genotyped in 91 samples from areas in and around the SoG, including four sponge reefs and nearby nonreef sites. The results show that sponge reefs are formed through sexual reproduction. Within a reef and across the SoG basin, the genetic distance between individuals does not vary with geographic distance (r = ?0.005 to 0.014), but populations within the SoG basin are genetically distinct from populations in Barkley Sound, on the west coast of Vancouver Island. Population structure was seen across all sample sites (global F_ST = 0.248), especially between SoG and non‐SoG locations (average pairwise F_ST = 0.251). Our results suggest that genetic mixing occurs across sponge reefs via larvae that disperse widely.     

Cryptic speciation, genetic diversity and gene flow in the California turret spider Atypoides riversi (Araneae: Antrodiaetidae)

Populations of the turret spider Atypoides riversi from eight central Californian sites were compared based on variation at ten allozyme loci. Multidimensional scaling of interpopulation genetic distances defined four population units (Coast Range, Sierran, Valley, Jenness Camp), corroborating the distinctness of Coast Range and Sierran populations indicated by a prior study. While the species status of these units has yet to be determined, Jenness Camp is the most likely to represent a new species, given its clear genetic uniqueness (two fixed allelic differences). Populations in all units were generally in Hardy–Weinberg equilibrium with no evidence of inbreeding, though variability was minimal (mean H _o = 2.8%, mean P  = 15.4%). Reduced variability in these populations may be the result of repeated bottlenecks, environmental homogeneity, and/or directional selection. Interpopulation differentiation within units was significant in the absence of intervening forest habitat and was substantially less in its presence, indicating that gene flow is likely only when forest corridors exist. To foster preservation of the existing gene pool and enhance participation in it, management of the units of At. riversi should focus on maintaining as many populations in situ as possible and facilitating connections between them, while also creating or restoring habitat for potential colonization.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 27–37.     

ECOLOGICAL BARRIERS TO GENE FLOW BETWEEN RIPARIAN AND FOREST SPECIES OF AINSLIAEA (ASTERACEAE)

Understanding the role of habitat‐associated adaptation in reducing gene flow resulting in population differentiation and speciation is a major issue in evolutionary biology. We demonstrate a significant role for habitat divergence in species isolation between two naturally hybridizing riparian and nonriparian plants, Ainsliaea faurieana and A. apiculata (Asteraceae), on Yakushima Island, Japan. By analyzing the fine‐scale population structure at six sympatric sites, we found that variations in leaf shape, geography, light conditions, and genotype were strongly correlated across riverbank–forest transitions. No evidence of effective gene flow was found between the two species across the majority of the transition zones, although the NewHybrid clustering analysis confirmed interspecific hybridization. However, a relatively high level of gene flow was observed across one zone with a more diffuse ecotone and intermediate flooding and light conditions, possibly generated by human disturbances. These results suggest that the barriers to gene flow between the riparian and forest species are primarily ecological. Additional common garden experiments indicated that the two species are adaptively differentiated to contrasting flooding and light environments. Overall, our study suggests that adaptations to different habitats can lead to the formation of reproductive isolating barriers and the maintenance of distinct species boundaries.     

Genetic diversity of captive forest musk deer (Moschus berezovskii) inferred from the mitochondrial DNA control region

Forest musk deer ( Moschus berezovskii ) were once distributed widely in China. However, wild populations have declined dramatically because of poaching and habitat loss. Captive breeding populations have been established for several decades, but the genetic backgrounds of most captive populations were unclear and the population sizes increased very slowly. To provide useful information for conservation and management of this species, we investigated the genetic diversity and population structure of forest musk deer by analysing a 582-bp fragment of the mitochondrial DNA (mtDNA) control region (CR) in three captive breeding populations in Sichuan Province, China. Ninety-four variable sites and 27 haplotypes were observed in 109 individuals, and the nucleotide and haplotype diversities were relatively high compared with those of other endangered mammals. Of the three investigated populations, the Maerkang population had the highest nucleotide diversity ( π  = 0.0568), haplotype diversity ( h  =   0.836) and average intra-population genetic distance (0.062). The analysis of molecular variance demonstrated that most variation occurred within samples and that there was significant differentiation of the three populations. Estimates of gene flow indicated that there were few genetic exchanges among the three populations. Building pedigree records and increasing gene flow between populations will be helpful for conserving these populations and this species.     

Gene flow among native bush rat,Rattus fuscipes (Rodentia: Muridae), populations in the fragmented subtropical forests of south‐east Queensland

Abstract Many natural populations in areas of continuous habitat exhibit some form of local genetic structure. Anthropogenic habitat fragmentation can also strongly influence the dynamics of gene flow between populations. We used eight microsatellite markers to investigate the population genetic structure of an abundant forest species, the Australian bush rat (Rattus fuscipes), in the subtropical forests of south‐east Queensland. Five sites were sampled, allowing pairwise comparisons within continuous habitat and across clearings. Weak, but significant population differentiation and a significant pattern of isolation by distance was detected over the small scale (<10 km) of this study. Fine‐scale analysis at a single site (<1 km) showed a significant correlation between individual female genetic distance and geographical distance, but no similar pattern among male individuals. There was no evidence of increased population differentiation across clearings relative to comparisons within continuous forest. This was attributed to dispersal within corridors of remnant and revegetated habitat between the forested areas. We concluded that an inherently restricted dispersal ability, female philopatry and natural habitat heterogeneity play an important part in the development of genetic structure among populations of R. fuscipes. It is important to understand the relationship between landscape features and the pattern of gene flow among continuous populations, as this allows us to predict the impact of fragmentation on natural populations.     

Social organization and genetic structure: insights from codistributed bat populations

The impact of ecology and social organization on genetic structure at landscape spatial scales, where gene dynamics shape evolution as well as determine susceptibility to habitat fragmentation, is poorly understood. Attempts to assess these effects must take into account the potentially confounding effects of history. We used microsatellites to compare genetic structure in seven bat species with contrasting patterns of roosting ecology and social organization, all of which are codistributed in an ancient forest habitat that has been exceptionally buffered from radical habitat shifts. Over one thousand individuals were captured at foraging sites and genotyped at polymorphic microsatellite loci. Analyses of spatially explicit genotype data revealed interspecies differences in the extent of movement and gene flow and genetic structure across continuous intact forest. Highest positive genetic structure was observed in tree-roosting taxa that roost either alone or in small groups. By comparison, a complete absence of genetic autocorrelation was noted in the cave-roosting colonial species across the study area. Our results thus reveal measurable interspecies differences in the natural limits of gene flow in an unmodified habitat, which we attribute to contrasting roosting ecology and social organization. The consequences of ecology and behaviour for gene flow have important implications for conservation. In particular, tree-roosting species characterized by lower vagility and thus gene flow will be disproportionally impacted by landscape-scale forest clearance and habitat fragmentation, which are prevalent in the study region. Our method also highlights the usefulness of rapid sampling of foraging bats for assaying genetic structure, particularly where roosting sites are not always known.     

Fine-scale population structure in a desert amphibian: landscape genetics of the black toad (Bufo exsul)

Environmental variables can strongly influence a variety of intra- and inter-population processes, including demography, population structure and gene flow. When environmental conditions are particularly harsh for a certain species, investigating these effects is important to understanding how populations persist under difficult conditions. Furthermore, species inhabiting challenging environments present excellent opportunities to examine the effects of complex landscapes on population processes because these effects will often be more pronounced. In this study, I use 16 microsatellite loci to examine population structure, gene flow and demographic history in the black toad, Bufo exsul , which has one of the most restricted natural ranges of any amphibian. Bufo exsul inhabits four springs in the Deep Springs Valley high desert basin and has never been observed more than several meters from any source of water. My results reveal limited gene flow and moderately high levels of population structure ( F _ST = 0.051–0.063) between all but the two closest springs. I found that the geographic distance across the arid scrub habitat between springs is significantly correlated with genetic structure when distance accounts for topography and barriers to dispersal. I also found very low effective population sizes ( N _e = 7–30) and substantial evidence for historical population bottlenecks in all four populations. Together, these results suggest that the desert landscape and B.   exsul 's high habitat specificity contribute significantly to population structure and demography in this species and emphasize the importance of considering behavioural and landscape data in conservation genetic studies of natural systems.     

Spatial genetic structuring in a vagile species, the European wood mouse

We examined the genetic structure of natural populations of the European wood mouse Apodemus sylvaticus at the microgeographic (<3 km) and macrogeographic (>30 km) scales. Ecological and behavioural studies indicate that this species exhibits considerable dispersal relative to its home-range size. Thus, there is potential for high gene flow over larger geographic areas. As levels of population genetic structure are related to gene flow, we hypothesized that population genetic structuring at the microgeographic level should be negligible, increasing only with geographic distance. To test this, four sites were sampled within a microgeographic scale with two additional samples at the macrogeographic level. Individuals ( n =415) were screened and analysed for seven polymorphic microsatellite loci. Contrary to our hypothesis, significant levels of population structuring were detected at both scales. Comparing genetic differentiation with geographic distance suggests increasing genetic isolation with distance. However, this distance effect was non-significant being confounded by surprisingly high levels of differentiation among microgeographic samples. We attribute this pattern of genetic differentiation to the effect of habitat fragmentation, splitting large populations into components with small effective population sizes resulting in enhanced genetic drift. Our results indicate that it is incorrect to assume genetic homogeneity among populations even where there is no evidence of physical barriers and dispersal can occur freely. In the case of A. sylvaticus , it is not clear whether dispersal does not occur across habitat barriers or behavioural dispersal occurs without consequent gene flow.     

Strong population genetic structure and contrasting demographic histories for the small-spotted catshark (Scyliorhinus canicula) in the Mediterranean Sea

Coastal and demersal chondrichthyans, such as the small-spotted catshark, are expected to exhibit genetic differentiation in areas of complex geomorphology like the Mediterranean Basin because of their limited dispersal ability. To test this hypothesis, we used a fragment of the mitochondrial cytochrome c oxidase subunit I gene and 12 nuclear microsatellite loci in order to investigate the genetic structure and historical demography of this species, and to identify potential barriers to gene flow. Samples were collected from the Balearic Islands, the Algerian Basin, the Ionian Sea, the Corinthian Gulf and various locations across the Aegean Sea. Additional sequences from the Atlantic and the Levantine Basin retrieved from GenBank were included in the mitochondrial DNA analysis. Both mitochondrial and nuclear microsatellite DNA data revealed a strong genetic subdivision, mainly between the western and eastern Mediterranean, whereas the Levantine Basin shared haplotypes with both areas. The geographic isolation of the Mediterranean basins seems to enforce the population genetic differentiation of the species, with the deep sea acting as a strong barrier to its dispersal. Contrasting historical demographic patterns were also observed in different parts of the species'' distribution, most notably a population growth trend in the western Mediterranean/Atlantic area and a slight decreasing one in the Aegean Sea. The different effects of the Pleistocene glacial periods on the habitat availability may explain the contrasting demographic patterns observed. The current findings suggest that the small-spotted catshark exhibits several genetic stocks in the Mediterranean, although further study is needed.     

Comparison of population-genetic structuring in congeneric kelp- versus rock-associated snails: a test of a dispersal-by-rafting hypothesis

Phylogeographic studies indicate that many marine invertebrates lacking autonomous dispersal ability are able to achieve trans-oceanic colonization by rafting on buoyant macroalgae. However, less is known about the impact of rafting on on-going population-genetic connectivity of intertidal species associated with buoyant macroalgae. We hypothesize that such species will have higher levels of population-genetic connectivity than those exploiting nonbuoyant substrates such as rock. We tested this hypothesis by comparing nuclear multilocus population-genetic structuring in two sister topshell species, which both have a planktonic larval phase but are fairly well segregated by their habitat preference of low-tidal bull-kelp holdfasts versus mid-to-low tidal bare rock. We analyzed population samples from four sympatric sites spanning 372 km of the east coast of southern New Zealand. The sampled region encompasses a 180 km wide habitat discontinuity and is influenced by a stable, northward coastal current. The level of connectivity was high in both species, and neither of them showed significant correlation between genetic and geographic distances. However, a significant negative partial correlation between genetic distance and habitat discontinuity was found in the rock-associated species, and estimates of migrant movement between sites were somewhat different between the two species, with the kelp-associated species more often yielding higher estimates across the habitat discontinuity, whereas the rock-associated species more often exhibited higher estimates between sites interspersed by rock habitats. We conclude that for species with substantial means of autonomous dispersal, the most conspicuous consequence of kelp dwelling may be enhanced long-distance dispersal across habitat discontinuities rather than a general increase of gene flow.     

Landscape scale genetic effects of habitat fragmentation on a high gene flow species: Speyeria idalia (Nymphalidae)

Detection of the genetic effects of recent habitat fragmentation in natural populations can be a difficult task, especially for high gene flow species. Previous analyses of mitochondrial DNA data from across the current range of Speyeria idalia indicated that the species exhibited high levels of gene flow among populations, with the exception of an isolated population in the eastern portion of its range. However, some populations are found on isolated habitat patches, which were recently separated from one another by large expanses of uninhabitable terrain, in the form of row crop agriculture. The goal of this study was to compare levels of genetic differentiation and diversity among populations found in relatively continuous habitat to populations in both recently and historically isolated habitat. Four microsatellite loci were used to genotype over 300 individuals from five populations in continuous habitat, five populations in recently fragmented habitat, and one historically isolated population. Results from the historically isolated population were concordant with previous analyses and suggest significant differentiation. Also, microsatellite data were consistent with the genetic effects of habitat fragmentation for the recently isolated populations, in the form of increased differentiation and decreased genetic diversity when compared to nonfragmented populations. These results suggest that given the appropriate control populations, microsatellite markers can be used to detect the effects of recent habitat fragmentation in natural populations, even at a large geographical scale in high gene flow species.     

Population structure on breeding grounds of Lake Malawi’s ‘twilight zone’ cichlid fishes

Aim Free‐ranging benthopelagic fishes often have large population sizes and high rates of dispersal. These traits can act to homogenize population structure across the distributional range of a species and to reduce the likelihood of allopatric speciation. The apparent absence of any barriers to gene flow among populations, together with prior molecular evidence for panmixia across the ranges of three species, has resulted in Diplotaxodon, a genus of benthopelagic cichlid fishes of Lake Malawi, being proposed as a candidate case of sympatric speciation. Our aim was to further investigate this possibility by testing for intraspecific genetic subdivision among breeding populations, and intraspecific differences in breeding habitat. Location Lake Malawi, central‐east Africa. Methods We analysed eight microsatellite DNA loci to test for spatial genetic differences among populations on breeding grounds of eight Diplotaxodon species. We also tested for temporal population genetic differences within breeding grounds of three species. Records of ripe Diplotaxodon encountered during sampling were analysed to test if spatial variation in assemblage structure was linked to nearshore water depth and geographic proximity of sampling sites. Results Consistent with previous molecular evidence, within four of the eight species tested we found no evidence of spatial genetic structuring among breeding populations. However, within the other four species we found slight yet significant spatial genetic differences, indicating restricted gene flow among breeding grounds. There was no evidence of temporal genetic differences within sites. Analyses of the distributions of ripe Diplotaxodon revealed differences in assemblage structure linked to nearshore water depth. Main conclusions Together, these results demonstrate both the evolution of fidelity to deep‐water breeding locations in some Diplotaxodon species, and differences in breeding habitat among species. These findings are consistent with a role for divergence of breeding habitat in speciation of these cichlids, possibly promoted by dispersal limitation among geographically segregated spawning aggregations.