Patterns Of Geographic Differentiation Research Articles

Habitat fragmentation and the structure of genetic diversity within disjunct isolates of Anthericum ramosum L. (Anthericaceae) in Scandinavia

The lilioid herb, Anthericum ramosum, occurs in four geographically-isolated regions (Sjælland, Skåne, Öland and Gotland) in Denmark and southern Sweden. We investigated allozyme variation at nine polymorphic loci in A. ramosum from 16 sites (33 populations) in the four regions. There was no clear overall geographic pattern of differentiation between the regions, but the southernmost Gotland and the Öland populations had similar allele frequencies, suggesting that they have had a common history. The total genetic diversity (Htot) was 0.458 and the between-region, site- and population components of diversity accounted, respectively, for 13%, 10% and 2% of the total diversity. The species is restricted to grassland habitats. Such habitats have become increasingly rare in the Sjælland and Skåne regions, where A. ramosum now has a highly fragmented distribution. Within three of the regions (Sjælland, Skåne and Öland) there was a negative relationship between the extent of grassland habitat and the between-site components of genetic diversity. Öland, with its extensive grassland habitats and low levels of population disjunction, showed little allelic differentiation between sites and the lowest between-site component of diversity (3%), suggesting that there is (or has been) extensive gene flow between sites. The between-site components of diversity were higher within Skåne (7%) and Sjælland (12%). The high within-region GST(25%) for the fourth region, Gotland, cannot be explained in terms of recent habitat disjunction but is, instead, interpreted in terms of the restricted distribution of limestone bedrock on Gotland and the fact that the southern and northern Gotland populations appear to have had different origins.

MAMMALS OF THE RIO JURUÁ AND THE EVOLUTIONARY AND ECOLOGICAL DIVERSIFICATION OF AMAZONIA

We describe the nonvolant mammal fauna of the Rio Juruá of the western Amazon of Brazil, based on collections made during a year-long survey of the river. We, along with our colleagues Drs. Claude Gascon and Carlos Peres, designed the field project to examine the effects of the river on the differentiation among terrestrial vertebrates (mammals, birds, and amphibians and reptiles) at both the community and population levels. This monograph examines only the patterns of geographic variation and community structure of the small-bodied mammals. Species inventories were made at 16 primary trapping localities divided into eight pairs of cross-river sites, with two pairs in each of four regions from near the mouth to the headwaters of the Rio Juruá. A total of 81 species of nonvolant mammals were obtained, including nine new to science. Four of these are described herein; the others have been described elsewhere. We used a standardized trapping protocol to assess community structure at each of the 16 localities that included terrestrial and canopy trap stations in floodplain (várzea) and upland (terra firme) forest formations. Supplemental trapping was done in secondary habitats at all sites. We describe these sites, the trap effort expended, and the placement of trap stations relative to local habitats. We also describe each species of marsupial, sciurid rodent, murid rodent, and echimyid rodent encountered; comment on their systematics; and summarize aspects of habitat use, life history, geographic distribution, and geographic differentiation based on morphological and molecular traits. We examine patterns of differentiation in the mitochondrial cytochrome-b gene for samples of 41 of the 45 species of marsupials and rodents obtained within the Rio Juruá Basin, and discuss these patterns from the perspective of the entire Amazon and, in some cases, the Mata Atlântica of coastal Brazil. We also examine patterns of community organization within the Rio Juruá basin and throughout Amazonia, drawing attention to the geographic distribution of what appear to be major faunal units that are independent of habitat differences. Finally, we use principles of phylogeography to analyze patterns of geographic differentiation among the nonvolant mammals with regard to the Riverine Barrier Hypothesis. We show that, while there are few examples of taxa for which the Rio Juruá is apparently a barrier, most taxa either are largely undifferentiated throughout the basin or are sharply divided into reciprocally monophyletic mtDNA haplotype clades separable into upriver and downriver units. We argue that the concordance in the geographic placement of clade boundaries suggests a common history; moreover, both the age of these clades and their geographic position in relation to underlying geological features suggest that landform evolution has been an important, but underappreciated component of diversification within western Amazonia.

Population structure of the Dover sole, Solea solea L., in a background of high gene flow

To investigate the genetic population structure of the Dover sole, Solea solea L., allozyme electrophoresis was performed on 303 fish collected from seven locations ranging from Cumbria, Great Britain, to Greece. A total of 22 enzyme systems were analysed, coded by 33 loci. Of these, 27 loci were polymorphic using the P 99 criterion. A phenogram using Prevosti's Distance generated by the Wagner method exhibited a geographic pattern in the clustering of populations. Estimates of N m (effective number of migrants per generation between populations) were sufficiently high to imply near-panmixia between the North Sea, Bay of Biscay and the Irish Coast populations, indicating a probable movement of migrants through the English Channel. However, despite this high level of gene flow, striking patterns of geographic differentiation were observed at a few loci. Allele frequencies at loci ACOH, EST-I-1, PEP-I-2 exhibited genetic patchiness on both local and range-wide (within the northern and southern European basins) scales. This pattern of genetic patchiness could be the result of localised selection, genetic drift or single-generation sampling effects. Estimates of mean heterozygosity ( H) were inversely related to latitude. Evolutionary processes such as genetic drift and founder effect, and/or selection, may have produced the observed difference in the number of alleles between the basins. Moreover, the absence of isolation by distance provides support for a model of geographic isolation. Such a pattern of genetic patchiness, revealing a slight reduction of genetic variability in the northern European basin, may suggest a population bottleneck, or local reduction in population size. Various physical parameters, especially water temperature during the reproductive period, vary within the range of the species, and may produce or maintain this genetic differentiation. These results indicate the role of both ecological and evolutionary structuring mechanisms in determining the genetic population structure of S. solea.

Allozyme variation in natural and cultured populations in two tilapia species: Oreochromis niloticus and Tilapia zillii

We investigated the electrophoretic polymorphism at 30 protein loci in 27 wild and cultured populations of two tilapia species, Tilapia zillii and Oreochromis niloticus. Single and joint segregations were analysed at 12 polymorphic loci in full-sib families. All the loci showed single segregation in agreement with the Mendelian expectations, and two nonrandom joint segregations were found. In total, 26 loci were polymorphic and 12 were diagnostic between the two species. A pronounced differentiation was observed between the Ivory Coast and Nilo-Sudanian T. zillii populations (average Nei's standard genetic distance = 0.13). Within the Nilo-Sudanian region, the level of variation observed in O. niloticus and T. zillii was rather low when compared with other freshwater species analysed at similar macrogeographical scales. The same pattern of geographical differentiation was found between the west and east African populations in both species suggesting that this structuring mainly reflected the same palaeo-geographical events. Most of the geographical variation observed in the Nile tilapia populations analysed here was present in the cultured stocks. In contrast to fish-farmed stocks analysed in other countries, those from the Ivory Coast and Niger displayed neither evidence of loss of genetic diversity nor any trace of introgression with other cultured tilapia species, indicating that these stocks have been properly managed.

Chance events, habitat age, and the genetic structure of pond populations

We surveyed the genetic structure of zooplankton populations in small natural ponds to measure geographic patterns of differentiation and to evaluate the rates of genetic exchange. Populations of eight taxa (the calanoid Aglaodiaptomus clavipoides, A. conipedatus, A. stagnalis, Aglaodiaptomus sp., Onychodiaptomus sanguineus, and Osphranticum labronectum, the cladoceran Daphnia laevis, and the conchostracan Lynceus gracilicornis) were sampled from 26 ponds in the uplands and alluvial terraces of the Savannah River Site near Aiken, South Carolina, USA. Genetic divergence ○÷ was high, with values of 0.053-0.297 for three calanoid copepods and one cladoceran

Population genetics of the blue crabCallinectes sapidus: modest population structuring in a background of high gene flow

To determine the genetic population structure of blue crabs (Callinectes sapidus Rathbun), electrophoretic allozyme analysis was performed on 750 individuals collected from 16 nearshore locations ranging from New York to Texas, USA. Twenty enzymes and non-enzymatic proteins coded by 31 presumptive loci were examined. Twenty-two loci were either monomorphic or polymorphic at less than theP95 level; alleles for these polymorphic loci were geographically dispersed. Allele frequencies for three of the remaining polymorphic loci were homogeneous over all populations, as were levels of polymorphism and heterozygosity. Phenograms generated by the UPGMA (unweighted pair-group method using arithmetic averages) and distance Wagner methods exhibited no geographic pattern in the clustering of populations. Estimates ofNem (effective number of migrants per generation between populations) indicated substantial gene flow, with aalues sufficiently high to infer panmixia between all blue crab populations from New York to Texas. However, despite this high level of gene flow, two striking patterns of geographic differentiation occurred: genetic patchiness and clinal variation. Allele frequencies atEST-2, GP-1, IDHP-2, DPEP-1, DPEP-2, andTPEP exhibited genetic patchiness on local and range-wide geographic scales, and allele frequencies atEST-2 varied temporally. Genetic patchiness in blue crabs is likely to be the result of the pre-settlement formation and subsequent settlement of genetically heterogeneous patches of larvae; allele frequencies of those larval patches may then be further modified through ontogeny by localized selection. In the Atlantic Ocean, a regional latitudinal cline ofEST-2 allele frequencies was superimposed on the range-wide genetic patchiness exhibited by that locus. This pattern against a background of high gene flow is highly likely to be maintained by selection. In estuaries along the Atlantic Ocean coast, a combination of low adult long-distance migration and a high retention rate of locally spawned larvae could serve to segregate populations and allow for the development of the geographic cline inEST-2. The Gulf of Mexico showed no apparent cline, perhaps due to long-distance migration of females in some regions of the Gulf, or to masking by genetic patchiness. These results emphasize the importance of both ecological and evolutionary time scales and structuring mechanisms in determining genetic population structure.

Geographic Variation in Coloration and Morphology of the Acorn Woodpecker

Although geographic variation in coloration and morphology of Acorn Woodpeckers (Melanerpes formicivorus) is thought to be slight, a sedentary species restricted to a particular habitat across a broad geographic range may be expected to exhibit some geographic differentiation. I analyzed seven morphometric and five plumage coloration characters on 1,531 study skins from across the species' geographic range. Dividing the specimens examined into 25 population samples, I found highly significant geographic differences in most of the characters analyzed, including clinal patterns and discrete differences in both morphometric and plumage characters. Patterns of geographic differentiation in the two character sets were not significantly concordant. It is possible to identify seven geographic units within the species, each of which is diagnosable based on combinations of discrete and continuous character states. Resumen. A pesar de que se ha pensado que la variación geográfica en el Carpintero Arlequín (Melanerpes formicivorus) es mínima, se puede esperar que una especie sedentaria y restringida a un hábitat particular que se distribuye sobre un área de distribución muy amplia, mostrará diferenciación en alguna escala geográfica. En este trabajo, se analizaron siete caracteres morfométricos y cinco de coloración del plumaje en 1,531 especímenes de toda el área geográfica de la especie. Se encontró variación geográfica significativa en la mayoría de los caracteres analizados, incluyendo patrones clinales y diferencias discretas entre poblaciones. Basándose en estos patrones, es posible identificar siete unidades geográficas en la especie, cada una diagnosticable con estados de caracteres discretos.

Biochemical Polymorphism and Its Relationship to Chromosomal and Morphological Variation in Peromyscus leucopus and Peromyscus gossypinus

Allozymic variation was used to examine patterns of geographic differentiation and variability in Peromyscus leucopus and P. gossypinus . The pattern of relationships based on genetic similarity was not concordant with either recognized subspecihc status of either species or with the major chromosomal subdivision in P. leucopus . The pattern was consistent with the isolation-by-distance model as tested by Mantel analysis, and differentiation at the variable loci was generally spatially autocorrelated. Differences between the species were indicative of complete speciation with little or no hybridization in areas of sympatry.

GEOGRAPHIC VARIATION IN THE MILKFISH CHANOS CHANOS. I. BIOCHEMICAL EVIDENCE.

Since the first use of protein gel electrophoresis for the assessment of genic variation in natural populations (Lewontin and Hubby, 1966), a large number of plants and animals across many phylogenetic lines has been surveyed for electrophoretic variation. Results of these surveys and synopses of the related evolutionary hypotheses have been summarized in Lewontin (1974), Ayala (1976), and Nevo (1978). In reviewing these reports, it is apparent that one group of organisms has been generally overlooked: the marine fishes. For instance, many workers have examined the population structure of freshwater fishes (Avise and Selander, 1972; Avise and Smith, 1974; Echelle et al., 1975; Allendorf et al., 1976; Vrijenhoek et al., 1977; and others), yet there are virtually no data available for widely distributed marine fishes. In comparable studies of marine fishes, the work has dealt with diadromous fishes (Koehn and Williams, 1978; Utter et al., 1980), estuarine fishes (Johnson, 1975), or fishes from restricted geographic areas (Gorman et al., 1976; Gorman and Kim, 1977; Ward and Beardmore, 1977). In only one report were genetic differences assessed for a marine fish found in separate localities across the tropics, but this study remains unpublished (Soule, reference in Ehrlich, 1975). It is generally assumed that because of a planktonic larval stage, marine fish have a high potential for gene flow and hence may show little interlocality differentiation, but this topic remains unexplored. It would be interesting, therefore, to assess patterns of geographic differentiation of fish populations within and among archipelagos across an ocean basin. It has long been realized that small and/ or isolated populations may undergo considerable genetic changes. Semi-isolated populations of cavefish Astyanax mexicanus had reduced levels of electrophoretic variation relative to conspecific surface populations (Avise and Selander, 1972), and stream populations of Etheostoma radiosum, separated in different drainages of the same tributary, exhibited high levels of population differentiation (Echelle et al., 1975). In a marine habitat the epitome of isolation of an ichthyofauna is the Hawaiian inshore fish community. Randall (1976) estimates the level of endemism for the inshore fishes is 29%-the highest value for any oceanic island. Yet, there have been no electrophoretic estimates of the degree of isolation of the fishes in Hawaii. Thus, using electrophoretic data, the following questions were examined for the milkfish Chanos chanos: 1) What is the level of genic variation in this species and how does it change over large geographic distances? 2) What is the genetic population structure for a marine fish, such as Chanos, that has a planktonic larval stage? 3) What factors affect the observed pattern of geographic variation and how does the level of geographic differentiation compare to that of other species-terrestrial and marine? 4) How genetically isolated is the Hawaiian population of milkfish relative to other milkfish populations? Life history of milkfish. -Milkfish, Chanos chanos (Forskal), are tropical marine fish distributed throughout the Pacific 1 Hawaii Institute of Marine Biology Contribution No. 586.

Biochemical Polymorphism and Systematics in the Genus Peromyscus VII. Geographic Differentiation in Members of the Truei and Maniculatus Species Groups

Allozymic variation at 22 gene loci in populations of Peromyscus truei, P. difficilis, P. melanotis , and P. maniculatus is used to examine patterns of geographic differentiation. Samples of P. maniculatus collected throughout most of its immense range, from southern Mexico to northern Canada and from the Pacific to Atlantic coasts, are very similar in genic composition. Evidence from morphology, chromosomal configurations, ecology, and patterns of reproductive isolation argue that contemporary gene flow among P. maniculatus populations is not sufficient to account for the similarity in their allelic configurations. The macrogeographic conservatism in level of genic divergence conceivably results from relatively recent separation of populations, coupled with a genetic inertia resulting from a selected cohesion of the genome. Nonetheless, significant intersample heterogeneity of allele frequencies at polymorphic loci may certainly result from stochastic effects. The relative geographic uniformity in P. maniculatus contrasts somewhat with the geographic differences observed in P. truei and in P. difficilis . Members of the truei and maniculatus species groups are added to a biochemical dendrogram, which now includes 20 named species of Peromyscus .