Inverting the null-hypothesis of speciation: a marine snail perspective

Speciation is currently an intensely debated topic, much more so than 20–30 years ago when most biologists held the view that new species (at least of animals) were formed through the split of evolutionary lineages by the appearance of physical barriers to gene flow. Recent advances have, however, lent both theoretical and empirical support to speciation in the presence of gene flow. Nevertheless, the allopatric hypothesis of speciation is still the default model. The consequence of this is that to support sympatric and parapatric modes of speciation all allopatric alternatives must be rejected, while an allopatric explanation is usually accepted without rejecting possible non-allopatric alternatives. However, classical cases of allopatric speciation can be challenged by alternative non-allopatric explanations, and this begs for a more respectful view of how to deal with all models of speciation. An appealing approach is studying parallel evolution of reproductive barriers, which allows for comparative approaches to distinguish between allopatric and non-allopatric events, and explicit tests of a suitable null-hypothesis. Parallel evolution of reproductive isolation in a strongly polymorphic marine snail species serves as an illustrative example of such an approach. In conclusion, a more balanced debate on allopatric and non-allopatric speciation is needed and an urgent issue is to treat both allopatric and nonallopatric hypotheses critically, rather than using allopatry as the default model of speciation.     

Latitude, elevational climatic zonation and speciation in New World vertebrates

Many biodiversity hotspots are located in montane regions, especially in the tropics. A possible explanation for this pattern is that the narrow thermal tolerances of tropical species and greater climatic stratification of tropical mountains create more opportunities for climate-associated parapatric or allopatric speciation in the tropics relative to the temperate zone. However, it is unclear whether a general relationship exists among latitude, climatic zonation and the ecology of speciation. Recent taxon-specific studies obtained different results regarding the role of climate in speciation in tropical versus temperate areas. Here, we quantify overlap in the climatic distributions of 93 pairs of sister species of mammals, birds, amphibians and reptiles restricted to either the New World tropics or to the Northern temperate zone. We show that elevational ranges of tropical- and temperate-zone species do not differ from one another, yet the temperature range experienced by species in the temperate zone is greater than for those in the tropics. Moreover, tropical sister species tend to exhibit greater similarity in their climatic distributions than temperate sister species. This pattern suggests that evolutionary conservatism in the thermal niches of tropical taxa, coupled with the greater thermal zonation of tropical mountains, may result in increased opportunities for allopatric isolation, speciation and the accumulation of species in tropical montane regions. Our study exemplifies the power of combining phylogenetic and spatial datasets of global climatic variation to explore evolutionary (rather than purely ecological) explanations for the high biodiversity of tropical montane regions.     

Uptake, distribution, and speciation of chromium in Brassica juncea

Brassica juncea (Indian mustard) has been widely used in phytoremediation because of its capacity to accumulate high levels of chromium (Cr) and other metals. The present study was conducted to investigate mechanism(s) involved in Cr binding and sequestration by B. juncea. The plants were grown under greenhouse conditions in field-moist or air-dried soils, amended with 100 mg kg(-1) of Cr (III) or VI). The plant concentrated Cr mainly in the roots. B. juncea removed an average of 48 and 58 microg Cr per plant from Cr (III) and Cr (VI)-treated soils, respectively. The uptake of Cr was not affected by the moisture status of the soils. X-ray absorption near-edge spectroscopy measurements showed only Cr (III) bound predominantly to formate and acetate ligands, in the bulk and rhizosphere soils, respectively. In the plant tissues, Cr (III) was detected, primarily as acetate in the roots and oxalate in the leaves. X-ray microprobe showed the sites of Cr localization, and probably sequestration, in epidermal and cortical cells in the roots and epidermal and spongy mesophyll cells in the leaves. These findings demonstrate the ability of B. juncea to detoxify more toxic Cr (VI), thereby making this plant a potential candidate for phytostabilization.     

Patterns of ecological diversity in fossil and modern mammalian faunas

Ecological diversity provides a means of analysing the community structure of fossil mammalian faunas in order to obtain information on the habitat of the fauna. As a basis for the analysis, 23 modern mammalian communities from distinct habitats have been used to establish patterns of community structure for tropical African habitats according to their species diversity by taxonomic group, size, locomotor zonal adaptation, and feeding adaptation. All the communities tested were in tropical Africa, but additional analyses on tropical forest communities in Australia, Malaya, and Panama have shown that these communities, which all have completely different species composition, nevertheless have community structures very similar to each other and to those of the African forest communities.     

西沙群岛海域造礁石珊瑚物种多样性与分布特点

2006年3-5月,采用国际通用的截线样带法对海南省西沙群岛主要珊瑚礁海域的18个岛礁造礁石珊瑚进行了实地调查,共记录13科45属188种,其中62个新记录种.整合文献资料,该海域共记录造礁石珊瑚13科50属204种,基本包括分布在我国(不含台湾省)的所有造礁石珊瑚科属和绝大多数种类.造礁石珊瑚以生长速度快的分枝状珊瑚为主.物种多样性最高的是华光礁海域、金银岛海域和永兴岛海域.聚类分析可以把18个调查岛礁分成3个类群:有人类居住的大型环礁群、独立岛礁群和无人居住的大型环礁群(华光礁).为了有效保护珍稀的造礁石珊瑚资源,按照其种类多样性和分布特点重点保护关键海域是十分必要的.     

Relative effects of time for speciation and tropical niche conservatism on the latitudinal diversity gradient of phyllostomid bats

Determinants of contemporary patterns of diversity, particularly those spanning extensive latitudinal gradients, are some of the most intensely debated issues in ecology. Recently, focus has shifted from a contemporary environmental perspective to a historical one in an attempt to better understand the construction of latitudinal gradients. Although the vast majority of research on historical mechanisms has focused on tropical niche conservatism (TNC), other historical scenarios could produce similar latitudinal gradients. Herein, I formalize predictions to distinguish between two such historical processes--namely time for speciation (TFS) and TNC--and test relative support based on diversity gradients of New World bats. TFS and TNC are distinctly spatial and environmental mechanisms, respectively. Nonetheless, because of the way that environmental characteristics vary spatially, these two mechanisms are hard to distinguish. Evidence provided herein suggests that TNC has had a more important effect than TFS in determining diversity gradients of New World bats. Indeed, relative effects of different historical mechanisms, as well as relative effects of historical and contemporary environmental determinants, are probably context-dependent. Future research should move away from attempting to identify the mechanism with primacy and instead attempt to understand the particular contexts in which different mechanisms have greater influence on diversity gradients.     

The distribution of pond snail communities across a landscape: separating out the influence of spatial position from local habitat quality for ponds in south-east Northumberland,UK

Ponds support a rich biodiversity because the heterogeneity of individual ponds creates, at the landscape scale, a diversity of habitats for wildlife. The distribution of pond animals and plants will be influenced by both the local conditions within a pond and the spatial distribution of ponds across the landscape. Separating out the local from the spatial is difficult because the two are often linked. Pond snails are likely to be affected by both local conditions, e.g. water hardness, and spatial patterns, e.g. distance between ponds, but studies of snail communities struggle distinguishing between the two. In this study, communities of snails were recorded from 52 ponds in a biogeographically coherent landscape in north-east England. The distribution of snail communities was compared to local environments characterised by the macrophyte communities within each pond and to the spatial pattern of ponds throughout the landscape. Mantel tests were used to partial out the local versus the landscape respective influences. Snail communities became more similar in ponds that were closer together and in ponds with similar macrophyte communities as both the local and the landscape scale were important for this group of animals. Data were collected from several types of ponds, including those created on nature reserves specifically for wildlife, old field ponds (at least 150 years old) primarily created for watering livestock and subsidence ponds outside protected areas or amongst coastal dunes. No one pond type supported all the species. Larger, deeper ponds on nature reserves had the highest numbers of species within individual ponds but shallow, temporary sites on farm land supported a distinct temporary water fauna. The conservation of pond snails in this region requires a diversity of pond types rather than one idealised type and ponds scattered throughout the area at a variety of sites, not just concentrated on nature reserves. Handling editor: B. Oertli     

Spatial distribution and morphologic diversity of virioplankton in Lake Donghu, China

The spatial distribution and morphological diversity of virioplankton were determined in Lake Donghu which contains three trophic regions: hypertrophic, eutrophic and mesotrophic region. Virioplankton abundance measured by transmission electron microscope (TEM) ranged from 7.7 × 10⁸ to 3.0 × 10⁹ ml^–1, being among the highest observed in any natural aquatic system examined so far. The spatial distribution of virioplankton was correlated significantly with chlorophyll a concentration (r = 0.847; P < 0.01) at the sampling sites in Lake Donghu. 76 morphotypes were observed. Most morphotypes have tails, belonging to Siphoviridae, Myoviridae and Podoviridae. The majority of tailed phages in the lake were Myoviridae. Morphotypes which were rarely reported, such as prolate-headed virus-like particles, lemon-shaped virus-like particle, and viruses resembling Tectiviridae and Corticoviridae were all observed in the lake. It is concluded that the high viral abundance might be associated with high density of phytoplankton including algae and cyanobacteria. There was high viral diversity in this eutrophic shallow lake. In addition, cyanophage represented an important fraction of the virioplankton community in Lake Donghu.     

Plasmids from Lactobacillus helveticus: distribution and diversity among natural isolates

AIMS: To investigate the distribution and the level of diversity of extrachromosomal molecules in Lactobacillus helveticus strains in relation to their different ecological niches. METHODS AND RESULTS: The plasmid profile of 22 Lact. helveticus strains, isolated from five different Italian cheeses, was determined. Among the tested strains, there was a variable presence of plasmids: eight plasmid-free strains and the remaining with several plasmids that could be differentiated on the basis of number and molecular weight. The profiles showed between one and five plasmid bands, which size ranged between 2.3 and 31 kb. Four of these plasmids were further analyzed by restriction digestion and compared with the plasmids from Lact. helveticus ATCC 15009(T). Analyses and comparison of their primary structures and hybridization experiments revealed the presence of different DNA homology groups. CONCLUSIONS: This study indicates that within Lact. helveticus species, there is a high degree of variability in relation to the presence of plasmid molecules. Moreover, the structural diversity found among some of these plasmids allows to hypothesize the presence of different evolutionary lineages. SIGNIFICANCE AND IMPACT OF THE STUDY: Studies on plasmid distribution and diversity should be considered as an essential component in a continuing effort to explore microbial diversity as well as to understand the real role of plasmids in the flow of genetic information in natural bacterial communities.     

Progressive invasion and allopatric speciation can also explain distribution patterns of rock-dwelling cichlids from southern Lake Victoria: a comment on Seehausen and van Alphen (1999)

I show that distribution patterns of rock-dwelling haplochromine cichlids in the south-eastern part of Lake Victoria can be explained by allopatric speciation alone. Observed patterns are consistent with a progressive invasion of different lineages from older ( = deeper) parts of the lake, after which some lineages speciate more frequently than others. Based on a quantitative analysis of distribution data, Seehausen and van Alphen conclude that a high percentage of sympatric speciation (17.5%) is essential to explain the patterns. In my view this conclusion has its origins in an unjustifiable assumption. Seehausen and van Alphen assume that the unresolved phylogeny is balanced. However, allopatric speciation would typically result in imbalanced phylogenies.     

Methods of studying the distribution, diversity and abundance of birds in East Africa—some quantitative approaches

In this paper we compare the use of transect counts with a simpler method of investigating bird diversity and numbers, particularly in terrestrial habitats, both natural and non‐natural. Transect Counts (TCs) have long been widely used, whereas Timed Species Counts (TSCs), which estimate relative abundance, are comparatively untried. We find that TSCs give results which are comparable to those from TCs in most respects, except that they can only be used indirectly for estimating population densities, and they give different measures of diversity. However, TSCs generate data on many more species much faster than do TCs and are therefore more cost‐effective in most situations. In particular, TSCs are useful for community studies. We show, for example, that in natural habitats bird populations are positively correlated with the amount of woody vegetation, but not with rainfall. Diversity too increases with woody vegetation. Because TSCs are simple, more of them can be made for a given input of time, and hence more distributional data are obtained as an additional benefit.     

The development of diversity in the land snail fauna of the Madeiran archipelago

The diversity and distribution of the Madeiran land snail fauna is analysed in relation to the geological and environmental history of the archipelago. High levels of single-island endemism, the varying balance of families and genera between islands and the local differentiation of populations within islands all suggest that speciation has been predominantly a within island phenomenon. Restricted patterns of distribution, nevertheless, suggest that speciation is predominantly allopatric and that it is driven by repeated and sometimes devastating environmental changes. An equilibrium model of faunal diversity is rejected; the fauna is supersaturated at archipelago level, yet individual sites are not species rich, nor are all niches full. The consequences for the success and effects of invasions and introductions are discussed, and questions still to be answered raised, in particular about the timing of colonization events and their number, and about the great range of rates of radiation apparently achieved by different stocks.     

Assessing abundance and diversity patterns of soil microarthropod assemblages in northern Victoria Land (Antarctica)

This study aimed to: (1) assess differences between two quantitative sampling methods of soil microarthropods (visual census vs. stone washing) in ice-free areas located along a latitudinal gradient (from 72°37′S to 74°42′S) in northern Victoria Land (Antarctica); (2) furnish preliminary results on the abundance and diversity of mites and springtails in the studied areas. Visual census yielded reliable density estimates for adult collembolans and larger prostigmatic mites but did not detect small species. The study updates the distribution of several mites, including the southernmost record of an Oribatida species at global scale. Species composition was correlated with latitude but the uneven abundance distribution and local high beta-diversity probably reflect habitat fragmentation and population isolation. Under this circumstance nested sampling design should be usefully employed. Priorities and suitable methods for studying terrestrial microarthropod communities in continental Antarctica are discussed.     

Dispersal and speciation of skinks among archipelagos in the tropical Pacific Ocean

Summary We examined the potential effects of geography on the distribution and speciation of skinks on tropical Pacific archipelagos. The entire tropical Pacific skink fauna was divided into continental (found also in continental areas), Pacific (endemic to the study area but found within more than one archipelago) and endemic (found within only one archipelago) species categories. The number and proportion of skinks within each species category were determined for each of the 27 archipelagos in the study area. Nine geographic variables reflecting archipelago size, isolation and elevation were estimated for each archipelago. Principal components analysis was used to reduce the nine variables to three uncorrelated composite variables that were interpreted as representing archipelago size, isolation and elevation. Numbers and proportions of skinks in each category within an archipelago were related to the composite geographic variables using multiple linear regression analysis. Archipelago size and isolation were important predictors of both skink diversity and endemism. Results were then compared to diversity and endemism of birds within the study area. Skinks showed an archipelago-wide level of endemism similar to that of birds. On an archipelago by archipelago basis, however, large differences between birds and skinks were evident. In particular, the New Caledonia skink fauna was much more endemic than that of birds. The bird faunas of Hawaii and the Marquesas were nearly completely endemic, while no endemic skinks occurred in these two archipelagos. These differences presumably reflect the relative dispersal powers of skinks and birds and, consequently, rates of colonization and speciation. Differences may also be due partly to morphological conservatism among isolated skink populations and the occurrence of cryptic species that have not yet been identified as separate species. The discovery of such cryptic species, however, is unlikely to increase the endemic skink fauna of Hawaii and other distant archipelagos to a level commensurate with that of birds. Differences in endemism between skinks and birds may also be due to unknown local ecological interactions.     

Spatial distribution and diversity of chironomid assemblages in a small hilly stream (Diptera: Chironomidae)

In 2006–2007, spatial variations of chironomid assemblages of a small chalky watercourse were investigated in the hilly region of Balaton Upland, Hungary. Samplings were carried out every three weeks at seven characteristically different sites along the whole length of the watercourse. Altogether 2966 specimens were collected and identified. The chironomid fauna of the stream was relatively rich with 40 taxa belonging to four subfamilies (eight Tanypodinae, one Prodiamesinae, 17 Orthocladiinae, 14 Chironominae). The lowest number of species and number of individuals was found at the spring, and the highest at the middle reach. Based on multivariate analysis the different sections of the creek were characterised by distinctive chironomid communities. Although longitudinal changes in the chironomid community of the Örvényesi creek were detectable due to changes in natural environmental factors, land-use changes also significantly influenced the chironomid community composition and diversity. The natural and anthropogenic effects were hardly distinguishable. Furthermore, we found that the response of different diversity indices to environmental variability was different. This draws attention to the difficulties in evaluating the relationships between stream communities and environmental variables using different diversity indices. The taxonomic diversity, which incorporates the taxonomical relationships between the specimens, proved to be one of the most useful diversity indices to characterise the stream chironomid assemblages.     

Floristic diversity in the Cape Flora of South Africa

Comprising a land area of ca 90000 km2, less than 4% of the total land area for the Southern African subcontinent, the Cape Floristic Province is one of the world's richest areas in terms of botanical diversity for its size. An estimated 8650 species of vascular plants occur in this area, about 65% of which are endemic. This is about 42% of the estimated total for all of southern Africa. The number of species packed into so small an area is remarkable for the temperate zone, and compares closely with species totals for areas of comparable size in the wet tropics. The Cape Region consists of a mosaic of sandstone and shale substrates that give rise to soils of quite different types, and in addition local areas of limestone add to the edaphic diversity. Climates across the region are extremely variable, and the predominant orographic rainfall pattern ranges from 2000mm locally to less than 100mm, often with extremely steep gradients, the result of a mountainous landscape. The edaphic diversity resulting from a mosaic of different soils is compounded by sharp local gradients in precipitation that creates an unusual number of local habitats. A feature of the some of the soils in the Region is low nutrient levels and many of the plants on such soils have low seed dispersal capabilities, a factor important in explaining the high levels of local endemism. Species richness in the Cape Region is hypothesized to have resulted from the presence of a complex mosaic of diverse habitats and steep ecological gradients against a background of relatively stable climate and geology after the mediterranean climate was established there sometime after the beginning of the Pliocene. A local or ecological mode of speciation may have been more important under these conditions than allopatric speciation.     

Phylogeny,speciation, and distribution of the Japanese dwarf bamboos of genus Sasa and allies

Development of the concept of the subfamily Bambusoideae during the past three decades is briefly reviewed. Phylogeny, putative hybrid taxa, a new, simplified key to explicit identification, and the revision of the distribution range among the Japanese dwarf bamboos of genus Sasa and its close allies are discussed. Based on analyses of the random amplified polymorphic DNA and morphological data and of FLOWERING LOCUS T (FT) homologs, parsimonious trees resolved that genus Sasa sec. Macrochlamys were the most basal and sect. Monilicladae were the next most basal, followed by the terminal sister clades of sects. Sasa and Crassinodi.