Species-level phylogeography and evolutionary history of the hyperdiverse marine gastropod genus Conus

Phylogenetic and paleontological analyses are combined to reveal patterns of species origination and divergence and to define the significance of potential and actual barriers to dispersal in Conus, a species-rich genus of predatory gastropods distributed throughout the world's tropical oceans. Species-level phylogenetic hypotheses are based on nucleotide sequences from the nuclear calmodulin and mitochondrial 16S rRNA genes of 138 Conus species from the Indo-Pacific, eastern Pacific, and Atlantic Ocean regions. Results indicate that extant species descend from two major lineages that diverged at least 33 mya. Their geographic distributions suggest that one clade originated in the Indo-Pacific and the other in the eastern Pacific + western Atlantic. Impediments to dispersal between the western Atlantic and Indian Oceans and the central and eastern Pacific Ocean may have promoted this early separation of Indo-Pacific and eastern Pacific + western Atlantic lineages of Conus. However, because both clades contain both Indo-Pacific and eastern Pacific + western Atlantic species, migrations must have occurred between these regions; at least four migration events took place between regions at different times. In at least three cases, incursions between regions appear to have crossed the East Pacific Barrier. The paleontological record illustrates that distinct sets of Conus species inhabited the Indo-Pacific, eastern Pacific + western Atlantic, and eastern Atlantic + former Tethys Realm in the Tertiary, as is the case today. The ranges of <1% of fossil species (N=841) spanned more than one of these regions throughout the evolutionary history of this group.     

Comparative phylogeography of reef fishes from the Gulf of Aden to the Arabian Sea reveals two cryptic lineages

The Arabian Sea is a heterogeneous region with high coral cover and warm stable conditions at the western end (Djibouti), in contrast to sparse coral cover, cooler temperatures, and upwelling at the eastern end (southern Oman). We tested for barriers to dispersal across this region (including the Gulf of Aden and Gulf of Oman), using mitochondrial DNA surveys of 11 reef fishes. Study species included seven taxa from six families with broad distributions across the Indo-Pacific and four species restricted to the Arabian Sea (and adjacent areas). Nine species showed no significant genetic partitions, indicating connectivity among contrasting environments spread across 2000 km. One butterflyfish (Chaetodon melannotus) and a snapper (Lutjanus kasmira) showed phylogenetic divergences of d = 0.008 and 0.048, respectively, possibly indicating cryptic species within these broadly distributed taxa. These genetic partitions at the western periphery of the Indo-Pacific reflect similar partitions recently discovered at the eastern periphery of the Indo-Pacific (the Hawaiian and the Marquesan Archipelagos), indicating that these disjunctive habitats at the ends of the range may serve as evolutionary incubators for coral reef organisms.

     

The historical bridge between the Amazon and the Atlantic Forest of Brazil: a study of molecular phylogeography with small mammals

Abstract Aim To examine how the genetic diversity of selected taxa of forest‐dwelling small mammals is distributed between and within the major rain forest domains of Amazonia and Atlantic Forest and the intervening interior forests of Brazil, as inferred by the relationships between gene genealogies and geography. I also addressed the historical importance of the central Brazilian forests in connecting Amazon and Atlantic Forest populations of rodents and marsupials. Methods I evaluated variation in the mitochondrial cytochrome b gene to estimate the levels of sequence divergence between those taxa occurring throughout the Amazon, Atlantic Forest, and forests in the Cerrado and Caatinga regions. I inferred the hierarchical relationships between haplotypes, populations and formal taxa using the cladistic approach of maximum parsimony. I compared areas and the clades identified by superimposing cladograms on the geographical distribution of samples. The degree of concordance both in phylogeny and the depth of the nodes in these phylogenies, in addition to patterns of geographical distribution of clades, permitted me to make inferences on how, when and where the taxa differentiated. Results Sequence similarity is often greater between samples from the Atlantic Forest and either Amazon or central Brazilian forests than it is within each of the two rain forest domains. The Atlantic Forest clades are either not reciprocally monophyletic or are the sister group to all the other clades. There is some indication of northern and southern components in the Atlantic Forest. Given the geographical distribution of clades and the relatively deep levels of divergence, the central Brazilian area does not behave as a separate region but is complementary to either Amazon or Atlantic Forest. Patterns of area relationships differ across taxa, suggesting that different processes and/or historic events affected the diversification within each lineage. Main conclusions The Amazon and the Atlantic forests are not exclusive in terms of their small mammal faunas; both overlap broadly with taxa occurring in gallery forests and dry forests in central Brazil. Central Brazilian forests are an integral part of the evolutionary scenario of lowland small mammals, playing an important role as present and past habitats for rain forest species. Therefore, representatives from this area should always be included in analyses of the evolutionary history of lowland rain forest faunas. The incongruence of branching patterns among areas is in agreement with recent results presented for Neotropical passerine birds and indicates that a single hypothesis of Neotropical area relationships is unlikely. These findings reinforce the idea that speciation in the Neotropics will not be explained by any single model of vicariance or climatic changes.     

Phylogeny of Hypselodoris (Nudibranchia: Chromodorididae) with a review of the monophyletic clade of Indo-Pacific species, including descriptions of twelve new species

This work provides an account of the systematics and phylogeny of Hypselodoris . Aspects of the morphology of 42 species are described and the systematic status of an additional 11 species is discussed. Twelve new species are described: Hypselodoris alboterminata, H. bertschi, H. bollandi, H. fucata, H. iacula, H. insulana, H. krakatoa, H. paulinae, H. reidi, H. rudmani, H. violabranchia and H. zephyra. A phylogenetic analysis supports the monophyly of Hypselodoris and Risbecia . Two distinct clades of Hypselodoris are present. One contains species from the Atlantic and eastern Pacific while the other contains species limited to the Indo-Pacific tropics and adjacent temperate regions. Species from the Atlantic and eastern Pacific are bluish in body colour and have a plesiomorphically large receptaculum seminis while Indo-Pacific taxa are variably coloured and all have a minute receptaculum seminis. The distribution and size of mantle glands provides a wealth of morphological characters. With few exceptions, mantle glands vary in closely related species and are important for distinguishing members of smaller clades. Mantle gland distribution is therefore useful in identifying preserved material that is difficult to identify to species in the absence of the pigment of living specimens. Similar colour patterns found in sympatric species of Hypselodoris appear to be a result of both common descent and convergence between less closely related lineages. Biogeographic distributions of sister taxa provide several examples of vicariance. Examination of these cases shows that no single vicariant pattern is present, but vicariance appears to occur at the margins of the Indo-Pacific rather than centrally. Some vicariance occurs even within archipelagos such as the Hawaiian Islands. These cases largely refute the generality of the hypothesis of Springer (1982), that Pacific Plate and Australasian Plate endemic sister taxa should predominate.     

Phylogeographic analysis reveals a deep lineage split within North Atlantic Littorina saxatilis

Phylogeographic studies provide critical insight into the evolutionary histories of model organisms; yet, to date, range-wide data are lacking for the rough periwinkle Littorina saxatilis, a classic example of marine sympatric speciation. Here, we use mitochondrial DNA (mtDNA) sequence data to demonstrate that L. saxatilis is not monophyletic for this marker, but is composed of two distinct mtDNA lineages (I and II) that are shared with sister species Littorina arcana and Littorina compressa. Bayesian coalescent dating and phylogeographic patterns indicate that both L. saxatilis lineages originated in the eastern North Atlantic, around the British Isles, at approximately 0.64 Ma. Both lineages are now distributed broadly across the eastern, central and western North Atlantic, and show strong phylogeographic structure among regions. The Iberian Peninsula is genetically distinct, suggesting prolonged isolation from northeastern North Atlantic populations. Western North Atlantic populations of L. saxatilis lineages I and II predate the last glacial maximum and have been isolated from eastern North Atlantic populations since that time. This identification of two distinct, broadly distributed mtDNA lineages further complicates observed patterns of repeated incipient ecological speciation in L. saxatilis, because the sympatric origins of distinct ecotype pairs on eastern North Atlantic shores may be confounded by admixture of divergent lineages.     

NICHE AND RANGE SIZE PATTERNS SUGGEST THAT SPECIATION BEGINS IN SMALL,ECOLOGICALLY DIVERGED POPULATIONS IN NORTH AMERICAN MONKEYFLOWERS (MIMULUS SPP.)

Closely related species (e.g., sister taxa) often occupy very different ecological niches and can exhibit large differences in geographic distributions despite their shared evolutionary history. Budding speciation is one process that may partially explain how differences in niche and distribution characteristics may rapidly evolve. Budding speciation is the process through which new species form as initially small colonizing populations that acquire reproductive isolation. This mode of species formation predicts that, at the time of speciation, sister species should have highly asymmetrical distributions. We tested this hypothesis in North American monkeyflowers, a diverse clade with a robust phylogeny, using data on geographical ranges, climate, and plant community attributes. We found that recently diverged sister pairs have highly asymmetrical ranges and niche breadths, relative to older sister pairs. Additionally, we found that sister species occupy distinct environmental niche positions, and that 80% of sister species have completely or partially overlapping distributions (i.e., are broadly sympatric). Together, these results suggest that budding speciation has occurred frequently in Mimulus, that it has likely taken place both inside the range and on the range periphery, and that observed divergences in habitat and resource use could be associated with speciation in small populations.     

Genetic variation in Simocephalus (Anomopoda: Daphniidae) in North America: patterns and consequences

The presumption of intercontinental distributions of many species of Anomopoda has been reinforced by their considerable morphological uniformity and ease of passive dispersal via ephippial eggs. To test the validity of this dogma, genetic variation among taxa in the cladoceran genus Simocephalus was examined on a continental scale. Genetic variability (percentage polymorphic loci = 8.8%, individual heterozygosity = 3.4%) was comparable to that determined on a local scale in Simocephalus and somewhat less than for other zooplankton groups. Four species complexes were distinguished allozymically with unique allelic substitutions found for at least two loci between these taxa. Eight species in North America were differentiated within these complexes (Nei s genetic distance, D>0.30), all of which were clearly separated from two European species. Both S. cf. vetulus and S. cf. serrulatus are broadly distributed in North America, and intraspecific divergence is relatively low. In contrast, S. cf. congener and S. cf. exspinosus represent species complexes, both consisting of several species, differentiated allozymically and morphologically. The concept of generalist species with moderate genetic variation throughout a broad range and specialist species, genetically depauperate, with restricted distributions, is explored.     

Origins of species richness in the Indo‐Malay‐Philippine biodiversity hotspot: evidence for the centre of overlap hypothesis

The Indo‐Malay‐Philippine (IMP) biodiversity hotspot, bounded by the Philippines, the Malay Peninsula and New Guinea, is the epicentre of marine biodiversity. Hypotheses to explain the source of the incredible number of species found there include the centre of overlap hypothesis, which proposes that in this region the distinct faunas of the Pacific and Indian Oceans overlap. Here we review the biogeographical evidence in support of this hypothesis. We examined tropical reef fish distributions, paying particular attention to sister species pairs that overlap in the IMP hotspot. We also review phylogeographical studies of wide‐ranging species for evidence of lineage divergence and overlap in the IMP region. Our synthesis shows that a pattern of isolation between the Pacific and the Indian Ocean faunas is evident across a wide range of taxa. The occurrence of sister species, with one member in each ocean, indicates that the mechanism(s) of isolation has been in effect since at least the Miocene, while phylogeographical studies indicate more recent divergences in the Pleistocene. Divergence in isolation followed by population expansion has led to an overlap of closely related taxa or genetic lineages in the hotspot, contributing to diversity and species richness in the region. These findings are consistent with the centre of overlap hypothesis and highlight the importance of this process in generating biodiversity within the IMP.     

Patterns of persistence and isolation indicate resilience to climate change in montane rainforest lizards

Globally, montane tropical diversity is characterized by extraordinary local endemism that is not readily explained by current environmental variables indicating a strong imprint of history. Montane species often exist as isolated populations under current climatic conditions and may have remained isolated throughout recent climatic cycles, leading to substantial genetic and phenotypic divergence. Alternatively, populations may have become contiguous during colder climates resulting in less divergence. Here we compare responses to historical climate fluctuation in a montane specialist skink, Lampropholis robertsi, and its more broadly distributed congener, L. coggeri, both endemic to rainforests of northeast Australia. To do so, we combine spatial modelling of potential distributions under representative palaeoclimates, multi‐locus phylogeography and analyses of phenotypic variation. Spatial modelling of L. robertsi predicts strong isolation among disjunct montane refugia during warm climates, but with potential for localized exchange during the most recent glacial period. In contrast, predicted stable areas are more widespread and connected in L. coggeri. Both species exhibit pronounced phylogeographic structuring for mitochondrial and nuclear genes, attesting to low dispersal and high persistence across multiple isolated regions. This is most prominent in L. robertsi, for which coalescent analyses indicate that most populations persisted in isolation throughout the climate cycles of the Pleistocene. Morphological divergence, principally in body size, is more evident among isolated populations of L. robertsi than L. coggeri. These results highlight the biodiversity value of isolated montane populations and support the general hypothesis that tropical montane regions harbour high levels of narrow‐range taxa because of their resilience to past climate change.     

High connectivity across the fragmented chemosynthetic ecosystems of the deep Atlantic Equatorial Belt: efficient dispersal mechanisms or questionable endemism?

Chemosynthetic ecosystems are distributed worldwide in fragmented habitats harbouring seemingly highly specialized communities. Yet, shared taxa have been reported from highly distant chemosynthetic communities. These habitats are distributed in distinct biogeographical regions, one of these being the so‐called Atlantic Equatorial Belt (AEB). Here, we combined genetic data (COI) from several taxa to assess the possible existence of cryptic or synonymous species and to detect the possible occurrence of contemporary gene flow among populations of chemosynthetic species located on both sides of the Atlantic. Several Evolutionary Significant Units (ESUs) of Alvinocarididae shrimp and Vesicomyidae bivalves were found to be shared across seeps of the AEB. Some were also common to hydrothermal vent communities of the Mid‐Atlantic Ridge (MAR), encompassing taxa morphologically described as distinct species or even genera. The hypothesis of current or very recent large‐scale gene flow among seeps and vents was supported by microsatellite analysis of the shrimp species Alvinocaris muricola/Alvinocaris markensis across the AEB and MAR. Two nonmutually exclusive hypotheses may explain these findings. The dispersion of larvae or adults following strong deep‐sea currents, possibly combined with biochemical cues influencing the duration of larval development and timing of metamorphosis, may result in large‐scale effective migration among distant spots scattered on the oceanic seafloor. Alternatively, these results may arise from the prevailing lack of knowledge on the ocean seabed, apart from emblematic ecosystems (chemosynthetic ecosystems, coral reefs or seamounts), where the widespread classification of endemism associated with many chemosynthetic taxa might hide wider distributions in overlooked parts of the deep sea.     

A role for nonphysical barriers to gene flow in the diversification of a highly vagile seabird, the masked booby (Sula dactylatra)

To test the hypothesis that nonphysical barriers to gene flow play a role in the divergence of low-latitude seabird populations, we applied phylogeographic methods to mitochondrial control region sequence variation in a global sample of masked boobies (Sula dactylatra). In accord with previous studies, we found that Indo-Pacific and Atlantic haplotypes form two divergent lineages, excluding one haplotype previously attributed to secondary contact between the Indian Ocean and the Caribbean Sea. Within the Indo-Pacific and the Atlantic, we found a relatively large number of haplotypes, many of which were unique to a single population. Although haplotypes from most populations were found in more than one higher-level clade, nested clade analysis revealed a significant association between clades and geography for the majority of higher-level clades, most often interpreted as a consequence of isolation by distance. We found low levels of gene flow within Indo-Pacific and Atlantic populations, and a significant correlation between gene flow and geographical distance among Indo-Pacific populations. We estimate that Indo-Pacific masked boobies experienced rapid population growth approximately 180,000 years ago and that the majority of Indo-Pacific and Atlantic populations diverged within the last approximately 115,000 years. These combined data suggest that the predominant pattern between Indo-Pacific and Atlantic populations is long-term isolation by physical barriers to gene flow. In contrast, populations within these regions appear to have diverged despite few obvious physical barriers to gene flow, perhaps as a consequence of limited natal dispersal combined with local adaptation and/or genetic drift.     

Snails in the desert: Species diversification of Theba (Gastropoda: Helicidae) along the Atlantic coast of NW Africa

The spatial subdivision of species often plays a pivotal role in speciation. Across their entire range, species are rarely panmictic and crucial consequences of spatial subdivision are (1) random genetic drift including historical factors, (2) uniform selection, and (3) divergent selection. Each of these consequences may result in geographic variation and eventually reproductive isolation, but their relative importance in speciation is still unclear. In this study, we used a combination of genetic, morphological, and climatic data to obtain a comprehensive picture of differentiation among three closely related, parapatrically distributed taxa of the land snail genus Theba occurring along the Atlantic coasts of South Morocco and Western Sahara. We conducted Mantel and partial Mantel tests to relate phenotypic and genotypic variation of these species to geography and/or climate. As null hypothesis for an evolutionary scenario, we assumed nonadaptive speciation and expected a pattern of isolation by distance among taxa. Rejection of the null hypothesis would indicate isolation by environment due to adaptation. Generally, genetic drift plays an important role but is rarely considered as sole driver of speciation. It is the combination of drift and selection that predominantly drives speciation. This study, however, provides a potential example, in which nonadaptive speciation, that is, genetic drift, is apparently the main driver of shaping the diversity of Theba in NW Africa. Restriction of gene flow between populations caused by geographic isolation probably has played an important role. Climate oscillations during the Plio‐ and Pleistocene may have led to repeated ecological changes in NW Africa and disruptions of habitats promoting differentiation by geographic isolation. The inferred evolutionary scenario, however, did not fully explain the incongruence between the AFLP‐ and mtDNA‐tree topologies. This incongruence might indicate past hybridization among the studied Theba forms.     

A revision of Atlantic species of Photostomias (Teleostei: Stomiidae: Malacosteinae), with a description of a new species

While one or possibly two species of the genus Photostomias have been recognized, an unpublished revision of the Malacosteinae suggested that there may be as many as six species worldwide. Our review of museum material revealed three taxa in the Atlantic alone: Photostomias atrox Alcock, 1890; Photostomias guernei Collett, 1889; and a new species described herein. Because of a paucity of Indo-Pacific material and a need to better document Atlantic biodiversity, we treat only the Atlantic species at this time. A key to the identification of Atlantic Photostomias is given.     

Evolutionary Patterns in Pearl Oysters of the Genus <Emphasis Type="Italic">Pinctada</Emphasis> (Bivalvia: Pteriidae)

Pearl oysters belonging to the genus Pinctada (Bivalvia: Pteriidae) are widely distributed between the Indo-Pacific and western Atlantic. The existence of both widely distributed and more restricted species makes this group a suitable model to study diversification patterns and prevailing modes of speciation. Phylogenies of eight out of the 11 currently recognised Pinctada species using mitochondrial (cox1) and nuclear (18S rRNA) data yielded two monophyletic groups that correspond to shell size and presence/absence of hinge teeth. Character trace of these morphological characters onto the molecular phylogeny revealed a strong correlation. Pinctada margaritifera appears polyphyletic with specimens from Mauritius grouping in a different clade from others of the French Polynesia and Japan. Hence, P. margaritifera might represent a species complex, and specimens from Mauritius could represent a different species. Regarding the putative species complex Pinctada fucata/Pinctada martensii/Pinctada radiata/Pinctada imbricata, our molecular analyses question the taxonomic validity of the morphological characters used to discriminate P. fucata and P. martensii that exhibited the lowest genetic divergence and are most likely conspecific as they clustered together. P. radiata and P. imbricata were recovered as monophyletic. The absence of overlapping distributions between sister lineages and the observed isolation by distance suggests that allopatry is the prevailing speciation mode in Pinctada. Bayesian dating analysis indicated a Miocene origin for the genus, which is consistent with the fossil record. The northward movement of the Australian plate throughout the Miocene played an important role in the diversification process within Pinctada.     

An analysis of species boundaries and biogeographic patterns in a cryptic species complex: the rotifer--Brachionus plicatilis

Since the advent of molecular phylogenetics, there is increasing evidence that many small aquatic and marine invertebrates--once believed to be single, cosmopolitan species--are in fact cryptic species complexes. Although the application of the biological species concept is central to the identification of species boundaries in these cryptic complexes, tests of reproductive isolation do not frequently accompany phylogenetic studies. Because different species concepts generally identify different boundaries in cryptic complexes, studies that apply multiple species concepts are needed to gain a more detailed understanding of patterns of diversification in these taxa. Here we explore different methods of empirically delimiting species boundaries in the salt water rotifer Brachionus plicatilis by comparing reproductive data (i.e., the traditional biological species concept) to phylogenetic data (the genealogical species concept). Based on a high degree of molecular sequence divergence and largely concordant genetic patterns in COI and ITS1, the genealogical species hypothesis indicates the existence of at least 14 species--the highest estimate for the group thus far. A test of the genealogical species concept with biological crosses shows a fairly high level of concordance, depending on the degree of reproductive success used to draw boundaries. The convergence of species concepts in this group suggests that many of the species within the group may be old. Although the diversity of the group is higher than previously understood, geographic distributions remain broad. Efficient passive dispersal has resulted in global distributions for many species with some evidence of isolation by distance over large geographic scales. These patterns concur with expectations that micro-meiofauna (0.1-1mm) have biogeographies intermediate to microbial organisms and large vertebrates. Sympatry of genetically distant strains is common.     

Molecular diversity of fungal phylotypes co-amplified alongside nematodes from coastal and deep-sea marine environments

Nematodes and fungi are both ubiquitous in marine environments, yet few studies have investigated relationships between these two groups. Microbial species share many well-documented interactions with both free-living and parasitic nematode species, and limited data from previous studies have suggested ecological associations between fungi and nematodes in benthic marine habitats. This study aimed to further document the taxonomy and distribution of fungal taxa often co-amplified from nematode specimens. A total of 15 fungal 18S rRNA phylotypes were isolated from nematode specimens representing both deep-sea and shallow water habitats; all fungal isolates displayed high pairwise sequence identities with published data in Genbank (99-100%) and unpublished high-throughput 454 environmental datasets (>95%). BLAST matches indicate marine fungal sequences amplified in this study broadly represent taxa within the phyla Ascomycota and Basidiomycota, and several phylotypes showed robust groupings with known taxa in phylogenetic topologies. In addition, some fungal phylotypes appeared to be present in disparate geographic habitats, suggesting cosmopolitan distributions or closely related species complexes in at least some marine fungi. The present study was only able to isolate fungal DNA from a restricted set of nematode taxa; further work is needed to fully investigate the taxonomic scope and function of nematode-fungal interactions.     

Biogeographical analysis of two Polypodium species complexes (Polypodiaceae) in Mexico and Central America

We analyzed the geographical and elevational distributions of two Polypodium complexes from Mexico and Central America. Distribution data of nine species of the Polypodium colpodes complex and the Polypodium plesiosorum complex were obtained from almost 1500 herbarium specimens, field collections in Mexico and Costa Rica, and literature studies. The presence of each species was recorded for each Mesoamerican country, in 1° × 1° grid‐cells and biogeographical provinces. The rarity of species was also evaluated. Although the two complexes show extensive overlap, the P. colpodes complex is distributed mainly along the Pacific versant of Mexico and Central America, whereas the P. plesiosorum complex occurs mainly along the Atlantic versant. Those biogeographical provinces with maximum species diversity are Chiapas (seven species), Sierra Madre del Sur (six species), and the Trans‐Mexican Volcanic belt (six species). Grid‐cells with more species are located mainly in the mountains of central‐southern Mexico and northern Central America. Richness does not decrease or increase with latitude. Elevation distributions showed that most Polypodium species are concentrated in the montane interval and three species groups were recognized based on elevational preferences. Polypodium colpodes and P. plesiosorum are the most widely distributed species, whereas Polypodium castaneum and Polypodium flagellare are the only two species that possess the three attributes of rarity (narrow geographical distribution, high habitat specificity, and scarce local populations). Polypodium species of both complexes are present mainly in the montane regions of the study area and show some degree of geographical sympatry, especially in southern Mexico and northern Central America. This overlapping is explained by the elevation tolerance within montane systems and because most species inhabit three or more vegetation types. The distributional patterns of these complexes coincided with the three regional highlands of Mesoamerica, which are separated from each other by the Isthmus of Tehuantepec and by the lowlands of Nicaragua. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Timing the Eastern Asian–Eastern North American Floristic Disjunction: Molecular Clock Corroborates Paleontological Estimates

Sequence data of the chloroplast gene rbcL were used to estimate the time of the well-known eastern Asian–eastern North American floristic disjunction. Sequence divergence of rbcL was examined for 22 species of 11 genera (Campsis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella, Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse array of flowering plants occurring disjunctly in eastern Asia and eastern North America. Divergence times of putative disjunct species pairs were estimated from synonymous substitutions, using rbcL molecular clocks calibrated for Cornus. Relative rate tests were performed to assess rate constancy of rbcL evolution among lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 ± 4.30 million years to recent (<0.31 million years), with most within the last 10 million years (in the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This estimate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian–eastern North American floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the Northern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribution of taxa with low divergence, such as Menispermum.     

Molecular Phylogeny and Biogeography of the Marine ShrimpPenaeus

The evolutionary relationships among 13 species representing all six subgenera of the shrimp genusPenaeuswere examined using 558 bp of mitochondrial (mt) DNA from the cytochrome oxidase subunit I gene. Analyses of this sequence revealed high genetic divergence between species (d = 8–24%), a finding which contrasts with previous work, which indicated that genetic diversity, based on electrophoretic analysis of allozymes, was extremely low inPenaeus.Three tree-building methods (maximum parsimony, neighbor joining, and maximum likelihood) were concordant in indicating that current subgenera assignments do not reflect evolutionary partitions within the genusPenaeus.While the molecular phylogenies cast doubt on the validity of subgenera, the observed relationships are concordant with biogeographic boundaries across the tropical range ofPenaeus.Both the western Atlantic and eastern Pacific contain monophyletic species pairs which cluster together in all analyses. The Indo-Pacific contains a putative basal taxa (P. indicus), the deepest mtDNA lineages, and the highest diversity, including representatives of all three primary lineages observed inPenaeus.These data are consistent with the suggestion by Dallet al.(1990) thatPenaeusarose in the Indo-Pacific and radiated eastward and westward to account for the current circumtropical distribution of the genus. This phylogenetic framework forPenaeuswill enhance the scientific foundations for wildlife resource management and breeding experiments (hybridization and related manipulations) designed to improve the commercial value of captive strains.