Mitochondrial pseudogenes are pervasive and often insidious in the snapping shrimp genus Alpheus

Here we show that multiple DNA sequences, similar to the mitochondrial cytochrome oxidase I (COI) gene, occur within single individuals in at least 10 species of the snapping shrimp genus Alpheus. Cloning of amplified products revealed the presence of copies that differed in length and (more frequently) in base substitutions. Although multiple copies were amplified in individual shrimp from total genomic DNA (gDNA), only one sequence was amplified from cDNA. These results are best explained by the presence of nonfunctional duplications of a portion of the mtDNA, probably located in the nuclear genome, since transfer into the nuclear gene would render the COI gene nonfunctional due to differences in the nuclear and mitochondrial genetic codes. Analysis of codon variation suggests that there have been 21 independent transfer events in the 10 species examined. Within a single animal, differences between the sequences of these pseudogenes ranged from 0.2% to 20.6%, and those between the real mtDNA and pseudogene sequences ranged from 0.2% to 18.8% (uncorrected). The large number of integration events and the large range of divergences between pseudogenes and mtDNA sequences suggest that genetic material has been repeatedly transferred from the mtDNA to the nuclear genome of snapping shrimp. Unrecognized pseudogenes in phylogenetic or population studies may result in spurious results, although previous estimates of rates of molecular evolution based on Alpheus sister taxa separated by the Isthmus of Panama appear to remain valid. Especially worrisome for researchers are those pseudogenes that are not obviously recognizable as such. An effective solution may be to amplify transcribed copies of protein-coding mitochondrial genes from cDNA rather than using genomic DNA.     

ACCELERATED EVOLUTION OF LAND SNAILS MANDARINA IN THE OCEANIC BONIN ISLANDS: EVIDENCE FROM MITOCHONDRIAL DNA SEQUENCES

An endemic land snail genus Mandarina of the oceanic Bonin (Ogasawara) Islands shows exceptionally rapid evolution not only of morphological and ecological traits, but of DNA sequence. A phylogenetic relationship based on mitochondrial DNA (mtDNA) sequences suggests that morphological differences equivalent to the differences between families were produced between Mandarina and its ancestor during the Pleistocene. The inferred phylogeny shows that species with similar morphologies and life habitats appeared repeatedly and independently in different lineages and islands at different times. Sequential adaptive radiations occurred in different islands of the Bonin Islands and species occupying arboreal, semiarboreal, and terrestrial habitat arose independently in each island. Because of a close relationship between shell morphology and life habitat, independent evolution of the same life habitat in different islands created species possesing the same shell morphology in different islands and lineages. This rapid evolution produced some incongruences between phylogenetic relationship and species taxonomy. Levels of sequence divergence of mtDNA among the species of Mandarina is extremely high. The maximum level of sequence divergence at 16S and 12S ribosomal RNA sequence within Mandarina are 18.7% and 17.7%, respectively, and this suggests that evolution of mtDNA of Mandarina is extremely rapid, more than 20 times faster than the standard rate in other animals. The present examination reveals that evolution of morphological and ecological traits occurs at extremely high rates in the time of adaptive radiation, especially in fragmented environments.     

Mining ESTs to Determine the Usefulness of SNPs Across Shrimp Species

Expressed sequence tag (EST) libraries from members of the Penaeidae family and brine shrimp (Artemia franciscana) are currently the primary source of sequence data for shrimp species. Penaeid shrimp are the most commonly farmed worldwide, but selection methods for improving shrimp are limited. A better understanding of shrimp genomics is needed for farmers to use genetic markers to select the best breeding animals. The ESTs from Litopenaeus vannamei have been previously mined for single nucleotide polymorphisms (SNPs). This present study took publicly available ESTs from nine shrimp species, excluding L. vannamei, clustered them with CAP3, predicted SNPs within them using SNPidentifier, and then analyzed whether the SNPs were intra- or interspecies. Major goals of the project were to predict SNPs that may distinguish shrimp species, locate SNPs that may segregate in multiple species, and determine the genetic similarities between L. vannamei and the other shrimp species based on their EST sequences. Overall, 4,597 SNPs were predicted from 4,600 contigs with 703 of them being interspecies SNPs, 735 of them possibly predicting species' differences, and 18 of them appearing to segregate in multiple species. While sequences appear relatively well conserved, SNPs do not appear to be well conserved across shrimp species.     

Determination of mitochondrial genetic diversity in mammals

Mitochondrial DNA (mtDNA) is one of the most popular population genetic markers. Its relevance as an indicator of population size and history has recently been questioned by several large-scale studies in animals reporting evidence for recurrent adaptive evolution, at least in invertebrates. Here we focus on mammals, a more restricted taxonomic group for which the issue of mtDNA near neutrality is crucial. By analyzing the distribution of mtDNA diversity across species and relating it to allozyme diversity, life-history traits, and taxonomy, we show that (i) mtDNA in mammals does not reject the nearly neutral model; (ii) mtDNA diversity, however, is unrelated to any of the 14 life-history and ecological variables that we analyzed, including body mass, geographic range, and The World Conservation Union (IUCN) categorization; (iii) mtDNA diversity is highly variable between mammalian orders and families; (iv) this taxonomic effect is most likely explained by variations of mutation rate between lineages. These results are indicative of a strong stochasticity of effective population size in mammalian species. They suggest that, even in the absence of selection, mtDNA genetic diversity is essentially unpredictable, knowing species biology, and probably uncorrelated to species abundance.     

Phylogenetic relationships and remarkable radiation in Parartemia (Crustacea: Anostraca), the endemic brine shrimp of Australia: evidence from mitochondrial DNA sequences

The Australian continent is notable for the faunal radiations which have occurred in its saline inland waters. The endemic brine shrimp genus Parartemia inhabits many of these habitats. The complex pattern of morphological variation in parartemiids has impeded the establishment of a sound scheme of species relationships. The present study provides an explicit hypothesis of relationships for the genus based on nucleotide sequence data from a segment of mitochondrial DNA coding for the large subunit rRNA gene. Phylogenetic analyses indicated that the eight known species are genetically distinct, and revealed the existence of at least two new species. The molecular data support certain morphology-based relationships among species, but are inconsistent with other hypotheses. There is evidence that most members of the genus arose in a short interval, followed by remarkable genetic divergence. Comparisons of levels of mt DNA sequence divergence between lineages from saline inland waters and freshwaters using representative crustacean groups from Australia that included parartemiids indicated profound differences in rates of evolution, with halophiles exhibiting greater rates of change than their counterparts from freshwaters.     

Molecular evidence for sequential colonization and taxon cycling in freshwater decapod shrimps on a Caribbean island

Taxon cycling, i.e. sequential phases of expansions and contractions in species' distributions associated with ecological or morphological shifts, are postulated to characterize dynamic biogeographic histories in various island faunas. The Caribbean freshwater shrimp assemblage is mostly widespread and sympatric throughout the region, although one species (Atyidae: Atya lanipes) is geographically restricted and ecologically and morphologically differentiated from other Atya species. Using patterns of nucleotide variation at the COI mtDNA gene in five species of freshwater shrimp (A. lanipes, A. scabra, A. innocuous; Xiphocarididae: Xiphocaris elongata; Palaemonidae: Macrobrachium faustinum) from Puerto Rico, we expected to detect a signature of sequential colonization in these shrimp, consistent with the concept of taxon cycling, and expected that A. lanipes would be at a different taxon stage (i.e. an early stage species) to all other species. We also examined patterns of genetic population structure in each species expected with poor, intermediate and well-developed abilities for among-river dispersal. Population expansions were detected in all species, although the relative timing of the expansions varied among them. Assuming that population expansions followed colonization of Puerto Rico by freshwater shrimp, results bear the hallmarks of sequential colonization and taxon cycling in this fauna. A. lanipes had a star phylogeny, low mean pairwise nucleotide differences and recent (Holocene) estimates for an in situ population expansion in Puerto Rico, and it was inferred as an early stage species in the taxon cycle undergoing a secondary phase of expansion. All other species were inferred as late stage species undergoing regional population expansions, as their mean pairwise nucleotide differences were relatively high and phylogenetic patterns were more complex than A. lanipes. High rates of gene flow without isolation by distance among rivers were detected in all species, although results should be treated cautiously as some populations are unlikely to be in mutation-drift equilibrium. Nested clade analysis produced inconsistent results among species that all have high rates of gene flow and expanding populations.     

Unequal rates of Y chromosome gene divergence during speciation of the family Ursidae

Evolution of the bear family Ursidae is well investigated in terms of morphological, paleontological, and genetic features. However, several phylogenetic ambiguities occur within the subfamily Ursinae (the family Ursidae excluding the giant panda and spectacled bear), which may correlate with behavioral traits of female philopatry and male-biased dispersal which form the basis of the observed matriarchal population structure in these species. In the process of bear evolution, we investigate the premise that such behavioral traits may be reflected in patterns of variation among genes with different modes of inheritance: matrilineal mitochondrial DNA (mtDNA), patrilineal Y chromosome, biparentally inherited autosomes, and the X chromosome. In the present study, we sequenced 3 Y-linked genes (3,453 bp) and 4 X-linked genes (4,960 bp) and reanalyzed previously published sequences from autosome genes (2,347 bp) in ursid species to investigate differences in evolutionary rates associated with patterns of inheritance. The results describe topological incongruence between sex-linked genes and autosome genes and between nuclear DNA and mtDNA. In more ancestral branches within the bear phylogeny, Y-linked genes evolved faster than autosome and X-linked genes, consistent with expectations based on male-driven evolution. However, this pattern changes among branches leading to each species within the lineage of Ursinae whereby the evolutionary rates of Y-linked genes have fewer than expected substitutions. This inconsistency between more recent nodes of the bear phylogeny with more ancestral nodes may reflect the influences of sex-biased dispersal as well as molecular evolutionary characteristics of the Y chromosome, and stochastic events in species natural history, and phylogeography unique to ursine bears.     

Molecular phylogeny of penaeid shrimps inferred from two mitochondrial markers

Penaeid shrimps are an important resource in crustacean fisheries, representing more than the half of the gross production of shrimp worldwide. In the present study, we used a sample of wide-ranging diversity (41 shrimp species) and two mitochondrial markers (758 bp) to clarify the evolutionary relationships among Penaeidae genera. Three different methodologies of tree reconstruction were employed in the study: maximum likelihood, neighbor joining and Bayesian analysis. Our results suggest that the old Penaeus genus is monophyletic and that the inclusion of the Solenocera genus within the Penaeidae family remains uncertain. With respect to Metapenaeopsis monophyly, species of this genus appeared clustered, but with a nonsignificant bootstrap value. These results elucidate some features of the unclear evolution of Penaeidae and may contribute to the taxonomic characterization of this family.     

The incomplete history of mitochondrial lineages between two rockfishes, Sebastes longispinis and Sebastes hubbsi (Scorpaeniformes: Scorpaenidae)

The genetic divergence between two closely related rockfishes, Sebastes longispinis and Sebastes hubbsi, was inferred from both mitochondrial DNA (mtDNA) sequence variations and amplified fragment length polymorphism (AFLP) markers. The two species were placed into two distinct clades in a neighbour-joining tree based on the AFLP data, clearly indicating that they represented separate species. Although this evidence, together with a previous morphological study, revealed clear differences between the two species, no obvious clustering of haplotypes by species was detected in the minimum spanning network inferred from sequence variations in the mtDNA control region (c. 500 base pairs). In fact, the significant Φ(ST) estimates indicated only a restriction of gene flow between the two species. Uncorrected pairwise sequence differences in mtDNA between two species were small (1·8% at maximum, on the lower end of the range of control region divergence between previously studied sister species pairs), suggesting their speciation event as having been fairly recent. The incongruent results of AFLP and mtDNA phylogenies suggested incomplete lineage sorting and introgression of mtDNA in the course of the evolution of the two species. Differences in their main distributional ranges and the small level of sequence divergence in mtDNA suggests that speciation and dispersal may have been associated with glacio-eustatic sea level fluctuations between the Japanese Archipelago and the Korean Peninsula during the past 0·4 million years.     

Morphological stasis and phylogenetic relationships in Tadpole shrimps, Triops (Crustacea: Notostraca)

The tadpole shrimp (Triops) is a well-known 'living fossil' whose fundamental morphology has been unchanged for over 170 million years. Thus, tadpole shrimps are suitable subjects for the study of morphological stasis. We were able to obtain samples of three species of Triops (T. granarius, T longicaudatus and T. cancriformis) from four regions in Japan. Taxonomic species were identified by diagnostic morphology. We inferred phylogenetic relationships between individual samples using mitochondrial 16S rRNA. Carapace shapes were compared among populations using shape coordinate methods. The phylogeny inferred from mtDNA shows that T. granarius is phylogenetically more similar to T. longicaudatus than to T. cancriformis. mtDNA sequences did not differ among the populations of T granarius. However, there were two distinct phylogenetic species within T. longicaudatus. In spite of the similarity in fundamental morphological characteristics among Triops species, mtDNA sequences of Triops showed marked differences among the four phylogenetic species. Among the populations of T. granarius , the carapace shape of the Fukuoka population was significantly different from those of other populations of T. granarius. The carapace shape of the Kagawa population of T. longicaudatus was more similar to those of the Shizuoka and Kagawa populations of T. granarius. The shape of the carapace of T. cancriformis was significantly different from those of T. granarius and T longicaudatus. Thus, taxonomic species, phylogenetic species and populations with similar carapace morphology did not correspond with each other. The present results indicate that most of the morphological change did not occur at the time of speciation (lineage separation) and that morphological stases are important evolutionary patterns, but they are not species-level properties.     

Molecular phylogenetics at the population/species interface in cave spiders of the southern Appalachians (Araneae:Nesticidae:Nesticus)

This paper focuses on the relationship between population genetic structure and speciation mechanisms in a monophyletic species group of Appalachian cave spiders (Nesticus). Using mtDNA sequence data gathered from 256 individuals, I analyzed patterns of genetic variation within and between populations for three pairs of closely related sister species. Each sister-pair comparison involves taxa with differing distributional and ecological attributes; if these ecological attributes are reflected in basic demographic differences, then speciation might proceed differently across these sister taxa comparisons. Both frequency-based and gene tree analyses reveal that the genetic structure of the Nesticus species studied is characterized by similar and essentially complete population subdivision, regardless of differences in general ecology. These findings contrast with results of prior genetic studies of cave-dwelling arthropods that have typically revealed variation in population structure corresponding to differences in general ecology. Species fragmentation through both extrinsic and intrinsic evolutionary forces has resulted in discrete, perhaps independent, populations within morphologically defined species. Large sequence divergence values observed between populations suggest that this independence may extend well into the past. These patterns of mtDNA genealogical structure and divergence imply that species as morphological lineages are currently more inclusive than basal evolutionary or phylogenetic units, a suggestion that has important implications for the study of speciation mechanisms.      

Genetic differentiation among recently diverged delphinid taxa determined using AFLP markers

In the mid-1990s, a new common dolphin species (Delphinus capensis) was defined in the northeast Pacific using morphological characters and mitochondrial DNA (mtDNA) sequences. This species is sympatric with a second species, Delphinus delphis; morphological differences between the two are slight and it is clear they are closely related. Does the phenotypic distinction result from only a few important genes or from large differences between their nuclear genomes? We used amplified fragment length polymorphism (AFLP) markers to broadly survey the nuclear genomes of these two species to examine the levels of nuclear divergence and genetic diversity between them. Furthermore, to create an evolutionary context in which to compare the level of interspecific divergence found between the two Delphinus taxa, we also examined two distinct morphotypes of the bottlenose dolphin (Tursiops truncatus). A nonmetric multidimensional scaling analysis clearly differentiated both Delphinus species, indicating that significant nuclear genetic differentiation has arisen between the species despite their morphological similarity. However, the AFLP data indicated that the two T. truncatus morphotypes exhibit greater divergence than D. capensis and D. delphis, suggesting that they too should be considered different species.     

Inferring speciation modes in a clade of Iberian chafers from rates of morphological evolution in different character systems

Background  

Studies of speciation mode based on phylogenies usually test the predicted effect on diversification patterns or on geographical distribution of closely related species. Here we outline an approach to infer the prevalent speciation mode in Iberian Hymenoplia chafers through the comparison of the evolutionary rates of morphological character systems likely to be related to sexual or ecological selection. Assuming that mitochondrial evolution is neutral and not related to measured phenotypic differences among the species, we contrast hypothetic outcomes of three speciation modes: 1) geographic isolation with subsequent random morphological divergence, resulting in overall change proportional to the mtDNA rate; 2) sexual selection on size and shape of the male intromittent organs, resulting in an evolutionary rate decoupled to that of the mtDNA; and 3) ecological segregation, reflected in character systems presumably related to ecological or biological adaptations, with rates decoupled from that of the mtDNA.     

Protein evolution in different cellular environments: cytochrome b in sharks and mammals

DNA sequences for the mitochondrial cytochrome b gene were determined for 13 species of sharks. Rates and patterns of amino acid replacement are compared for sharks and mammals. Absolute rates of cytochrome b evolution are six times slower in sharks than in mammals. Bivariate plots of the number of nonsynonymous and silent transversions are indistinguishable in the two groups, however, suggesting that the differences in amino acid replacement rates are due primarily to differences in DNA substitution rates. Patterns of amino acid replacement are also similar in the two groups. Conserved and variable regions occur in the same parts of the cytochrome b gene, and there is little evidence that the types of amino acid changes are significantly different between the groups. Similarity in the relative rates and patterns of protein change between the two groups prevails despite dramatic differences in the cellular environments of sharks and mammals. Poor penetrance of physiological differences through to rates of protein evolution provides support for the neutral theory and suggests that, for cytochrome b, patterns of evolution have been relatively constant throughout much of vertebrate history.      

Excess amino acid polymorphism in mitochondrial DNA: contrasts among genes from Drosophila, mice, and humans

Recent studies of mitochondrial DNA (mtDNA) variation in mammals and Drosophila have shown an excess of amino acid variation within species (replacement polymorphism) relative to the number of silent and replacement differences fixed between species. To examine further this pattern of nonneutral mtDNA evolution, we present sequence data for the ND3 and ND5 genes from 59 lines of Drosophila melanogaster and 29 lines of D. simulans. Of interest are the frequency spectra of silent and replacement polymorphisms, and potential variation among genes and taxa in the departures from neutral expectations. The Drosophila ND3 and ND5 data show no significant excess of replacement polymorphism using the McDonald-Kreitman test. These data are in contrast to significant departures from neutrality for the ND3 gene in mammals and other genes in Drosophila mtDNA (cytochrome b and ATPase 6). Pooled across genes, however, both Drosophila and human mtDNA show very significant excesses of amino acid polymorphism. Silent polymorphisms at ND5 show a significantly higher variance in frequency than replacement polymorphisms, and the latter show a significant skew toward low frequencies (Tajima's D = -1.954). These patterns are interpreted in light of the nearly neutral theory where mildly deleterious amino acid haplotypes are observed as ephemeral variants within species but do not contribute to divergence. The patterns of polymorphism and divergence at charge-altering amino acid sites are presented for the Drosophila ND5 gene to examine the evolution of functionally distinct mutations. Excess charge-altering polymorphism is observed at the carboxyl terminal and excess charge-altering divergence is detected at the amino terminal. While the mildly deleterious model fits as a net effect in the evolution of nonrecombining mitochondrial genomes, these data suggest that opposing evolutionary pressures may act on different regions of mitochondrial genes and genomes.      

Morphological stasis in the Eurytemora affinis species complex (Copepoda: Temoridae)

Morphological stasis has long been regarded as one of the most challenging problems in evolutionary biology. This study focused on the copepod species complex, Eurytemora affinis, as a model system to determine pattern and degree of morphological stasis. This study revealed discordant rates of morphological differentiation, molecular evolution, and reproductive isolation, where speciation was accompanied by lack of morphological differentiation in secondary sex characters. Comparisons were made among phylogenies based on morphometrics, nuclear (allozyme) loci, and mitochondrial DNA (mtDNA) sequences from cytochrome oxidase I, for a total of 43 populations within the complex. These systematic relationships were also compared to patterns of reproductive isolation. In addition, genetic subdivision of nuclear molecular (allozyme) markers (G _ST) and quantitative (morphological) characters (Q _ST) were determined to infer evolutionary forces driving morphological differentiation. The morphometric phylogeny revealed that all clades, excluding the European clade, were morphologically undifferentiated and formed a polytomy (multifurcation). Morphometric distances were not correlated with mtDNA distances, or with patterns of reproductive isolation. In contrast, nuclear and mtDNA phylogenies were mostly congruent. Reproductive isolation proved to be the most sensitive indicator of speciation, given that two genetically and morphologically proximate populations showed evidence of hybrid breakdown. Quantitative genetic (morphological) subdivision (Q _ST = 0.162) was lower than nuclear genetic subdivision (G _ST = 0.617) for four laboratory-reared North American populations, indicating retarded evolution of morphological characters. This result contrasts with most other species, where Q _ST typically exceeds G _ST as a result of directional selection. Thus, in all but the European populations, evolution of the secondary sex characters was marked by morphological stasis, even between reproductively-isolated populations.     

Testing paleolimnological predictions with molecular data: the origins of Holarctic Eubosmina

Zooplankton of the family Bosminidae have a unique paleolimnological record in many Holarctic lakes that provides a near continuous record of morphological change for thousands of years. If this morphological change could be interpreted reliably, then a rarely achieved direct observation of macroevolution would be feasible. We tested paleolimnological predictions derived from morphological variation found in the genus Eubosmina using mtDNA and nuclear DNA sequence variation from geographically distant Holarctic sites. The mtDNA and nDNA trees were congruent but genetic divergence was inversely associated with morphological divergence. The three most genetically divergent groups belonged to Eubosmina longispina, whose phylogeography and genetic divergence was consistent with glacial vicariance. The genetic evidence also supported the hypothesis that at least two Nearctic species were recent European introductions. Finally, the genetic evidence was consistent with paleolimnology in the finding of several proposed species undergoing rapid morphological evolution and being post-glacially derived from European E. longispina. The results suggested that lacustrine bosminids are susceptible to geographic speciation processes, and that morphological interpretation of diversity in paleolimnology can be markedly improved by genetic studies.     

Number of ancestral human species: a molecular perspective

Despite the remarkable developments in molecular biology over the past three decades, anthropological genetics has had only limited impact on systematics in human evolution. Genetics offers the opportunity to objectively test taxonomies based on morphology and may be used to supplement conventional approaches to hominid systematics. Our analyses, examining chromosomes and 46 estimates of genetic distance, indicate there may have been only around 4 species on the direct line to modern humans and 5 species in total. This contrasts with current taxonomies recognising up to 23 species.

The genetic proximity of humans and chimpanzees has been used to suggest these species are congeneric. Our analysis of genetic distances between them is consistent with this proposal. It is time that chimpanzees, living humans and all fossil humans be classified in Homo. The creation of new genera can no longer be a solution to the complexities of fossil morphologies. Published genetic distances between common chimpanzees and bonobos, along with evidence for interbreeding, suggest they should be assigned to a single species.

The short distance between humans and chimpanzees also places a strict limit on the number of possible evolutionary side branches that might be recognised on the human lineage. All fossil taxa were genetically very close to each other and likely to have been below congeneric genetic distances seen for many mammals.

Our estimates of genetic divergence suggest that periods of around 2 million years are required to produce sufficient genetic distance to represent speciation. Therefore, Neanderthals and so-called H. erectus were genetically so close to contemporary H. sapiens they were unlikely to have been separate species. Thus, it is likely there was only one species of human (H. sapiens) for most of the last 2 million years. We estimate the divergence time of H. sapiensfrom 16 genetic distances to be around 1.7 Ma which is consistent with evidence for the earliest migration out of Africa. These findings call into question the mitochondrial «African Eve» hypothesis based on a far more recent origin for H. sapiens and show that humans did not go through a bottleneck in their recent evolutionary history.

Given the large offset in evolutionary rates of molecules and morphology seen in human evolution, Homo species are likely to be characterised by high levels of morphological variation and low levels of genetic variability. Thus, molecular data suggest the limits for intraspecific morphological variation used by many palaeoanthropologists have been set too low. The role of phenotypic plasticity has been greatly underestimated in human evolution.

We call into question the use of mtDNA for studies of human evolution. This DNA is under strong selection, which violates the assumption of selective neutrality. This issue should be addressed by geneticists, including a reassessment of its use for molecular clocks. There is a need for greater cooperation between palaeoanthropologists and anthropological geneticists to better understand human evolution and to bring palaeoanthropology into the mainstream of evolutionary biology.     

Molecular and morphological divergence in the butterfly genus Lycaeides (Lepidoptera: Lycaenidae) in North America: evidence of recent speciation

Male genital morphology, allozyme allele frequencies and mtDNA sequence variation were surveyed in the butterfly species Lycaeides idas and L. melissa from across much of their range in North America. Despite clear differences in male genital morphology, wing colour patterns and habitat characteristics, genetic variation was not taxonomically or geographically structured and the species were not identifiable by either genetic data set. Genetic distances (Nei's D=0.002–0.078, calculated from allozyme data) between all populations of both species were within the range commonly observed for conspecific populations of other butterflies. The most frequent mtDNA haplotype was present in individuals of both species in populations from southern California to Wisconsin. We conclude that speciation has probably happened recently and the lack of genetic differentiation between the species is the product of either (1) recent or ongoing gene flow at neutral loci, and/or (2) an insufficiency of time for lineage sorting. The evolution of male genital morphology, wing colour patterns and ecological characteristics has proceeded more rapidly than allozyme or mtDNA evolution.     

Random interbreeding between cryptic lineages of the Common Raven: evidence for speciation in reverse

DNA sequence studies frequently reveal evidence of cryptic lineages in morphologically uniform species, many of which turn out to be evolutionarily distinct species. The Common Raven (Corvus corax) includes two deeply divergent mtDNA lineages: one lineage seems restricted to western North America and the other is Holarctic in distribution. These deep clades hint of the possibility of cryptic species in the western United States. We tested this hypothesis in a population consisting of an equal proportion of both mtDNA clades, by quantifying mating patterns and associated fitness consequences with respect to mtDNA. We also tested for morphological, behavioural and ecological correlates of sex and mtDNA clade membership. Mate pairings were random with respect to mtDNA clades, and there were no differences in reproductive success between assortatively and nonassortatively mated pairs. We found no differences in survival or resource use between clades. There were no differences in morphological or behavioural characters between mtDNA clades, except one clade trended towards greater mobility. These results suggest there are no barriers to gene flow between mtDNA clades and argue that the mtDNA clades have remerged in this population, likely due to a lack of ecological or signal differentiation between individuals in each lineage. Hence, in Common Ravens, phylogeographic structure in mtDNA is a reflection of likely past isolation rather than currently differentiated species.