Ultraconserved elements improve resolution of marmot phylogeny and offer insights into biogeographic history

Marmots (Marmota spp.) comprise a lineage of large-bodied ground squirrels that diversified rapidly in the Pleistocene, when the planet quickly transitioned to a drier, colder, and highly seasonal climate-particularly at high latitudes. Fossil evidence indicates the genus spread from North America, across Beringia, and into the European Alps over the course of only a few million years, beginning in the late Pliocene. Marmots are highly adapted to survive long and severely cold winters, and this likely favored their expansion and diversification over this time period. Previous phylogenetic studies have identified two major subgenera of marmots, but the timing of important speciation events and some species relationships have been difficult to resolve. Here we use ultraconserved elements and mitogenomes, with samples from all 15 extant species, to more precisely retrace how and when marmots came to inhabit a vast Holarctic range. Our results indicate marmots arose in North America in the mid Miocene (∼16.3 Mya) and dispersed across the Bering Land Bridge in the late Pliocene (∼3-4 Mya); in addition, our fossil-calibrated timeline is suggestive of the rise and spread of open grasslands as being particularly important to marmot diversification. The woodchuck (M. monax) and the Alaska marmot (M. broweri) are found to be more closely related to the Eurasian species than to the other North American species. Paraphyly is evident in the bobak marmot (M. bobak) and the hoary marmot (M. caligata), and in the case of the latter the data are highly suggestive of a second, cryptic species in the Cascade Mountains of Washington.

Evidence of a population of leaf-eared mice Phyllotis vaccarum above 6,000 m in the Andes and a survey of high-elevation mammals

Biologists have long pondered the extreme limits of life on Earth, including the maximum elevation at which species can live and reproduce. Here we review evidence of a self-sustaining population of mice at an elevation that exceeds that of all previously reported for mammals. Five expeditions over 10 years to Volcán Llullaillaco on the Argentina/Chile border observed and collected mice at elevations ranging from 5,070 m at the mountain's base to the summit at 6,739 m (22,110 feet). Previously unreported evidence includes observations and photographs of live animals and mummified remains, environmental DNA, and a soil microbial community reflecting animal activity that are evaluated in combination with previously reported video recordings and capture of live mice. All of the evidence identifies the mouse as the leaf-eared mouse Phyllotis vaccarum, and it robustly places the population within a haplotype group containing individuals from the Chilean Atacama Desert and nearby regions of Argentina. A critical review of the literature affirms that this population is not only an elevational record for mammals but for all terrestrial vertebrates to date, and we further find that many extreme elevations previously reported for mammals are based on scant or dubious evidence.

Uncovering cryptic diversity does not end: a new species of leaf-eared mouse, genus Phyllotis (Rodentia, Cricetidae), from Central Sierras of Argentina

Based on previously published molecular (mitochondrial) and herein provided morphological (qualitative and quantitative data) evidence, we describe a new species of leaf-eared mouse of the genus Phyllotis. The new species is morphometrically distinct when compared with other phylogenetically or geographically close species of Phyllotis, showing several quantitative differences in their external and craniodental characters (e.g., proportionally broader nasals and interorbital region, and proportionally smaller tympanic bullae). The new species is endemic to central Argentina, occurring on rocky grasslands at elevations of 650–2,800 m a.s.l. This is the only species of Phyllotis inhabiting the Central Sierras, a mountain system of medium elevation, isolated from the Andes by low elevation arid and semiarid environments.

Phylogenetics And The Evolution Of Terrestriality In Mudskippers (Gobiidae: Oxudercinae)

The initial vertebrate conquest of land by stegocephalians (Sarcopterygia) allowed access to new resources and exploitation of untapped niches precipitating a major phylogenetic diversification. However, a paucity of fossils has left considerable uncertainties about phylogenetic relationships and the eco-morphological stages in this key transition in Earth history. Among extant actinopterygians, three genera of mudskippers (Gobiidae: Oxudercinae), Boleophthalmus, Periophthalmus and Periophthalmodon are the most terrestrialized, with vertebral, appendicular, locomotory, respiratory, and epithelial specializations enabling overland excursions up to 14 hours. Unlike early stegocephalians, the ecologies and morphologies of the 45 species of oxudercines are well known, making them viable analogs for the initial vertebrate conquest of land. Nevertheless, they have received little phylogenetic attention. We compiled the largest molecular dataset to date, with 29 oxudercine species, and 5 nuclear and mitochondrial loci. Phylogenetic and comparative analyses revealed strong support for two independent terrestrial transitions, and a complex suit of ecomorphological forms in estuarine environments. Furthermore, neither Oxudercinae nor their presumed sister-group the eel gobies (Amblyopinae, a group of elongated gobies) were monophyletic with respect to each other, requiring a merging of these two subfamilies and revealing an expansion of phenotypic variation within the "mudskipper" clade. We did not find support for the expected linear model of ecomorphological and locomotory transition from fully aquatic, to mudswimming, to pectoral-aided mudswimming, to lobe-finned terrestrial locomotion proposed by earlier morphological studies. This high degree of convergent or parallel transitions to terrestriality, and apparent divergent directions of estuarine adaptation, promises even greater potential for this clade to illuminate the conquest of land. Future work should focus on these less-studied species with "transitional" and other mud-habitat specializations to fully resolve the dynamics of this diversification.

A rodent anchored hybrid enrichment probe set for a range of phylogenetic utility: From order to species

Rodents are the largest order of mammals and contain several model organisms important to scientific research in a variety of fields, yet no large set of genomic markers have been designed for this group to date, hindering evolutionary studies into relationships of the group as a whole. Here we present a genomic probe set designed and optimized for rodents with a protocol that is easy to replicate with little laboratory investment. This design utilizes an anchored hybrid enrichment approach specifically targeting rodents to generate longer loci with a higher substitution rate than existing vertebrate probes to provide utility at various taxonomic levels. Using a test set of rodents from all five suborders, we successfully obtained alignments for 416 of the 418 target loci with an average of 1379 bp per locus and a total alignment of more than half a million base pairs. This genomic data set performed well in all phylogenetic analyses, especially in recent phylogenetic splits, with ample parsimony-informative sites within genera and even within species, showing more than four times as many single nucleotide polymorphisms per locus than a recent vertebrate ultraconserved elements study. Additional support is provided in resolving deeper clades in Rodentia. By providing this probe design, we hope that more laboratories can easily generate data for answering questions in rodents from species delimitation to understanding relationships among families in rapid radiations.

Tempo and mode of evolution of oryzomyine rodents (Rodentia, Cricetidae, Sigmodontinae): A phylogenomic approach

The tribe Oryzomyini is an impressive group of rodents, comprising 30 extant genera and an estimated 147 species. Recent remarkable advances in the understanding of the diversity, taxonomy and systematics of the tribe have mostly derived from analyses of single or few genetic markers. However, the evolutionary history and biogeography of Oryzomyini, its origin and diversification across the Neotropics, remain unrevealed. Here we use a multi-locus dataset (over 400 loci) obtained through anchored phylogenomics to provide a genome-wide phylogenetic hypothesis for Oryzomyini and to investigate the tempo and mode of its evolution. Species tree and supermatrix analyses produced topologies with strong support for most branches, with all genera confirmed as monophyletic, a result that previous studies failed to obtain. Our analyses also corroborated the monophyly and phylogenetic relationship of three main clades of Oryzomyini (B, C and D). The origin of the tribe is estimated to be in the Miocene (8.93-5.38 million years ago). The cladogenetic events leading to the four main clades occurred during the late Miocene and early Pliocene and most speciation events in the Pleistocene. Geographic range estimates suggested an east of Andes origin for the ancestor of oryzomyines, most likely in the Boreal Brazilian region, which includes the north bank of Rio Amazonas and the Guiana Shield. Oryzomyini rodents are an autochthonous South America radiation, that colonized areas and dominions of this continent mainly by dispersal events. The evolutionary history of the tribe is deeply associated with the Andean cordillera and the landscape history of Amazon basin.

Discovery of the world's highest-dwelling mammal

Environmental limits of animal life are invariably revised when the animals themselves are investigated in their natural habitats. Here we report results of a scientific mountaineering expedition to survey the high-altitude rodent fauna of Volcán Llullaillaco in the Puna de Atacama of northern Chile, an effort motivated by video documentation of mice (genus Phyllotis) at a record altitude of 6,205 m. Among numerous trapping records at altitudes of >5,000 m, we captured a specimen of the yellow-rumped leaf-eared mouse (Phyllotis xanthopygus rupestris) on the very summit of Llullaillaco at 6,739 m. This summit specimen represents an altitudinal world record for mammals, far surpassing all specimen-based records from the Himalayas and other mountain ranges. This discovery suggests that we may have generally underestimated the altitudinal range limits and physiological tolerances of small mammals simply because the world's high summits remain relatively unexplored by biologists.

Evolutionary journey of the retroviral restriction gene Fv1

Both exogenous and endogenous retroviruses have long been studied in mice, and some of the earliest mouse studies focused on the heritability of genetic factors influencing permissivity and resistance to infection. The prototypic retroviral restriction factor, Fv1, is now understood to exhibit a degree of control across multiple retroviral genera and is highly diverse within Mus To better understand the age and evolutionary history of Fv1, a comprehensive survey of the Muroidea was conducted, allowing the progenitor integration to be dated to ∼45 million years. Intact coding potential is visible beyond Mus, and sequence analysis reveals strong signatures of positive selection also within field mice, ApodemusFv1's survival for such a period implies a recurring and shifting retroviral burden imparting the necessary selective pressures-an influence likely also common to analogous factors. Regions of Fv1 adapt cooperatively, highlighting its preference for repeated structures and suggesting that this functionally constrained aspect of the retroviral capsid lattice presents a common target in the evolution of intrinsic immunity.

Muroid rodent phylogenetics: 900-species tree reveals increasing diversification rates

We combined new sequence data for more than 300 muroid rodent species with our previously published sequences for up to five nuclear and one mitochondrial genes to generate the most widely and densely sampled hypothesis of evolutionary relationships across Muroidea. An exhaustive screening procedure for publically available sequences was implemented to avoid the propagation of taxonomic errors that are common to supermatrix studies. The combined data set of carefully screened sequences derived from all available sequences on GenBank with our new data resulted in a robust maximum likelihood phylogeny for 900 of the approximately 1,620 muroids. Several regions that were equivocally resolved in previous studies are now more decisively resolved, and we estimated a chronogram using 28 fossil calibrations for the most integrated age and topological estimates to date. The results were used to update muroid classification and highlight questions needing additional data. We also compared the results of multigene supermatrix studies like this one with the principal published supertrees and concluded that the latter are unreliable for any comparative study in muroids. In addition, we explored diversification patterns as an explanation for why muroid rodents represent one of the most species-rich groups of mammals by detecting evidence for increasing net diversification rates through time across the muroid tree. We suggest the observation of increasing rates may be due to a combination of parallel increases in rate across clades and high average extinction rates. Five increased diversification-rate-shifts were inferred, suggesting that multiple, but perhaps not independent, events have led to the remarkable species diversity in the superfamily. Our results provide a phylogenetic framework for comparative studies that is not highly dependent upon the signal from any one gene.

PHYLOGENETIC ANALYSIS OF PHENOTYPIC COVARIANCE STRUCTURE. I. CONTRASTING RESULTS FROM MATRIX CORRELATION AND COMMON PRINCIPAL COMPONENT ANALYSES

Applications of quantitative techniques to understanding macroevolutionary patterns typically assume that genetic variances and covariances remain constant. That assumption is tested among 28 populations of the Phyllotis darwini species group (leaf-eared mice). Phenotypic covariances are used as a surrogate for genetic covariances to allow much greater phylogenetic sampling. Two new approaches are applied that extend the comparative method to multivariate data. The efficacy of these techniques are compared, and their sensitivity to sampling error examined. Pairwise matrix correlations of correlation matrices are consistently very high (> 0.90) and show no significant association between matrix similarity and phylogenetic relatedness. Hierarchical decomposition of common principal component (CPC) analyses applied to each clade in the phylogeny rejects the hypothesis that common principal component structure is shared in clades more inclusive than subspecies. Most subspecies also lack a common covariance structure as described by the CPC model. The hypothesis of constant covariances must be rejected, but the magnitudes of divergence in covariance structure appear to be small. Matrix correlations are very sensitive to sampling error, while CPC is not. CPC is a powerful statistical tool that allows detailed testing of underlying patterns of covariation.

Emergence and evolution of Zfp36l3

In most mammals, the Zfp36 gene family consists of three conserved members, with a fourth member, Zfp36l3, present only in rodents. The ZFP36 proteins regulate post-transcriptional gene expression at the level of mRNA stability in organisms from humans to yeasts, and appear to be expressed in all major groups of eukaryotes. In Mus musculus, Zfp36l3 expression is limited to the placenta and yolk sac, and is important for overall fecundity. We sequenced the Zfp36l3 gene from more than 20 representative species, from members of the Muridae, Cricetidae and Nesomyidae families. Zfp36l3 was not present in Dipodidae, or any families that branched earlier, indicating that this gene is exclusive to the Muroidea superfamily. We provide evidence that Zfp36l3 arose by retrotransposition of an mRNA encoded by a related gene, Zfp36l2 into an ancestral rodent X chromosome. Zfp36l3 has evolved rapidly since its origin, and numerous modifications have developed, including variations in start codon utilization, de novo intron formation by mechanisms including a nested retrotransposition, and the insertion of distinct repetitive regions. One of these repeat regions, a long alanine rich-sequence, is responsible for the full-time cytoplasmic localization of Mus musculus ZFP36L3. In contrast, this repeat sequence is lacking in Peromyscus maniculatus ZFP36L3, and this protein contains a novel nuclear export sequence that controls shuttling between the nucleus and cytosol. Zfp36l3 is an example of a recently acquired, rapidly evolving gene, and its various orthologues illustrate several different mechanisms by which new genes emerge and evolve.

Molecular phylogenetics and historical biogeography amid shifting continents in the cockles and giant clams (Bivalvia: Cardiidae)

Reconstructing historical biogeography of the marine realm is complicated by indistinct barriers and, over deeper time scales, a dynamic landscape shaped by plate tectonics. Here we present the most extensive examination of model-based historical biogeography among marine invertebrates to date. We conducted the largest phylogenetic and molecular clock analyses to date for the bivalve family Cardiidae (cockles and giant clams) with three unlinked loci for 110 species representing 37 of the 50 genera. Ancestral ranges were reconstructed using the dispersal-extinction-cladogenesis (DEC) method with a time-stratified paleogeographic model wherein dispersal rates varied with shifting tectonics. Results were compared to previous classifications and the extensive paleontological record. Six of the eight prior subfamily groupings were found to be para- or polyphyletic. Cardiidae originated and subsequently diversified in the tropical Indo-Pacific starting in the Late Triassic. Eastern Atlantic species were mainly derived from the tropical Indo-Mediterranean region via the Tethys Sea. In contrast, the western Atlantic fauna was derived from Indo-Pacific clades. Our phylogenetic results demonstrated greater concordance with geography than did previous phylogenies based on morphology. Time-stratifying the DEC reconstruction improved the fit and was highly consistent with paleo-ocean currents and paleogeography. Lastly, combining molecular phylogenetics with a rich and well-documented fossil record allowed us to test the accuracy and precision of biogeographic range reconstructions.

Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation

Background

Crocodilians have dominated predatory niches at the water-land interface for over 85 million years. Like their ancestors, living species show substantial variation in their jaw proportions, dental form and body size. These differences are often assumed to reflect anatomical specialization related to feeding and niche occupation, but quantified data are scant. How these factors relate to biomechanical performance during feeding and their relevance to crocodilian evolutionary success are not known.

Methodology/Principal Findings

We measured adult bite forces and tooth pressures in all 23 extant crocodilian species and analyzed the results in ecological and phylogenetic contexts. We demonstrate that these reptiles generate the highest bite forces and tooth pressures known for any living animals. Bite forces strongly correlate with body size, and size changes are a major mechanism of feeding evolution in this group. Jaw shape demonstrates surprisingly little correlation to bite force and pressures. Bite forces can now be predicted in fossil crocodilians using the regression equations generated in this research.

Conclusions/Significance

Critical to crocodilian long-term success was the evolution of a high bite-force generating musculo-skeletal architecture. Once achieved, the relative force capacities of this system went essentially unmodified throughout subsequent diversification. Rampant changes in body size and concurrent changes in bite force served as a mechanism to allow access to differing prey types and sizes. Further access to the diversity of near-shore prey was gained primarily through changes in tooth pressure via the evolution of dental form and distributions of the teeth within the jaws. Rostral proportions changed substantially throughout crocodilian evolution, but not in correspondence with bite forces. The biomechanical and ecological ramifications of such changes need further examination.

Developmental constraint on the evolution of marsupial forelimb morphology

Compared with the placental mammals, marsupials are born at an almost embryonic stage, but nearly all of these neonates immediately climb or crawl to one of their mother’s teats using precociously developed forelimbs. Marsupial adults also exhibit limited forelimb shape diversity relative to the members of their sister group. That the functional requirements of this natal climb have imposed a developmental constraint on marsupial forelimb evolution represents a compelling and widely accepted hypothesis, yet its resulting predictions for the comparative patterns of mammal limb shape diversity have never been tested. In order to perform such tests we conducted extensive taxonomic sampling of mammal limb morphology (including fossil specimens), and then examined these data using morphometric methods, non-parametric analyses of anatomical disparity, and phylogenetic comparative analyses of evolutionary rates. Our results strongly support the constraint hypothesis, and indicate that the highly significant differences between marsupial and placental forelimb shape diversity has been strongly influenced by different rates of morphological evolution among the distal forelimb elements in these two important mammal lineages.

Phylogenomic evidence for multiple losses of flight in ratite birds

Ratites (ostriches, emus, rheas, cassowaries, and kiwis) are large, flightless birds that have long fascinated biologists. Their current distribution on isolated southern land masses is believed to reflect the breakup of the paleocontinent of Gondwana. The prevailing view is that ratites are monophyletic, with the flighted tinamous as their sister group, suggesting a single loss of flight in the common ancestry of ratites. However, phylogenetic analyses of 20 unlinked nuclear genes reveal a genome-wide signal that unequivocally places tinamous within ratites, making ratites polyphyletic and suggesting multiple losses of flight. Phenomena that can mislead phylogenetic analyses, including long branch attraction, base compositional bias, discordance between gene trees and species trees, and sequence alignment errors, have been eliminated as explanations for this result. The most plausible hypothesis requires at least three losses of flight and explains the many morphological and behavioral similarities among ratites by parallel or convergent evolution. Finally, this phylogeny demands fundamental reconsideration of proposals that relate ratite evolution to continental drift.

Ancient hybridization and mitochondrial capture between two species of chipmunks

Models that posit speciation in the face of gene flow are replacing classical views that hybridization is rare between animal species. We use a multilocus approach to examine the history of hybridization and gene flow between two species of chipmunks (Tamias ruficaudus and T. amoenus). Previous studies have shown that these species occupy different ecological niches and have distinct genital bone morphologies, yet appear to be incompletely isolated reproductively in multiple areas of sympatry. We compared data from four sequenced nuclear loci and from seven microsatellite loci to published cytochrome b sequences. Interspecific gene flow was primarily restricted to introgression of the T. ruficaudus mitochondrial genome into a sympatric subspecies of T. amoenus, T. a. canicaudus, with the four sequenced nuclear loci showing little to no interspecific allele sharing. Microsatellite data were consistent with high levels of differentiation between the species and also showed no current gene flow between broadly sympatric populations of T. a. canicaudus and T. ruficaudus. Coalescent analyses date the mtDNA introgression event from the mid-Pleistocene to late Pliocene. Overall, these data indicate that introgression has had a minimal impact on the nuclear genomes of T. amoenus and T. ruficaudus despite multiple independent hybridization events. Our findings challenge long-standing assumptions on patterns of reproductive isolation in chipmunks and suggest that there may be other examples of hybridization among the 23 species of Tamias that occur in western North America.

Ancient hybridization and mitochondrial capture between two species of chipmunks

Models that posit speciation in the face of gene flow are replacing classical views that hybridization is rare between animal species. We use a multilocus approach to examine the history of hybridization and gene flow between two species of chipmunks (Tamias ruficaudus and T. amoenus). Previous studies have shown that these species occupy different ecological niches and have distinct genital bone morphologies, yet appear to be incompletely isolated reproductively in multiple areas of sympatry. We compared data from four sequenced nuclear loci and from seven microsatellite loci to published cytochrome b sequences. Interspecific gene flow was primarily restricted to introgression of the T. ruficaudus mitochondrial genome into a sympatric subspecies of T. amoenus, T. a. canicaudus, with the four sequenced nuclear loci showing little to no interspecific allele sharing. Microsatellite data were consistent with high levels of differentiation between the species and also showed no current gene flow between broadly sympatric populations of T. a. canicaudus and T. ruficaudus. Coalescent analyses date the mtDNA introgression event from the mid-Pleistocene to late Pliocene. Overall, these data indicate that introgression has had a minimal impact on the nuclear genomes of T. amoenus and T. ruficaudus despite multiple independent hybridization events. Our findings challenge long-standing assumptions on patterns of reproductive isolation in chipmunks and suggest that there may be other examples of hybridization among the 23 species of Tamias that occur in western North America.

Multigene phylogeny of the Old World mice, Murinae, reveals distinct geographic lineages and the declining utility of mitochondrial genes compared to nuclear genes

Despite its great diversity and biomedical importance, the rodent subfamily Murinae is poorly resolved phylogenetically. We present the first cladistic analysis sampling multiple representatives of most major groups based on DNA sequence for three nuclear (GHR, RAG1, and AP5) and one mitochondrial (COII and parts of COI and ATPase 8) fragments. Analyzed separately, the four partitions agree broadly with each other and the combined analysis. The basal split is between a clade of Philippine Old Endemics and all remaining murines. Within the latter, rapid radiation led to at least seven geographically distinct lineages, including a Southeast Asian Rattus clade; a diverse Australo-Papuan and Philippine clade; an African arvicanthine group including the otomyines; an African Praomys group; and three independent genera from Africa and Asia, Mus, Apodemus, and Malacomys. The murines appear to have originated in Southeast Asia and then rapidly expanded across all of the Old World. Both nuclear exons provide robust support at all levels. In contrast, the bootstrap proportions from mitochondrial data decline rapidly with increasing depth in the tree, together suggesting that nuclear genes may be more useful even for relatively recent divergences (< 10MYA).

Seeing the Forest for the Trees

Assembling the Tree of Life . Joel Cracraft and Michael J. Donoghue, Eds. . Oxford University Press, New York, 2004. 592 pp. $59.95, £36.50. ISBN 0-19-517234-5.

The contributors summarize our current understanding of phylogenetic relationships among and within the major evolutionary branches of life on Earth and argue for the importance of such understanding.

Nuclear DNA phylogeny of the squirrels (Mammalia: Rodentia) and the evolution of arboreality from c-myc and RAG1

Although the family Sciuridae is large and well known, phylogenetic analyses are scarce. We report on a comprehensive molecular phylogeny for the family. Two nuclear genes (c-myc and RAG1) comprising approximately 4500 bp of data (most in exons) are applied for the first time to rodent phylogenetics. Parsimony, likelihood, and Bayesian analyses of the separate gene regions and combined data reveal five major lineages and refute the conventional elevation of the flying squirrels (Pteromyinae) to subfamily status. Instead, flying squirrels are derived from one of the tree squirrel lineages. C-myc indels corroborate the sequence-based topologies. The common ancestor of extant squirrels appears to have been arboreal, confirming the fossil evidence. The results also reveal an unexpected clade of mostly terrestrial squirrels with African and Holarctic centers of diversity. We present a revised classification of squirrels. Our results demonstrate the phylogenetic utility of relatively slowly evolving nuclear exonic data even for relatively recent clades.

Molecular phylogeny of the marmots (Rodentia: Sciuridae): tests of evolutionary and biogeographic hypotheses

There are 14 species of marmots distributed across the Holarctic, and despite extensive systematic study, their phylogenetic relationships remain largely unresolved. In particular, comprehensive studies have been lacking. A well-supported phylogeny is needed to place the numerous ecological and behavioral studies on marmots in an evolutionary context. To address this situation, we obtained complete cytochrome (cyt) b sequences for 13 of the species and a partial sequence for the 14th. We applied a statistical approach to both phylogeny estimation and hypothesis testing, using parsimony and maximum likelihood-based methods. We conducted statistical tests on a suite of previously proposed hypotheses of phylogenetic relationships and biogeographic histories. The cyt b data strongly support the monophyly of Marmota and a western montane clade in the Nearctic. Although some other scenarios cannot be rejected, the results are consistent with an initial diversification in North America, followed by an invasion and subsequent rapid diversification in the Palearctic. These analyses reject the two major competing hypotheses of M. broweri's phylogenetic relationships--namely, that it is the sister species to M. camtschatica of eastern Siberia, and that it is related closely to M. caligata of the Nearctic. The Alaskan distribution of M. broweri is best explained as a reinvasion from the Palearctic, but a Nearctic origin can not be rejected. Several other conventionally recognized species groups can also be rejected. Social evolution has been homoplastic, with large colonial systems evolving in two groups convergently. The cyt b data do not provide unambiguous resolution of several basal nodes in the Palearctic radiation, leaving some aspects of pelage and karyotypic evolution equivocal.