Animal Speciation Research Articles

Simulating a hybridization event of a range-expanding sexual cannibal (Argiope bruennichi)

Abstract Hybridization plays an important role in animal speciation. Mediterranean populations of the Palaearctic wasp spider Argiope bruennichi have recently undergone a range expansion northwards. Adaptation to colder winters was facilitated by introgressed alleles derived from Asian populations, hence the resulting hybrids were able to expand geographically. Argiope bruennichi exhibits a highly complex mating system that includes strong sexual conflict and predisposes the species to behaviours that could facilitate directed hybridization and hybrid stabilization. To test this, we simulated a hybridization event with European and Asian spiders and a backcrossing event with southern and northern European spiders by conducting mate choice experiments and mating trials. We found pronounced differences in the cannibalism and monopolization successes between the populations. Asian male spiders were more successful in monopolizing European females than their European conspecifics and more successful than European males with Asian females. This would have led to an increased proportion of these specific offspring in the hybrid population. Our results suggest the mating system as a driving factor in the directional hybridization of Asian males and European females and the establishment of new northern European populations of A. bruennichi. We found no influences of the mating system on the stabilization of the resulting hybrid populations, probably owing to cold survival adaptations being a more important factor.

The microbiome impacts host hybridization and speciation.

Microbial symbiosis and speciation profoundly shape the composition of life’s biodiversity. Despite the enormous contributions of these two fields to the foundations of modern biology, there is a vast and exciting frontier ahead for research, literature, and conferences to address the neglected prospects of merging their study. Here, we survey and synthesize exemplar cases of how endosymbionts and microbial communities affect animal hybridization and vice versa. We conclude that though the number of case studies remain nascent, the wide-ranging types of animals, microbes, and isolation barriers impacted by hybridization will likely prove general and a major new phase of study that includes the microbiome as part of the functional whole contributing to reproductive isolation. Though microorganisms were proposed to impact animal speciation a century ago, the weight of the evidence supporting this view has now reached a tipping point.

Conserved genetic signatures parcellate cardinal spinal neuron classes into local and projection subsets.

Motor and sensory functions of the spinal cord are mediated by populations of cardinal neurons arising from separate progenitor lineages. However, each cardinal class is composed of multiple neuronal types with distinct molecular, anatomical, and physiological features, and there is not a unifying logic that systematically accounts for this diversity. We reasoned that the expansion of new neuronal types occurred in a stepwise manner analogous to animal speciation, and we explored this by defining transcriptomic relationships using a top-down approach. We uncovered orderly genetic tiers that sequentially divide groups of neurons by their motor-sensory, local-long range, and excitatory-inhibitory features. The genetic signatures defining neuronal projections were tied to neuronal birth date and conserved across cardinal classes. Thus, the intersection of cardinal class with projection markers provides a unifying taxonomic solution for systematically identifying distinct functional subsets.

Mitochondrial DNA Variation and Selfish Propagation Following Experimental Bottlenecking in Two Distantly Related Caenorhabditis briggsae Isolates.

Understanding mitochondrial DNA (mtDNA) evolution and inheritance has broad implications for animal speciation and human disease models. However, few natural models exist that can simultaneously represent mtDNA transmission bias, mutation, and copy number variation. Certain isolates of the nematode Caenorhabditis briggsae harbor large, naturally-occurring mtDNA deletions of several hundred basepairs affecting the NADH dehydrogenase subunit 5 (nduo-5) gene that can be functionally detrimental. These deletion variants can behave as selfish DNA elements under genetic drift conditions, but whether all of these large deletion variants are transmitted in the same preferential manner remains unclear. In addition, the degree to which transgenerational mtDNA evolution profiles are shared between isolates that differ in their propensity to accumulate the nduo-5 deletion is also unclear. We address these knowledge gaps by experimentally bottlenecking two isolates of C. briggsae with different nduo-5 deletion frequencies for up to 50 generations and performing total DNA sequencing to identify mtDNA variation. We observed multiple mutation profile differences and similarities between C. briggsae isolates, a potentially species-specific pattern of copy number dysregulation, and some evidence for genetic hitchhiking in the deletion-bearing isolate. Our results further support C. briggsae as a practical model for characterizing naturally-occurring mtgenome variation and contribute to the understanding of how mtgenome variation persists in animal populations and how it presents in mitochondrial disease states.

Developmental pathways of periodontal tissue regeneration: Developmental diversities of tooth morphogenesis do also map capacity of periodontal tissue regeneration?

Nothing is known on the impact of developmental divergence on periodontal tissue regeneration in vertebrate animals. Molecularly, the induction of tooth morphogenesis is highly conserved deploying across animal phyla a constant and reproducible set of gene pathways, which result in morphogenesis of multiple odontode forms and shapes. Genetic mutations positively affect animal speciation via evolving biting and masticatory forces as well as dietary habits selectively imprinted in animal phyla during evolutionary speciation. The geometry of the attachment apparatus of a tooth is important for the interpretation of the induction of cementogenesis with de novo Sharpey's fibres as in thecodonty, ie, a tripartite attachment of alveolar bone, periodontal ligament and cementum. This review addresses the tooth implantation in different animal clades from the fibrous attachment of the Elasmobranch Carcharinus obscurus dusky shark, reviewing the evolution and functional significance of cementum with functionally inserted Sharpey's fibres. In sharks there is a continuous tooth replacement mechanistically supported by the continuously erupting dental lamina. We show that the arching of the continuously erupting dental lamina, a critical step for the selachians' tooth differentiation, is prominently characterized by transforming growth factor-β3 (TGF-β3 ) expression not only within the dental lamina but also in cellular condensations in the mesenchymal tissues of the erupting tooth. Such findings indicate the pleiotropic multifaceted activity of a highly conserved mammalian gene across genera, masterminding tooth morphogenesis in both selachians and mammals as well as periodontal tissue induction in the non-human primate Papio ursinus. In P.ursinus, the induction of cementogenesis entails the expression of TGF-β3 and osteocalcin with fine-tuning and regulation of bone morphogenetic proteins BMP-2 and BMP-7, and upregulation of TGF-β3 . TGF-β3 autoinduction and upregulation during the induction of cementogenesis and osteogenesis in P.ursinus provide novel insights into the induction of cementogenesis. It is hypothesized that the evolutionary expression and upregulation of the TGF-β3 gene may provide the mechanistic insights into the induction of extensive cementogenesis as seen in stem mammals and the induction of trabecular-like cementum formation in mosasaurs' tooth attachment. Aspidin, the precursor of cementum, was reported to appear 310-330million years ago (Ma) in Odontostraci armoured fish. Studies showed that the differentiation of cementum with inserted Sharpey's fibres is also present in lower amniotes such as Diatectomorpha or Diadectidae, the first herbivorous tetrapods, 323Ma. In mosasaurs, 168-165Ma, there is the induction of extensive trabeculation of cementum though nothing is known on the phylogenetic temporo-spatial evolution of cementum before Diadectidae and stem mammals. The large trabeculations of cementum as seen in the attachment of extinct mosasaurs invocates a pleiotropic capacity of cemental growth previously unknown. The appearance of cementum facing a vascularized and innervated periodontal ligament space with Sharpey's fibres inserting on to mineralized cementum provides a multiform pleiotropic masticatory apparatus adapted to multiple biting and lacerating forces as well as finely tuned and controlled forces beyond mastication and deglutition. The remarkable cementogenesis as seen in stem mammals but particularly in mosasaurs with cemental trabeculations across the ligament space invocates the developmental capacity of cementum. The large cemental trabeculations as seen in mosasaurs and the cemental growth in stem mammals, together with regenerating scenarios in P.ursinus with large seams of cellular cementum and cementoid populated by contiguous cementoblasts indicate the continuous molecular cross-talk between cementum, newly formed cementoid matrix, cementoblasts and extracellular matrix soluble molecular signals. This molecular cross-talk may control the biomolecular homeostasis of both cementum and periodontal ligament, including angiogenesis. A further molecular scenario is invocated by the tight and exquisite anatomical relationships between the cementoid surfaces and the newly formed capillaries. The primitiveness of the craniate masticatory mineralized craniofacial apparatus has been controlled by several yet ancestral common genes not lastly the TGF-β3 gene. The TGF-β3 might have been responsible for the induction of cementogenesis not only in extant P.ursinus but also in Diatectomorpha and mosasaurs, thus providing continuous evolutionary mechanisms for the induction of tissue morphogenesis across animal phyla for almost a billion years of evolution, epitomizing Nature's parsimony in controlling tissue induction and morphogenesis. TGF-β receptor II regulates osterix expression via Smad-dependent pathways indicating that TGF-β signalling acts as an upstream regulator of osterix during cementoblast differentiation. The presence of morphogenetic signals within the cemental matrix capable of inducing bone formation needs now to be assigned: bone induction initiated by extracted and partially purified cemental matrices may be the result of a slow release of embryonic remnants of osteogenic signals required and deployed during cementogenesis. The cementum may thus rule the periodontal ligament space homeostasis, remodelling and repair by releasing sequestered morphogenetic signals that were deployed during embryogenesis.

Mitochondrial DNA Variation in Human Radiation and Disease

Environmental adaptation, predisposition to common diseases, and, potentially, speciation may all be linked through the adaptive potential of mitochondrial DNA (mtDNA) alterations of bioenergetics. This Perspective synthesizes evidence that human mtDNA variants may be adaptive or deleterious depending on environmental context and proposes that the accrual of mtDNA variation could contribute to animal speciation via adaptation to marginal environments.

Mitonuclear coevolution as the genesis of speciation and the mitochondrial DNA barcode gap.

Mitochondrial genes are widely used in taxonomy and systematics because high mutation rates lead to rapid sequence divergence and because such changes have long been assumed to be neutral with respect to function. In particular, the nucleotide sequence of the mitochondrial gene cytochrome c oxidase subunit 1 has been established as a highly effective DNA barcode for diagnosing the species boundaries of animals. Rarely considered in discussions of mitochondrial evolution in the context of systematics, speciation, or DNA barcodes, however, is the genomic architecture of the eukaryotes: Mitochondrial and nuclear genes must function in tight coordination to produce the complexes of the electron transport chain and enable cellular respiration. Coadaptation of these interacting gene products is essential for organism function. I extend the hypothesis that mitonuclear interactions are integral to the process of speciation. To maintain mitonuclear coadaptation, nuclear genes, which code for proteins in mitochondria that cofunction with the products of mitochondrial genes, must coevolve with rapidly changing mitochondrial genes. Mitonuclear coevolution in isolated populations leads to speciation because population‐specific mitonuclear coadaptations create between‐population mitonuclear incompatibilities and hence barriers to gene flow between populations. In addition, selection for adaptive divergence of products of mitochondrial genes, particularly in response to climate or altitude, can lead to rapid fixation of novel mitochondrial genotypes between populations and consequently to disruption in gene flow between populations as the initiating step in animal speciation. By this model, the defining characteristic of a metazoan species is a coadapted mitonuclear genotype that is incompatible with the coadapted mitochondrial and nuclear genotype of any other population.

Gamete production patterns and mating systems in water frogs of the hybridogenetic Pelophylax esculentus complex in north-eastern Ukraine

Hybridization and polyploidy play an important role in animal speciation. European water frogs of the Pelophylax esculentus complex demonstrate unusual genetic phenomena associated with hybridization, clonality and polyploidy which presumably indicate an initial stage of reticulate speciation. The Seversky Donets River drainage in north-eastern Ukraine is inhabited by both sexes of the diploid and triploid hybrid P. esculentus and only one parental species Pelophylax ridibundus. Based on the presence of various types of hybrids, all populations studied can be divided into three geographical groups: I) P. ridibundus–P. esculentus without triploids; II) P. ridibundus–P. esculentus without diploid hybrids; and III) P. ridibundus–P. esculentus with a mixture of diploids and triploids. A study of gametogenesis revealed that diploid P. esculentus in populations of the first type usually produced haploid gametes of P. ridibundus and a mixture of haploid gametes that carried one or another parental genome (hybrid amphispermy). In populations of the second type, hybrids are derived from crosses of P. ridibundus males with triploid hybrid females producing haploid eggs with a genome of P. lessonae. Therefore, we suggest that clonal genome duplication in these eggs might be the result of suppression of second polar body formation or extra precleavage endoreduplication. In populations of the third type, some diploid females can produce diploid gametes. Fertilization of these eggs with haploid sperm can result in triploid hybrids. Other hybrids here produce haploid gametes with one or another parental genome or their mixture giving rise to new diploid hybrids.

Speciation by Symbiosis: the Microbiome and Behavior.

ABSTRACTSpecies are fundamental units of comparison in biology. The newly discovered importance and ubiquity of host-associated microorganisms are now stimulating work on the roles that microbes can play in animal speciation. We previously synthesized the literature and advanced concepts of speciation by symbiosis with notable attention to hybrid sterility and lethality. Here, we review recent studies and relevant data on microbes as players in host behavior and behavioral isolation, emphasizing the patterns seen in these analyses and highlighting areas worthy of additional exploration. We conclude that the role of microbial symbionts in behavior and speciation is gaining exciting traction and that the holobiont and hologenome concepts afford an evolving intellectual framework to promote research and intellectual exchange between disciplines such as behavior, microbiology, genetics, symbiosis, and speciation. Given the increasing centrality of microbiology in macroscopic life, microbial symbiosis is arguably the most neglected aspect of animal and plant speciation, and studying it should yield a better understanding of the origin of species.

Negligible nuclear introgression despite complete mitochondrial capture between two species of chipmunks.

The idea that species boundaries can be semipermeable to gene flow is now widely accepted but the evolutionary importance of introgressive hybridization remains unclear. Here we examine the genomic contribution of gene flow between two hybridizing chipmunk species, Tamias ruficaudus and T. amoenus. Previous studies have shown that ancient hybridization has resulted in complete fixation of introgressed T. ruficaudus mitochondrial DNA (mtDNA) in some populations of T. amoenus, but the extent of nuclear introgression is not known. We used targeted capture to sequence over 10,500 gene regions from multiple individuals of both species. We found that most of the nuclear genome is sorted between these species and that overall genealogical patterns do not show evidence for introgression. Our analysis rules out all but very minor levels of interspecific gene flow, indicating that introgressive hybridization has had little impact on the overall genetic composition of these species outside of the mitochondrial genome. Given that much of the evidence for introgression in animals has come from mtDNA, our results underscore that unraveling the importance introgressive hybridization during animal speciation will require a genome-wide perspective that is still absent for many species.

Cryptic diversity, diversification and vicariance in two species complexes ofTomocerus(Collembola,Tomoceridae) fromChina

Springtails (Collembola) are a group of arthropods that are found in terrestrial ecosystems throughout the world. Two species complexes,Tomocerus ocreatusandT. nigruscomplexes, are widely distributed in the southern and northern parts of easternChina, respectively. There is a poor understanding of the species diversity within these complexes and of the factors affecting their diversification and dispersal. Species delimitation using a general mixedYule coalescent model and aBayesian multilocus approach recognized 22DNA‐based species. This supports the presence of extensive cryptic diversity in species that are geographically widespread. In addition to genetic differences, we discovered corresponding morphological differences in jumping organs among the major clades. Analyses of divergence times and historical biogeographical processes revealed thatocreatusandnigruscomplexes originated in southern and northernChina, respectively. We estimated their divergence at 27.8–44.9 Mya during theEocene–Oligocene, at the time when the transmeridionalQinling–DabieMountains uplifted and formed the north–south geographical boundary of easternChina. Diversification analyses suggest that the subsequent orogenesis of theQinghai–TibetanPlateau in westernChina had little impact on divergences within the two species complexes so that they maintained their geographical patterns from the Paleogene to the present day. Our findings also point to a potentially important influence of theQinling–Dabie Mountains on patterns of animal speciation and distribution inChina.

Rapid phenotypic evolution during incipient speciation in a continental avian radiation

Adaptive radiations have helped shape how we view animal speciation, particularly classic examples such as Darwin's finches, Hawaiian fruitflies and African Great Lakes cichlids. These 'island' radiations are comparatively recent, making them particularly interesting because the mechanisms that caused diversification are still in motion. Here, we identify a new case of a recent bird radiation within a continentally distributed species group; the capuchino seedeaters comprise 11 Sporophila species originally described on the basis of differences in plumage colour and pattern in adult males. We use molecular data together with analyses of male plumage and vocalizations to understand species limits of the group. We find marked phenotypic variation despite lack of mitochondrial DNA monophyly and few differences in other putatively neutral nuclear markers. This finding is consistent with the group having undergone a recent radiation beginning in the Pleistocene, leaving genetic signatures of incomplete lineage sorting, introgressive hybridization and demographic expansions. We argue that this apparent uncoupling between neutral DNA homogeneity and phenotypic diversity is expected for a recent group within the framework of coalescent theory. Finally, we discuss how the ecology of open habitats in South America during the Pleistocene could have helped promote this unique and ongoing radiation.

THE ROLES OF HOST EVOLUTIONARY RELATIONSHIPS (GENUS: NASONIA) AND DEVELOPMENT IN STRUCTURING MICROBIAL COMMUNITIES

The comparative structure of bacterial communities among closely related host species remains relatively unexplored. For instance, as speciation events progress from incipient to complete stages, does divergence in the composition of the species' microbial communities parallel the divergence of host nuclear genes? To address this question, we used the recently diverged species of the parasitoid wasp genus Nasonia to test whether the evolutionary relationships of their bacterial microbiotas recapitulate the Nasonia phylogenetic history. We also assessed microbial diversity in Nasonia at different stages of development to determine the role that host age plays in microbiota structure. The results indicate that all three species of Nasonia share simple larval microbiotas dominated by the γ-proteobacteria class; however, bacterial species diversity increases as Nasonia develop into pupae and adults. Finally, under identical environmental conditions, the relationships of the microbial communities reflect the phylogeny of the Nasonia host species at multiple developmental stages, which suggests that the structure of an animal's microbial community is closely allied with divergence of host genes. These findings highlight the importance of host evolutionary relationships on microbiota composition and have broad implications for future studies of microbial symbiosis and animal speciation.

Mechanical Reproductive Isolation Facilitates Parallel Speciation in Western North American Scincid Lizards

Mechanical reproductive barriers have been dismissed as a major driver of animal speciation, yet the extent to which such barriers cause reproductive isolation in most animal groups is largely unknown and rarely tested. In this study, we used hierarchical Bayesian modeling of mate compatibility experiments to show that body size divergence in lizards of the Plestiodon skiltonianus complex contributes to reproductive isolation in at least three ways: males preferably court females that are more similar in size, females reject males that are highly divergent in size, and the size difference of a male and female in copula constrains the ability to align the genitalia for intromission. We used a predictive model to estimate the contributions of behavioral and mechanical barriers to reproductive isolation between populations with differing degrees of size divergence. This model shows that the mechanical barrier is more important than behavioral barriers at small and intermediate degrees of size divergence, suggesting that it acts earlier during speciation when body morphology is more similar between diverging lineages. Given that correlated divergence in size and ecology is common in animals, similar constraints imposed by the geometry of the mating posture may apply to a variety of major animal lineages and merit further attention in speciation research.

Omissions in the synthetic theory of evolution.

The Synthetic Theory of Evolution is the most unifying theory of life science. This theory has dominated scientific thought in explaining the mechanisms involved in speciation. However, there are some omissions that have delayed the understanding of some aspects of the mechanisms of organic evolution, principally: 1) the bridge between somatic and germinal cells, especially in some phylum of invertebrates and vertebrates; 2) horizontal genetic transferences and the importance of viruses in host adaptation and evolution; 3) the role of non-coding DNA and non-transcriptional genes; 4) homeotic evolution and the limitations of gradual evolution; and 5) excessive emphasis on extrinsic barriers to animal speciation. This paper reviews each of these topics in an effort to contribute to a better comprehension of organic evolution. Molecular findings suggest the need for a new evolutionary synthesis.

Speciation genes in plants

Analyses of speciation genes--genes that contribute to the cessation of gene flow between populations--can offer clues regarding the ecological settings, evolutionary forces and molecular mechanisms that drive the divergence of populations and species. This review discusses the identities and attributes of genes that contribute to reproductive isolation (RI) in plants, compares them with animal speciation genes and investigates what these genes can tell us about speciation. Forty-one candidate speciation genes were identified in the plant literature. Of these, seven contributed to pre-pollination RI, one to post-pollination, prezygotic RI, eight to hybrid inviability, and 25 to hybrid sterility. Genes, gene families and genetic pathways that were frequently found to underlie the evolution of RI in different plant groups include the anthocyanin pathway and its regulators (pollinator isolation), S RNase-SI genes (unilateral incompatibility), disease resistance genes (hybrid necrosis), chimeric mitochondrial genes (cytoplasmic male sterility), and pentatricopeptide repeat family genes (cytoplasmic male sterility). The most surprising conclusion from this review is that identities of genes underlying both prezygotic and postzygotic RI are often predictable in a broad sense from the phenotype of the reproductive barrier. Regulatory changes (both cis and trans) dominate the evolution of pre-pollination RI in plants, whereas a mix of regulatory mutations and changes in protein-coding genes underlie intrinsic postzygotic barriers. Also, loss-of-function mutations and copy number variation frequently contribute to RI. Although direct evidence of positive selection on speciation genes is surprisingly scarce in plants, analyses of gene family evolution, along with theoretical considerations, imply an important role for diversifying selection and genetic conflict in the evolution of RI. Unlike in animals, however, most candidate speciation genes in plants exhibit intraspecific polymorphism, consistent with an important role for stochastic forces and/or balancing selection in development of RI in plants.

Genetic evidence for hybrid trait speciation in heliconius butterflies.

Homoploid hybrid speciation is the formation of a new hybrid species without change in chromosome number. So far, there has been a lack of direct molecular evidence for hybridization generating novel traits directly involved in animal speciation. Heliconius butterflies exhibit bright aposematic color patterns that also act as cues in assortative mating. Heliconius heurippa has been proposed as a hybrid species, and its color pattern can be recreated by introgression of the H. m. melpomene red band into the genetic background of the yellow banded H. cydno cordula. This hybrid color pattern is also involved in mate choice and leads to reproductive isolation between H. heurippa and its close relatives. Here, we provide molecular evidence for adaptive introgression by sequencing genes across the Heliconius red band locus and comparing them to unlinked wing patterning genes in H. melpomene, H. cydno, and H. heurippa. 670 SNPs distributed among 29 unlinked coding genes (25,847bp) showed H. heurippa was related to H. c. cordula or the three species were intermixed. In contrast, among 344 SNPs distributed among 13 genes in the red band region (18,629bp), most showed H. heurippa related with H. c. cordula, but a block of around 6,5kb located in the 3′ of a putative kinesin gene grouped H. heurippa with H. m. melpomene, supporting the hybrid introgression hypothesis. Genealogical reconstruction showed that this introgression occurred after divergence of the parental species, perhaps around 0.43Mya. Expression of the kinesin gene is spatially restricted to the distal region of the forewing, suggesting a mechanism for pattern regulation. This gene therefore constitutes the first molecular evidence for adaptive introgression during hybrid speciation and is the first clear candidate for a Heliconius wing patterning locus.

THE EVOLUTIONARY UNDERPINNINGS OF MACROECOLOGY

Review of: Butlin, R., Bridle, J. and Schluter, D. 2009. Speciation and Patterns of Diversity. Cambridge University Press. 346 pp. ISBN 978-0-521-70963-7 Paper. The fields of adaptive speciation and macroecology have made many significant advances in recent years. Research in adaptive speciation has elucidated the mechanisms of ecological speciation and sought to explain how selection can divide a single lineage into multiple independent lineages (e.g., Schluter 2000; Rundle and Nosil 2005; Nosil et al. 2009). Concomitantly, macroecologists have characterized the large-scale distributional patterns of the earth’s biodiversity and advanced theory to explain the turnover of taxa within space and time (e.g., Gaston and Blackburn 2000; Blackburn and Gaston 2003; Smith et al. 2008). Although seemingly directed at the same goal, understanding the origin and maintenance of diversity, these two fields have developed along somewhat independent lines. There is an obvious connection between the two and no evolutionary biologist would doubt the importance of ecology in facilitating speciation and no ecologist would doubt the importance of speciation in facilitating diversity. Beyond that, however, there has been little dialogue about how these two fields can integrate and form a much stronger understanding of the genesis of biodiversity. Butlin, Bridle, and Schluter designed the book, Speciation and Patterns of Diversity, to stimulate thought and discussion about the interconnectedness of macroecology and evolutionary biology. The book is a result of the 2007 Annual Symposium of the British Ecological Society entitled Speciation and Ecology. The goal of the symposium was to foster ties between the fields of macroecology and adaptive speciation. The resulting edited volume goes a long way toward enlightening the reader of the importance of blending evolution with macroecology. The main thesis of the book is to take the evolutionary mechanisms that are responsible for generating species and examine their consequences for patterns of diversity—a melding of mechanism and pattern that will lead to an understanding of process. The editors assembled leaders in the fields of macroecology and adaptive speciation to provide perspectives of where we are both ecologically and evolutionarily with regards to patterns of diversity. This is no small feat given that the topic encompasses all of the earth’s biodiversity—past and present. These leaders have offered their views and challenges for understanding the evolutionary ecology of speciation and how mechanisms of speciation may determine patterns of diversity. The result is a careful examination of the key issues currently being studied with regards to speciation and its role in generating patterns of diversity. This book is not an in-depth look at one particular aspect of diversity, nor is it a cursory look at all topics. Instead, it is a well-balanced book that explores mechanisms of speciation and the consequences for the patterns of diversity that we observe. The taxa discussed in the book range from microbes to mammals. If I have one complaint about the breadth of taxa, it is that plants are sorely missing. They are featured on the cover, but are only mentioned in passing in one of the chapters. As has historically been the case in the development of speciation theory, plant speciation mechanisms and patterns have rarely been integrated with animal speciation. Perhaps one day that will be remedied.

Omissions in the synthetic theory of evolution

The Synthetic Theory of Evolution is the most unifying theory of life science. This theory has dominated scientific thought in explaining the mechanisms involved in speciation. However, there are some omissions that have delayed the understanding of some aspects of the mechanisms of organic evolution, principally: 1) the bridge between somatic and germinal cells, especially in some phylum of invertebrates and vertebrates; 2) horizontal genetic transferences and the importance of viruses in host adaptation and evolution; 3) the role of non-coding DNA and non-transcriptional genes; 4) homeotic evolution and the limitations of gradual evolution; and 5) excessive emphasis on extrinsic barriers to animal speciation. This paper reviews each of these topics in an effort to contribute to a better comprehension of organic evolution. Molecular findings suggest the need for a new evolutionary synthesis.

Assessment of gene flow across a hybrid zone in red‐tailed chipmunks (Tamias ruficaudus)

The role of hybridization in animal speciation is controversial and recent research has challenged the long-standing criterion of complete reproductive isolation to attain species status. The speciation-with-gene-flow model posits that the genome is semi-permeable and hybridization may be a phase in the process of divergence. Here, we apply these concepts to a previously identified zone of mtDNA introgression between the two strongly morphologically differentiated subspecies of red-tailed chipmunk (Tamias ruficaudus) in the US Inland Northwest. Using multilocus genotype data from the southern, older contact zone, we demonstrate that neutral gene flow is unusually low between the subspecies across the Lochsa River. This is geographically congruent with the discontinuity in bacular morphology, indicating that the cline of mitochondrial DNA (mtDNA) haplotypes is displaced. Furthermore, we elucidate the evolutionary forces responsible by testing hypotheses of lineage sorting and hybridization. We determined that introgressive hybridization is the cause of mtDNA/morphology incongruence because there are non-zero levels of migration and gene flow. Although our estimate of the age of the hybrid zone has wide credibility intervals, the hybridization events occurred in the Late Pleistocene and the divergence occurred in the Middle Pleistocene. Finally, we assessed substructure within and adjacent to the hybrid zone and found that the hybrid zone constitutes a set of populations that are genetically differentiated from parental sets of populations; therefore, hybridization in this system is not likely an evolutionary sink, but has generated novel combinations of genotypes.