Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation

Bats (Chiroptera) represent one of the largest and most diverse radiations of mammals, accounting for one-fifth of extant species. Although recent studies unambiguously support bat monophyly and consensus is rapidly emerging about evolutionary relationships among extant lineages, the fossil record of bats extends over 50 million years, and early evolution of the group remains poorly understood. Here we describe a new bat from the Early Eocene Green River Formation of Wyoming, USA, with features that are more primitive than seen in any previously known bat. The evolutionary pathways that led to flapping flight and echolocation in bats have been in dispute, and until now fossils have been of limited use in documenting transitions involved in this marked change in lifestyle. Phylogenetically informed comparisons of the new taxon with other bats and non-flying mammals reveal that critical morphological and functional changes evolved incrementally. Forelimb anatomy indicates that the new bat was capable of powered flight like other Eocene bats, but ear morphology suggests that it lacked their echolocation abilities, supporting a 'flight first' hypothesis for chiropteran evolution. The shape of the wings suggests that an undulating gliding-fluttering flight style may be primitive for bats, and the presence of a long calcar indicates that a broad tail membrane evolved early in Chiroptera, probably functioning as an additional airfoil rather than as a prey-capture device. Limb proportions and retention of claws on all digits indicate that the new bat may have been an agile climber that employed quadrupedal locomotion and under-branch hanging behaviour.     

Cladogenesis and morphological diversification in passerine birds

Morphological diversity tends to increase within evolving lineages over time, but the relative roles of gradual evolutionary change (anagenesis) and abrupt shifts associated with speciation events (cladogenesis, or 'punctuated equilibrium') have not been resolved for most groups of organisms. However, these two modes of evolution can be distinguished by the fact that morphological variance increases in proportion to time under anagenesis, and in proportion to the logarithm of the number of species under cladogenesis. Although species and time are themselves correlated, multiple regression analysis provides a statistical framework for partitioning their relative contributions. In this study, I use multiple regressions to evaluate the effects of time and species number on morphological diversity within clades of passerine birds. The results show clearly that number of species exerts a strong influence on morphological variance independent of time, but that time has no unique effect. Thus, morphological evolution in birds seems to be associated with cladogenesis. How lineage splitting promotes morphological diversification poses an important challenge to ecologists and evolutionary biologists.     

Comparative genomic analysis reveals more functional nasal chemoreceptors in nocturnal mammals than in diurnal mammals

Natural selection has a critical role in the diversity of morphological traits. However, the genetic basis underlying the evolution and diversity of morphological characteristics, particularly in the context an organism’s behavior, lifestyle, and environment, is not well understood. The discovery of nasal chemoreceptors in mammals provided an opportunity to address this question. Here, we identify 4 nasal chemoreceptor gene families (V1R, V2R, OR, and TAAR) from horse, guinea pig, marmoset and orangutan genome sequences, respectively. Together with previously described mammalian nasal chemoreceptor gene repertoires, we found a significant positive correlation between functional gene number and morphological complexity, both in the main olfactory system and the vomeronasal system. The combined analysis of morphological data, behavioral data, and gene repertoires suggests that nocturnal mammals tend to possess more species-specific chemoreceptor genes and more complicated olfactory organs than diurnal mammals. Moreover, analysis of evolutionary forces revealed the existence of positive selection on the species-specific genes, likely reflecting the species-specific detection of odors and pheromones. Taken together, these results reflect a rare case of adaptation to circadian rhythm activity at the genome scale, and strongly suggest that the complexity of morphological olfactory organs and the diversification of nasal chemoreceptors in nocturnal mammals are under selection for the ability to perceive the variety of odors that nocturnal mammals may encounter in their particular dark environments.     

Accelerated speciation in colour-polymorphic birds

Colour polymorphism exemplifies extreme morphological diversity within populations. It is taxonomically widespread but generally rare. Theory suggests that where colour polymorphism does occur, processes generating and maintaining it can promote speciation but the generality of this claim is unclear. Here we confirm, using species-level molecular phylogenies for five families of non-passerine birds, that colour polymorphism is associated with accelerated speciation rates in the three groups in which polymorphism is most prevalent. In all five groups, colour polymorphism is lost at a significantly greater rate than it is gained. Thus, the general rarity and phylogenetic dispersion of colour polymorphism is accounted for by a combination of higher speciation rate and higher transition rate from polymorphism to monomorphism, consistent with theoretical models where speciation is driven by fixation of one or more morphs. This is corroborated by evidence from a species-level molecular phylogeny of passerines, incorporating 4,128 (66.5%) extant species, that polymorphic species tend to be younger than monomorphic species. Our results provide empirical support for the general proposition, dating from classical evolutionary theory, that colour polymorphism can increase speciation rates.     

Developmental constraints versus flexibility in morphological evolution

Evolutionary developmental biology has encouraged a change of research emphasis from the sorting of phenotypic variation by natural selection to the production of that variation through development. Some morphologies are more readily generated than others, and developmental mechanisms can limit or channel evolutionary change. Such biases determine how readily populations are able to respond to selection, and have been postulated to explain stasis in morphological evolution and unexplored morphologies. There has been much discussion about evolutionary constraints but empirical data testing them directly are sparse. The spectacular diversity in butterfly wing patterns is suggestive of how little constrained morphological evolution can be. However, for wing patterns involving serial repeats of the same element, developmental properties suggest that some directions of evolutionary change might be restricted. Here we show that despite the developmental coupling between different eyespots in the butterfly Bicyclus anynana, there is great potential for independent changes. This flexibility is consistent with the diversity of wing patterns across species and argues for a dominant role of natural selection, rather than internal constraints, in shaping existing variation.     

Transformation and diversification in early mammal evolution

Evolution of the earliest mammals shows successive episodes of diversification. Lineage-splitting in Mesozoic mammals is coupled with many independent evolutionary experiments and ecological specializations. Classic scenarios of mammalian morphological evolution tend to posit an orderly acquisition of key evolutionary innovations leading to adaptive diversification, but newly discovered fossils show that evolution of such key characters as the middle ear and the tribosphenic teeth is far more labile among Mesozoic mammals. Successive diversifications of Mesozoic mammal groups multiplied the opportunities for many dead-end lineages to iteratively evolve developmental homoplasies and convergent ecological specializations, parallel to those in modern mammal groups.     

Fast evolution of growth hormone receptor in primates and ruminants

Pituitary growth hormone (GH) evolves very slowly in most of mammals, but the evolutionary rates appear to have increased markedly on two occasions during the evolution of primates and ruminants. To investigate the evolutionary pattern of growth hormone receptor (GHR), we sequenced the extracellular domain of GHR genes from four primate species. Our results suggested that GHR in mammal also shows an episodic evolutionary pattern, which is consistent with that observed in pituitary growth hormone. Further analysis suggested that this pattern of rapid evolution observed in primates and ruminants is likely the result of coevolution between pituitary growth hormone and its receptor.     

广州市区翼手类物种多样性的研究

2004年5月至2006年4月对广州市区内翼手类的物种多样性进行了调查.结果表明,广州市区有犬蝠(Cynopterus sphinx)、东亚伏翼(Pipistrellus abramus)、中华山蝠(Nyctalus velutinus)、扁颅蝠(Tylonycteris pachy-pus)、小黄蝠(Scotophilus kuhlii)、大黄蝠(Scotophilus heathii)、彩蝠(Kerivoula picta)等7种翼手类物种.分析了广州市区内翼手类的物种组成、区系特点和分布特征,其中东洋界种类占优势(71.42%),小黄蝠为优势种,东亚伏翼分布最广泛.     

The molecular clock runs more slowly in man than in apes and monkeys

The molecular clock hypothesis postulates that the rate of molecular evolution is approximately constant over time. Although this hypothesis has been highly controversial in the past, it is now widely accepted. The assumption of rate constancy has often been taken as a basis for reconstructing the phylogenetic relationships among organisms or genes and for dating evolutionary events. Further, it has been taken as strong support for the neutral mutation hypothesis, which postulates that the majority of molecular changes in evolution are due to neutral or nearly neutral mutations. For these reasons, the validity of the rate constancy assumption is a vital issue in molecular evolution. Recent studies using DNA sequence data have raised serious doubts about the hypothesis. These studies provided support for the suggestion made from immunological distance and protein sequence data that a rate slowdown has occurred in hominoid evolution, and showed, in agreement with DNA hybridization studies, that rates of nucleotide substitution are significantly higher in rodents than in man. Here, rates of nucleotide substitution in rodents are estimated to be 4-10 times higher than those in higher primates and 2-4 times higher than those in artiodactyls. Further, this study provides strong evidence for the hominoid slowdown hypothesis and suggests a further rate-slowdown in hominoid evolution. Our results suggest that the variation in rate among mammals is primarily due to differences in generation time rather than changes in DNA repair mechanisms. We also propose a method for estimating the divergence times between species when the rate constancy assumption is violated.     

Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont

The transference of post-dentary jaw elements to the cranium of mammals as auditory ossicles is one of the central topics in evolutionary biology of vertebrates. Homologies of these bones among jawed vertebrates have long been demonstrated by developmental studies; but fossils illuminating this critical transference are sparse and often ambiguous. Here we report the first unambiguous ectotympanic (angular), malleus (articular and prearticular) and incus (quadrate) of an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China. The ectotympanic and malleus have lost their direct contact with the dentary bone but still connect the ossified Meckel's cartilage (OMC); we hypothesize that the OMC serves as a stabilizing mechanism bridging the dentary and the detached ossicles during mammalian evolution. This transitional mammalian middle ear narrows the morphological gap between the mandibular middle ear in basal mammaliaforms and the definitive mammalian middle ear (DMME) of extant mammals; it reveals complex changes contributing to the detachment of ear ossicles during mammalian evolution.     

Common mammals drive the evolutionary increase of hypsodonty in the Neogene

During the past 20 million years, herbivorous mammals of numerous lineages have evolved hypsodont, or high-crowned, cheek teeth. Hypsodonty is informative ecologically because it is well developed in mammals eating fibrous and abrasive foods that are most abundant in open and generally or seasonally dry environments. Here we report that in the Neogene of Europe mammals with the greatest locality coverages showed an increase in hypsodonty. We used a data set of 209 localities to measure whether large mammals occurring in many fossil localities show a similar increase in hypsodonty to mammals occurring in single or few localities. Taxonomic and morphological groupings show a low average hypsodonty in the early Miocene epoch. From the middle Miocene onwards, only the hypsodonty of commonly found mammals shows a marked increase. Therefore, in the drying Europe of the late Miocene, only increasingly hypsodont mammals may have been able to expand their share of habitats and food resources. These results suggest that the relatively small number of species known from multiple localities are palaeoecologically informative by themselves, irrespective of the rest of the known species.     

Genetic mechanisms and constraints governing the evolution of correlated traits in drosophilid flies

Some morphological traits differ greatly between related species, but it is not clear whether diversity evolves through changes in the same genes and whether similar, independent (that is, convergent) changes occur by the same mechanism. Pigmentation in fruitflies presents an attractive opportunity to explore these issues because pigmentation patterns are diverse, similar patterns have arisen in independent clades, and numerous genes governing their formation have been identified in Drosophila melanogaster. Here we show that both evolutionary diversification and convergence can be due to evolution at the same locus, by comparing abdominal pigmentation and trichome patterns and the expression of Bric-à-brac2 (Bab2), which regulates both traits in D. melanogaster, in 13 species representing the major clades of the subfamily Drosophilinae. Modifications of Bab2 expression are frequently correlated with diverse pigmentation and trichome patterns that evolved independently in multiple lineages. In a few species, Bab2 expression is not correlated with changes in pigmentation but is correlated with a conserved pattern of trichomes, indicating that this locus can be circumvented to evolve new patterns when a correlated trait is under different constraints.     

Eocene evolution of whale hearing

The origin of whales (order Cetacea) is one of the best-documented examples of macroevolutionary change in vertebrates. As the earliest whales became obligately marine, all of their organ systems adapted to the new environment. The fossil record indicates that this evolutionary transition took less than 15 million years, and that different organ systems followed different evolutionary trajectories. Here we document the evolutionary changes that took place in the sound transmission mechanism of the outer and middle ear in early whales. Sound transmission mechanisms change early on in whale evolution and pass through a stage (in pakicetids) in which hearing in both air and water is unsophisticated. This intermediate stage is soon abandoned and is replaced (in remingtonocetids and protocetids) by a sound transmission mechanism similar to that in modern toothed whales. The mechanism of these fossil whales lacks sophistication, and still retains some of the key elements that land mammals use to hear airborne sound.     

Is the guinea-pig a rodent?

The guinea-pig (Cavia porcellus), traditionally classified as a New World hystricomorph rodent, often shows anomalous morphological and molecular features in comparison with other eutherian mammals. For example, its insulin differs from that of other mammals in anabolic and growth-promoting activities and in its capability to form hexamers. Indeed, the literature about the molecular evolution of guinea-pigs abounds in references to 'convergent evolution', 'extremely rapid rates of substitution', and 'unique evolutionary mechanisms'. These claims are based on the assumption that the guinea-pig is a rodent. Our phylogenetic analyses of amino-acid sequence data, however, imply that the guinea-pig diverged before the separation of the primates and the artiodactyls from the myomorph rodents (rats and mice). If true, then the myomorphs and the caviomorphs do not constitute a natural clade, and the Caviomorpha (or the Histricomorpha) should be elevated in taxonomical rank and regarded as a separate mammalian order distinct from the Rodentia. If, as suggested by recent data, the myomorphs branched off before the divergence among the carnivores, lagomorphs, artiodactyls and primates, then the new order would represent an early divergence in eutherian radiation.     

基于色木枫转录组数据的核低拷贝基因引物开发

基于色木枫的转录组数据初步设计166对引物,并根据琼脂糖凝胶电泳与Sanger测序结果筛选出其中的26对核低拷贝基因引物．在东亚63个种群的182个个体中大批量扩增所开发引物,单倍型多样性范围为0.45~0.97,核苷酸多样性为(1.90~18.03)×10?3;9~15对引物可在金钱槭、茶条槭、青榨槭和鸡爪槭中扩增;STRUCTURE聚类结果与之前核微卫星研究结果类似,且展示了更多的谱系地理结构,结果表明引物多态性高、拓展性强、具有可靠性．本研究的转录组数据为槭属的研究提供重要遗传信息,所用方法被证明可以有效开发大量核低拷贝基因引物,已开发出的26对引物将为色木枫的进化历史和槭属系统发育研究提供丰富有效的分子标记．      

Strong population substructure is correlated with morphology and ecology in a migratory bat

Examining patterns of inter-population genetic diversity can provide valuable information about both historical and current evolutionary processes affecting a species. Population genetic studies of flying and migratory species such as bats and birds have traditionally shown minimal population substructure, characterized by high levels of gene flow between populations. In general, strongly substructured mammalian populations either are separated by non-traversable barriers or belong to terrestrial species with low dispersal abilities. Species with female philopatry (the tendency to remain in or consistently return to the natal territory) might show strong substructure when examined with maternally inherited mitochondrial DNA, but this substructure generally disappears when biparentally inherited markers are used, owing to male-mediated gene flow. Male-biased dispersal is considered typical for mammals, and philopatry in both sexes is rare. Here we show strong population substructure in a migratory bat species, and philopatry in both sexes, as indicated by concordance of nuclear and mtDNA findings. Furthermore, the genetic structure correlates with local biomes and differentiation in wing morphology. There is therefore a close correlation of genetic and morphological differentiation in sympatric subspecific populations of this mammalian species.     

Birds have paedomorphic dinosaur skulls

The interplay of evolution and development has been at the heart of evolutionary theory for more than a century. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals, including humans. Birds are the most speciose land vertebrates, with more than 10,000 living species representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.