Taxonomic changes for Neotropical pinhole borer ambrosia beetles (Coleoptera: Curculionidae: Platypodinae).

Stephen L. Wood re-defined Platypus such that its members are native to realms outside of the Americas and transferred most Neotropical species out of that genus. I have come across 44 species that still remain, though, and these are treated here. In total, I report 49 new generic assignments, 30 of which are transfers out of Platypus. I propose 22 new synonymies, eight of which are Platypus species that are synonymized with previously transferred species. Six Neotropical species are left in Platypus, for reasons detailed in the text. These taxonomic acts affect the compositions of eight of the 11 Neotropical genera of core Platypodinae. The following species are transferred from Platypus Herbst, 1793: Cenocephalus dubiosus (Schedl, 1933) comb. nov., Cenocephalus neotruncatus (Schedl, 1972) comb. nov.; Costaroplatus barbosai (Schedl, 1972) comb. nov., Costaroplatus devius (Schedl, 1976) comb. nov., Costaroplatus mixtus (Schedl, 1976) comb. nov., Costaroplatus roppai (Schedl, 1978) comb. nov.; Epiplatypus bicaudatulus (Schedl, 1935) comb. nov., Epiplatypus carduus (Schedl, 1936) comb. nov., Epiplatypus complanus (Schedl, 1967) comb. nov., Epiplatypus grandiporus (Schedl, 1961) comb. nov., Epiplatypus insculptus (Schedl, 1967) comb. nov., Epiplatypus macroporus (Chapuis, 1865) comb. nov., Epiplatypus perforans (Schedl, 1961) comb. nov., Epiplatypus propinquus (Schedl, 1959) comb. nov., Epiplatypus quadrispinatus (Chapuis, 1865) comb. nov., Epiplatypus sallei (Chapuis, 1865) comb. nov., Epiplatypus sequius (Schedl, 1935) comb. nov.; Euplatypus detectus (Schedl, 1976) comb. nov., Euplatypus erraticus (Schedl, 1972) comb. nov., Euplatypus longulus (Chapuis, 1865) comb. nov., Euplatypus perplexus Bright, 1972 comb. nov., Euplatypus rugosifrons (Schedl, 1933) comb. nov., Euplatypus vexans (Schedl, 1972) comb. nov.; Megaplatypus asperatus (Schedl, 1976) comb. nov., Megaplatypus carinifer (Schedl, 1970), Megaplatypus durus (Schedl, 1936) comb. nov., Megaplatypus eversus (Wood, 1971) comb. nov., Megaplatypus gagates (Schedl, 1976) comb. nov., Megaplatypus irrepertus (Schedl, 1936) comb. nov., Megaplatypus lineaticornis (Schedl, 1936) comb. nov., Megaplatypus paramonovi (Schedl, 1972) comb. nov., Megaplatypus schedli (Wood, 1966) comb. nov., Megaplatypus vafer (Schedl, 1972) comb. nov.; Teloplatypus caligatus (Schedl, 1972) comb. nov. Costaroplatus bidens (Schedl, 1970) comb. nov. and Costaroplatus darlingtoni (Reichardt, 1965) comb. nov. are transferred from Megaplatypus Wood, 1993. Costaroplatus vonfaberi (Reichardt, 1962) comb. nov. is transferred from Platyphysus Wood, 1993. Epiplatypus striatus (Chapuis, 1865) comb. nov., Megaplatypus contextus (Schedl, 1963) comb. nov., Megaplatypus decorus (Schedl, 1936) comb. nov. and Megaplatypus dignatus (Schedl, 1936) comb. nov. are removed from Euplatypus Wood, 1993. Epiplatypus ornatus (Schedl, 1936) comb. nov. is transferred from Teloplatypus Wood, 1993. Euplatypus jamaicensis Bright, 1972 comb. nov., Megaplatypus discolor (Blandford, 1896) comb. nov., Teloplatypus brasiliensis (Nunberg, 1959) comb. nov., Teloplatypus nudus (Schedl, 1936) comb. nov. and Teloplatypus pernudus (Schedl, 1936) comb. nov. are transferred from Epiplatypus Wood, 1993. Costaroplatus ornatus (Schedl, 1936) comb. nov., is transferred from Cenocephalus Chapuis, 1865. Megaplatypus acutidens (Blandford, 1895) comb. nov. and Megaplatypus despectus (Schedl, 1971) comb. nov. are transferred from Tesserocerus Saunders, 1837. New synonymies are proposed as follows: Cenocephalus rugicollis Schedl, 1952 (= Cenocephalus epistomalis Wood, 1966 syn. nov.); Tesserocerus forcipatus Schedl, 1972 (= Platypus aplanatus Schedl, 1976 syn. nov.); Tesserocerus retusus Gurin-Mneville, 1838 (= Tesserocerus guerini ssp. montanus Schedl, 1960 syn. nov.); Tesserocerus simulatus Schedl, 1936 (= Platypus bilobus Schedl, 1961 syn. nov.); Tesserocerus spinax Blandford, 1896 (= Tesserocephalus forficula Schedl, 1936 syn. nov.); Costaroplatus carinulatus (Chapuis, 1865) (= Platypus umbrosus Schedl, 1936 syn. nov.); Costaroplatus shenefelti Nunberg (1963) (= Platypus abditulus Wood, 1966 syn. nov.); Costaroplatus vonfaberi (Reichardt, 1962) (= Platypus convexus Schedl, 1972 syn. nov.); Epiplatypus sallei (Chapuis, 1865) (= Platypus quadricaudatulus Schedl, 1934 syn. nov. and = Platypus filaris Wood, 1971 syn. nov.); Euplatypus longulus (Chapuis, 1865) (= Platypus dimidiatus Chapuis, 1865 syn. nov. = Platypus mulsanti Chapuis, 1865 syn. nov. and = Platypus pseudolongulus Schedl, 1963 syn. nov. ); Megaplatypus acutidens (Blandford, 1895) (= Tesserocerus alternantes Schedl, 1977 syn. nov.); Megaplatypus durus (Schedl, 1936) (= Platypus arcuatus Schedl, 1976 syn. nov.); Megaplatypus fuscus (Chapuis, 1865) (= Platypus marginatus Chapuis, 1865 syn. nov., = Platypus granarius Schedl, 1952 syn. nov., and = Platypus obsitus Schedl, 1976 syn. nov.); Megaplatypus irrepertus (Schedl, 1936) (= Platypus sulcipennis Schedl, 1976 syn. nov.); Neotrachyostus abbreviatus (Chapuis, 1865) (= Platypus concavus Chapuis, 1865 syn. nov.); Teloplatypus enixus (Schedl, 1936) (= Platypus interponens Schedl, 1978 syn. nov.); Teloplatypus ratzeburgi (Chapuis, 1865) (= Platypus pallidipennis Blandford, 1896 syn. nov.). Platypus simpliciformis Wood, 1966 had been transferred by Wood (1993) to both Megaplatypus and Euplatypus by mistake; I propose keeping it in Megaplatypus. Six Neotropical species are left in the genus Platypus with the status incertae sedis: Platypus armatus Chapuis, 1865; Platypus dorsalis Schedl, 1972; Playpus quadrilobus Blandford, 1895; Platypus squamifer Schedl, 1963; Platypus subaequalispinosus Schedl, 1936; and Platypus trispinosus Chapuis, 1965. These taxonomic changes prepare the foundations for future revisionary work on the American Platypodinae.

Development and application of a second-generation multilingual tool for invasion risk screening of non-native terrestrial plants.

Under the increasing threat to native ecosystems posed by non-native species invasions, there is an urgent need for decision support tools that can more effectively identify non-native species likely to become invasive. As part of the screening (first step) component in non-native species risk analysis, decision support tools have been developed for aquatic and terrestrial organisms. Amongst these tools is the Weed Risk Assessment (WRA) for screening non-native plants. The WRA has provided the foundations for developing the first-generation WRA-type Invasiveness Screening Kit (ISK) tools applicable to a range of aquatic species, and more recently for the second-generation ISK tools applicable to all aquatic organisms (including plants) and terrestrial animals. Given the most extensive usage of the latter toolkits, this study describes the development and application of the Terrestrial Plant Species Invasiveness Screening Kit (TPS-ISK). As a second-generation ISK tool, the TPS-ISK is a multilingual turnkey application that provides several advantages relative to the WRA: (i) compliance with the minimum standards against which a protocol should be evaluated for invasion process and management approaches; (ii) enhanced questionnaire comprehensiveness including a climate change component; (iii) provision of a level of confidence; (iv) error-free computation of risk scores; (v) multilingual support; (vi) possibility for across-study comparisons of screening outcomes; (vii) a powerful graphical user interface; (viii) seamless software deployment and accessibility with improved data exchange. The TPS-ISK successfully risk-ranked five representative sample species for the main taxonomic groups supported by the tool and ten angiosperms previously screened with the WRA for Turkey. The almost 20-year continuous development and evolution of the ISK tools, as opposed to the WRA, closely meet the increasing demand by scientists and decision-makers for a reliable, comprehensive, updatable and easily deployable decision support tool. For terrestrial plant screening, these requirements are therefore met by the newly developed TPS-ISK.

The rare genus Myoplatypus (Coleoptera: Curculionidae: Platypodinae): three new species, new reports, and first records for South America.

We describe three new species of Myoplatypus Wood, 1993: from Peru, Myoplatypus petrovi Kirkendall new species; from Honduras, M. quadricornis Kirkendall new species; from Nicaragua (M. nicaraguensis Kirkendall new species). We transfer Platypus biprorus Blandford, 1896 and Platypus sicarius Wood, 1971 into Myoplatypus, and we synonymize Platypus querceus Wood, 1971 with M. biprorus. The net result of these actions is a genus comprising nine tropical and one temperate American species. Males of all species are illustrated by photographs and a key to all species is provided. The collections reported here include the first South American records of Myoplatypus, a genus hitherto known only from North and Central America. Most Myoplatypus species are known from just one or a few collections and none of the tropical species are very widespread; only five tropical species have any known hosts (Quercus [Fagaceae] for three of them). The paucity of specimens could be because of peculiarities of biology that lead to them being under-collected (such as restriction to high elevations), but it also could be that they are narrow endemics, in which case these pinhole borer species deserve conservation attention. Nosotros describimos tres nuevas especies de Myoplatypus Wood, 1993: de Per, Myoplatypus petrovi Kirkendall espcie nueva; de Honduras, M. quadricornis Kirkendall espcie nueva; de Nicaragua, M. nicaraguensis Kirkendall espcie nueva. Platypus biprorus Blandford, 1896 y Platypus sicarius Wood, 1971 se transfieren a Myoplatypus, y Platypus querceus Wood, 1971 se sinonimiza con M. biprorus Blandford. El resultado neto de estas acciones es un gnero que consiste en nueve espcies tropicales y una templada. Los machos de todas las especies estn ilustrados con fotografas y se da una clave para todas las especies. Las colecciones reportadas aqu incluyen los primeros registros sudamericanos de Myoplatypus, un gnero hasta ahora conocido solo en Amrica del Norte y Amrica Central. La mayora de las especies de Myoplatypus se conocen a partir de solo una o unas pocas colecciones y ninguna de las especies tropicales est muy extendida; slo cinco especies tropicales tienen hospedadores conocidos (tres de ellos son robles). La escasez de especmenes podra deberse a algunas peculiaridades de la biologa que los llevan a una recoleccin insuficiente (como la restriccin a grandes alturas), pero tambin podra ser que sean endmicos estrechos, en cuyo caso estas especies de barrenadores merecen atencin de conservacin.

Bark beetles and pinhole borers recently or newly introduced to France (Coleoptera: Curculionidae, Scolytinae and Platypodinae).

We present an annotated list of 11 Scolytinae and Platypodinae species newly or recently introduced to France. Four species are recorded for the first time as interceptions: Euplatypus hintzi (Schaufuss), Euplatypus parallelus (Fabricius), Xyleborus affinis Eichhoff and Xyleborus ferrugineus (Fabricius). Two are possibly naturalised: Xyleborus bispinatus Eichhoff and Cryphalus dilutus Eichhoff, while Cyclorhipidion distinguendum (Eggers) and Xyloterinus politus (Say) are confirmed as species newly established in Europe. Moreover, an unidentified species of Amasa Lea, collected previously in Spain, is recorded for the first time in France: Amasa sp. near truncata (Erichson). We point out that literature references to Amasa truncata as an invasive species in New Zealand and South America are incorrect, as the photographs of these non-native populations do not match the holotype of A. truncata. For each species we have updated its global distribution, detailed all French records, and summarized biology, ecology, host trees and potential risks as pests.

Rainforest and cloud forest Scolytodes (Curculionidae, Scolytinae, Hexacolini) from the Arthropods of La Selva inventory in Costa Rica: new species, new synonymy, new records.

Quantitative collecting efforts over the last several decades in Costa Rica have resulted in many new species of insects. The Arthropods of La Selva projects included collecting from a typical lowland Neotropical forest and up an altitudinal transect, and has provided many valuable samples of insects, spiders and mites potentially new to science. We describe 18 new species in the bark beetle genus Scolytodes Ferrari, 1867, 14 of which were collected during this project: S.angulus Jordal & Kirkendall, sp. nov., S.sufflatus Jordal & Kirkendall, sp. nov., S.squamatifrons Jordal & Kirkendall, sp. nov., S.comosus Jordal & Kirkendall, sp. nov., S.spatulatus Jordal & Kirkendall, sp. nov., S.seriatus Jordal & Kirkendall, sp. nov., S.profundus Jordal & Kirkendall, sp. nov., S.catinus Jordal & Kirkendall, sp. nov., S.fimbriatus Jordal & Kirkendall, sp. nov., S.sulcifrons Jordal & Kirkendall, sp. nov., S.planifrons Jordal & Kirkendall, sp. nov., S.porosus Jordal & Kirkendall, sp. nov., S.mundus Jordal & Kirkendall, sp. nov., S.callosus Jordal & Kirkendall, sp. nov., S.parvipilus Jordal & Kirkendall, sp. nov., S.plenus Jordal & Kirkendall, sp. nov., S.niger Jordal & Kirkendall, sp. nov., and S.simplex Jordal & Kirkendall, sp. nov. One species, Scolytodesminutissimus Schedl, 1952, is redescribed to match the holotype. We give new Costa Rica records for S.costabilis Wood, 1974, which is the correct name for S.obesus Wood, 1975 (syn. nov.). We report Costa Rica as a new country record for six species: Scolytodesclusiacolens Wood, 1967, S.crinalis Wood, 1978, S.culcitatus (Blandford, 1897), S.libidus Wood, 1978, S.reticulatus (Wood, 1961), and S.spadix (Blackman, 1943). From a closely related genus, we provide the first record for Central America (and only the second collection) of Pycnarthrumfulgidum Wood, 1977.

Biological factors contributing to bark and ambrosia beetle species diversification.

The study of species diversification can identify the processes that shape patterns of species richness across the tree of life. Here, we perform comparative analyses of species diversification using a large dataset of bark beetles. Three examined covariates-permanent inbreeding (sibling mating), fungus farming, and major host type-represent a range of factors that may be important for speciation. We studied the association of these covariates with species diversification while controlling for evolutionary lag on adaptation. All three covariates were significantly associated with diversification, but fungus farming showed conflicting patterns between different analyses. Genera that exhibited interspecific variation in host type had higher rates of species diversification, which may suggest that host switching is a driver of species diversification or that certain host types or forest compositions facilitate colonization and thus allopatric speciation. Because permanent inbreeding is thought to facilitate dispersal, the positive association between permanent inbreeding and diversification rates suggests that dispersal ability may contribute to species richness. Bark beetles are ecologically unique; however, our results indicate that their impressive species diversity is largely driven by mechanisms shown to be important for many organism groups.

Globally distributed Xyleborus species reveal recurrent intercontinental dispersal in a landscape of ancient worldwide distributions.

BACKGROUND: Invasive species can have devastating effects on native ecosystems and therefore impose a significant threat to human welfare. The introduction rate of invasive species has accelerated dramatically in recent times due to human activity (anthropogenic effects), with a steadily growing pool of widespread tramp species. We present an in-depth analysis of four pantropical species of Xyleborus ambrosia beetles (Xyleborus volvulus, Xyleborus perforans, Xyleborus ferrugineus, and Xyleborus affinis) with similar ecology (fungus cultivation in dead wood), reproductive biology (permanent inbreeding) and genetic system (haplodiploidy). The unique combination of reproductive traits and broad host plant usage pre-adapts these beetles for colonizing of new areas. RESULTS: We found that all four species were broadly distributed long before human-assisted dispersal became common, and that the impact of anthropogenic effects varied among the species. For X. volvulus, X. perforans, and X. affinis there was evidence of ancient establishment in numerous regions, but also of abundant recent introductions into previously colonized areas. For X. ferrugineus, we found clear biogeographical structuring of old clades, but little evidence for recent successful introductions. CONCLUSIONS: Our results indicate that current human-aided transoceanic dispersal has strongly affected the genetic makeup of three of the species in this study. However, current biogeographical patterns of all four species are equally, if not more strongly, influenced by ancient establishment on different continents.

The costs of sex: facing real-world complexities.

Understanding the maintenance of sexual reproduction constitutes a difficult problem for evolutionary biologists because of the immediate costs that sex seems to incur. Typically, general benefits to sex and recombination are investigated that might outweigh these costs. However, several factors can strongly influence the complex balance between costs and benefits of sex; these include constraints on the evolution of asexuality, ecological differentiation, and certain lif-history traits. We review these factors and their empirical support for the first time in a unified framework and find that they can reduce the costs of sex, circumvent them, or make them inapplicable. These factors can even tip the scales to a net benefit for sex. The reviewed factors affect species and species groups differently, and we conclude consequently that understanding the maintenance of sex could turn out to be more species-specific than commonly assumed. Interestingly, our study suggests that, in some species, no general benefits to sex and recombination might be needed to understand the maintenance of sex, as in our case study of dandelions.

Improbable but true: the invasive inbreeding ambrosia beetle Xylosandrus morigerus has generalist genotypes.

The wide distribution and dominance of invasive inbreeding species in many forest ecosystems seems paradoxical in face of their limited genetic variation. Successful establishment of invasive species in new areas is nevertheless facilitated by clonal reproduction: parthenogenesis, regular self-fertilization, and regular inbreeding. The success of clonal lineages in variable environments has been explained by two models, the frozen niche variation (FNV) model and the general-purpose genotype (GPG) model. We tested these models on a widely distributed forest pest that has been recently established in Costa Rica-the sibling-mating ambrosia beetle Xylosandrus morigerus. Two deeply diverged mitochondrial haplotypes coexist at multiple sites in Costa Rica. We find that these two haplotypes do not differ in their associations with ecological factors. Overall the two haplotypes showed complete overlap in their resource utilization; both genotypes have broad niches, supporting the GPG model. Thus, probable or not, our findings suggest that X. morigerus is a true ecological generalist. Clonal aspects of reproduction coupled with broad niches are doubtless important factors in the successful colonization of new habitats in distant regions.

Evidence for a new lineage of primary ambrosia fungi in Geosmithia Pitt (Ascomycota: Hypocreales).

Geosmithia is a genus of mitosporic filamentous fungi typically associated with phloeophagous bark beetles world-wide. During this study, the fungal associates of ambrosia beetles Cnesinus lecontei, Eupagiocerus dentipes, and Microcorthylus sp. from Costa Rica, were studied using morphology and DNA sequences. Fungal associates belonged to four undescribed Geosmithia species. Geosmithia eupagioceri sp. nov. and G. microcorthyli sp. nov. are evidently primary ambrosia fungi of their respective vectors E. dentipes and Microcorthylus species. They both have convergently evolved distinct morphological adaptations including the production of large, solitary and globose conidia, and yeast-like cells. Tunnels of C. lecontei contained an undescribed Geosmithia species, but its nutritional importance for its vector is unclear. An auxiliary ambrosia fungus, Geosmithia rufescens sp. nov., was found associated with both G. eupagioceri and the Geosmithia species associated with C. lecontei. G. microcorthyli is genetically quite similar to the phloem-associated Geosmithia sp. 8 from Europe. Large differences in morphology between these two species suggest the rapid co-evolution resulting from the close symbiosis of the former with its beetle host. The ITS rDNA sequences of G. microcorthyli and Geosmithia sp. 8 were not diagnostic, suggesting that alternative markers such as EF-1α, IGS rDNA or ß-tubulin should be used, together with morphological and ecological data, for species delimitation in this genus. The primary ambrosia fungi described here are derived from phloem-associated ancestors, and represent two independent lineages of ambrosia fungi in the Hypocreales and a new ecological strategy within Geosmithia.

Phylogeny of a Macaronesian radiation: host-plant use and possible cryptic speciation in Liparthrum bark beetles.

The Macaronesian islands are well known for their unique endemic floras of woody plants. Many of these unusual plant groups provide important novel resources for bark and wood boring beetles which breed in dead or moribund parts of their host plants. The bark beetle genus Liparthrum exploits a wide range of unusual host plants and has its highest proportion of species living on the Macaronesian Islands. We used DNA sequences of the mitochondrial Cytochrome Oxidase I gene and the nuclear Elongation Factor 1 alpha gene, and morphological characters, to estimate the phylogenetic relationships among species endemic to these archipelagos, and to trace the evolution of host-plant use. All parsimony and Bayesian analyses of the combined data, and maximum likelihood analyses of the molecular data, showed that species associated with Euphorbia are monophyletic. We also found genetic and subtle morphological evidence for three cases of cryptic speciation in one polyphyletic species associated with different Euphorbia plants, showing that high levels of host specialisation can occur also in insects breeding in older and very dry, dead plant tissues.

Molecules and morphology: evidence for cryptic hybridization in African Hyalomma (Acari: Ixodidae).

The role of natural hybridization and introgression as part of the evolutionary process is of increasing interest to zoologists, particularly as more examples of gene exchange among species are identified. We present mitochondrial and nuclear sequence data for Hyalomma dromedarii, Hyalomma truncatum, and Hyalomma marginatum rufipes (Acari: Ixodidae) collected from one-humped camels in Ethiopia. These species are well differentiated morphologically and genetically; sequence data from the mitochondrial DNA (mtDNA) cytochrome oxidase I gene indicates 10-14% divergence between the species. However, incongruence between morphology and the mtDNA phylogeny was observed, with multiple individuals of H. dromedarii and H. truncatum present on the same mtDNA lineage as H. marginatum rufipes. Thus, individuals with morphology of H. dromedarii and H. truncatum are indistinguishable from H. marginatum rufipes on the basis of mtDNA. Multiple copies of ITS-2 were subsequently cloned and sequenced for a subset of individuals from the mtDNA phylogeny, representing both 'normal' and 'putative hybrid' individuals. Very low sequence divergence (0.3%) was observed within 'normal' individuals of both H. dromedarii and H. truncatum relative to the 'putative hybrid' individuals (6 and 2.7%, respectively). The pattern of intra-individual variation in ITS-2 within 'putative hybrid' individuals, particularly in H. dromedarii, strongly suggests that gene flow has occurred among these Hyalomma species, but no indication of this is given by the morphology of the individuals.

Extraordinary haplotype diversity in haplodiploid inbreeders: phylogenetics and evolution of the bark beetle genus Coccotrypes.

Regular inbreeding by sib-mating is one of the most successful ecological strategies in the bark beetle family Scolytinae. Within this family, the many species (119) in Coccotrypes are found breeding in an exceptional variety of untraditional woody tissues different from bark and phloem. Species delineation by morphological criteria is extremely difficult, however, as in most other inbreeding groups of beetles, perhaps due to the unusual evolutionary dynamics characterizing sib-mating organisms. Hence, we here performed a phylogenetic analysis using molecular data in conjunction with morphological data to better understand morphological and ecological evolution in this sib-mating group. We used partial DNA sequences from the nuclear gene EF-alpha and the mitochondrial genes 12S and CO1 to elucidate patterns of morphological evolution, haplotype variation, and evolutionary pathways in resource use. Sequence variation was high among species and far above that expected at the species level (e.g., 19% for CO1 within Coccotrypes advena). The tendency for exhaustive sequence variation at deeper nodes resulted in ambiguous reconstructions of the deepest splits. However, all results suggested that species with the broadest diets were clustered in a single derived position-another piece of evidence against specialization as a derived evolutionary feature.

ECOLOGICAL AND EVOLUTIONARY STABILITY OF SPERM-DEPENDENT PARTHENOGENESIS: EFFECTS OF PARTIAL NICHE OVERLAP BETWEEN SEXUAL AND ASEXUAL FEMALES.

Pseudogamous females reproduce parthenogenetically but require sperm. We analyze a density- and frequency-dependent model for the ecological and evolutionary stability of bisexual populations exposed to invasion by pseudogamous clones. In particular, we examine the effects of partial niche overlap and asymmetric competition between sexual and asexual forms. The model predicts that for a variety of relative fitness values for asexual females, pseudogamous forms can successfully invade bisexual populations. The probability of successful invasion increases as niche overlap decreases. Furthermore, invaded populations are often likely to be stable; for the parameter values analyzed, only combinations of nearly complete niche overlap and high asexual fitness will lead to extinction. Even such combinations will be stable under pronounced asymmetric competition. Asymmetric competition does not, however, affect the invadability of bisexual populations. The model predicts that stable populations cannot have more than three or four females per male; populations with more biased sex ratios are expected to be unstable. We analyze available sex ratio data for pseudogamous insects, fish, and salamanders, and find significant changes in roughly one-half of the asexual-dominated populations, but in only one sexual-dominated population. This analysis includes previously unpublished data on population sex ratios in a pseudogamous bark beetle, Ips acuminatus. Some asexual-dominated populations have far more than four females per male, contrary to predictions of the model.

ON THE EVOLUTION OF PSEUDOGAMY.

A. density- and frequency-dependent model for the evolution and maintenance of pseudogamous females is developed and analyzed. Ecological as well as evolutionary aspects of pseudogamy are discussed. Criteria are described for the stable coexistence of sexual females and pseudogamous females under natural conditions. The conditions for invasion of a normal bisexual population by pseudogamous females are less stringent than the conditions for stable coexistence. Hence, we expect that some populations will be characterized by unstable sex ratios over time (with the resulting local extinction due to lack of males) while other populations will be characterized by stable sex ratios over time. If high population sex ratios (i.e., many females to few males) are to be stable, the net population growth rate must be large, and there can be no successful male preference for sexual females.