Showing papers by "Edward O. Wilson published in 2015"

Adaptive Shift and Dispersal in a Tropical ant Fauna

Abstract: The tropics have long been recognized as the site of maximum evolutionary activity on land. In tropical rain forests, evolution proceeds simultaneously in the largest number of species. A growing amount of evidence of diverse kinds also points to the rain forests, particularly those of continents and the large islands, as the "center" of evolution of a majority of major animal and plant groups, where these groups have diversified maximally in various stages of their phyletic history and out of which they have tended to spread into adjacent temperate and arid regions (Richards, 1952; Darlington, 1957). According to the theory stressed by Darlington it is the Old World tropics specifically that form the principal evolutionary center of the vertebrates. Successful groups, achieving "general adaptation," send emigrant species out of the Old World tropics into temperate zones, where they may diversify secondarily and in time come to show zonation and radial dispersal of their own. From time to time faunal drift occurs across the region of the present-day Bering Straits. Movement across this barrier has been predominantly out of the Old World into the New. Once emigrant species reach the New World, further diversification may take place; this becomes especially probable if members of the group succeed in penetrating the Neotropical forests. The nature of general adaptation and the dispersal mechanisms underlying major biotic n-iovement is clearly one of the great problems of modern evolutionary theory. It appears that our knowledge has now reached the stage where finer analyses of these causal processes can and should be undertaken. There is a need for a "biogeography of the species," oriented with respect to the broad background of biogeographic theory but drawn at the species level and correlated with studies on ecology, speciation, and genetics. In the present paper one such analysis is attempted in very preliminary form, dealing with the ponerine ants 1 of Melanesia. This fauna is unusually suitable for a study of the kind proposed. Melanesia, including New Guinea, has proven to be a peripheral or "recipient" zoogeographic and for ants, i.e., most of the present fauna has been derived ultimately from immigrations from southeastern Asia and Australia, while proportionately few Melanesian-centered groups have emigrated into these adjacent source areas. Hence dominant, successful groups can conceivably be distinguished in their early stages of expansion as they first enter Melanesia, and older resident elements can be studied to piece together details of the later history of invading groups. Theoretically, it should then be possible to characterize the early invading groups ecologically and to infer some of the attributes that have contributed to their successful dispersal, in short, the "attributes of success" associated with general adaptation.

Breaking out of biogeographical modules: range expansion and taxon cycles in the hyperdiverse ant genus Pheidole

Abstract: Aim We sought to reconstruct the biogeographical structure and dynamics of a hyperdiverse ant genus, Pheidole, and to test several predictions of the taxon cycle hypothesis. Using large datasets on Pheidole geographical distributions and phylogeny, we (1) inferred patterns of biogeographical modularity (clusters of areas with similar faunal composition), (2) tested whether species in open habitats are more likely to be expanding their range beyond module boundaries, and (3) tested whether there is a bias of lineage flow from high- to low-diversity areas. Location The Old World. Methods We compiled and jointly analysed a comprehensive database of Pheidole geographical distributions, the ecological affinities of different species, and a multilocus phylogeny of the Old World radiation. We used network modularity methods to infer biogeographical structure in the genus and comparative methods to evaluate the hypotheses. Results The network analysis identified eight biogeographical modules, and a suite of species with anomalous ranges that are statistically more likely to occur in open habitat, supporting the hypothesis that open habitats promote range expansion. Phylogenetic analysis shows evidence for a cascade pattern of colonization from Asia to New Guinea to the Pacific, but no ‘upstream’ colonization in the reverse direction. Main conclusions The distributions of Pheidole lineages in the Old World are highly modular, with modules generally corresponding to biogeographical regions inferred in other groups of organisms. However, some lineages have expanded their ranges across module boundaries, and these species are more likely to be adapted to open habitats rather than interior forest. In addition, there is a cascade pattern of dispersal from higher to lower diversity areas during these range expansions. Our findings are consistent with the taxon cycle hypothesis, although they do not rule out alternative interpretations.

The True Army Ants of the Indo-Australian Area (Hymenoptera: Formicidae: Dorylinae)

Abstract: All of the known Indo-Australian species of Dorylinae, 4 in Dorylus and 34 in Aenictus, are included in this revision. Eight of the Aenictus species are described as new: artipus, chapmani, doryloides, exilis, huonicus, nganduensis, philiporum and schneirlai. Phylogenetic and numerical analyses resulted in the discarding of two extant subgenera of Aenictus (Typhlatta and Paraenictus) and the loose clustering of the species into 5 informal " groups" within the unified genus Aenictus. A consistency test for phylogenetic characters is discussed. The African and Indo-Australian doryline species are compared, and available information in the biology of the Indo-Australian species is summarized. The " true " army ants are defined here as equivalent to the subfamily Dorylinae. Not included are species of Ponerinae which have developed legionary behavior independently (see Wilson, E. O., 1958, Evolution 12: 24-31) or the subfamily Leptanillinae, which is very distinct and may be independent in origin. The Dorylinae are not as well developed in the Indo-Australian area as in Africa and the New World tropics. Dorylus itself, which includes the famous driver ants, is centered in Africa and sends only four species into tropical Asia. Of these, the most widespread reaches only to Java and the Celebes. Aenictus, on the other hand, is at least as strongly developed in tropical Asia and New Guinea as it is in Africa, with 34 species being known from the former regions and only about 15 from Africa. Aenictus is nevertheless notably poor in dispersal ability, as one might expect in army ants. No species is known from east of New Guinea, while only three species have reached eastern and northern Australia. Dorylines are unknown from the rich fossil ant deposits of the Eocene and Miocene of Europe and North America. This fact is consistent with the apparently youthful character of the distribution of the subfamily in the Indo-Australian area. The Asian Dorylus form very large colonies as do the African species, and the one species on which we have any information (Z>. orientalis) appears to be a general predator. The studied species of Aenictus also form large colonies, consisting of tens of even hundreds of thousands of workers. They are apparently specialized predators of other ant species. This monograph is a continuation of my revisionary studies of the Indo-Australian ant fauna. I have been stimulated and encouraged in this work by Dr T. C. Schneirla's field studies of the behavior of Aenictus colonies in Thailand and the Philippines. Both Dr Schneirla and Dr James W. Chapman, before the latter's recent death, contributed gen428 Pacific Insects Vol. 6, no. 3 erously from their collections and field notes to supply a fuller biological background to what otherwise would have been a much more purely taxonomic treatment. The work has been supported by grant no. GB 1634 from the National Science Foundation. GLOSSARY OF SPECIAL TERMS AND MEASUREMENTS The following terms, which are useful in the description of Dorylinae, either are not found in the ordinary entomological glossary or else require special definition with reference to ants. Basal face (of propodeum). The dorsal surface of the propodeum. Basal tooth. The posteriormost tooth of the masticatory border of the mandible. In IndoAustralian dorylines it is usually as well developed as the apical and preapical teeth. CL Cephalic index. (HW X 100)/HL. Declivitous face (of propodeum). The posterior face of the propodeum, extending to the petiolar junction. 15116 level. The set of species that share at least 15 of the 16 binary characters used (q. v.) with the species under consideration. HL. Head length. As conventionally used in myrmecology, the maximum distance, taken along the midline between the points of intersection of the midline with the lines, drawn perpendicular to the midline, that touch the anteriormost and posteriormost parts of the head exclusive of the mandibles (the posteriormost and anteriormost points need not be on or near the midline). HW. Head width. When the head is viewed in full face, the maximum width taken perpendicular to the midline. This measurement is made across the eyes in the & but above or below the eyes (if these break the profile) in the queen and worker. Mesosoma. The alitrunk, or middle tagma, including the thorax and propodeum together. Microreticulum. A very fine reticulum, where the reticular diameters are of the order of 0.01 mm. For most cases in Aenictus, this is one particular form of the microsculpture generically referred to as " shagreening " by many authors. Parafrontal ridges. Thin, low, bilaterally placed ridges running (in some Aenictus) from the posterior margin of the clypeus longitudinally just laterad to the antennal sockets. Propodeal junction. Line of junction of the basal and declivitous faces (q.v.). SL Scape index (SLxl00) /HW. SL. Scape length. The maximum length of this appendage that can be taken. Subopaque. Four arbitrary degrees in the spectrum of " shininess " are recognized: shining, feebly shining, subopaque, opaque. The first and last are self-explanatory, and the middle two are meant to be (subjectively) two equidistant intermediate steps. Thus subopaque means mostly opaque but with a few points of reflection under ordinary reflected light. Typhlatta spot. The two, symmetrically placed patches of yellow located on the occipital corners or upper genae or overlapping both. They usually contrast with a much darker pigment in surrounding areas and characterize certain species of Aenictus. Typical mandible. In Aenictus, the triangular mandible with more than four teeth on the masticatory border, characterizing species such as pachycerus and peguensis (q. v.) ; contrasted with the thin, 3or 4-toothed mandible of brevicornis, ceylonicus, etc. The designation as " typica l" is arbitrary. 1964 Wilson: Indo-Australian true army ants 429 ANALYSIS OF RELATIONSHIPS In the case of Dorylus, the Indo-Australian and African species together are few enough and the characters clear enough to make firm subjective comparisons. In Aenictus, however, there are many more species to be considered: 34 from the Indo-Australian area and at least 12 from Africa. There are moreover about 20 more or less independent characters available to separate these entities. Thus in evaluating the relationships of the Aenictus species, one has to handle over 900 pieces of information. Subjective evaluation becomes exceedingly difficult unless certain characters are chosen in advance as " more important" than others and used exclusively. The latter crude weighting method is in common usage and is justified provided these characters can be assumed to be the ones that do not show convergence in their evolution. However, in Aenictus, I chose initially to use a " numerical " method consisting simply of matching species according to a list of binary characters, i. e., characters that exist in two states. Sixteen such characters were found convenient in the classification of the genus. I used them, together with a smaller number of continuously varying characters, both in the key and routine verbal descriptions. Most involve the simple presence or absence of structures; several involve a 2-state condition of a structure always present; and a single one, ground coloration, is continuous but still allows a sharp partition of the species. The 16 binary characters are as follows: 1. Antenna 9or 10segmented. 2. Mandible thin, with 3 or 4 teeth; vs. mandible broad, subtriangular, with more than 4 teeth. 3. A gap remaining between the mandibles and clypeus when the mandibles are closed; or not. 4. Clypeus entire vs. emarginate. 5. Clypeus armed with teeth; or not. 6. Parafrontal ridge present vs. absent. 7. Occiput tumulose (as in gracilis) ; or not. 8. Occiput bearing a well-developed collar (as in dentatus) ; or not. 9. Head mostly smooth and shining vs. head mostly microreticulate and opaque. 10. Propodeum mostly smooth and shining vs. propodeum mostly microreticulate and opaque. 11. Pronotum armed (as in cornutus) ; or not. 12. Mesonotum and mesopleuron separated by a ridge; or not. 13. Propodeal junction angulate vs. smoothly rounded. 14. Subpetiolar process large and angulate vs. low and rounded or absent. 15. Typhlatta spots present vs. absent. 16. Ground color yellow vs. light reddish brown or darker. The results are shown in Table 1. These data should be very useful for making a rapid assessment of the purely morphological affinities of any given species, and I have used them as a measure of " relationships " in individual taxonomic descriptions. Can the binary-character counts be used to set up an objective subdivision of the genus ? Applying the methods of numerical taxonomy (R. R. Sokal & R. H. A. Sneath, 1963, Principles of numerical taxonomy, Freeman and Co.) it should be possible to set up objective groupings reproducible on the basis of " phenons." For instance, an 80-phenon connotes a group affiliated at no lower than 8 0 ^ of the similarity scale used in the analysis. Den430 Pacific Insects Vol. 6, no. 3 Table I. The number of binary characters, out of a maximum possible total of 16, held in Twelve African species, their names preceded by a star Cit'), are then added. All of the species Two of those from Africa are apparently undescribed and are indicated here by code numbers