ECOLOGICAL DIVERSIFICATION AND COMMUNITY STRUCTURE IN THE OLD WORLD LEAF WARBLERS (GENUS PHYLLOSCOPUS): A PHYLOGENETIC PERSPECTIVE

I investigated the historical basis for variation in regional species diversity. I used a molecular phylogenetic analysis within a single genus of birds (the Old World leaf warblers, genus Phylloscopus) in conjunction with ecological studies in Europe, the Himalayas, and Japan to evaluate the importance of historical events in shaping the present constellation of morphology and behavior in the three different regions. The relatively depauperate assemblages have different histories. In Japan, there was invasion of several lineages, which have more closely related species elsewhere in Asia, whereas in Europe there was also limited in situ speciation. Much of the structure of peripheral communities is attributable to invasions from species-rich Asia, with little in situ morphological diversification. Within the Phylloscopus there are several phylogenetic clades with nonoverlapping size distributions. Major ecological and morphological shifts occurred early in the history of diversification within the group, and rarely since.     

THE EVOLUTION OF FEMALE-BIASED SEXUAL SIZE DIMORPHISM: A POPULATION-LEVEL COMPARATIVE STUDY IN HORNED LIZARDS (PHRYNOSOMA)

Female-biased sexual size dimorphism is uncommon among vertebrates and traditionally has been attributed to asymmetric selective pressures favoring large fecund females (the fecundity-advantage hypothesis) and/or small mobile males (the small-male advantage hypothesis). I use a phylogenetically based comparative method to address these hypotheses for the evolution and maintenance of sexual size dimorphism among populations of three closely related lizard species (Phrynosoma douglasi, P. ditmarsi, and P. hernandezi). With independent contrasts I estimate evolutionary correlations among female body size, male body size, and sexual size dimorphism (SSD) to determine whether males have become small, females have become large, or both sexes have diverged concurrently in body size during the evolutionary Xhistory of this group. Population differences in degree of SSD are inversely correlated with average male body size, but are not correlated with average female body size. Thus, variation in SSD among populations has occurred predominantly through changes in male size, suggesting that selective pressures on small males may affect degree of SSD in this group. I explore three possible evolutionary mechanisms by which the mean male body size in a population could evolve: changes in size at maturity, changes in the variance of male body sizes, and changes in skewness of male body size distributions. Comparative analyses indicate that population differentiation in male body size is achieved by changes in male size at maturity, without changes in the variance or skewness of male and female size distributions. This study demonstrates the potential of comparative methods at lower taxonomic levels (among populations and closely related species) for studying microevolutionary processes that underlie population differentiation.     

EVOLUTION OF PARASITISM AMONG CLOSELY RELATED SPECIES: PHYLOGENETIC RELATIONSHIPS AND THE ORIGIN OF INQUILINISM IN GALL WASPS (HYMENOPTERA,CYNIPIDAE)

A new term, agastoparasitism, is proposed for parasitism among closely related species. Cynipid inquilines are typical agastoparasites. They cannot induce galls; instead their larvae live inside the galls formed by other cynipids. As in many other groups of agastoparasites, there are two competing hypotheses for the evolutionary origin of cynipid inquilines: either they arose from one of their cynipid hosts, and later radiated to exploit other gall-inducing cynipids (monophyletic origin), or they arose repeatedly, each inquiline from its host (polyphyletic origin). These hypotheses for the origin of cynipid inquilines were tested by a phylogenetic analysis of representative species of cynipid gall inducers and inquilines based on adult morphological characters. The analysis supported the monophyly of the inquilines and indicated an origin from gall inducers related to the genus Diastrophus, one of the current host groups. To examine whether the result of the analysis was influenced by convergent similarities among inquilines because of their similar mode of life, all putative apomorphies shared by some or all of the inquilines but not occurring in any of the gall inducers were removed. Despite this, the phylogenetic conclusions essentially remained the same, that is, the support for inquiline monophyly was not caused by convergent evolution. Based on these results, adaptive aspects of the evolutionary origin and maintenance of cynipid inquilinism are discussed, as well as general patterns in the evolution of agastoparasitism.     

TROPICS ACCELERATE THE EVOLUTION OF HYBRID MALE STERILITY IN DROSOPHILA

Understanding the evolutionary mechanisms that facilitate speciation and explain global patterns of species diversity has remained a challenge for decades. The most general pattern of species biodiversity is the latitudinal gradient, whereby species richness increases toward the tropics. Although such a global pattern probably has a multitude of causes, recent attention has focused on the hypothesis that speciation and the evolution of reproductive isolation occur faster in the tropics. Here, I tested this prediction using a dataset on premating and postzygotic isolation between recently diverged Drosophila species. Results showed that while the evolution of premating isolation was not greater between tropical Drosophila relative to nontropical species, postzygotic isolation evolved faster in the tropics. In particular, hybrid male sterility was much greater among tropical Drosophila compared to nontropical species pairs of similar genetic age. Several testable explanations for the novel pattern are discussed, including greater role for sterility‐inducing bacterial endosymbionts in the tropics and more intense sperm–sperm competition or sperm–egg sexual conflict in the tropics. The results imply that processes of speciation in the tropics may evolve at different rates or may even be somewhat different from those at higher latitudes.     

GENETIC STRUCTURE AND EVOLUTION OF SPECIES IN THE MANGROVE GENUS AVICENNIA (AVICENNIACEAE) IN THE INDO-WEST PACIFIC

Allozyme variation in species of the mangrove genus Avicennia was screened in 25 populations collected from 22 locations in the Indo-West Pacific and eastern North America using 11 loci. Several fixed gene differences supported the specific status of Avicennia alba, A. integra, A. marina, and A. rumphiana from the Indo-West Pacific, and A. germinans from the Atlantic-East Pacific. The three varieties of A. marina, var. marina, var. eucalyptifolia, and van australasica, had higher genetic similarities (Nei's I) and no fixed gene differences, confirming their conspecific status. Strong genetic structuring was observed in A. marina, with sharp changes in gene frequencies at the geographical margins of varietal distributions. The occurrence of alleles found otherwise in only one variety, in only immediately adjacent populations of another variety, provided evidence of introgession between varieties. The varieties appear to have diverged recently in the Pleistocene and are apparently not of ancient Cretaceous origin, as suggested earlier. Despite evidence of high degrees of outcrossing, gene flow among populations was relatively low (N_em < 1–2), except where populations were geographically continuous, questioning assumptions that these widespread mangrove species achieve high levels of long-distance dispersal.     

THE EVOLUTION OF HOST-PLANT USE AND SEQUESTRATION IN THE LEAF BEETLE GENUS PHRATORA (COLEOPTERA: CHRYSOMELIDAE)

Leaf beetles in the genus Phratora differ in host plant use and in the chemical composition of their larval defensive secretion. Most species specialize on either poplars or willows (family Salicaceae), but two species feed on birch (family Betulaceae). Phratora vitellinae utilizes salicylates from the host plant to produce its larval secretion, which contains salicylaldehyde, while other Phratora species produce an autogenous secretion. To reconstruct the evolutionary history of host plant use and the larval secretion chemistry in this genus, we sequenced 1383 base pairs of the mt cytochrome oxidase I gene for six European and one North American Phratora species and three outgroup taxa. Bootstrap values of the complete nucleotide sequence were 99-100% for six of eight nodes in the maximum parsimony tree. They were 71% and 77% for the two other nodes. The maximum parsimony tree and the maximum likelihood tree based on nucleotide sequence showed the same relationships as a maximum parsimony tree based on the amino acid sequence. Beetle phylogeny overlapped broadly with host plant taxonomy and chemistry, and it revealed historical constraints influencing host plant use. However, there was one host shift from the willow family (Salicaceae) to the birch family (Betulaceae). The use of host plant phenol glycosides for the larval defensive secretion evolved along the lineage that led to P. vitellinae. Phratora vitellinae feeds on the taxonomically widest range of host plants, which are characterized by moderate to high levels of salicylates. The results support the hypothesis that the use of salicylates for the larval secretion evolved twice independently in chrysomeline leaf beetles.     

EVOLUTION OF LIFE CYCLE,COLONY MORPHOLOGY,AND HOST SPECIFICITY IN THE FAMILY HYDRACTINIIDAE (HYDROZOA,CNIDARIA)

Biased transitions are common throughout the tree of life. The class hydrozoa is no exception, having lost the feeding medusa stage at least 70 times. The family hydractiniidae includes one lineage with pelagic medusae (Podocoryna) and several without (e.g., Hydractinia). The benthic colony stage also varies widely in host specificity and in colony form. The five‐gene phylogeny presented here requires multiple transitions between character states for medusae, host specificity, and colony phenotype. Significant phylogenetic correlations exist between medusoid form, colony morphology, and host specificity. Species with nonfeeding medusae are usually specialized on a single host type, and reticulate colonies are correlated with nonmotile hosts. The history of feeding medusae is less certain. Podocoryna is nested within five lineages lacking medusae. This requires either repeated losses of medusae, or the remarkable re‐evolution of a feeding medusa after at least 150 million years. Traditional ancestral reconstruction favors medusa regain, but a likelihood framework testing biased transitions cannot distinguish between multiple losses versus regain. A hypothesis of multiple losses of feeding medusae requires transient selection pressure favoring such a loss. Populations of species with feeding medusae are always locally rare and lack of feeding medusae does not result in restricted species distribution around the world.