Showing papers in "Annual Review of Ecology, Evolution, and Systematics in 1986"

Fluctuating Asymmetry: Measurement, Analysis, Patterns

Abstract: With these words Darwin opened a brief paragraph citing observations antithetical to his supposition: anecdotal reports of the inheritance of characters missing from one side of the body. His initial hunch, however, has stood the test of time: Genetic studies have confirmed that where only small, random deviations from bilateral symmetry exist, the deviations in a particular direction have little or no measurable heritability (17, 47, 51, 65a, 73a, 74, 91). The genetic basis of bilateral symmetry thus appears to differ fundamentally from that of virtually all other morphological features. Why, then, all the recent (and not so recent) interest in such minor, nondirectional deviations from bilateral symmetry [fluctuating asymmetry (FA); 60 cited in 99]? Four reasons. First, FA relates in a curious way to what is perhaps the major unsolved general problem in modem biology: the orderly expression of genotypes as complex, three-dimensional phenotypes. As was emphasized in a flurry of activity in the mid to late 1950s, and many times since, FA provides an appealing measure of 'developmental noise,' or minor environmentally induced departures from some ideal developmental program (101). Its appeal derives from an a priori knowledge of the ideal: perfect bilateral symmetry. For unilateral characters, the ideal is rarely known (but see 1, 2, and 59 for one possible approach). A second reason for interest in

Ecology of Tropical Dry Forest

Abstract: widespread, usually transitional between semidesert or savanna and moist forest. About 40% of the earth's tropical and subtropical landmass is dominated by open or closed forest. Of this, 42% is dry forest, 33% is moist forest, and only 25% is wet and rain forest (sensu Holdridge, 42; 15). We will never know the true original or potential extent of dry forest because many savannas and scrub or thorn woodlands are thought to be derived from disturbed dry forest. Walter (110), for example, considers most or all of the grassland in India to have been derived from seasonal or dry forest. Some of the processes that cause this conversion are addressed later in this review. The largest proportion of dry forest ecosystems is in Africa and the world's tropical islands, where they account for 70-80% of the forested area. In South America they

Vascular plant breakdown in freshwater ecosystems

Abstract: That major flows of energy occur along detrital pathways in all ecosystems is a recent recognition. In freshwater ecosystems, detritus or dead organic matter (217) has two possible sources: autochthonous detritus generated within the ecosystem and allochthonous detritus generated externally. This review is concerned with the breakdown of vascular plant detritus whether autochthonous, from aquatic vascular plants, or allochthonous, derived from riparian trees and herbs. The importance to the energetics of streams of vascular plant material from riparian vegetation was recognized in early studies by Nelson & Scott (184), Egglishaw (85), and Minshall (175). Organic matter budgets for various streams have provided quantitative data to support these early observations (96, 132, 182, 254). However, many low-order streams that lack canopies of riparian vegetation may be dominated by autochthonous primary production of nonvascular plant origin. (72, 176). Theoretical models (256) predict increasing importance of autochthonous production by periphyton and aquatic vascular plants for middle-order streams but less importance of these sources in very large streams, mainly due to light limitation. The relative dominance of allochthonous vs autochthonous sources has been shown to vary between stream systems and with local conditions within streams (72, 178). While vascular plant leaves have received most attention in stream research, there is growing recognition that wood is also important. The direct contribution of wood to stream energy budgets is minimal because wood is resistant to breakdown (e.g. 8, 251). However, woody debris is indirectly important because it creates habitat for aquatic organisms (5), promotes

Nutrient cycling in moist tropical forest

Abstract: Early studies of nutrient cycling in moist tropical forests described productive forests rich in nutrients (98, 114, 176) in which rates of primary production and the amounts of nutrients cycled clearly exceeded those in temperate zone forests. Reviews of global-scale patterns in biomass, production, and nutrient cycling reported these results as representative of tropical forests (130, 175). At the same time, tropical forest soils were described as acid, infertile clays that harden irreversibly to "laterite" when cleared (106), or as bleached quartz sands low in mineral nutrients (88). This apparent paradox was crystallized by Whittaker (174) in the statement "The tropical rain forest thus has a relatively rich nutrient economy perched on a nutrient-poor substrate" (p. 271). Reviews of more recent research on overall patterns of mineral cycling in the tropics (78, 124), and of important components such as biomass (20, 21), litterfall nutrients (123, 164), and decomposition (5), clearly show that patterns of nutrient cycling in tropical forests are diverse. It makes no more sense to describe a 'typical' tropical forest than a 'typical' temperate forest (33, 151). Variations in mineral cycling nonetheless follow coherent, explicable patterns in tropical forests. Our purposes in undertaking this review are: (a) to illustrate the patterns of nutrient cycling in moist tropical forests; (b) to identify the mechanisms which regulate those patterns; and (c) to show how those patterns affect the productivity, physiology, and population biology of tropical forests and their large-scale linkages with aquatic ecosystems and the atmosphere. We emphasize the cycling of nitrogen, phosphorus, potassium, calcium, and magnesium; these are the best-studied soil-derived nutrients, and they are the nutrients most likely to limit primary production and other ecosystem functions.

The Evolution of Phenotypic Plasticity in Plants

Abstract: Phenotypic plasticity is the ability of an individual organism to alter its physiology/morphology in response to changes in environmental conditions. This ability is particularly important in plants, whose sessile life-style requires them to deal with ambient conditions. This review focuses on issues related to the evolution of phenotypic plasticity in plants-factors responsible for differences in plasticity between populations or species (e.g. selection, drift), the presence of plasticity variation among taxa, the relationship between character correlations and character plasticities, and the assessment of the ecological role of phenotypic plasticity. In addition, a section is provided to introduce methods and problems of analyzing data from plasticity experiments. As a general rule, references are provided as an entrance to the literature and do not represent an exhaustive list. Bradshaw's review (20) remains the best introduction to the subject; this review is intended to complement his, not to replace it. The notion that plants respond morphologically to changes in their environment has a long tradition. Work by Turesson (139), Clausen et al (27), and others (see 20 for review) firmly documented the nature and extent of phenotypic responses to varied environments. Classic examples such as sun vs shade leaves (143), heterophylly (29, 30, 35), environmental control of cleistogamy (153), and the responses to herbivory (58, 84), mowing (147), and competition (140) all represent cases of phenotypic plasticity. These and other good examples support the notion that plastic responses are adaptive. Note, however, that the definition of phenotypic plasticity does not imply that such responses are necessarily adaptive. Plant response to an environmental

Sexual selection and the evolution of song

Abstract: Songs are complex acoustic signals produced mainly during the mating season. Such signals are found in many taxa but are most typical of birds, frogs, and insects. Darwin (38) noted that in these groups males are the predominant singers, and for this reason he attributed the evolution of song to sexual selection, that is, selection due to competition for mates. Darwin could muster no direct evidence that song or other display influences mating suc­ cess; only recently has such evidence begun to accumulate. Our fIrst goal here is to review and evaluate evidence that song influences mating success, via either female choice or male contests. Although convinced that sexual selection shapes display behavior, Darwin could not explain the evolution of a relationship between display and mating success. Fisher (50) first gave such an explanation in his discussion of female choice and male threat display. Our second goal is to review ideas and evidence on the evolution of relationships between song and female choice, and between song and success in male contests. We focus on the question of why listeners have evolved to respond to song in the contexts of female choice and male contest.

Genetic polymorphism in heterogeneous environments: a decade later

Abstract: Ten years ago, Joseph Felsenstein (35) and I (56) both wrote reviews discussing the maintenance of genetic polymorphisms in heterogeneous environments. I addressed the question of the importance in maintaining genetic variation of diversifying selection (sometimes called the niche-variation hypothesis), i.e. selection for different alleles in different environments. My focus was on the evidence and theory supporting the connection of a variable environment with genetic variation, particularly as an explanation for the large amounts of allozymic variation. At that time, the empirical evidence suggesting a genetic-environment connection for allozymic variation was mostly correlative and not in general experimentally based. Theory suggested, however, that polymorphism maintenance was possible in a variable environment; it was most likely when there was variable selection in space and limited gene flow and/or habitat selection. The purpose of this review is to evaluate the research of the intervening decade on genetic polymorphism in variable environments. The topics of this review are divided in a fashion similar to that of the earlier review. First, I mention some recent work in which there has been detailed investigation of the relationship of different environments and the maintenance of genetic variation. Second, I review the theoretical developments in the past decade. Finally, I discuss recent experimental work in some detail. An emphasis in the review is to evaluate these experimental studies, a number of which were designed specifically to determine the importance of heterogeneous environments in maintaining genetic variation.

The Role of Competition in Plant Communities in Arid and Semiarid Regions

Abstract: The importance, and even the existence, of competition among plants in arid ecosystems has often been questioned. An influential statement of Shreve (113) asserted that interspecific competition does not occur in deserts, and Went (145) denied that competition between desert plants occurs at all. Neither provided evidence for his assertions, although Shreve pointed out the diversity of habits and phenologies found among desert species. He may have been responding to the strong emphasis placed on competition by Clements and his followers (e.g. 27). The importance of competition in natural communities has recently been debated (28, 109, 127). These reviews suggested that terrestrial plant communities are among the communities in which competition is relatively important. However, the majority of studies upon which this conclusion is based were made in humid regions. Grime (53) suggested that competition is less important in "high stress" habitats (in which he included dry habitats), but he presented little evidence from true arid or semiarid environments. This paper reviews the available evidence for competition in plant communities in arid and semiarid regions; as is demonstrated, competition certainly occurs in these communities and involves many different species. In several instances it appears to be important in the determination of community structure. Competition may be less frequent in these communities, though not less important on that account. This review also addresses several other questions concerning the role of competition in these communities, including: the role of competition in determining the absence, or presence and abundance, of species in a community, and their spatial arrangement; which

The Ecology of Coralline Algal Crusts: Convergent Patterns and Adaptative Strategies

Abstract: Although crustose coralline algae (Rhodophyta, Corallinaceae) are among the most abundant marine organisms to live on hard substratum within the photic zone, relatively little is known about their ecology at the species level. This is due in part to difficulties in identification; strong similarities in appearance exist among phylogenetically distant taxa. The study of such convergent morphological and anatomical properties of corallines provides insight into interpreting the adaptive value of some characters or the "strategies" (55) of some of these plants. Crustose corallines are nonarticulated calcareous red algae that commonly grow prostrate on hard substrata and as epibionts on other plants and animals. Although these algae are globally abundant (18, 109), until recently most coralline research has been taxonomic. Early ecological accounts were largely confined to distributional studies. In the 1960s and 1970s, interest in the ecology of corallines grew as problems with their taxonomy were resolved. The corallines were generally recognized to be abundant (26, 43, 44, 69, 71, 96) and geologically important (14, 18, 74). It was suggested that "once obstacles arising from taxonomic problems are generally overcome, it seems likely that the group will be included more often in ecological studies or may even become 'popular subjects' " (18). However,

The Socioecology of Primate Groups

Abstract: Of all vertebrates, primates are among the most amenable to study in the wild. They are mostly large diurnal creatures that can readily be habituated to close observation, often to the point that the observer can be spatially integrated into the group under study. Consequently, primate behavior and feeding can be observed in extraordinary detail. Variation in facial features permits individual recognition in many species; this opens the door to studies of the ontogeny of behavior, dominance structure of groups, dispersal, and even lifetime reproductive success. Sometimes individuals or whole groups can be captured repeatedly, making possible precise measurements of growth rates, tooth wear, etc, and the taking of fecal and blood samples for the analysis of parasite loads and genetic relationships. Few nonprimates offer such a multiplicity of advantages to the student of animals in the wild. Within the past two years there have appeared a plethora of volumes devoted entirely or in part to the socioecology of primates; these books review the subject far more thoroughly than is possible in the space allotted here (9, 60a, 87, 122, 129, 138). We thus do not attempt a broad treatment of the subject but instead focus on the development of theory pertaining to the possible selective links between environmental variables and the social organization of primate groups. This subject is touched upon in all the works mentioned above, but in none of them is it given the in-depth treatment we think is due.

Ecology of african grazing and browsing mammals

Abstract: Africa is the earth's second largest continent, comprising 20% of its surface. Largely tropical, Africa extends as well into temperate zones to 37?N and 35?S. Eastern and southern Africa display steep elevation gradients due to the prevalence there of volcanic orogeny and rifting (29). Local landscapes are distinguished by substantial geological heterogeneity, dissected land forms, and resultant steep gradients of precipitation and vegetation. The consequent pronounced fragmentation of habitats and sharp juxtaposition of distinct vegetation types, combined with climatic oscillations in geological time, contributed to major adaptive radiations of the mammalian fauna (102, 120). Early zoological expeditions recorded that habitat fragmentation and wide spatial variation of animal densities and diversities were distinctive features of African ecosystems (92, 138, 162, 226). Those early records provided the bases of natural history information on animal distributions, habitat preferences, feeding habits, and general ecology; scientific research followed only much later (201). Modern scientific study of African savanna-grassland mammals began in the 1950s (23, 24, 107, 108, 148, 149, 197, 203, 204, 210, 230), long after the distributions and densities of the major game animals had been affected by growing human populations, colonial land and hunting policies, and virulent exotic diseases that affected the animals both directly and indirectly (57). The mammalian fauna has been increasingly isolated and fragmented within game reserves of varying size, habitat diversity, and animal species diversity; the ability to sustain it in the absence of active management is increasingly questioned (112, 187). For species with population sizes greater than 100 individuals, game reserve area (A) and faunal

Risk and foraging in stochastic environments

Abstract: Foraging animals confront problems conceptually similar to those facing an economically minded consumer (46, 47), and foraging theory shares a methodology in common with economics. Indeed, the last 20 years have seen wide application of economic models in biology. A growing consensus suggests that ecological and economic theories are ultimately indistinguishable (6, 30). Economic analyses begin with an economic agent that chooses from alternative objects or activities. The analyst extracts a set of measurable attributes from this collection of objects (e.g. energy content, costs, etc) that are combined into a single index. This index has the property that objects whose attributes generate higher values are preferred by the consumer over objects generating lower values. In economics, this index (the 'utility function') is the basic organizing principle (36). Behavioral ecologists use this technique to learn what attributes should be included in an organism's utility function to determine how various attributes should be combined and weighted, and then to predict choices by maximizing the utility function under applicable constraints. The organism's utility function is, therefore, both an object of empirical study and a predictive tool. Economic analysis usually stops here, but evolutionary biologists go farther. They assume that the organisms' preferences are related to evolutionary fitness and that the options

Parasite mediation in ecological interactions

Abstract: The interaction between two populations, whether of the same or different species, can often be influenced by a third population of a separate species. For example, the outcome of competition between two species can be modified by a predator, parasite, or pathogen (57, 88, 105). Consider Figure 1. Competitor A affects competitor B via three routes. The direct effect (-a) is called interference competition, and the effect via impact on a common resource (-g x +j) is called resource competition. The effect via a common

Seed Dispersal by Adhesion

Abstract: Seed dispersal by animals has received much recent attention (42, 44), with most of it focused on seed dispersal by frugivores (39, 41, 53, 70, 71), by ants (8, 21), and by scatterhoarding birds and mammals (19, 81, 85). Adhesive seed dispersal by means of animal fur and feathers has been largely neglected, although anecdotes about this dispersal mode abound (46, 62, 84). This review focuses on the evolutionary ecology of adhesive seed dispersal (epizoochory). The central question asked is: What circumstances favor the evolution of epizoochory? Using data from 10 regional floras and from the literature, I examine hypotheses about the habitat distribution, life history, growth form, seed dispersal patterns, and morphology of adhesive fruits. In addition, I survey the ways that fruits and plant parts are modified to adhere to animals. I also put forth new hypotheses predicting how morphological characteristics of adhesive fruits influence both the grooming behavior of dispersal agents and the fruit retention times. Experimental data are provided to support these hypotheses. Suggestions for tests of hypotheses and future work on adhesive-fruit dispersal are provided wherever possible. For convenience, all dispersal units are calledfruits. [A fruit is the matured gynoecium with or without other floral organs or parts of organs. A seed is either the embryo embedded in endosperm, or the embryo and digested endosperm within the megasporangium, surrounded by one or more integuments (42).] Most adhesive fruits adhere to animals by barbs, hooks, or viscid outgrowths. In this review, fruits that adhere by barbs or hooks are called burrs. Fruits that adhere by viscid outgrowths are called viscidfruits. I

Designing Agricultural Ecosystems for the Humid Tropics

Abstract: Warmth, moisture, and constancy are the environmental factors responsible for the potentially high productivity of the humid tropical lowlands. Ironically, they are also the biome's greatest ecological constraints to agriculture. Chemical weathering increases with temperature and is three to six times faster in the tropics than in the temperate zone (125, 250). This frequently leads to the development of deeply weathered soil with scant opportunity for the addition of fresh supplies of nutrients from the bedrock. High temperatures also mean potentially lower net photosynthesis; respiration approximately doubles with a temperature increase of 100 (114, 122, 178), and the oxygen inhibition of photosynthesis in C3 plants increases with temperature (34, 166). Potential evapotranspiration at 280 is about 1.6 m yr-1 (106, 241), yet annual rainfall in the humid tropics commonly exceeds 3 m. The excess runs off across the surface or infiltrates the soil and, if the site is not fully vegetated, may carry nitrate and cations with it. Moreover, fungi, bacteria, and epiphylls, many of which are crop pests, thrive in warm, humid climates. The lack of a harsh season comparable to temperate-zone winter or the annual drought of the seasonally dry tropics means that life, including agricultural pests, can flourish year-round. A seasonal shutdown of growthwhich also depresses pest populationsis an important agricultural subsidy, one that is missing in the less seasonal tropics (36, 117, 242). Many of the constraints imposed by the humid tropical environment can be overcome by supplying fertilizers and pest controls (207), but these amenities require locally scarce and expensive fossil fuels. Given the reality of current

The Future of Tropical Ecology

Abstract: Whether there is a future for tropical ecology, and of what it will consist, does not lie in the unveiling of yet another intricate animal-plant interaction, in the application of technological marvels, or in the discovery of a crop plant that can be grown with high yield on rainforest soils. The answer does not lie in meticulous analysis of what we know to date. Yes, we need these things. But the real future of tropical ecology lies in whether, within our generation, the academic, social and commercial sectors can collaboratively preserve even small portions of tropical wildlands to be studied and used for understanding, for material gain, and for the intellectual development of the society in which the wildland is embedded. The tropical ecologist has a clear mandate to be a prominent guide and glue in this collaboration. Ecologists are specialists at understanding interactions between complex units and their environments; the future of tropical ecology lies, above all, in the interface between humanity and the tropical nature that humanity has corralled. It is this generation of ecologists who will determine whether the tropical agroscape is to be populated only by humans and their mutualists, commensals, and parasites, or whether it will also contain some islands of the greater nature-the nature that spawned humans yet has been vanquished by them. An ocean of oil palm plantations, no matter how sustained the yield and no matter how well-fed the caretakers, is no more human destiny, nor is it of more ecological interest, than is any other assembly line. My goal here, then, is not to review the trajectory of the literature generated by studies of tropical ecology and biology, but rather to outline what is clearly becoming the structure of tropical ecology's future. My inferences and guid-

Perspectives for an Etiology of Stand-Level Dieback

Abstract: When trees are dying in large numbers in a forest stand, it is usually assumed they are dying from a disease. Therefore, the study of stand-level dieback has traditionally been the concern of the forest pathologist and pest entomologist. However, stand-level dieback is not always caused by biotic agents, and the term as defined in this paper refers to stands of dead or dying trees whose dieback cause is not obvious and that typically occur in several locations of a larger forest ecosystem. In such stands, adjacent trees as well as spatially separated trees die more or less simultaneously, and loss of foliage usually begins from the top downwards. Dieback trees may be identified either as standing snags or as still living but stag-headed trees. Such dieback stands are common in many indigenous Pacific forests (41), and they have received attention also in several North American (32) and European forests (9). The recent emphasis on the dieback of forests in industrial countries has enlarged the group of researchers to include soil scientists, foresters, and pollution specialists. In both the pathological and air pollution research of dieback, the structure of the forests with dieback, their spatial and temporal patterns, their habitat relationships, and their associated vegetation and successional re­ sponses have received less attention than the more immediate symptoms and assumed causes. However, to arrive at a more complete etiology, such vegetation structural aspects should be analyzed with equal effort. Without such effort, it is difficult to distinguish commonalities and discrepancies in stand-level dieback of different forest biomes. For example, Jimenez et al (21) described stand-level dieback in mangrove forests as "mass mortality" and suggested that mass diebacks always have an abiotic rather than biotic cause. Their distinction between mass mortality and normal mortality is helpful in understanding natural dieback, but when larger and smaller areas of

PHENETIC TAXONOMY: Theory and Methods

Abstract: Phenetic taxonomy is a system of classification based on the overall similarity of the organisms being classified. Phenetic relationships are defined by Cain & Harrison (10) as "arrangement by overall similarity, based on all available characters without any weighting." (The weighting implied here is a priori, i.e. before the classification is established.) This review focuses on numerical phenetics (59), in which the phenetic arrangement of the taxa is developed with numerical procedures applied to the character states of the organisms classified or to distance matrixes among them obtained by various techniques, principally applied to molecular data. I propose to summarize developments subsequent to the publication of the most recent comprehensive review of the subject (90); some of the technical terms not elaborated in this review can be found there. Phenetics is discussed as a basis for the system of classifying organisms and is briefly contrasted with two alternative current approaches in taxonomy-evolutionary taxonomy (the syncretistic approach of the Simpson-Mayr school) and phylogenetic systematics sensu Hennig and his disciples, also known as cladistics. This comparison is based in part on the notion of producing optimal classifications (defined later). Recent developments in the techniques of phenetic taxonomy are considered separately from the theory. These techniques are applied in wide areas of systematics and ecology, not just to produce biological classifications. The review also touches upon the as-yet-unrealized potential of phenetic methods as tools in biosystematic research.

Rates of molecular evolution

Abstract: Many patterns in the evolution of proteins and DNA are now well established and have been adequately reviewed in recent years (23, 37, 38, 52, 55, 61, 73). However, there are several gaps in our understanding of the process of molecular evolution, the most conspicuous of which is our ignorance of the forces responsible for most of the evolution of DNA sequences. Our ignorance is due, in part, to the lack of an adequate statistical description of molecular evolution. Whereas a few years ago modelling molecular evolution with a constant rate Poisson process seemed adequate (52, 76), today it is clear that the dynamics of molecular evolution are much more complex than those suggested by such a naive model. The statistical description of molecular evolution is hampered by the fact that we cannot observe the process directly. Instead, our only record is the accumulated changes in the sequences of species whose common ancestors lived many millions of years ago. With suitable data we can infer the number of changes that have occurred on a lineage, but we cannot infer the times when the changes occurred. Unfortunately, the times of the changes are needed to understand the forces responsible for molecular evolution. For this reason, much of the theoretical work in molecular evolution is directed toward the reconstruction of these past events. A critique of this work is presented in this review. The most compelling generalization to come out of molecular evolution studies is the concept of the molecular clock (76, 73). This is an abstraction from the common observation that the number of substitutions on a lineage is roughly proportional to the length of the lineage. (The constant of propor-

Information, Economics, and Evolution

Abstract: Cet article examine la proposition selon laquelle l'evolution est le resultat de l'interaction entre les entites biologiques impliquees dans les 2 grandes classes de processus biologiques ― les transferts de matiere et d'energie et le maintien ― la transmission et la modification de l'information genetique

Biology of birds of paradise and bowerbirds

Abstract: which include the most elabo­ rately decorated structures erected by any animal other than Homo sapiens. Darwin discussed both of these families because they furnish extreme ex­ amples of sexual selection. However, BOPs and BBs also pose other ques­ tions that have only recently attracted attention. BOPs provide a little-studied example of adaptive radiation in bills and diets which rivals that of Darwin's finches. Because of this range of diets, as well as of social systems, BOPs and BBs interest sociobiologists concerned with ecological determinants of social systems. Both families provide rich data for studying speciation. Male BBs decorating their bowers with objects of different colors are beginning to interest animal behaviorists.

Soil-plant relationships in the tropics

Abstract: As commonly defined, the tropics comprise that region of the earth lying between the Tropics of Cancer and Capricorn, constituting 38% of the land surface (5 x 109 ha). The tropics may also be defined as those areas with mean temperatures greater than 18?C in all months of the year. A third definition of the tropics specifies those areas with soils in which the temperature at a depth of 50 cm varies less than 5?C between summer and winter (39). Compared to the geographical definition, these latter definitions reduce the area considered to be tropical. In contrast to the temperate region where low temperature restricts plant growth at some period of the year, in the tropical regions the length of the growing season is determined by the amount of rainfall and its temporal distribution. Dudal (13) has separated the tropics into five major ecological zones based on growing periods: humid, subhumid, semiarid, arid, and wetland. The growing season varies from 12 months in the humid zone to nearly 0 months in the arid regions. In the tropics a wide range of climate and vegetation exists, owing mainly to variations in amount and duration of rainfall.

The ecology of tropical arthropod-borne viruses

Abstract: Arthropod-borne viruses (arboviruses) are ubiquitous in the tropics of the world and are significant causes of human and animal disease. The term arbovirus is epidemiologic rather than taxonomic. Arboviruses infect both arthropods and vertebrates. Arboviruses occur in 8 families and are found in the tropics around the world (Tables 1, 2). For the tropical areas of the world, 145 distinct viruses have been described in the Americas, 99 in Africa, 45 Asia, and 40 in Australia and New Guinea. This list grows longer each year, as new viruses are isolated. Hematophagous arthropods of all the major groups are associated with arbovirus transmission. Mere isolation of viruses from arthropods, as with many viruses in Table 2, does not constitute proof of vector status. There are four generally accepted criteria for establishing arthropod vector status: (a) the virus must be frequently isolated from wild-caught arthropods, (b) the virus must be shown to replicate in the arthropod species after ingestion of an infectious blood meal, (c) the arthropod must transmit the virus by bite after a suitable extrinsic incubation period, and (d) field evidence must establish the association of arthropods with infected vertebrate hosts during transmission periods in nature. Although arthropods may serve as "flying pin" mechanical vectors, their major role in virus transmission and maintenance is as biological vectors. There are three critical events that determine the efficiency of an arthropod as an arbovirus vector (35). (a) The arthropod must ingest a sufficient amount of viremic blood to infect gut cells. A threshold of infection (a minimum

Molecular Aspects of the Species Barrier

Abstract: The discovery that several molecular mechanisms can rapidly alter genome organization has led to speculation about their possible role in speciation (129). Speciation mechanisms have the effect of establishing barriers to genetic exchange; we use the term "species barriers" in this sense. At least one molecular mechanism, dysgenesis due to high levels of transposition, appears to have deleterious effects on the fitness of hybrids and thence could play some role in the establishment of species barriers (21, 52). Another point of recent interest is the possibility of genetic exchange across species barriers, or across lineage boundaries in asexual forms. Prokaryotic gene transfer has been known for quite some time; its different modalitiestransduction (145, 164), transformation (143), and conjugation (41)-are key parts of biotechnology. While these systems stand some distance from complete outcrossing, they can nevertheless shuffle genes between "asexual" lineages. The molecular mechanisms acting on the genome are extremely diverse in their specific pathways (136) but can be reduced to a few generic categories, which we discuss here. Knowledge of the particular features of these molecular mechanisms is essential to the evaluation of possible roles for them in speciation and population divergence. Our purpose is to present the various theories concerning molecular aspects of species barriers together with the available arguments, theoretical and empirical.