Patterns of species ranges, speciation, and extinction

The exact nature of the relationship among species range sizes, speciation, and extinction events is not well understood. The factors that promote larger ranges, such as broad niche widths and high dispersal abilities, could increase the likelihood of encountering new habitats but also prevent local adaptation due to high gene flow. Similarly, low dispersal abilities or narrower niche widths could cause populations to be isolated, but such populations may lack advantageous mutations due to low population sizes. Here we present a large-scale, spatially explicit, individual-based model addressing the relationships between species ranges, speciation, and extinction. We followed the evolutionary dynamics of hundreds of thousands of diploid individuals for 200,000 generations. Individuals adapted to multiple resources and formed ecological species in a multidimensional trait space. These species varied in niche widths, and we observed the coexistence of generalists and specialists on a few resources. Our model shows that species ranges correlate with dispersal abilities but do not change with the strength of fitness trade-offs; however, high dispersal abilities and low resource utilization costs, which favored broad niche widths, have a strong negative effect on speciation rates. An unexpected result of our model is the strong effect of underlying resource distributions on speciation: in highly fragmented landscapes, speciation rates are reduced.     

Distribution and dispersal in populations capable of resource depletion

Summary Theoretical predictions from a simulation model of insect distributions and dispersal among isolated food plants have been tested with data gathered from 13 Cinnabar moth populations. Agreement with the predictions was good. Egg batch size was equal to or slightly larger than the number which could be supported by the average food plant. Egg batch distribution was more clumped when density was high and when egg batch size was small relative to food plant size. The tendency for larval dispersal was lower in populations in areas where plants were widely spaced. These findings indicate that the Cinnabar moth has the genetic or phenotypic flexibility to adjust egg batch size, egg distribution, and larval dispersal to characteristics of the habitat. A hypothesized model is proposed to describe the interactions between larval dispersal, food plant response to defoliation, and population fluctuations for the Cinnabar moth and its food plant, tansy ragwort.     

Interacting maternal and spatial cues influence natal – dispersal out of social groups

Proximate cues for animal dispersal are complex and varied. Multiple cues may provide information about different aspects of habitat quality, and these aspects may interact with each other, as well as with population density in different ways. We examined how individuals incorporate multiple cues in their decisions to emigrate and immigrate in the colonial orb‐weaving spider, Cyrtophora citricola. We manipulated maternal feeding as a cue for prey abundance and measured the size of the maternal web, which provides a limited space for philopatric offspring and a second potential dispersal cue. In addition, we recorded all immigration events to determine dispersal distances and the cues juveniles may use in settlement. Dispersal increased when mothers were poorly fed, web sizes were small and clutch sizes were large. In addition to these overall effects, maternal feeding also interacted with web size, indicating that offspring from well‐fed mothers were more tolerant of high sibling densities. We also detected a threshold for the effect of clutch size on dispersal for the first egg sac: below 20 offspring, there was no effect of clutch size, but dispersal increased with clutch size for larger clutches. Dispersal distances were often short, and immigrants preferred sheltered trees and those occupied by adult females. Dispersal not only depended on multiple cues, but these cues interacted, and the importance of web size suggested that saturation of the natal web might force dispersal, at least for spiders with poorly‐fed mothers. How one aspect of habitat quality influences dispersal can therefore depend on the state of other aspects of habitat quality. In particular, some natal resources, such as a nest or territory, may become saturated and limit group size, but this limit will also depend on other factors, such as prey availability.     

Synergistic selection between ecological niche and mate preference primes diversification

The ecological niche and mate preferences have independently been shown to be important for the process of speciation. Here, we articulate a novel mechanism by which ecological niche use and mate preference can be linked to promote speciation. The degree to which individual niches are narrow and clustered affects the strength of divergent natural selection and population splitting. Similarly, the degree to which individual mate preferences are narrow and clustered affects the strength of divergent sexual selection and assortative mating between diverging forms. This novel perspective is inspired by the literature on ecological niches; it also explores mate preferences and how they may contribute to speciation. Unlike much comparative work, we do not search for evolutionary patterns using proxies for adaptation and sexual selection, but rather we elucidate how ideas from niche theory relate to mate preference, and how this relationship can foster speciation. Recognizing that individual and population niches are conceptually and ecologically linked to individual and population mate preference functions will significantly increase our understanding of rapid evolutionary diversification in nature. It has potential to help solve the difficult challenge of testing the role of sexual selection in the speciation process. We also identify ecological factors that are likely to affect individual niche and individual mate preference in synergistic ways and as a consequence to promote speciation. The ecological niche an individual occupies can directly affect its mate preference. Clusters of individuals with narrow, differentiated niches are likely to have narrow, differentiated mate preference functions. Our approach integrates ecological and sexual selection research to further our understanding of diversification processes. Such integration may be necessary for progress because these processes seem inextricably linked in the natural world.     

The relationship between pelagic larval duration and range size in tropical reef fishes: a synthetic analysis

We address the conflict in earlier results regarding the relationship between dispersal potential and range size. We examine all published pelagic larval duration data for tropical reef fishes. Larval duration is a convenient surrogate for dispersal potential in marine species that are sedentary as adults and that therefore only experience significant dispersal during their larval phase. Such extensive quantitative dispersal data are only available for fishes and thus we use a unique dataset to examine the relationship between dispersal potential and range size. We find that dispersal potential and range size are positively correlated only in the largest ocean basin, the Indo-Pacific, and that this pattern is driven primarily by the spatial distribution of habitat and dispersal barriers. Furthermore, the relationship strengthens at higher taxonomic levels, suggesting an evolutionary mechanism. We document a negative correlation between species richness and larval duration at the family level in the Indo-Pacific, implying that speciation rate may be negatively related to dispersal potential. If increased speciation rate within a taxonomic group results in smaller range sizes within that group, speciation rate could regulate the association between range size and dispersal potential.     

Sympatric speciation: a simulation model

A model of mating and population growth dependent on competition that suggests circumstances under which sympatric speciation might occur is described. The model is similar to one in a companion paper by Rosenzweig in that a heterozygote genotype, involving a new allele, is first selected by virtue of its ability to exploit a new niche and is then eliminated through competition. The superior competitor, which eliminates the heterozygote, is the homozygote for the new allele. For this process to occur the heterozygote must be sufficiently fit to exploit and invade a new niche, but not so fit that a classical polymorphism results from heterozygous advantage. This process of speciation is most likely to occur when there are vacant niches. When and where these might occur are discussed.     

Repertoire Sharing and Song Similarity between Great Tit Males Decline with Distance between Forest Fragments

Birdsong can play a critical role in establishing a territory and finding a mate among individuals from local and foreign populations. Variation in birdsong among populations can be influenced by habitat fragmentation and might affect successful dispersal among habitat fragments. We studied variation in great tit song in a long‐term study population distributed over nine forest fragments. All individual males recorded had a known dispersal history within the fragmented forest habitat. We found spatial structure of declining song‐type sharing with distance, with a marked drop from an individual’s own forest fragment to another across a habitat gap. We also found decreasing song similarity among increasingly distant fragments in terms of temporal and spectral characteristics of shared song types. The change in acoustic structure was more gradual and seemed less affected by habitat discontinuity but also showed a tight correlation with dispersal index among forest fragments. Immigrant birds shared fewer song types with neighbouring birds that were born within the same forest fragment, but not less compared to birds born in another forest fragment within the study area. Our data provide detailed insight into the relationship between song differentiation and male dispersal and contribute to our understanding of the potential role of song in reproductive exchange and avian speciation. The fact that birds in small forest fragments shared more songs than birds in larger forest fragments confirms that song analysis has potential as a tool for conservation in rare species.     

THE EVOLUTION OF REPRODUCTIVE ISOLATION AS A CORRELATED CHARACTER UNDER SYMPATRIC CONDITIONS: EXPERIMENTAL EVIDENCE

A set of experiments is described that tests the general hypothesis that sympatric speciation is genetically feasible whenever reproductive isolation evolves indirectly as a correlated character. We specifically test the hypothesis that disruptive selection on habitat preference can lead to sympatric speciation when individuals mate locally within their selected habitat. Drosophila melanogaster was used as a model system. A 35-generation experiment using a complex habitat maze led to complete reproductive isolation between subpopulations using different spatiotemporal habitats. The reproductive isolation that developed over the course of the experiment was a result of offspring returning to mate in the habitat type selected by their parents, i.e., a gradual breakdown in migration between habitats.     

Isolation by Distance in a Quantitative Trait

Random genetic drift in a quantitative character is modeled for a population with a continuous spatial distribution in an infinite habitat of one or two dimensions. The analysis extends Wright's concept of neighborhood size to spatially autocorrelated sampling variation in the expected phenotype at different locations. Weak stabilizing selection is assumed to operate toward the same optimum phenotype in every locality, and the distribution of dispersal distances from parent to offspring is a (radially) symmetric function. The equilibrium pattern of geographic variation in the expected local phenotype depends on the neighborhood size, the genetic variance within neighborhoods, and the strength of selection, but is nearly independent of the form of the dispersal function. With all else equal, geographic variance is smaller in a two-dimensional habitat than in one dimension, and the covariance between expected local phenotypes decreases more rapidly with the distance separating them in two dimensions than in one. The equilibrium geographic variance is less than the phenotypic variance within localities, unless the neighborhood size is small and selection is extremely weak, especially in two dimensions. Nevertheless, dispersal of geographic variance created by random genetic drift is an important mechanism maintaining genetic variance within local populations. For a Gaussian dispersal function it is shown that, even with a small neighborhood size, a population in a two-dimensional habitat can maintain within neighborhoods most of the genetic variance that would occur in an infinite panmictic population.     

Feeding ecology and seed dispersal of sympatric cheirogaleid lemurs (Microcebus murinus, Cheirogaleus medius, Cheirogaleus major) in the littoral rainforest of south-east Madagascar

The lemurs of Madagascar are known for their extraordinary species diversity. The mechanisms that allow the coexistence of these species are still poorly known. Here feeding patterns were investigated for three small nocturnal lemur species of Cheirogaleidae ( Microcebus murinus , Cheirogaleus medius and Cheirogaleus major ) occurring sympatrically in a littoral rainforest in south-east Madagascar. During three rainy seasons, the plant species eaten by these three lemurs were described in relation to morphological and biochemical characteristics. All three species were mainly frugivorous and fed on 68 different plant species with small- and medium-sized fruits. A total of 91% of these forage plant species was visited by all three lemur species. Fruits larger than 25–30 mm were avoided. Seeds of a total of 51 food plant species were swallowed and passed the gut unharmed. Thus, even these smaller lemur species play an important role in seed dispersal. There were no differences in the morphological and biochemical characteristics of fruits eaten between the three species, but the feeding height was significantly different between the species. Thus, competition avoidance and niche separation are presumably not based on different feeding patterns of M. murinus , C. medius and C. major in the littoral rainforest, but on different habitat utilization.     

Cross-generational environmental effects and the evolution of offspring size in the Trinidadian guppy Poecilia reticulata

Abstract The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full‐sib sisters were exposed to either a low‐ or high‐food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low‐ and high‐food mothers in either low‐ or high‐food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low‐food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low‐resource environment or in an environment that selects for lower reproductive effort     

EVOLUTION OF DISPERSAL RATES IN METAPOPULATION MODELS: BRANCHING AND CYCLIC DYNAMICS IN PHENOTYPE SPACE

We study the evolution of dispersal rates in a two patch metapopulation model. The local dynamics in each patch are given by difference equations, which, together with the rate of dispersal between the patches, determine the ecological dynamics of the metapopulation. We assume that phenotypes are given by their dispersal rate. The evolutionary dynamics in phenotype space are determined by invasion exponents, which describe whether a mutant can invade a given resident population. If the resident metapopulation is at a stable equilibrium, then selection on dispersal rates is neutral if the population sizes in the two patches are the same, while selection drives dispersal rates to zero if the local abundances are different. With non-equilibrium metapopulation dynamics, non-zero dispersal rates can be maintained by selection. In this case, and if the patches are ecologically identical, dispersal rates always evolve to values which induce synchronized metapopulation dynamics. If the patches are ecologically different, evolutionary branching into two coexisting dispersal phenotypes can be observed. Such branching can happen repeatedly, leading to polymorphisms with more than two phenotypes. If there is a cost to dispersal, evolutionary cycling in phenotype space can occur due to the dependence of selection pressures on the ecological attractor of the resident population, or because phenotypic branching alternates with the extinction of one of the branches. Our results extend those of Holt and McPeek (1996), and suggest that phenotypic branching is an important evolutionary process. This process may be relevant for sympatric speciation.     

Effect of dispersal at range edges on the structure of species ranges

Range edges are of particular interest to ecology because they hold key insights into the limits of the realized niche and associated population dynamics. A recent feature of Oikos summarized the state of the art on range edge ecology. While the typical question is what causes range edges, another important question is how range edges influence the distribution of abundances across a species geographic range when dispersal is present. We used a single species population dynamics model on a coupled-lattice to determine the effects of dispersal on peripheral populations as compared to populations at the core of the range. In the absence of resource gradients, the reduced neighborhood and thus lower connectivity or higher isolation among populations at the range edge alone led to significantly lower population sizes in the periphery of the range than in the core. Lower population sizes mean higher extinction risks and lower adaptability at the range edge, which could inhibit or slow range expansions, and thus effectively stabilize range edges. The strength of this effect depended on the potential population growth rate and the maximum dispersal distance. Lower potential population growth rates led to a stronger effect of dispersal resulting in a higher difference in population sizes between the two areas. The differential effect of dispersal on population sizes at the core and periphery of the range in the absence of resource gradients implies that traditional, habitat-based distribution models could result in misleading conclusions about the habitat quality in the periphery. Lower population sizes at the periphery are also relevant to conservation, because habitat removal not only eliminates populations but also creates new edges. Populations bordering these new edges may experience declines, due to their increased isolation.     

The effects of ecological and genetic neighbourhood size on the evolution of two competing species

Summary Individual-based simulations were conducted to examine the effect of a small ecological neighbourhood (an area in which ecological processes such as density-dependent factors operate) and the genetic neighbourhood size (the size of an area from which the parents may be assumed to be drawn at random) on the coevolution of two competing species. For the simulations, individuals of two consumer species compete for two types of food organisms. Different genotypes (one locus and two alleles) have different efficiencies of food acquisition for different food types. Individual consumer organisms search for food within their home ranges and reproduce depending on the amount of food eaten. The dispersal distance of the offspring follows a normal distribution with a zero mean and _d standard deviation. Simulations were conducted by varying the home range size, mating area (area from where individuals choose their mates), standard deviation of dispersal distance, food generation time, the reproductive rates of food populations and the sizes and number of independent food populations. Food organisms reproduce either within one population or independently within 16 spatially divided populations. For all the simulations, competitive exclusion was the most frequent outcome and character displacement was the least frequent outcome. Through a 200-generation simulation, the two consumer species could co-exist longer and maintain a polymorphic resource use longer when the home range and mating size were small in 16 spatially divided populations than when random mating and homogeneous interaction occurred within a community (perfect mixing population). For perfect mixing populations, the frequency of character displacement increased as the food generation time became short and the reproductive rates of food decreased. It follows from the results that the sizes of the genetic and ecological neighbourhoods and the mode of resource dynamics can affect the evolution of two competing species.     

Demographic consequences of population subdivision on the long-furred woolly mouse opossum (Micoureus paraguayanus) from the Atlantic Forest

Habitat destruction and fragmentation severely affected the Atlantic Forest. Formerly contiguous populations may become subdivided into a larger number of smaller populations, threatening their long-term persistence. The computer package VORTEX was used to simulate the consequences of habitat fragmentation and population subdivision on Micoureus paraguayanus, an endemic arboreal marsupial of the Atlantic Forest. Scenarios simulated hypothetical populations of 100 and 2000 animals being partitioned into 1–10 populations, linked by varying rates of inter-patch dispersal, and also evaluated male-biased dispersal. Results demonstrated that a single population was more stable than an ensemble of populations of equal size, irrespective of dispersal rate. Small populations (10–20 individuals) exhibited high instability due to demographic stochasticity, and were characterized by high rates of extinction, smaller values for metapopulation growth and larger fluctuations in population size and growth rate. Dispersal effects on metapopulation persistence were related to the size of the populations and to the sexes that were capable of dispersing. Male-biased dispersal had no noticeable effects on metapopulation extinction dynamics, whereas scenarios modelling dispersal by both sexes positively affected metapopulation dynamics through higher growth rates, smaller fluctuations in growth rate, larger final metapopulation sizes and lower probabilities of extinction. The present study highlights the complex relationships between metapopulation size, population subdivision, habitat fragmentation, rate of inter-patch dispersal and sex-biased dispersal and indicates the importance of gaining a better understanding of dispersal and its interactions with correlations between disturbance events.     

Genetic population structure as indirect measure of dispersal ability in a Lake Tanganyika cichlid

Diversification and speciation processes are influenced by intrinsic (ecological specialization, dispersal) and extrinsic (habitat structure and instability) factors, but the effect of ecological characteristics on dispersal is difficult to assess. This study uses mitochondrial control region sequences to investigate the population structure and demographic history of the endemic Lake Tanganyika cichlid Neolamprologus caudopunctatus with a preference for the rock-sand interface along two stretches of continuous, rocky shoreline, and across a sandy bay representing a potential dispersal barrier. Populations along uninterrupted habitat were not differentiated; whereas, the sandy bay separated two reciprocally monophyletic clades. The split between the two clades between 170,000 and 260,000 years BP coincides with a period of rising water level following a major lowstand, and indicates that clades remained isolated throughout subsequent lake level fluctuations. Low long-term effective population sizes were inferred from modest genetic diversity estimates, and may be due to recent population expansions starting from small population sizes 45,000–60,000 years BP. Comparisons with available data from specialized rock-dwelling species of the␣same area suggest that habitat structure and lake level fluctuations determine phylogeographic patterns on large scales, while fine-scale population structure and demography are modulated by species-specific ecologies.     

The role of environment and core‐margin effects on range‐wide phenotypic variation in a montane grasshopper

The integration of genetic information with ecological and phenotypic data constitutes an effective approach to gain insight into the mechanisms determining interpopulation variability and the evolutionary processes underlying local adaptation and incipient speciation. Here, we use the Pyrenean Morales grasshopper (Chorthippus saulcyi moralesi) as study system to (i) analyse the relative role of genetic drift and selection in range‐wide patterns of phenotypic differentiation and (ii) identify the potential selective agents (environment, elevation) responsible for variation. We also test the hypothesis that (iii) the development of dispersal‐related traits is associated with different parameters related to population persistence/turnover, including habitat suitability stability over the last 120 000 years, distance to the species distribution core and population genetic variability. Our results indicate that selection shaped phenotypic differentiation across all the studied morphological traits (body size, forewing length and shape). Subsequent analyses revealed that among‐population differentiation in forewing length was significantly explained by a temperature gradient, suggesting an adaptive response to thermoregulation or flight performance under contrasting temperature regimes. We found support for our hypothesis predicting a positive association between the distance to the species distribution core and the development of dispersal‐related morphology, which suggests an increased dispersal capability in populations located at range edges that, in turn, exhibit lower levels of genetic variability. Overall, our results indicate that range‐wide patterns of phenotypic variation are partially explained by adaptation in response to local environmental conditions and differences in habitat persistence between core and peripheral populations.     

Host–parasitoid spatial models: the interplay of demographic stochasticity and dynamics

Host–parasitoid metapopulation models have typically been deterministic models formulated with population numbers as a continuous variable. Spatial heterogeneity in local population abundance is a typical (and often essential) feature of these models and means that, even when average population density is high, some patches have small population sizes. In addition, large temporal population fluctuations are characteristic of many of these models, and this also results in periodically small local population sizes. Whenever population abundances are small, demographic stochasticity can become important in several ways. To investigate this problem, we have reformulated a deterministic, host–parasitoid metapopulation as an integer-based model in which encounters between hosts and parasitoids, and the fecundity of individuals are modelled as stochastic processes. This has a number of important consequences: (1) stochastic fluctuations at small population sizes tend to be amplified by the dynamics to cause massive population variability, i.e. the demographic stochasticity has a destabilizing effect; (2) the spatial patterns of local abundance observed in the deterministic counterpart are largely maintained (although the area of ''spatial chaos'' is extended); (3) at small population sizes, dispersal by discrete individuals leads to a smaller fraction of new patches being colonized, so that parasitoids with small dispersal rates have a greater tendency for extinction and higher dispersal rates have a larger competitive advantage; and (4) competing parasitoids that could coexist in the deterministic model due to spatial segregation cannot now coexist for any combination of parameters.     

Is female-biased sex determination in lemmings caused by staying together for warmth?

Two kinds of lemming have evolved genetic modifications of sex determination that result in surpluses of daughters. Female-biased sex ratios can evolve when mating occurs between neighbouring individuals who are more related than if mating occurred randomly. Two proposed sources of such 'viscous' gene flow in lemmings arc cyclical changes in population density and mosaic habitat. Alternatively, perhaps cold climate favours winter aggregation and inhibits the dispersal of winter-born offspring, which would Nature and mate with close relatives; dispersal and outbreeding would occur during the warm months. Thus the episodes of dispersal and inbreeding would be seasonal rather than density-dependent and the supposition of discontinuous habitat is obviated.     

Not going with the flow: a comprehensive time‐calibrated phylogeny of dragonflies (Anisoptera: Odonata: Insecta) provides evidence for the role of lentic habitats on diversification

Ecological diversification of aquatic insects has long been suspected to have been driven by differences in freshwater habitats, which can be classified into flowing (lotic) waters and standing (lentic) waters. The contrasting characteristics of lotic and lentic freshwater systems imply different ecological constraints on their inhabitants. The ephemeral and discontinuous character of most lentic water bodies may encourage dispersal by lentic species in turn reducing geographical isolation among populations. Hence, speciation probability would be lower in lentic species. Here, we assess the impact of habitat use on diversification patterns in dragonflies (Anisoptera: Odonata). Based on the eight nuclear and mitochondrial genes, we inferred species diversification with a model‐based evolutionary framework, to account for rate variation through time and among lineages and to estimate the impact of larval habitat on the potentially nonrandom diversification among anisopteran groups. Ancestral state reconstruction revealed lotic fresh water systems as their original primary habitat, while lentic waters have been colonized independently in Aeshnidae, Corduliidae and Libellulidae. Furthermore, our results indicate a positive correlation of speciation and lentic habitat colonization by dragonflies: speciation rates increased in lentic Aeshnidae and Libellulidae, whereas they remain mostly uniform among lotic groups. This contradicts the hypothesis of inherently lower speciation in lentic groups and suggests species with larger ranges are more likely to diversify, perhaps due to higher probability of larger areas being dissected by geographical barriers. Furthermore, larger range sizes may comprise more habitat types, which could also promote speciation by providing additional niches, allowing the coexistence of emerging species.