Recent studies have started to unravel the structure of large networks of ecological interactions. This has provided valuable information on network dynamics, coevolution, and responses to human-induced perturbations. What remains to be done is to bring a spatial dimension to these ecological networks. As an example, I will consider two cases. First, I will explore how network structure is molded by spatial processes in a large Caribbean food web. Second, I will consider how the structure of mutualistic networks affects species responses to habitat destruction, a major cause of biodiversity loss and mutualism disruption.
Coevolution is widely accepted as one of the dominant forces driving the creation of biodiversity, however the way in which coevolution promotes speciation is not well understood. I will show that divergent selection as the result of a coevolutionary arms race between red crossbills (Loxia curvirostra complex) and Rocky Mountain lodgepole pine (Pinus contorta latifolia) in the South Hills, Idaho promotes ecological speciation in crossbills. Less than one percent of 1285 breeding South Hills crossbills paired with non-South Hills crossbills indicating considerable reproductive isolation. The low frequency of heterotypic pairing was the result of at least three factors. One was related to enhanced seed defenses of lodgepole pine in the South Hills and adaptation of each call type to alternative resources with South Hills crossbills depressing seed availability so that few of the other less well adapted call types persisted in the South Hills (competitive exclusion causing habitat isolation). Another pertained to temporal isolation. When crossbills of other call types moved into the South Hills late in the breeding season, feeding conditions were deteriorating because of seed depletion by crossbills (another competitive effect) so that relatively few non-South Hills crossbills bred. Finally, among those crossbills that bred, pairing was strongly assortative by call type (behavioral isolation) further contributing to reproductive isolation between South Hills crossbills and the two other call types most common in the South Hills (call types 2 and 5), with total reproductive isolation summing to 0.999 on a scale of zero to one. This extremely high level of reproductive isolation indicates that the divergent selection resulting from the coevolutionary arms race between crossbills and pine has not only favored the evolution of a South Hills crossbill, but is also causing it to speciate. Because divergent selection is the result of a coevolutionary arms race between crossbills and lodgepole pine, it provides an example of how a geographic mosaic of coevolution gives rise to divergent selection causing ecological speciation. Coevolution may often drive ecological speciation if coevolutionary trajectories vary among populations causing divergent selection as envisioned in the geographic mosaic theory of coevolution. Indeed, many recent studies have demonstrated that divergent selection between populations may be a common outcome of geographically structured coevolution, including studies of other populations of crossbills. Because ecologically based divergent natural selection is thought to be an important process promoting speciation and coevolution is likely to vary across the range of a species, coevolution could play a prominent role in generating new species via ecological speciation.
Coevolutionary interactions between species take place over a wide geographic scale. Population subdivision across that range and spatially variable selection within it may lead to a mix of local adaptation and maladaptation for a pair of interacting species. Toxic newts of the genus Taricha and their resistant garter snake predators in the genus Thamnophis illustrate this general pattern throughout their concurrent ranges in western North America. Understanding of the mechanisms of toxicity and resistance in this system allows us to evaluate the degree of ecologically relevant phenotype matching at any given locality. The resultant picture suggests that nearly half of localities are so phenotypically mismatched as to prevent direct reciprocal selection at present. In at least some of these populations, snake predators seem to have 'won' the arms race by evolving sufficiently high levels of resistance to withstand the effects of any observed level of toxicity. The genetic basis of resistance in garter snakes is at least partly understood and suggests that these mismatches may result from single amino acid substitutions in the sodium channels of resistant snake populations.
Adaptation to environmental gradients has received much attention recently in two contexts: understanding range margins and their response to environmental change, and evolution of reproductive isolation in parapatry. These two issues are linked by common features in the behaviour of marginal populations and hybrid zones. The rocky shore snail, Littorina saxatilis, has evolved distinct morphotypes at different points on the steep intertidal environmental gradient. This has apparently happened independently at least three times in Europe. AFLP-based approaches have allowed us to investigate the genetic architecture of these adaptations and the barrier to gene flow that they generate. I will also discuss some results from an individual-based simulation of adaptation at range margins. This work has focused on the consequences of introducing factors such as mating dispersal and finite population size into the framework developed by Kirkpatrick and Barton. Adding these real-world features increases the range of parameter space in which stable range margins occur.
Historically, a great deal research in theoretical evolutionary ecology has modeled biological populations by supposing that the population size can take on any of a continuum of values. This assumption is reasonable so long as the population size is relatively large. Much of this research has ignored the consequences of the spatial distribution of populations, but the last couple of decades have seen an increased interest in developing explicitly spatial models for ecological and evolutionary processes. Interestingly, many of these models continue to assume that population sizes at each spatial location can take on a continuum of values. This assumption is often questionable because, although many real biological populations are relatively large, they are often distributed across a spatial range such local population sizes are quite small. I will discuss these issues in more detail, and present some theoretical results illustrating how such finite local population sizes can influence evolutionary change. This will involve an interesting application of ideas related to kin selection theory.
The range of most species is not constant over time, but appears influenced by several factors including inter-specific competition and climatic changes. I shall describe here the consequences of different types of range expansions on several aspects of neutral molecular diversity within and between populations. These results have been mainly obtained by simulations. I will first report on the consequences of a spatial expansion in an empty and homogeneous environment modeled as a 2D stepping-stone, showing that the pattern of genetic diversity within deme mainly depends on the age of the expansion, as well as on the amount of migrants exchanged between neighboring demes. Analytical results obtained under an infinite-island model support these conclusions. I will also introduce a model of spatial expansion into an occupied environment, with an explicitly modeling of inter-population competition. An interesting prediction of this model is that invading populations should have their gene pool invaded by the resident population if interbreeding is possible between the two competing populations. Previous studies have shown that range expansions can also propagate new mutations over a wide geographical area, and that these mutations can sometimes reach large frequencies. We find that the final area occupied by these surfing mutations as well as their final frequency depends to a large extent on local deme size. Finally, I shall introduce a new metapopulation model allowing one to study the effect of spatial and temporal heterogeneity of the environment on molecular diversity.
Evolutionary biologists have identified several factors that could explain the widespread phenomena of sex and recombination. One hypothesis is that host-parasite interactions favor sex and recombination because they favor the production of rare genotypes. A problem with many of the early models of this so-called Red Queen hypothesis is that several factors are acting together: directional selection, fluctuating epistasis, and drift. It is thus difficult to identify what exactly is selecting for sex in these models. Is one factor more important than the others or is it the synergistic action of these different factors that really matters? Here we focus on the analysis of a simple model with a single mechanism that might select for sex: fluctuating epistasis. We first analyze the evolution of recombination when the temporal variation is driven by the abiotic environment. We then analyze the evolution of recombination in a specific two-species coevolution model. In this model there is no directional selection (allele frequencies remain fixed), and the temporal variation in epistasis is induced by the coevolution with an antagonist species. In both cases we contrast situations with weak or strong selection. In the single species model we derive an expression for the evolutionarily stable (ES) recombination rate. This ES strategy decreases with the speed of the fluctuations of epistasis, but even when fluctuations are very slow (period longer than 100 generations) some recombination rate (>0) can be selected for. In the two-species coevolution model we find that the evolutionary outcome is mainly governed by the maintenance of coevolutionary cycles. In both situations we discuss the effect of migration when recombination evolves in a metapopulation with an infinite number of large populations, using an island model of dispersal.
Adaptive radiation is defined as the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. When it occurs, adaptive radiation typically follows the colonization of a new environment or the establishment of a "key innovation" which opens new ecological niches and/or new paths for evolution. Here, we take advantage of recent developments in speciation theory and modern computing power to build and explore a large-scale, stochastic, spatially explicit, individual-based model of adaptive radiation driven by adaptation to multidimensional ecological niches. We are able to model evolutionary dynamics of populations with hundreds of thousands of sexual diploid individuals over a time span of 100,000 generations assuming realistic mutation rates and allowing for genetic variation in a large number of both selected and neutral loci. Our results provide theoretical support and explanation for a number of empirical patterns including "area effect", "overshooting effect", "least action effect", as well as for the idea of a "porous genome". Our findings suggest that the genetic architecture of traits involved in the most spectacular radiations might be rather simple. We show that a great majority of speciation events are concentrated early in the phylogeny. Our results emphasize the importance of ecological opportunity and genetic constraints in controlling the dynamics of adaptive radiation.
Species often range over heterogeneous selective environments which, relative to a comparable uniform environment, can have unique impacts on the fate of a new mutation . Different approximations have been developed to characterize the probability of fixation of a new mutation in spatially variable environments for different combinations of migration and selection parameters. However, no single method seems to be accurate for all parameter combinations, and there are some parameter ranges for which no accurate approximation is available. This talk will review the performance of several approximations for the probability of fixation and present a new approximation, based on separation of the time scales of selection and migration. Simulations we performed with symmetric migration suggest that heterogeneous selection never decreased---and at times substantially increased--- the fixation probability of a new mutation compared to a new mutation experiencing homogeneous selection with the same mean intensity.
Models of metapopulation ecology, genetics, and evolution have tended to assume a simple description of landscape structure, which has hindered the testing of models with empirical data. Recent work has attempted to link a more realistic description of landscape structure with modelling of the ecological metapopulation dynamics. It would be helpful to develop a comparable framework for genetic and evolutionary studies. I discuss some empirical results on a well-studied butterfly metapopulation, including coupling of the ecological and evolutionary dynamics in host plant selection and evolution of dispersal in fragmented landscapes.
Two papers published in 1986 set forth the notion that some hybrid zones might profitably be viewed as mosaics of populations or genotypes, reflecting an underlying habitat and/or resource template. I review the theoretical and empirical literature on mosaic hybrid zones that has accumulated in the past two decades, and discuss the insights that have emerged. I also summarize our current understanding of patterns of variation in a field cricket (Gryllus) hybrid zone that provided the initial motivation for thinking about habitat mosaics and their influence on interactions between hybridizing species.
Some species are responding to current global climate warming and shifting their distributions polewards and/or uphill. It is becoming clear that evolutionary changes are occurring as a consequences of this climate-driven range expansion. Evidence for increased dispersal ability, shifts onto novel host-plants and increased ability to tolerate poor larval hostplant quality in populations at expanding range margins suggest that some species may be able to keep track of environmental changes. However these changes are balanced by evolutionary trade-offs in fecundity, and most species are failing to expand due to loss of breeding habitat, regardless of any evolutionary adaptations. In addition, reduced genetic diversity in populations expanding through patchy habitats is also likely to affect species' ability to respond to novel environments. I discuss the implications of these findings for the future distribution of biodiversity.
We present a review of the attempt within the Tangled Nature [1,2] model to understand the effect of evolution and interaction on ecological and evolutionary observables. We report on the relation between the interaction structure in genotype space and the resulting Species Abundance Distribution. Ecological relevant SADs are only obtained if the genotype space allow for a potential high connectivity between species . We also study the relation between the degree of genotype interaction and species diversity . Furthermore we include spatial degrees of freedom to investigate the Species Area Relation from an evolutionary perspective.
The model has been generalised to include correlations in genotype (or phenotype) space and a conserved resource for which all existing types have to compete. This allows us to study, from an evolutionary perspective, the relation between community structure and availability of the conserved resource .
Although theoretical studies suggest sympatric and parapatric speciation can occur through disruptive natural or sexual selection, recent reevaluations of these speciation models indicated that conditions under which this happens are restrictive. Thus, it is important to investigate the probability of such speciation by using models based on explicit genetic mechanisms for female choice and male ornamentation. Here we first show that in simulations in which the evolution of visual pigments and color perception are explicitly modeled, sensory drive can promote speciation along a short selection gradient within a continuous habitat and population. We assumed that color perception of individuals evolves to adapt to the light environment and that females prefer to mate with males whose nuptial color they perceive most intensively. In our simulations color perception depends on the absorption spectra of an individual fs visual pigments. Speciation occurred most frequently when the steepness of the environmental light color gradient was intermediate and dispersal distance of offspring was small. In addition, our results predict that mutations that cause large shifts in the wavelength of peak absorption promote speciation. The genetic control for male nuptial color also affects the probability of speciation, but far less so then the genetics of female mating preference. We discuss putative cases of sympatric and parapatric speciation in fishes that might, at least partially, be explained by this model.
Chromosome inversions may play an important role in adaptation to local environmental conditions. I will discuss models for the evolution of inversions that capture locally-adapted alleles when two populations are exchanging migrants or hybridizing. By suppressing recombination between the loci, a new inversion can spread. Neither drift nor coadaptation between the alleles (epistasis) is needed, so this local adaptation mechanism may apply to a broader range of genetic and demographic situations than alternative hypotheses that have been widely discussed. The mechanism can explain many features observed in inversion systems. The mechanism can establish postzygotic barriers and thus contribute to speciation: it can establish underdominant inversions that decrease heterokaryotype fitness by several percent if the cause of fitness loss is structural, while if the cause is genic there is no limit to the strength of underdominance that can result. The mechanism is expected to cause loci responsible for adaptive species-specific differences to map to inversions, as seen in recent QTL studies.
After a general introduction, I will concentrate on cases of Lepidoptera speciation I know about. I will argue that many cases of intermediate speciation occur in sympatry, both just below the traditional species level, and just above. The coexistence of these intermediate stages in nature suggests that the whole process of speciation isn't as difficult as all that, especially given local spatial variation in ecological factors. Whether you call this "sympatry" is a matter of taste, but I'll attempt to persuade you that it is sensible to do so, at least if you want any natural populations to be classified as "sympatric" at all.
The idea that speciation in the presence of gene flow is difficult seems merely to be an artifact of a rigid and highly non-darwinian idea: that species are "real" (whatever that means). They are also regarded as "the only real taxon". This was proposed along with the "biological species concept" around 65 years ago, coupled with lots of naivete about about the supposed power of gene flow. Natural populations are telling us that "species reality" and the concomitant "difficulty of speciation" are both greatly overstated. Instead, species are demonstrably continuous with "varieties" in nature, and the evidence of continuous speciation processes is all around us. I believe it would solve a lot of problems to go back to a much more pragmatic view of species and speciation, closer to Darwin's own ideas, and dispense with all that mid-20thC mystical nonsense about "species reality" once and for all.
I discuss a systematic approach to the modeling of biological systems which starts from individual-based models, and then goes on to derive from these the corresponding deterministic equations which are valid when the size of the system is large. The formalism used to study the stochastic dynamics of the individual-based model is common to a large number of systems, such as models of epidemics, metapopulations, metabolic reactions, biodiversity --- including Hubbell's neutral theory --- as well as more conventional predator-prey and competition models. In contrast to most previous studies, these processes are modeled using master equations, which allows use to be made of well-established methods from the theory of these equations to analyze their behaviors. The formalism naturally generalizes to spatially explicit models, and I will compare the governing deterministic equations for these systems to those which are normally written down on phenomenological grounds. The consequences of these, and other novel aspects of the master equation description for the systems under consideration, will also be explored.
Empirical studies of host-parasite and predator-prey interactions commonly demonstrate local maladaptation in at least one of the component species. These empirical results are in line with theoretical predictions based upon models of host-parasite interactions mediated by simple genetic mechanisms of infection and resistance. The extent to which these theoretical results extend to host-parasite or predator-prey interactions mediated by quantitative traits is, however, unclear. I will present mathematical and numerical results for a model of spatially structured coevolution mediated by quantitative traits. The results demonstrate that local maladaptation is substantially less likely when coevolution is mediated by quantitative traits.
Long-term coevolution of species is an inherently geographic process. It is shaped by geographic selection mosaics that create spatially structured coadaptation among pairs and groups of species. It is further fueled by gene flow and by coevolutionary coldspots where one species falls outside the geographic range of the other species or by lack of reciprocal selection in some coexisting populations. In addition, the coevolutionary process is continually reshaped by the appearance of new tips on phylogenetic branches as some locally coevolving populations diverge into coevolving sibling species complexes. These dynamics of coeadaptation and speciation are the interface of microevolution and macroevolution in coevolutionary biology. Moreover, these are the collective processes that allow lineages to coevolve across millennia, despite the transient dynamics and lack of persistence of most locally coevolving populations. Current data and models suggest specific needs for future modeling on how the geographic mosaic coevolution drives adaptation and speciation, and, in turn, how adaptation and speciation collectively reshape the geographic mosaic of coevolution across millennia.