Interspecific Competition, Environmental Gradients, Gene Flow, and the Coevolution of Species' Borders

Trophic interactions and range limits: the diverse roles of predation

Interactions between natural enemies and their victims are a pervasive feature of the natural world. In this paper, we discuss trophic interactions as determinants of geographic range limits. Predators can directly limit ranges, or do so in conjunction with competition. Dispersal can at times permit a specialist predator to constrain the distribution of its prey-and thus itself-along a gradient. Conversely, we suggest that predators can also at times permit prey to have larger ranges than would be seen without predation. We discuss several ecological and evolutionary mechanisms that can lead to this counter-intuitive outcome.

Evolutionarily stable range limits set by interspecific competition

A combination of abiotic and biotic factors probably restricts the range of many species. Recent evolutionary models and tests of those models have asked how a gradual change in environmental conditions can set the range limit, with a prominent idea being that gene flow disrupts local adaptation. We investigate how biotic factors, explicitly competition for limited resources, result in evolutionarily stable range limits even in the absence of the disruptive effect of gene flow. We model two competing species occupying different segments of the resource spectrum. If one segment of the resource spectrum declines across space, a species that specializes on that segment can be driven to extinction, even though in the absence of competition it would evolve to exploit other abundant resources and so be saved. The result is that a species range limit is set in both evolutionary and ecological time, as the resources associated with its niche decline. Factors promoting this outcome include: (i) inherent gaps in the resource distribution, (ii) relatively high fitness of the species when in its own niche, and low fitness in the alternative niche, even when resource abundances are similar in each niche, (iii) strong interspecific competition, and (iv) asymmetric interspecific competition. We suggest that these features are likely to be common in multispecies communities, thereby setting evolutionarily stable range limits.

Geographic range limits of species

Understanding the forms that the geographic range limits of species take, their causes and their consequences are key issues in ecology and evolutionary biology. They are also topics on which understanding is advancing rapidly. This themed issue of Proc. R. Soc. B focuses on the wide variety of current research perspectives on the nature and determinants of the limits to geographic ranges. The contributions address important themes, including the roles and influences of dispersal limitation, species interactions and physiological limitation, the broad patterns in the structure of geographic ranges, and the fundamental question of why at some point species no longer evolve the ability to overcome the factors constraining their distributions and thus fail to continue to spread. In this introduction, these contributions are placed in the wider context of these broad themes.

Allee effects, invasion pinning, and species' borders

All species' ranges are the result of successful past invasions. Thus, models of species' invasions and their failure can provide insight into the formation of a species' geographic range. Here, we study the properties of invasion models when a species cannot persist below a critical population density known as an "Allee threshold." In both spatially continuous reaction-diffusion models and spatially discrete coupled ordinary-differential-equation models, the Allee effect can cause an invasion to fail. In patchy landscapes (with dynamics described by the spatially discrete model), range limits caused by propagation failure (pinning) are stable over a wide range of parameters, whereas, in an uninterrupted habitat (with dynamics described by a spatially continuous model), the zero velocity solution is structurally unstable and thus unlikely to persist in nature. We derive conditions under which invasion waves are pinned in the discrete space model and discuss their implications for spatially complex dynamics, including critical phenomena, in ecological landscapes. Our results suggest caution when interpreting abrupt range limits as stemming either from competition between species or a hard environmental limit that cannot be crossed: under a wide range of plausible ecological conditions, species' ranges may be limited by an Allee effect. Several example systems appear to fit our general model.

The response of two butterfly species to climatic variation at the edge of their range and the implications for poleward range shifts

To predict changes in species' distributions due to climate change we must understand populations at the poleward edge of species' ranges. Ecologists generally expect range shifts under climate change caused by the expansion of edge populations as peripheral conditions increasingly resemble the range core. We tested whether peripheral populations of two contrasting butterflies, a small-bodied specialist (Erynnis propertius) and a large-bodied generalist (Papilio zelicaon), respond favorably to warmer conditions. Performance of populations related to climate was evaluated in seven peripheral populations spanning 1.2 degrees latitude (160 km) using: (1) population density surveys, an indirect measure of site suitability; and (2) organismal fitness in translocation experiments. There was evidence that population density increased with temperature for P. zelicaon whose population density declined with latitude in 1 of 3 sample years. On the other hand, E. propertius showed a positive relationship of population density with latitude, apparently unrelated to climate or measured habitat variables. Translocation experiments showed increased larval production at increased temperatures for both species, and in P. zelicaon, larval production also increased under drier conditions. These findings suggest that both species may increase at their range edge with warming but the preference for core-like conditions may be stronger in P. zelicaon. Further, populations of E. propertius at the range boundary may be large enough to act as sources of colonists for range expansions, but range expansion in this species may be prevented by a lack of available host plants further north. In total, the species appear to respond differently to climate and other factors that vary latitudinally, factors that will likely affect poleward expansion.

Adaptation, migration or extirpation: climate change outcomes for tree populations

Species distribution models predict a wholesale redistribution of trees in the next century, yet migratory responses necessary to spatially track climates far exceed maximum post-glacial rates. The extent to which populations will adapt will depend upon phenotypic variation, strength of selection, fecundity, interspecific competition, and biotic interactions. Populations of temperate and boreal trees show moderate to strong clines in phenology and growth along temperature gradients, indicating substantial local adaptation. Traits involved in local adaptation appear to be the product of small effects of many genes, and the resulting genotypic redundancy combined with high fecundity may facilitate rapid local adaptation despite high gene flow. Gene flow with preadapted alleles from warmer climates may promote adaptation and migration at the leading edge, while populations at the rear will likely face extirpation. Widespread species with large populations and high fecundity are likely to persist and adapt, but will likely suffer adaptational lag for a few generations. As all tree species will be suffering lags, interspecific competition may weaken, facilitating persistence under suboptimal conditions. Species with small populations, fragmented ranges, low fecundity, or suffering declines due to introduced insects or diseases should be candidates for facilitated migration.

Species Richness and Evolutionary Niche Dynamics: A Spatial Pattern–Oriented Simulation Experiment

Evolutionary processes underlying spatial patterns in species richness remain largely unexplored, and correlative studies lack the theoretical basis to explain these patterns in evolutionary terms. In this study, we develop a spatially explicit simulation model to evaluate, under a pattern-oriented modeling approach, whether evolutionary niche dynamics (the balance between niche conservatism and niche evolution processes) can provide a parsimonious explanation for patterns in species richness. We model the size, shape, and location of species' geographical ranges in a multivariate heterogeneous environmental landscape by simulating an evolutionary process in which environmental fluctuations create geographic range fragmentation, which, in turn, regulates speciation and extinction. We applied the model to the South American domain, adjusting parameters to maximize the correspondence between observed and predicted patterns in richness of about 3,000 bird species. Predicted spatial patterns, which closely resemble observed ones (r2=0.795), proved sensitive to niche dynamics processes. Our simulations allow evaluation of the roles of both evolutionary and ecological processes in explaining spatial patterns in species richness, revealing the enormous potential of the link between ecology and historical biogeography under integrated theoretical and methodological frameworks.

The cane toad's (Chaunus [Bufo] marinus) increasing ability to invade Australia is revealed by a dynamically updated range model

Invasive species threaten biological diversity throughout the world. Understanding the dynamics of their spread is critical to mitigating this threat. In Australia, efforts are underway to control the invasive cane toad (Chaunus [Bufo] marinus). Range models based on their native bioclimatic envelope suggest that the cane toad is nearing the end of its invasion phase. However, such models assume a conserved niche between native and invaded regions and the absence of evolution to novel habitats. Here, we develop a dynamically updated statistical model to predict the growing extent of cane toad range based on their current distribution in Australia. Results demonstrate that Australian cane toads may already have the ability to spread across an area that almost doubles their current range and that triples projections based on their native distribution. Most of the expansion in suitable habitat area has occurred in the last decade and in regions characterized by high temperatures. Increasing use of extreme habitats may indicate that novel ecological conditions have facilitated a broader realized niche or that toad populations at the invasion front have evolved greater tolerance to extreme abiotic conditions. Rapid evolution to novel habitats combined with ecological release from native enemies may explain why some species become highly successful global invaders. Predicting species ranges following invasion or climate change may often require dynamically updated range models that incorporate a broader realization of niches in the absence of natural enemies and evolution in response to novel habitats.

Genetic differentiation across a latitudinal gradient in two co-occurring butterfly species: revealing population differences in a context of climate change

Genetic differentiation within a species' range is determined by natural selection, genetic drift, and gene flow. Selection and drift enhance genetic differences if populations are sufficiently isolated, while gene flow precludes differentiation and local adaptation. Over large geographical areas, these processes can create a variety of scenarios, ranging from admixture to a high degree of population differentiation. Genetic differences among populations may signal functional differences within a species' range, potentially leading to population or ecotype-specific responses to global change. We investigated differentiation within the geographical range of two butterfly species along a broad latitudinal gradient. This gradient is the primary axis of climatic variation, and many ecologists expect populations at the poleward edge of this gradient to expand under climate change. Our study species inhabit a shared ecosystem and differ in body size and resource specialization; both also find their poleward range limit on an island. We find evidence for divergence of peripheral populations from the core in both taxa, suggesting the potential for genetic distinctiveness at the leading edge of climate change. We also find differences between the species in the extent of peripheral differentiation with the smaller and more specialized species showing greater population divergence (microsatellites and mtDNA) and reduced gene flow (mtDNA). Finally, gene flow estimates in both species differed strongly between two marker types. These findings suggest caution in assuming that populations are invariant across latitude and thus will respond as a single ecotype to climatic change.

Species’ Borders and Dispersal Barriers

Range limits of species are determined by combined effects of physical, historical, ecological, and evolutionary forces. We consider a subset of these factors by using spatial models of competition, hybridization, and local adaptation to examine the effects of partial dispersal barriers on the locations of borders between similar species. Prompted by results from population genetic models and biogeographic observations, we investigate the conditions under which species' borders are attracted to regions of reduced dispersal. For borders maintained by competition or hybridization, we find that dispersal barriers can attract borders whose positions would otherwise be either neutrally stable or moving across space. Borders affected strongly by local adaptation and gene flow, however, are repelled from dispersal barriers. These models illustrate how particular biotic and abiotic factors may combine to limit species' ranges, and they help to elucidate mechanisms by which range limits of many species may coincide.

Testing the role of interspecific competition in the evolutionary origin of elevational zonation: an example with Buarremon brush-finches (Aves, Emberizidae) in the neotropical mountains

Interspecific competition might drive the evolution of ecological niches and result in pairs of formerly competing species segregating along ecological gradients following a process of character displacement. This mechanism has been proposed to account for replacement of related species along gradients of elevation in many areas of the world, but the fundamental issue of whether competition is responsible for the origin of elevational replacements has not been tested. To test hypotheses about the role of interspecific competition in the origin of complementary elevational ranges, I combined molecular phylogenetics, phylogeography, and population genetic analyses on Buarremon torquatus and B. brunneinucha (Aves, Emberizidae), whose patterns of elevational distribution suggest character displacement or ecological release. The hypothesis that elevational distributions in these species changed in opposite directions as a result of competition is untenable because: (1) a historical expansion of the range of B. brunneinucha into areas occupied by B. torquatus was not accompanied by a shift in the elevational range of the former species; (2) when B. brunneinucha colonized the range of B. torquatus, lineages of the latter distributions had already diverged; and (3) historical trends in effective population size do not suggest populations with elevational ranges abutting those of putative competitors have declined as would be expected if competition caused range contractions. However, owing to uncertainty in coalescent estimates of historical population sizes, the hypothesis that some populations of B. torquatus have declined cannot be confidently rejected, which suggests asymmetric character displacement might have occurred. I suggest that the main role of competition in elevational zonation may be to act as a sorting mechanism that allows the coexistence along mountain slopes only of ecologically similar species that differ in elevational distributions prior to attaining sympatry. The contrasting biogeographic histories of B. brunneinucha and B. torquatus illustrate how present-day ecological interactions can have recent origins, and highlights important challenges for testing the hypothesis of character displacement in the absence of data on population history and robust reconstructions of the evolution of traits and geographic ranges.

Gene flow reverses an adaptive cline in a coevolving host-parasitoid interaction

Many natural populations are characterized by clinal patterns of adaptation, but it is unclear how gene flow and environmental gradients interact to drive such clines. We addressed this question by directly manipulating dispersal and productivity in an experimental landscape containing a microbial parasitoid, the bacteriophage T7, and its host, the bacterium Escherichia coli. We observed that the adaptation of parasitoids increased on hosts originating from lower-productivity communities in the absence of gene flow. However, adaptation decreased along the same productivity gradient with experimentally imposed gene flow of the host and parasitoid. This occurred despite relatively low rates of gene flow.

Resource-based adaptive divergence in the freshwater fish Telmatherina from Lake Matano, Indonesia

Adaptive radiations are an important source of biodiversity, but resolving which ecological pressures seed these processes in natural systems remains difficult. Here the adaptive radiation among Telmatherina, a genus of freshwater fish endemic to an ancient lake in central Sulawesi, Indonesia, was examined to determine its causal root. We demonstrate that all Telmatherina in this lake can be categorized into three lineages each possessing specialized skull shapes and pharyngeal jaw bones allowing them to exploit different resources. These data demonstrate a natural example of how resource partitioning has likely initiated adaptive radiation in a resource limited environment.

Limits to evolution at range margins: when and why does adaptation fail?

What stops populations expanding into new territory beyond the edge of a range margin? Recent models addressing this problem have brought together population genetics and population ecology, and some have included interactions among species at range edges. Here, we review these models of adaptation at environmental or parapatric margins, and discuss the contrasting effects of migration in either swamping local adaptation, or supplying the genetic variation that is necessary for adaptation to continue. We illustrate how studying adaptation at range margins (both with and without hybridization) can provide insight into the genetic and ecological factors that limit evolution more generally, especially in response to current rates of environmental change.

Evolving metacommunities: toward an evolutionary perspective on metacommunities

The metacommunity framework predicts that local coexistence depends on the outcome of local species interactions and regional migration. In analogous fashion, spatial structure among populations can shape species interactions through evolutionary mechanisms. Yet, most metacommunity theories assume that populations do not evolve. Here, we evaluate how evolution shapes local species coexistence and exclusion within the multiscale and multispecies context embodied by the metacommunity framework. In general, coexistence in joint ecological-evolutionary models requires low to intermediate dispersal rates that can promote maintenance of both regional species and genetic diversity. These conditions support a set of key mechanisms that modify patterns of species coexistence including local adaptation, gene storage effects, genetic rescue effects, spatial genetic subsidies, and metacommunity evolution. Multispecies extensions indicate that correlated selection can further alter the outcome of interspecific interactions depending on the magnitude and direction of correlations and shape of fitness trade-offs. We suggest that an evolving metacommunity perspective has the potential to generate novel predictions about community structure and function by incorporating the genetic and species diversity that characterize natural communities. In adopting such a perspective, we seek to facilitate understanding about the interactions between evolutionary and metacommunity dynamics.

ECOLOGICAL FACTORS LIMITING THE DISTRIBUTION OFGILIA TRICOLORIN A CALIFORNIA GRASSLAND MOSAIC

Recent evolutionary models of range limits emphasize the importance of ecological and demographic factors operating at species' margins. This study aims to establish the ecological context driving population boundaries in Gilia tricolor, a native California annual restricted to distinct habitat patches in the coastal range of California. A transplant experiment in one hillside G. tricolor population examined the roles of competition and soil chemistry as well as litter and biomass accumulation in setting local population boundaries. Results indicate that boundaries are maintained primarily by inhibition of seedling emergence by vegetation and litter, and that upslope and downslope population boundaries are heterogeneous in litter biomass and transplant performance. Consistent emergence inhibition in undisturbed, peripheral sites maintains limits to the distribution of G. tricolor in this population. Fine-scaled ecological heterogeneity and heterogeneous boundary conditions likely play important roles in limiting adaptation and subsequent range expansion at population boundaries in G. tricolor.

Moving beyond assumptions to understand abundance distributions across the ranges of species

The assumption that species are most abundant in the center of their range and decline in abundance toward the range edges has a long history in the ecological literature. This assumption has driven basic and applied ecological and evolutionary hypotheses about the causes of species range limits and their responses to climate change. Here, we review recent studies that are taking biogeographical ecology beyond previously held assumptions by observing populations in the field across large parts of the species range. When these studies combine data on abundance, demographics, organismal physiology, genetics and physical factors, they provide a promising approach for teasing out ecological and evolutionary mechanisms of the patterns and processes underlying species ranges.

Does colonization asymmetry matter in metapopulations?

Despite the considerable evidence showing that dispersal between habitat patches is often asymmetric, most of the metapopulation models assume symmetric dispersal. In this paper, we develop a Monte Carlo simulation model to quantify the effect of asymmetric dispersal on metapopulation persistence. Our results suggest that metapopulation extinctions are more likely when dispersal is asymmetric. Metapopulation viability in systems with symmetric dispersal mirrors results from a mean field approximation, where the system persists if the expected per patch colonization probability exceeds the expected per patch local extinction rate. For asymmetric cases, the mean field approximation underestimates the number of patches necessary for maintaining population persistence. If we use a model assuming symmetric dispersal when dispersal is actually asymmetric, the estimation of metapopulation persistence is wrong in more than 50% of the cases. Metapopulation viability depends on patch connectivity in symmetric systems, whereas in the asymmetric case the number of patches is more important. These results have important implications for managing spatially structured populations, when asymmetric dispersal may occur. Future metapopulation models should account for asymmetric dispersal, while empirical work is needed to quantify the patterns and the consequences of asymmetric dispersal in natural metapopulations.

NICHE EVOLUTION AND ADAPTIVE RADIATION: TESTING THE ORDER OF TRAIT DIVERGENCE

In the course of an adaptive radiation, the evolution of niche parameters is of particular interest for understanding modes of speciation and the consequences for coexistence of related species within communities. We pose a general question: In the course of an evolutionary radiation, do traits related to within-community niche differences (alpha niche) evolve before or after differentiation of macrohabitat affinity or climatic tolerances (beta niche)? Here we introduce a new test to address this question, based on a modification of the method of independent contrasts. The divergence order test (DOT) is based on the average age of the nodes on a tree, weighted by the absolute magnitude of the contrast at each node for a particular trait. The comparison of these weighted averages reveals whether large divergences for one trait have occurred earlier or later in the course of diversification, relative to a second trait; significance is determined by bootstrapping from maximum-likelihood ancestral state reconstructions. The method is applied to the evolution of Ceanothus, a woody plant group in California, in which co-occurring species exhibit significant differences in a key leaf trait (specific leaf area) associated with contrasting physiological and life history strategies. Co-occurring species differ more for this trait than expected under a null model of community assembly. This alpha niche difference evolved early in the divergence of two major subclades within Ceanothus, whereas climatic distributions (beta niche traits) diversified later within each of the subclades. However, rapid evolution of climate parameters makes inferences of early divergence events highly uncertain, and differentiation of the beta niche might have taken place throughout the evolution of the group, without leaving a clear phylogenetic signal. Similar patterns observed in several plant and animal groups suggest that early divergence of alpha niche traits might be a common feature of niche evolution in many adaptive radiations.

Seed and pollen flow in expanding a species’ range

The distinct processes of gene flow via seeds and pollen in hermaphrodite plants provide a biological basis for interpreting their different roles in expanding a species' range. A species' range is primarily expanded through the colonization process by seed dispersal and followed by the joint effects of both seed and pollen flow. Here we examined the effects of seed and pollen flow on shaping a species' distribution in one-dimensional space. Our results demonstrate that pollen flow can enhance range expansion when immigrating genes are adaptive to recipient populations, but can shrink a species' range when immigrating genes are maladaptive. The incompletely purging of maladaptive genes from immigrating pollen grains at the gametophyte stage can reinforce the biological barrier to range expansion. The linkage disequilibria attained by immigrating seeds and pollen grains indirectly amplify the effects of the reaction component and further limit a species' range. The cumulative effect from multiple loci each with a small effect can be substantial on altering a species' range when these genes are maladaptive. These theoretical predictions can help understand the role of pollen flow that is incapable of colonizing new habitats in range expansion.

Maladaptation and mass effects in a metacommunity: consequences for species coexistence

Metacommunity theories predict multispecies coexistence based on the interplay between local species interactions and regional migration. To date, most metacommunity models implicitly assume that evolution can be ignored. Yet empirical studies indicate a substantial potential for contemporary evolution. I evaluate how evolution alters species diversity in a simulated mass-effects (sink-source) metacommunity. Populations inhabiting source habitats became locally adapted, while subordinate competitors became maladapted because of assumed ecological and phenotypic trade-offs between habitats. This maladaptation decreased and leveled relative abundances among subordinate populations. These two effects produced two regions of departure from nonevolutionary predictions. Assuming low proportional migration, maladaptation reduced local species richness via an overall reduction in reproductive rates in sink populations. With intermediate proportional migration, a greater absolute reduction of reproductive rates in intermediate competitors leveled reproductive rates and thereby enhanced local species richness. Although maladaptation is usually viewed as a constraint on species coexistence, simulations suggest that its effects on diversity are manifold and dependent on interpatch migration and community context. Hence, metacommunity predictions often may profit from an evolutionary perspective. Results indicate that modifications of community connectivity, such as might occur during habitat fragmentation, could elicit rapid shifts in communities from regions of high to low biodiversity.

Divergent outcomes of reinforcement speciation: the relative importance of assortative mating and migration modification

Most studies of reinforcement speciation focus on the evolution of assortative mating, but R. A. Fisher argued that migration modification is likely to be a common alternative mechanism. Despite previous models showing that assortative mating and migration modification may both be involved in reinforcement, no one has determined their relative evolutionary importance. This is surprising because understanding the biological conditions favoring these mechanisms may explain why certain pairs of species exhibit abutting, nonoverlapping geographical ranges with habitat fidelity while other pairs coexist in sympatry with sexual isolation. In this article, we explicitly model the evolution of both mechanisms simultaneously. First, we explore how these mechanisms differ in their evolutionary dynamics. Second, we ask how they affect each other's evolution and whether the interaction alters their relative importance in reinforcement. Our results reveal that assortative mating may evolve faster and under a broader range of biological conditions than migration modification. However, direct evolutionary interactions favor migration modification when populations experience strong divergent selection. Depending on the nature of postmating isolation, these mechanisms may either interfere with each other's evolution or coevolve in the same system. These results illustrate the importance of studying multiple mechanisms of speciation simultaneously in future speciation models.

Is Evolution Necessary for Range Expansion? Manipulating Reproductive Timing of a Weedy Annual Transplanted beyond Its Range

Ecologists often consider how environmental factors limit a species' geographic range. However, recent models suggest that geographic distribution also may be determined by a species' ability to adapt to novel environmental conditions. In this study, we empirically tested whether further evolution would be necessary for northern expansion of the weedy annual cocklebur (Xanthium strumarium) in its native North American range. We transplanted seedlings beyond the northern border and photoperiodically manipulated reproductive timing, a trait important for adaptation to shorter growing seasons at higher latitudes within the range, to determine whether further evolution of this trait would result in a phenotype viable beyond the range. Earlier reproductive induction enabled plants to produce mature seeds beyond the range and to achieve a reproductive output similar to those grown within the range. Therefore, evolution of earlier reproduction in marginal populations would be necessary for northward range expansion. This study is the first to empirically show that evolution in an ecologically important trait would enable a species to survive and reproduce beyond its current range. These results suggest that relatively few traits may limit a species' range and that identifying evolutionary constraints on such traits could be important for predicting geographic distribution.

Geographical patterns of adaptation within a species' range: interactions between drift and gene flow

We use individual-based stochastic simulations and analytical deterministic predictions to investigate the interaction between drift, natural selection and gene flow on the patterns of local adaptation across a fragmented species' range under clinally varying selection. Migration between populations follows a stepping-stone pattern and density decreases from the centre to the periphery of the range. Increased migration worsens gene swamping in small marginal populations but mitigates the effect of drift by replenishing genetic variance and helping purge deleterious mutations. Contrary to the deterministic prediction that increased connectivity within the range always inhibits local adaptation, simulations show that low intermediate migration rates improve fitness in marginal populations and attenuate fitness heterogeneity across the range. Such migration rates are optimal in that they maximize the total mean fitness at the scale of the range. Optimal migration rates increase with shallower environmental gradients, smaller marginal populations and higher mutation rates affecting fitness.

Crossing species' range borders: interspecies gene exchange mediated by hybridogenesis

The distribution of species is limited by their ability to adapt to local environments. For adaptation by selection, genetic variability is crucial. As founder effects reduce genetic variability, extension of species' range borders is usually slow due to the reduced probability of successful colonization. However, the range limit might be extended by incorporating locally adapted genes. In western Palaearctic waterfrogs, interspecies hybrids show hemiclonal gametogenesis, are fertile and reproductively mimic one parental species. Genetic analysis, using allozyme loci, shows that they mediate gene exchange between the two parental species. Selection analysis provides evidence for local adaptation of single locus genotypes. This suggests that hybridogenesis presents a process which increases the number of neoform parental genotypes, exposing these to selection, and thereby revealing locally adapted genotypes which are essential for species range expansion.