Models Of Sympatric Speciation Research Articles

Ecological disruptive selection acting on quantitative loci can drive sympatric speciation

The process of speciation generates biodiversity. According to the null model of speciation, barriers between populations arise in allopatry, where, prior to biology, geography imposes barriers to gene flow. On the other hand, sympatric speciation requires that the process of speciation happen in the absence of a geographical barrier, where the members of the population have no spatial, temporal barriers. Several attempts have been made to theoretically identify the conditions in which speciation can occur in sympatry. However, these efforts suffer from several limitations. We propose a model for sympatric speciation based on adaptation for resource utilization. We use a genetics-based model to investigate the relative roles of prezygotic and postzygotic barriers, from the context of ecological disruptive selection, sexual selection, and genetic architecture, in causing and maintaining sympatric speciation. Our results show that sexual selection that acts on secondary sexual traits does not play any role in the process of speciation in sympatry and that assortative mating based on an ecologically relevant trait forces the population to show an adaptive response. We also demonstrate that understanding the genetic architecture of the trait under ecological selection is very important and that it is not required for the strength of ecological disruptive selection to be very high in order for speciation to occur in sympatry. Our results provide an insight into the kind of scenarios in which sympatric speciation can be demonstrated in the lab.

Perfect association between spatial swarm segregation and the X-chromosome speciation island in hybridizing Anopheles coluzzii and Anopheles gambiae populations

The sibling species An. coluzzii and An. gambiae s.s. are major malaria vectors thought to be undergoing sympatric speciation with gene flow. In the absence of intrinsic post-zygotic isolation between the two taxa, speciation is thought possible through the association of assortative mating and genomic regions protected from gene flow by recombination suppression. Such genomic islands of speciation have been described in pericentromeric regions of the X, 2L and 3L chromosomes. Spatial swarm segregation plays a major role in assortative mating between sympatric populations of the two species and, given their importance for speciation, genes responsible for such pre-mating reproductive barriers are expected to be protected within divergence islands. In this study 2063 male and 266 female An. coluzzii and An. gambiae s.s. individuals from natural swarms in Burkina Faso, West Africa were sampled. These were genotyped at 16 speciation island SNPs, and characterized as non-hybrid individuals, F1 hybrids or recombinant F1+n backcrossed individuals. Their genotypes at each speciation island were associated with their participation in An. coluzzii and An. gambiae-like swarms. Despite extensive introgression between the two species, the X-island genotype of non-hybrid individuals (37.6%), F1 hybrids (0.1%) and F1+n recombinants (62.3%) of either sex perfectly associated to each swarm type. Associations between swarm type and the 3L and 2L speciation islands were weakened or broken down by introgression. The functional demonstration of a close association between spatial segregation behaviour and the X speciation island lends further support to sympatric speciation models facilitated by pericentric recombination suppression in this important species complex.

Genomic differentiation and local adaptation on a microgeographic scale in a resident songbird.

Elucidating forces capable of driving species diversification in the face of gene flow remains a key goal in evolutionary biology. Song sparrows, Melospiza melodia, occur as 25 subspecies in diverse habitats across North America, are among the continent's most widespread vertebrate species, and are exemplary of many highly variable species for which the conservation of locally adapted populations may be critical to their range-wide persistence. We focus here on six morphologically distinct subspecies resident in the San Francisco Bay region, including three salt-marsh endemics and three residents in upland and riparian habitats adjacent to the Bay. We used reduced-representation sequencing to generate 2,773 SNPs to explore genetic differentiation, spatial population structure, and demographic history. Clustering separated individuals from each of the six subspecies, indicating subtle differentiation at microgeographic scales. Evidence of limited gene flow and low nucleotide diversity across all six subspecies further supports a hypothesis of isolation among locally adapted populations. We suggest that natural selection for genotypes adapted to salt marsh environments and changes in demography over the past century have acted in concert to drive the patterns of diversification reported here. Our results offer evidence of microgeographic specialization in a highly polytypic bird species long discussed as a model of sympatric speciation and rapid adaptation, and they support the hypothesis that conserving locally adapted populations may be critical to the range-wide persistence of similarly highly variable species.

Sympatric speciation based on pure assortative mating

Although geographic isolation has been shown to play a key role in promoting reproductive isolation, it is now believed that speciation can also happen in sympatry and with considerable gene flow. Here we present a model of sympatric speciation based on assortative mating that does not require a genetic threshold for reproduction, i.e., that does not directly associate genetic differences between individuals with reproductive incompatibilities. In the model individuals mate with the most similar partner in their pool of potential mates, irrespective of how dissimilar it might be. We show that assortativity alone can lead to the formation of clusters of genetically similar individuals. The absence of a minimal genetic similarity for mating implies the constant generation of hybrids and brings up the old problem of species definition. Here, we define species based on clustering of genetically similar individuals but allowing genetic flow among different species. We show that the results obtained with the present model are in good agreement with empirical data, in which different species can still reproduce and generate hybrids.

Neutral and niche forces as drivers of species selection

The evolutionary and ecological processes behind the origin of species are among the most fundamental problems in biology. In fact, many theoretical hypothesis on different type of speciation have been proposed. In particular, models of sympatric speciation leading to the formation of new species without geographical isolation, are based on the niche hypothesis: the diversification of the population is induced by the competition for a limited set of available resources. Interestingly, neutral models of evolution have shown that stochastic forces are sufficient to generate coexistence of different species. In this work, we put forward this dichotomy within the context of species formation, studying how neutral and niche forces contribute to sympatric speciation in a model ecosystem. In particular, we study the evolution of a population of individuals with asexual reproduction whose inherited characters or phenotypes are specified by both niche-based and neutral traits. We analyze the stationary state of the dynamics, and study the distribution of individuals in the whole phenotypic space. We show, both numerically and analytically, that there is a non-trivial coupling between neutral and niche forces induced by stochastic effects in the evolution of the population allowing the formation of clusters, that is, species in the phenotypic space. Remarkably, our framework can be generalized also to sexual reproduction or other type of population dynamics.

Sympatric speciation in the Simulium arcticum s. l. complex (Diptera: Simuliidae): The Rothfels model updated.

We tested the Rothfels sympatric speciation model for black flies by comparing all available data for sex‐chromosome diversity with the geographic locations of larval collection sites within the Simulium arcticum complex of black flies (Diptera: Simuliidae). Five separate data sets equaling about 20,000 larvae were included from throughout the geographic range of this complex. We record a total of 31 taxa having unique sex chromosomes, all of which demonstrate linkage disequilibrium with most taxa sharing autosomal polymorphisms. All siblings share portions of their distributions with S. negativum, the presumed oldest member of the complex. Twenty‐one of 22 cytotypes have distributions within the ranges of siblings thus supporting the sympatric speciation model of Rothfels. Chromosomally diverse sites may require analysis of as many as 200 larvae to be properly described. There is no effect of any inversions influencing the occurrence of other inversions. Finally, we report a new cytotype, Simulium arcticum IIL‐6, which we originally discovered in Alaska. Aspects of future genomic research are discussed as they relate to the main chromosomal structural/functional tenants of the model.OPEN RESEARCH BADGE This article has earned an Open Data Badge for making publicly available the digitally‐shareable data necessary to reproduce the reported results. The data are available at https://doi.org/10.6084/m9.figshare.7719398

Searching for Sympatric Speciation in the Genomic Era.

Sympatric speciation illustrates how natural and sexual selection may create new species in isolation without geographic barriers. However, recent genomic reanalyses of classic examples of sympatric speciation reveal complex histories of secondary gene flow from outgroups into the radiation. In contrast, the rich theoretical literature on this process distinguishes among a diverse range of models based on simple genetic histories and different types of reproductive isolating barriers. Thus, there is a need to revisit how to connect theoretical models of sympatric speciation and their predictions to empirical case studies in the face of widespread gene flow. Here, theoretical differences among different types of sympatric speciation and speciation-with-gene-flow models are reviewed and summarized, and genomic analysesare proposed for distinguishing which models apply to case studies based on the timing and function of adaptive introgression. Investigating whether secondary gene flow contributed to reproductive isolation is necessary to test whether predictions of theory are ultimately borne out in nature.

Dynamical model for sympatric speciation in an ecological niche.

The speciation phenomenon is the process used by the evolution to allow populations to become distinct species. The speciation is the primary cause of the complexity of the ecological network. Sympatric speciation concerns the rise of a new species from a surviving ancestral species while both continue to inhabit the same ecological niche or geographical region. In sympatric speciation, reproductive isolation evolves within a population in an ecological niche without the aid of geographic barriers. Different models have been proposed for alternative modes of sympatric speciation. The most popular was first put forward by John Maynard Smith in 1966 who suggested that in a given population homozygous individuals may, under particular environmental conditions, have a greater fitness than those with alleles heterozygous for a certain trait, eventually leading to speciation in the population. In this framework we assume an effective description of the speciation process based on a dynamical model for the populations in an ecological system. Our basic assumption is the existence of an ancestral population in an ecological niche that can express two phenotypes. In presence of certain environmental conditions one of the phenotypes has the propensity to separate from the original population in the reproduction process. Then new individuals may give rise to a new species in the ecosystem realizing a sympatric speciation. Due to the finite resources in the niche the populations are continuously competing each other's, and their numerousness fluctuates according to the changes of the environmental conditions. The effect of natural selection is introduced in the model by stochastic perturbations, that decrease the reproduction rate of the populations in the niche. We show some the dynamical properties of the system and we prove the existence of a threshold values in the environmental stress in order to observe the speciation process. We also discuss some biological implications of the model and the validation problem using empirical data.

Speciation and genetic divergence of three species of charr from ancient Lake El'gygytgyn (Chukotka) and their phylogenetic relationships with other representatives of the genusSalvelinus

The diversity of phenotypically different and often reproductively isolated lacustrine forms of charrs of the genus Salvelinus represents a substantial problem for taxonomists and evolutionary biologists. Based on the analysis of variability of ten microsatellite loci and two fragments of mitochondrial DNA (control region and cyt-b gene), the evolutionary history of three charr species from Lake El’gygytgyn was reconstructed, and phylogenetic relationships between the main representatives of the genus were revealed. Three species from Lake El’gygytgyn were strongly reproductively isolated. Long-finned charr described previously as Salvethymus svetovidovi ,a n ancient endemic form in the lake, originated 3.5 Mya (95% Bayesian credible intervals: 1.7, 6.1). Placement of this species in the phylogenetic tree of Salvelinus was not determined strictly, but it should be located in the basal part of the clade Salvelinus alpinus – S. malma species complex. The origin of small-mouth charr S. elgyticus and Boganida charr S. boganidae in Lake El’gygytgyn was related to allopatric speciation. Their ancestors were represented by two glacial lineages of Taranets charr S. alpinus taranetzi from Asia. In Lake El’gygytgyn, these lineages entered into secondary contact postglacially. A revision of the main phylogenetic groups within the Salvelinus alpinus – S. malma complex is conducted. The Boganida charrs from Lakes El’gygytgyn and Lama (Taimyr) belong to different phylogenetic groups of Arctic charr and should not be regarded as a single species S. boganidae. Using the charrs from Lakes El’gygytgyn and Lama as a case study, we show that a model of sympatric speciation, which seemed more probable based on previous empirical evidence, was rejected by other data. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116 ,6 3–85. ADDITIONAL KEYWORDS: microsatellites – mitochondrial DNA – molecular dating – Salvelinus boganidae – Salvelinus elgyticus – Salvethymus svetovidovi – species complex – systematics.

Experimental swap of Anopheles gambiae's assortative mating preferences demonstrates key role of X-chromosome divergence island in incipient sympatric speciation.

Although many theoretical models of sympatric speciation propose that genes responsible for assortative mating amongst incipient species should be associated with genomic regions protected from recombination, there are few data to support this theory. The malaria mosquito, Anopheles gambiae, is known for its sympatric cryptic species maintained by pre-mating reproductive isolation and its putative genomic islands of speciation, and is therefore an ideal model system for studying the genomic signature associated with incipient sympatric speciation. Here we selectively introgressed the island of divergence located in the pericentric region of the X chromosome of An. gambiae s.s. into its sister taxon An. coluzzii through 5 generations of backcrossing followed by two generations of crosses within the introgressed strains that resulted in An. coluzzii-like recombinant strains fixed for the M and S marker in the X chromosome island. The mating preference of recombinant strains was then tested by giving virgin recombinant individuals a choice of mates with X-islands matching and non-matching their own island type. We show through genetic analyses of transferred sperm that recombinant females consistently mated with matching island-type males thereby associating assortative mating genes with the X-island of divergence. Furthermore, full-genome sequencing confirmed that protein-coding differences between recombinant strains were limited to the experimentally swapped pericentromeric region. Finally, targeted-genome comparisons showed that a number of these unique differences were conserved in sympatric field populations, thereby revealing candidate speciation genes. The functional demonstration of a close association between speciation genes and the X-island of differentiation lends unprecedented support to island-of-speciation models of sympatric speciation facilitated by pericentric recombination suppression.

A comparative study of Gaussian mating preference functions: a key element of sympatric speciation models

Simulating the evolution of reproductive isolation under sympatric speciation scenarios is a complex process that requires modelling several phases, including evolution of phenotypes, demography, migration, fitness components and mating preference. The last has been shown to be a key parameter in several simulation studies, allowing the incorporation of assortative mating (premating isolation). Mating preference can be modelled by different mathematical functions but, as far as we know, a formal comparison of those functions has not yet been undertaken. In this work, we briefly review the main functions used in the literature and suggest a new one. In doing so, we also define three basic properties (monotonicity, proportionality and symmetry) that an ideal function should satisfy when generating assortative mating. We simulated several scenarios to compare how all these functions perform based on these properties. We also draw attention to the fact that the existing functions are affected distinctly by changing the scale of the preferred trait value. Some functions remain unaffected by scaling the trait, while in others assortative mating increases proportionally to the trait value. Most of the functions tested did not fulfil all the properties studied, and we find certain flaws in some of them that should be considered before being used in future studies. We provide some general recommendations for using the preference functions in simulation studies, and suggest that an unnoticed scaling effect could have underestimated the chance to obtain speciation under certain scenarios. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 642–657.

Repeated origins of social parasitism in allodapine bees indicate that the weak form of Emery's rule is widespread, yet sympatric speciation remains highly problematic

The evolutionary origins of social parasitism are very unevenly distributed among ants, bees and wasps, but social parasite lineages are frequently close relatives of their host lineages. Two explanations for these relationships have been proposed: (1) initially, social species are more likely to become parasitic on relatively closely related social species, because they share life history, physiological and behavioural traits that allow successful integration within the host colony; and (2) social parasites have evolved directly from their host lineage via sympatric speciation. Comparative approaches, covering multiple origins and intermediate evolutionary stages, are needed to determine which of these possibilities is more likely. We use molecular phylogenetics to examine multiple origins of parasitism in the bee tribe Allodapini. We identify seven origins resulting in obligate social parasitism (inquilinism), one origin of facultative social parasitism, which was followed by subsequent speciation and where both daughter species remained facultatively parasitic, and one case of frequent facultative heterospecific co-nesting that probably represents incipient social parasitism. All host–parasite lineage pairs show strong phylogenetic affinities, but only the case of facultative heterospecific nesting involves true sister species relationships. Our results are consistent with the range of parasitic relationships that are expected under an allopatric model for the origin of social parasitism, but are highly problematic for a sympatric speciation model. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 320–331.

Multiple Wolbachia infections in Rhagoletis pomonella

Rhagoletis pomonella Walsh (Diptera: Tephritidae) is a model species for sympatric speciation through host race formation on apple and hawthorn. The bacterial endosymbiont Wolbachia, a manipulator of arthropod reproduction, has been considered to contribute to speciation in several species. A potential role of Wolbachia in sympatric speciation of R. pomonella remains to be tested despite an earlier detection by PCR. In this study, we isolated Wolbachia from R. pomonella individuals from both host species using multi-locus sequence typing (MLST) and the surface protein wsp. By cloning and sequencing of 311 plasmids, we found sequence types of at least four wPom strains. A complete MLST profile was obtained only for wPom1, whereas MLST loci of the other putative strains were difficult to assign because of multiple infections and low sample numbers. wPom1 occurs in both host races, whereas different sequence types were found at low frequencies only in apple-infesting R. pomonella. This warrants further investigation as it cannot be excluded that Wolbachia plays a part in this model of sympatric speciation.

Effects of genetic architecture on the evolution of assortative mating under frequency-dependent disruptive selection

We consider a model of sympatric speciation due to frequency-dependent competition, in which it was previously assumed that the evolving traits have a very simple genetic architecture. In the present study, we numerically analyze the consequences of relaxing this assumption. First, previous models assumed that assortative mating evolves in infinitesimal steps. Here, we show that the range of parameters for which speciation is possible increases when mutational steps are large. Second, it was assumed that the trait under frequency-dependent selection is determined by a single locus with two alleles and additive effects. As a consequence, the resultant intermediate phenotype is always heterozygous and can never breed true. To relax this assumption, here we add a second locus influencing the trait. We find three new possible evolutionary outcomes: evolution of three reproductively isolated species, a monomorphic equilibrium with only the intermediate phenotype, and a randomly mating population with a steep unimodal distribution of phenotypes. Both extensions of the original model thus increase the likelihood of competitive speciation.

Genetics of male nuptial colour divergence between sympatric sister species of a Lake Victoria cichlid fish

The hypothesis of sympatric speciation by sexual selection has been contentious. Several recent theoretical models of sympatric speciation by disruptive sexual selection were tailored to apply to African cichlids. Most of this work concludes that the genetic architecture of female preference and male trait is a key determinant of the likelihood of disruptive sexual selection to result in speciation. We investigated the genetic architecture controlling male nuptial colouration in a sympatric sibling species pair of cichlid fish from Lake Victoria, which differ conspicuously in male colouration and female mating preferences for these. We estimated that the difference between the species in male nuptial red colouration is controlled by a minimum number of two to four genes with significant epistasis and dominance effects. Yellow colouration appears to be controlled by one gene with complete dominance. The two colours appear to be epistatically linked. Knowledge on how male colouration segregates in hybrid generations and on the number of genes controlling differences between species can help us assess whether assumptions made in simulation models of sympatric speciation by sexual selection are realistic. In the particular case of the two sister species that we studied a small number of genes causing major differences in male colouration may have facilitated the divergence in male colouration associated with speciation.

A model of sympatric speciation through reinforcement

Sympatric speciation, i.e. the evolutionary split of one species intotwo in the same environment, has been a highly troublesomeconcept. It has been a questioned if it is actually possible. Eventhough there have been a number of reported results both in the wildand from controlled experiments in laboratories, those findings areboth hard to get and hard to analyze, or even repeat. In the current studywe propose a mathematical model which addresses the question ofsympatric speciation and the evolution of reinforcement.Our aim has been to capture some of theessential features such as: phenotype, resources, competition,heritage, mutation, and reinforcement, in as simple a way aspossible. Still, the resulting model is not too easy to grasp withpurely analytical tools, so we have also complemented those studieswith stochastic simulations. We present a few results that bothillustrates the usefulness of such a model, but also rises newbiological questions about sympatric speciation and reinforcement inparticular.

Intranidal mating, gyne polymorphism, polygyny, and supercoloniality as factors for sympatric and parapatric speciation in ants

1. The syndrome of intranidal mating, morpho‐ethological gyne polymorphism, and polygyny is found in less than 5% of European ant species, but is considered as an important precondition for sympatric and parapatric speciation.2. This idea is discussed in three cases that represent example of evolution below the species level: (1) the polygynous—polydomous form of Lasius turcicus as a model of how speciation of the supercolonial Lasius neglectus could have taken place; (2) the microgyne form of Myrmica rubra as a model for sympatric speciation of socially parasitic species; and (3) hybridisation and rapid genotype selection in supercolonial wood ants of the Formica rufa group as a model for hybrid speciation.3. The first step of the evolutionary scenario is transition to polygyny followed by development of morpho‐ethological gyne polymorphism: smaller gynes with weaker wing muscles and body reserves shift to intranidal mating and reduce dispersal flight, while larger gynes with bigger wing muscles and body reserves retain nuptial and dispersal flight. This scenario leads to mating place separation and facilitates further genotype divergence.4. There is much less gene flow between conspecific supercolonies of Formica rufa group ants with dominance of intranidal or intracolonial mating than between conspecific monogynous colonies performing a nuptial flight. If two supercolonial species occasionally hybridise and if landscape structure provides some isolation of the resulting hybrid supercolony, very strong selection for genotypes takes place within this cohesive system of thousands of reproductive females eventually leading to novel genotypes and new species.

The youngest split in sympatric schizothoracine fish (Cyprinidae) is shaped by ecological adaptations in a Tibetan Plateau glacier lake

Although new empirical evidence shows that sympatric speciation has occurred in some species, there are few indisputable model organisms for this process of speciation. The two subspecies (Gymnocypris eckloni eckloni and G. e. scoliostomus) of the schizothoracine Gymnocypris fish species complex from a small glacier lake in the Tibetan Plateau, Lake Sunmcuo, fit several of the key characteristics of the sympatric speciation model. We used combined mitochondrial control region sequences and the cytochrome b gene (1894 bp) to address the phylogenetics and population genetics of 232 specimens of G. e. eckloni and G. e. scoliostomus, as well as all of its closely related sister species. We found that: (i) a total of four old lineages were uncovered in the widespread G. e. eckloni, of which only one was shown to be shared with all G. e. scoliostomus individuals and (ii) the new subspecies (G. e. scoliostomus) evolved in Lake Sunmcuo from the ancestral G. e. eckloni population within approximately 0.057 Ma. These two taxa of the species complex are morphologically distinct, and reproductive isolation is further suggested. Ecological disruptive selection based on morphological traits (e.g. mouth cleft characters) and food utilization may be a mechanism of incipient speciation of two sympatric populations within Lake Sunmcuo. This study provides the first genetic evidence for sympatric speciation in the schizothoracine fish.

Happy Birthday, Mr. Darwin

![Figure][1] CREDITS (TOP TO BOTTOM): A. KONINGS; M. LOSOS The diversification and speciation of living organisms are the broad theme for this special section, continuing our celebration of Charles Darwin's 200th birthday. The five Reviews in this section present multiple views on research on diversification at scales ranging from the macroevolutionary to the molecular. Benton (p. [728][2]) examines the extents to which biotic and abiotic factors have shaped species diversity in the fossil record. Gavrilets and Losos (p. 732) use theoretical predictions and empirical data to identify general patterns in the temporal, spatial, and genetic/morphological properties of adaptive radiation. Schluter (p. [737][3]) reviews how research on ecological speciation has shifted in focus from morphological evolution to reproductive isolation, tracing the links between Darwin's ideas and current thinking. Fraser et al. (p. [741][4]) discuss the contentious area of microbial species formation. Finally, using examples from studies of genes and mutations involved in evolutionary change, Stern and Orgogozo (p. 746) illustrate how developmental biology and evolutionary theory might combine to reveal new predictive principles of genetic evolution. The special section is accompanied by evolutionary coverage in all other sections of the magazine. The News section features the second in our series of monthly “origins” essays, on the origins of art and symbolic behavior. The Commentary section includes reviews of new evolutionary books. Science Careers carries a feature on researchers in the museum world, who play a vital role in evolutionary research. In Reports, Krug et al. (p. [767][5]) reveal the legacy of the end-Cretaceous mass extinction for the subsequent diversification of bivalves [see the accompanying Perspective by Crame (p. [720][6])]. With a focus on conservation, Carnaval et al. (p. [785][7]) model spatially explicit evolutionary processes in endemic tree-frog species in the Brazilian Atlantic Forest (the biodiversity hotspot that inspired Darwin during his South American landfall). Forbes et al. (p. 776) reveal how a recent host shift of the fly Rhagoletis pomonella , a model for sympatric speciation, has led to incipient speciation in a parasitoid wasp that attacks the fly. Rowland and Emlen (p. 773) show facultative male trimorphism in dung beetles, a hitherto unsuspected level of intraspecific variation. Tang and Presgraves (p. [779][8]) report a cellular and molecular mechanism of hybrid sterility, which has a key role in speciation in Drosophila . And in Science Express, there is an echo of another of Darwin's central interests: Anderson et al. show that a melanism mutation has been selected for in gray wolves, most likely after a hybridization event with domestic dogs. This is a sample, not a survey. Research on speciation and diversification is itself expanding and diversifying. Evolutionary topics have been covered more frequently in Science in the first decade of the 21st century than in any previous one (and an order of magnitude more than in the years of the Modern Synthesis in the mid-20th century). This reflects not only the continuing efforts to understand and document the selective forces leading to speciation, but also how genetic research is homing in on the molecular and cellular mechanisms that enable diversification to occur. Darwin, we hope, would be thrilled. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1157719 [3]: /lookup/doi/10.1126/science.1160006 [4]: /lookup/doi/10.1126/science.1159388 [5]: /lookup/doi/10.1126/science.1164905 [6]: /lookup/doi/10.1126/science.1169410 [7]: /lookup/doi/10.1126/science.1166955 [8]: /lookup/doi/10.1126/science.1169123

Sequential Sympatric Speciation Across Trophic Levels

A major cause for biodiversity may be biodiversity itself. As new species form, they may create new niches for others to exploit, potentially catalyzing a chain reaction of speciation events across trophic levels. We tested for such sequential radiation in the Rhagoletis pomonella (Diptera: Tephritidae) complex, a model for sympatric speciation via host plant shifting. We report that the parasitic wasp Diachasma alloeum (Hymenoptera: Braconidae) has formed new incipient species as a result of specializing on diversifying fly hosts, including the recently derived apple-infesting race of R. pomonella. Furthermore, we show that traits that differentially adapt R. pomonella flies to their host plants have also quickly evolved and serve as ecological barriers to reproduction, isolating the wasps. Speciation therefore cascades as the effects of new niche construction move across trophic levels.