How sympatric is speciation in the Howea palms of Lord Howe Island?

The two species of the palm genus Howea (Arecaceae) from Lord Howe Island, a minute volcanic island in the Tasman Sea, are now regarded as one of the most compelling examples of sympatric speciation, although this view is still disputed by some authors. Population genetic and ecological data are necessary to provide a more coherent and comprehensive understanding of this emerging model system. Here, we analyse data on abundance, juvenile recruitment, pollination mode and genetic variation and structure in both species. We find that Howea forsteriana is less abundant than Howea belmoreana . The genetic data based on amplified fragment length polymorphisms markers indicate similar levels of variation in the two species, despite the estimated census population size of H. belmoreana being three times larger than that of H. forsteriana . Genetic structure within species is low although some weak isolation by distance is detectable. Gene flow between species appears to be extremely limited and restricted to early-generation hybrids – only three admixed individuals, classified as F2s or first generation backcrosses to a parental species, were found among sampled palms. We conclude that speciation in Howea was indeed sympatric, although under certain strict definitions it may be called parapatric.     

A comparative analysis of the mechanisms underlying speciation on Lord Howe Island

On Lord Howe Island, speciation is thought to have taken place in situ in a diverse array of distantly related plant taxa (Metrosideros, Howea and Coprosma; Proc. Natl Acad. Sci. USA 108 , 2011, 13188). We now investigate whether the speciation processes were driven by divergent natural selection in each genus by examining the extent of ecological and genetic divergence. We present new and extensive, ecological and genetic data for all three genera. Consistent with ecologically driven speciation, outlier loci were detected using genome scan methods. This mechanism is supported by individual‐based analyses of genotype–environment correlations within species, demonstrating that local adaptation is currently widespread on the island. Genetic analyses show that prezygotic isolating barriers within species are currently insufficiently strong to allow further population differentiation. Interspecific hybridization was found in both Howea and Coprosma, and species distribution modelling indicates that competitive exclusion may result in selection against admixed individuals. Colonization of new niches, partly fuelled by the rapid generation of new adaptive genotypes via hybridization, appears to have resulted in the adaptive radiation in Coprosma – supporting the ‘Syngameon hypothesis’.     

被子植物同域物种形成研究进展

同域物种形成是指在缺少地理隔离的情况下分化出新种,相比异域物种形成更为罕见,存在较多的研究空白.该文分析了近十年来与被子植物同域物种形成相关的国内外研究,着重论述同域物种形成的影响因素和种对间的生殖隔离.考虑到历史上的地理隔离难以确定,加之种对间亲缘关系很近,同域物种的判定容易引发争议.其成因可分为生态因素和突变因素:...     

昆虫同域物种形成机制及检验

从生物学、生态和遗传的角度阐述昆虫同域物种形成过程中涉及到的可能性机制。昆虫同域种的分化与作用于同域初始种群的歧化选择密切相关,歧化选择间接导致种群生态特征和遗传特征的分化,促进同域近缘种群间的生殖隔离。同域物种形成的过程中涉及到性状替换、性选择、同型交配等机制。寄主专化型多见于昆虫同域种的分化过程中,一般以植食性昆虫为主。有关昆虫同域物种形成的检验机制有多种,归纳起来主要包括同型交配的检验、遗传漂流的量化、遗传分化程度和连锁不平衡(LD)的检测、杂交后代适合度的估算等。目前发现在许多昆虫种类中存在同域物种形成的可能性,但是有关其隔离机制并没有得到充分的解释。     

Ecological speciation in sympatric palms: 4. Demographic analyses support speciation of Howea in the face of high gene flow

The idea that populations must be geographically isolated (allopatric) to evolve into separate species has persisted for a long time. It is now clear that new species can also diverge despite ongoing genetic exchange, but few accepted cases of speciation in sympatry have held up when scrutinized using modern approaches. Here, we examined evidence for speciation of the Howea palms of Lord Howe Island, Australia, in light of new genomic data. We used coalescence‐based demographic models combined with double digest restriction site associated DNA sequencing of multiple individuals and provide support for previous claims by Savolainen et al. that speciation in Howea did occur in the face of gene flow.     

Mechanisms of rapid sympatric speciation by sex reversal and sexual selection in cichlid fish

Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic models of sexual selection on sex-determining genes associated with color polymorphisms. The models are based on a combination of laboratory experiments and field observations on the ecology, male and female mating behavior, and inheritance of sex-determination and color polymorphisms. The models explain why sex-reversal genes that change males into females tend to be X-linked and associated with novel colors, using the hypothesis of restricted recombination on the sex chromosomes, as suggested by previous theory on the evolution of recombination. The models reveal multiple pathways for rapid sympatric speciation through the origin of novel color morphs with strong assortative mating that incorporate both sex-reversal and suppressor genes. Despite the lack of geographic isolation or ecological differentiation, the new species coexists with the ancestral species either temporarily or indefinitely. These results may help to explain different patterns and rates of speciation among groups of cichlids, in particular the explosive diversification of rock-dwelling haplochromine cichlids.     

Polymorphism in heterogeneous environments,evolution of habitat selection and sympatric speciation: Soft and hard selection models

Summary The adaptation to a variable environment has been studied within soft and hard selection frameworks. It is shown that an epistatically determined habitat preference, following a Markovian process, always leads to the maintenance of an adaptive polymorphism, in a soft selection context. Although local mating does not alter the conditions for polymorphism maintenance, it is shown that, in that case, habitat selection also leads to the evolution of isolated reproductive units within each available habitat. Habitat selection, however, cannot evolve in the total absence of adaptive polymorphism. This represents a theoretical problem for all models assuming habitat selection to be an initially fixed trait, and means that within a soft selection framework, all the available habitats will be exploited, even the less favourable ones.On the other hand, polymorphism cannot be maintained when selection is hard, even when all individuals select their habitat. Here, the evolution of habitat selection does not need any prerequisite polymorphism, and always leads to the exploitation of only one habitat by the most specialized genotype. It appears then that hard selection can account for the existence of empty habitat and for an easier evolution of habitat specialization.     

The genetics and ecology of sympatric speciation: A case study

Mating occurs on the larval host plant in allRhagoletis species (Diptera: Tephritidae). We show how this attribute, when coupled with certain differences in other biological traits, strongly influences the mode of speciation. In species of thesuavis species group, host shifts have never occurred during speciation, and larvae feed in the husks of any walnut species(Juglans spp.), which are highly toxic. Taxa are allopatric or parapatric and exhibit deep phylogenetic nodes suggesting relatively ancient speciation events. Traits responsible for species and mate recognition, particularly in parapatric species, are morphologically distinct and strongly sexually dimorphic. All aspects of their biology, genetics and distribution are consistent with a slow rate of allopatric speciation followed by morphological divergence in secondary contact. In contrast, speciation in thepomonella species group has always involved a shift to a new, usually unrelated, non-toxic host, and all taxa within these groups are sympatric, monophagous and morphologically indistinguishable from one another. Phylogenetic nodes are very shallow, indicating recent sympatric speciation. Sympatric divergence is promoted by genetic variation which allows a portion of the original species to shift to a new habitat or host. Evidence suggests that changes in a few key loci responsible for host selection and fitness on a new host may initiate host shifts. By exploiting different habitats, competition for resources between diverging populations is reduced or avoided. We provide evidence that in phytophagous and parasitic insects sufficient intrinsic barriers to gene flow can evolve between sister populations as they adapt to different habitats or hosts to allow each population to establish independent evolutionary lineages in sympatry.     

Refining the conditions for sympatric ecological speciation

Can speciation occur in a single population when different types of resources are available, in the absence of any geographical isolation, or any spatial or temporal variation in selection? The controversial topics of sympatric speciation and ecological speciation have already stimulated many theoretical studies, most of them agreeing on the fact that mechanisms generating disruptive selection, some level of assortment, and enough heterogeneity in the available resources, are critical for sympatric speciation to occur. Few studies, however, have combined the three factors and investigated their interactions. In this article, I analytically derive conditions for sympatric speciation in a general model where the distribution of resources can be uni‐ or bimodal, and where a parameter controls the range of resources that an individual can exploit. This approach bridges the gap between models of a unimodal continuum of resources and Levene‐type models with discrete resources. I then test these conditions against simulation results from a recently published article (Thibert‐Plante & Hendry, 2011, J. Evol. Biol. 24 : 2186–2196) and confirm that sympatric ecological speciation is favoured when (i) selection is disruptive (i.e. individuals with an intermediate trait are at a local fitness minimum), (ii) resources are differentiated enough and (iii) mating is assortative. I also discuss the role of mating preference functions and the need (or lack thereof) for bimodality in resource distributions for diversification.     

Ecological speciation in sympatric palms: 3. Genetic map reveals genomic islands underlying species divergence in Howea

Although it is now widely accepted that speciation can occur in the face of continuous gene flow, with little or no spatial separation, the mechanisms and genomic architectures that permit such divergence are still debated. Here, we examined speciation in the face of gene flow in the Howea palms of Lord Howe Island, Australia. We built a genetic map using a novel method applicable to long‐lived tree species, combining it with double digest restriction site–associated DNA sequencing of multiple individuals. Based upon various metrics, we detected 46 highly differentiated regions throughout the genome, four of which contained genes with functions that are particularly relevant to the speciation scenario for Howea, specifically salt and drought tolerance.     

Little evidence for sympatric speciation in island birds

It has been suggested that the presence of sister species in small circumscribed areas, such as isolated lakes or islands, might imply that these species originated sympatrically. To investigate this possibility in birds, we searched for endemic, congeneric species on isolated islands in the ocean. Among 46 islands and small archipelagos chosen because they contain at least one species of endemic land bird, we identified seven pairs of endemic congeners (excluding flightless rails). Of these seven, only four pairs are potentially sister species and thus possible candidates for sympatric speciation. However, three of these four pairs have always been considered the results of double invasion from a mainland source (in two of these cases, molecular-phylogenetic work has either confirmed a double invasion or is ambiguous). The one remaining pair may have speciated allopatrically on a small archipelago. Additional phylogenetic studies are required to understand these cases, and our results should also be considered in light of the large number of island-bird extinctions in historic time. We conclude that, at present, there is little evidence for sympatric speciation in island birds.     

Female mate-choice behavior and sympatric speciation

Many models have investigated how the process of speciation may occur in sympatry. In these models, individuals are either asexual or mate choice is determined by very simple rules. Females, for example, may be assumed either to compare their phenotype to that of a potential mate, preferring to mate with similar males (phenotype matching), or to possess preference genes that determine which male phenotype they prefer. These rules often do not reflect the mate-choice rules found in empirical studies. In this paper, we compare these two modes of female choice with various types of sexual imprinting. We examine the efficacy of different mate-choice behavior in causing divergence in male traits under simple deterministic one-locus population genetic models as well as under polygenic, individual-based simulations based on the models of Dieckmann and Doebeli (1999). We find that the inheritance mechanism of mate choice can have a large effect on the ease of sympatric speciation. When females imprint on their mothers, the result of the model is similar to phenotype matching, where speciation can occur fairly easily. When females imprint on their fathers or imprint obliquely, speciation becomes considerably less likely. Finally, when females rely on preference genes, male trait evolution occurs easily, but the correlation between trait and preference can be weak, and interpreting these results as speciation may be suspect.     

Factors influencing progress toward sympatric speciation

Many factors could influence progress towards sympatric speciation. Some of the potentially important ones include competition, mate choice and the degree to which alternative sympatric environments (resources) are discrete. What is not well understood is the relative importance of these different factors, as well as interactions among them. We use an individual-based numerical model to investigate the possibilities. Mate choice was modelled as the degree to which male foraging traits influence female mate choice. Competition was modelled as the degree to which individuals with different phenotypes compete for portions of the resource distribution. Discreteness of the environment was modelled as the degree of bimodality of the underlying resource distribution. We find that strong mate choice was necessary, but not sufficient, to cause sympatric speciation. In addition, sympatric speciation was most likely when the resource distribution was strongly bimodal and when competition among different phenotypes was intermediate. Even under these ideal conditions, however, sympatric speciation occurred only a fraction of the time. Sympatric speciation owing to competition on unimodal resource distributions was also possible, but much less common. In all cases, stochasticity played an important role in determining progress towards sympatric speciation, as evidenced by variation in outcomes among replicate simulations for a given set of parameter values. Overall, we conclude that the nature of competition is much less important for sympatric speciation than is the nature of mate choice and the underlying resource distribution. We argue that an increased understanding of the promoters and inhibitors of sympatric speciation is best achieved through models that simultaneously evaluate multiple potential factors.     

Sympatric speciation by sexual selection alone is unlikely

According to Darwin, sympatric speciation is driven by disruptive, frequency-dependent natural selection caused by competition for diverse resources. Recently, several authors have argued that disruptive sexual selection can also cause sympatric speciation. Here, we use hypergeometric phenotypic and individual-based genotypic models to explore sympatric speciation by sexual selection under a broad range of conditions. If variabilities of preference and display traits are each caused by more than one or two polymorphic loci, sympatric speciation requires rather strong sexual selection when females exert preferences for extreme male phenotypes. Under this kind of mate choice, speciation can occur only if initial distributions of preference and display are close to symmetric. Otherwise, the population rapidly loses variability. Thus, unless allele replacements at very few loci are enough for reproductive isolation, female preferences for extreme male displays are unlikely to drive sympatric speciation. By contrast, similarity-based female preferences that do not cause sexual selection are less destabilizing to the maintenance of genetic variability and may result in sympatric speciation across a broader range of initial conditions. Certain groups of African cichlids have served as the exclusive motivation for the hypothesis of sympatric speciation by sexual selection. Mate choice in these fishes appears to be driven by female preferences for extreme male phenotypes rather than similarity-based preferences, and the evolution of premating reproductive isolation commonly involves at least several genes. Therefore, differences in female preferences and male display in cichlids and other species of sympatric origin are more likely to have evolved as isolating mechanisms under disruptive natural selection.     

A deterministic genetic model for sympatric speciation by sexual selection

A deterministic haploid genetic model confirms and explores in more detail the results of our previous individual-based simulation model for sympatric speciation by sexual selection. With the deterministic model, we are able to elucidate parameter dependence by phase plane analysis. We clarify how and why sympatric speciation by sexual selection can happen in a number of ways: (1) Female preferences for or against particular types of males have different effects. Whereas the former affects how readily speciation is invoked, the latter changes the stability of speciation equilibrium. (2) When there is no cost on male ornamentations, speciation is triggered regardless of initial haplotype frequencies if sufficient female preference is provided. (3) There exists a threshold for female initial frequencies for speciation to be invoked, but male initial frequencies have little effect. (4) A small cost on female mate choice does not cancel speciation, but when large, it greatly reduces the possibility of speciation.     

Gene flow between sexual and asexual strains of parasitic wasps: a possible case of sympatric speciation caused by a parthenogenesis-inducing bacterium

Sympatric speciation is strictly defined as the emergence of two species from a population in which mating has been random with respect to the place of birth of the mating partners. Mathematical models have shown that sympatric speciation is possible, but very few examples have been documented in nature. In this article, we demonstrate that arrhenotokous and thelytokous strains of a parasitic wasp, Neochrysocharis formosa, speciated sympatrically through infection by a symbiotic bacterium Rickettsia for the following reasons: First, Rickettsia infection was detected in all of the thelytokous strains collected throughout Japan. Second, the arrhenotokous and thelytokous strains have been collected sympatrically. Third, crossing experiments between the two strains did not result in fertilized offspring. In addition, the two strains were genetically isolated at the nuclear and mitochondrial genes. Fourth, the two strains showed a sister relationship in nuclear 28S rRNA gene. Finally, thelytokous females treated with antibiotics produced Rickettsia-free male offspring of the same reproductive form as arrhenotokous females indicating that the thelytokous strain could have speciated sympatrically from an individual of the arrhenotokous strain.     

Alternative adaptations, sympatric speciation and the evolution of parasitic, inquiline ants

Inquiline ant species are workerless social parasites whose queens reproduce in colonies of other species alongside the host queens. Inquilines arise either when one non-parasitic species evolves into an inquiline parasite of another non-parasitic species (the interspecific hypothesis), or by the speciation of intraspecific inquilines from their host stock (the intraspecific hypothesis): it is unlikely that inquilines evolve from other forms of social parasite. This paper reviews the evidence for and against the inter-and intraspecific hypotheses. All inquilines are close phylogenetic relatives of their host species (loose ‘Emery's rule’), and some are their host's closest relative (strict ‘Emery's rule’). A problem for the interspecific hypothesis is how to explain the strict Emery's rule, because phylogenetic constraints on host choice are probably quite weak. By contrast, the intraspecific hypothesis has difficulty accounting for the parasites' sympatric reproductive isolation. Facultative polygyny, in which queens may found colonies alone or by adoption into an existing multi-queen colony, should promote the evolution of small intraspecific inquilines. This is because small colony-founding queens should preferentially seek adoption, which provides the opportunity to produce a sexual-only brood. We suggest that microgynes, i.e. miniature queens found in some polygynous ants, represent such parasites. We review the evidence that inquiline species have evolved intraspecifically from microgynes in Myrmica ants. The coexistence within a species of a monogynous (singly-queened) and a polygynous form is probably a phenomenon usually unconnected with inquiline evolution. The reproductive isolation of intraspecific inquilines plausibly arises from divergent breeding behaviour associated with the parasites' small size. Such divergence could involve either a temporal separation in mating episodes, with small parasites maturing early, or a spatial separation, with small males being sexually-selected to mate near the nest with small queens seeking adoption, instead of in mating aggregations. We conclude that inquiline species strictly following Emery's rule could have evolved by the intraspecific route. If so, such species provide evidence for West-Eberhard's “alternative adaptation” hypothesis that between-species diversity frequently stems from diversity within species. They also represent likely cases of sympatric speciation. We suggest work on the parasites' phytogeny, genetics, behaviour and mating biology to test these conclusions further.     

Ecological speciation in sympatric palms: 2. Pre‐ and post‐zygotic isolation

We evaluated reproductive isolation in two species of palms (Howea) that have evolved sympatrically on Lord Howe Island (LHI, Australia). We estimated the strength of some pre‐ and post‐zygotic mechanisms in maintaining current species boundaries. We found that flowering time displacement between species is consistent across in and ex situ common gardens and is thus partly genetically determined. On LHI, pre‐zygotic isolation due solely to flowering displacement was 97% for Howea belmoreana and 80% for H. forsteriana; this asymmetry results from H. forsteriana flowering earlier than H. belmoreana and being protandrous. As expected, only a few hybrids (here confirmed by genotyping) at both juvenile and adult stages could be detected in two sites on LHI, in which the two species grow intermingled (the Far Flats) or adjacently (Transit Hill). Yet, the distribution of hybrids was different between sites. At Transit Hill, we found no hybrid adult trees, but 13.5% of younger palms examined there were of late hybrid classes. In contrast, we found four hybrid adult trees, mostly of late hybrid classes, and only one juvenile F1 hybrid in the Far Flats. This pattern indicates that selection acts against hybrids between the juvenile and adult stages. An in situ reciprocal seed transplant between volcanic and calcareous soils also shows that early fitness components (up to 36 months) were affected by species and soil. These results are indicative of divergent selection in reproductive isolation, although it does not solely explain the current distribution of the two species on LHI.