Parallel speciation: a key to sympatric divergence
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Keywords:
Ecological speciation
Genetic algorithm
Reproductive isolation
Incipient speciation
Disruptive selection
Genetic divergence
Divergence (linguistics)
Although verbal theories of speciation consider landscape changes, ecological speciation is usually modelled in a fixed geographical arrangement. Yet landscape changes occur, at different spatio-temporal scales, due to geological, climatic or ecological processes, and these changes result in repeated divisions and reconnections of populations. We examine the effect of such landscape dynamics on speciation. We use a stochastic, sexual population model with polygenic inheritance, embedded in a landscape dynamics model (allopatry–sympatry oscillations). We show that, under stabilizing selection, allopatry easily generates diversity, but species coexistence is evolutionarily unsustainable. Allopatry produces refuges whose persistence depends on the characteristic time scales of the landscape dynamics. Under disruptive selection, assuming that sympatric speciation is impossible due to Mendelian inheritance, allopatry is necessary for ecological differentiation. The completion of reproductive isolation, by reinforcement, then requires several sympatric phases. These results demonstrate that the succession of past, current and future geographical arrangements considerably influence the speciation process.
Genetic algorithm
Ecological speciation
Disruptive selection
Parapatric speciation
Incipient speciation
Reproductive isolation
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Cichlid
Ecological speciation
Reproductive isolation
Disruptive selection
Assortative mating
Parapatric speciation
Genetic algorithm
Adaptive Radiation
Incipient speciation
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Apparent cases of sympatric speciation may actually be due to micro-allopatric or micro-parapatric speciation. One way to distinguish between these models is to examine the existence and nature of genomic islands of divergence, wherein divergent DNA segments are interspersed with low-divergence segments. Such islands should be rare or absent under micro-allopatric speciation but common in cases of speciation with gene flow. Sympatric divergence of endemic fishes is known from isolated saline, crater, postglacial, and ancient lakes. Two morphologically distinct cyprinid fishes, Gymnocypris eckloni scoliostomus (GS) and G. eckloni eckloni (GE), in a small glacial lake on the Qinghai-Tibet Plateau, Lake Sunmcuo, match the biogeographic criteria of sympatric speciation. In this study, we examined genome-wide variation in 46 individuals from these two groups. The divergence time between the GS and GE lineages was estimated to be 20-60 Kya. We identified 54 large genomic islands (≥100 kb) of speciation, which accounted for 89.4% of the total length of all genomic islands. These islands harboured divergent genes related to olfactory receptors and olfaction signals that may play important roles in food selection and assortative mating in fishes. Although the genomic islands clearly indicated speciation with gene flow and rejected micro-allopatric speciation, they were too large to support the hypothesis of sympatric speciation. Theoretical and recent empirical studies suggested that continual gene flow in sympatry should give rise to many small genomic islands (as small as a few kilobases in size). Thus, the observed pattern is consistent with the extensive evidence on parapatric speciation, in which adjacent habitats facilitate divergent selection but also permit gene flow during speciation. We suggest that many, if not most, of the reported cases of sympatric speciation are likely to be micro-parapatric speciation.
Parapatric speciation
Genetic algorithm
Ecological speciation
Incipient speciation
Disruptive selection
Assortative mating
Genetic divergence
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Among the most debated subjects in speciation is the question of its mode. Although allopatric (geographical) speciation is assumed the null model, the importance of parapatric and sympatric speciation is extremely difficult to assess and remains controversial. Here I develop a novel approach to distinguish these modes of speciation by studying the evolution of reproductive isolation (RI) among taxa. I focus on the Drosophila genus, for which measures of RI are known. First, I incorporate RI into age-range correlations. Plots show that almost all cases of weak RI are between allopatric taxa whereas sympatric taxa have strong RI. This either implies that most reproductive isolation (RI) was initiated in allopatry or that RI evolves too rapidly in sympatry to be captured at incipient stages. To distinguish between these explanations, I develop a new "rate test of speciation" that estimates the likelihood of non-allopatric speciation given the distribution of RI rates in allopatry versus sympatry. Most sympatric taxa were found to have likely initiated RI in allopatry. However, two putative candidate species pairs for non-allopatric speciation were identified (5% of known Drosophila). In total, this study shows how using RI measures can greatly inform us about the geographical mode of speciation in nature.
Parapatric speciation
Genetic algorithm
Reproductive isolation
Ecological speciation
Incipient speciation
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Adaptive speciation with gene flow via the evolution of assortative mating has classically been studied in one of two different scenarios. First, speciation can occur if frequency-dependent competition in sympatry induces disruptive selection, leading to indirect selection for mating with similar phenotypes. Second, if a subpopulation is locally adapted to a specific environment, then there is indirect selection against hybridizing with maladapted immigrants. While both of these mechanisms have been modeled many times, the literature lacks models that allow direct comparisons between them. Here we incorporate both frequency-dependent competition and local adaptation into a single model and investigate whether and how they interact in driving speciation. We report two main results. First, we show that individually, the two mechanisms operate under separate conditions, hardly influencing each other when one of them alone is sufficient to drive speciation. Second, we also find that the two mechanisms can operate together, leading to a third speciation mode in which speciation is initiated by selection against maladapted migrants but completed by within-deme competition in a distinct second phase. While this third mode bears some similarity to classical reinforcement, it is considerably faster, and both newly formed species go on to coexist in sympatry.
Disruptive selection
Genetic algorithm
Assortative mating
Incipient speciation
Ecological speciation
Frequency-dependent selection
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Coyne and Orr found that mating discrimination (premating isolation) evolves much faster between sympatric than allopatric Drosophila species pairs. Their meta-analyses established that this pattern, expected under reinforcement, is common and that Haldane's rule is ubiquitous in Drosophila species divergence. We examine three possible contributors to the reinforcement pattern: intrinsic postzygotic isolation, dichotomized as to whether hybrid males show complete inviability/sterility; host-plant divergence, as a surrogate for extrinsic postzygotic isolation; and X chromosome size, whether roughly 20% or 40% of the genome is X-linked. We focus on "young" species pairs with overlapping ranges, contrasted with allopatric pairs. Using alternative criteria for "sympatry" and tests that compare either level of prezygotic isolation in sympatry or frequency of sympatry, we find no statistically significant effects associated with X chromosome size or our coarse quantifications of intrinsic postzygotic isolation or ecological differentiation. Although sympatric speciation seems very rare in animals, the pervasiveness of the reinforcement pattern and the commonness of range overlap for close relatives indicate that speciation in Drosophila is often not purely allopatric. It remains to determine whether increased premating isolation with sympatry results from secondary contact versus parapatric speciation and what drives this pattern.
Reproductive isolation
Ecological speciation
Parapatric speciation
Incipient speciation
Genetic algorithm
Character displacement
Drosophila pseudoobscura
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Ecological speciation
Genetic algorithm
Reproductive isolation
Incipient speciation
Disruptive selection
Genetic divergence
Divergence (linguistics)
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Abstract Sympatric and parapatric speciation refer to the origin of biological species in the absence of complete geographic isolation between the diverging taxa. Until recently, most biologists believed that geographic isolation was almost universal in the development of species, i.e. most species originated in allopatry. However, new empirical and theoretical studies have shown that speciation may occur despite the diverging populations having adjacent or overlapping geographic ranges and despite on‐going gene flow. Attention in speciation studies has shifted to the mechanisms responsible for reducing gene flow, regardless of the extent of geographic range overlap. Key Concepts: The generation of new species has historically been classified according to geographic context, based on the overlap of different populations. Geographic overlap is usually thought to influence gene flow between populations. Of all possible cases, diversification in sympatry has been considered the most unlikely. Theoretical models and examples from nature have shown sympatric speciation to be possible, but rare. A sympatric stage following allopatric divergence may be more common. Divergence in the presence of gene flow may be common, especially if disruptive selection and/or assortative mating are strong. A modern view of speciation focuses on processes that generate divergence, rather than a strict geographical classification. Speciation is usually the result of complex interactions between different genetic, environmental and geographic processes, and must be understood in terms of this more complicated reality.
Parapatric speciation
Ecological speciation
Genetic algorithm
Disruptive selection
Assortative mating
Incipient speciation
Reproductive isolation
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ABSTRACT Much of what we know about speciation comes from detailed studies of well‐known model systems. Although there have been several important syntheses on speciation, few (if any) have explicitly compared speciation among major groups across the Tree of Life. Here, we synthesize and compare what is known about key aspects of speciation across taxa, including bacteria, protists, fungi, plants, and major animal groups. We focus on three main questions. Is allopatric speciation predominant across groups? How common is ecological divergence of sister species (a requirement for ecological speciation), and on what niche axes do species diverge in each group? What are the reproductive isolating barriers in each group? Our review suggests the following patterns. ( i ) Based on our survey and projected species numbers, the most frequent speciation process across the Tree of Life may be co‐speciation between endosymbiotic bacteria and their insect hosts. ( ii ) Allopatric speciation appears to be present in all major groups, and may be the most common mode in both animals and plants, based on non‐overlapping ranges of sister species. ( iii ) Full sympatry of sister species is also widespread, and may be more common in fungi than allopatry. ( iv ) Full sympatry of sister species is more common in some marine animals than in terrestrial and freshwater ones. ( v ) Ecological divergence of sister species is widespread in all groups, including ~70% of surveyed species pairs of plants and insects. ( vi ) Major axes of ecological divergence involve species interactions (e.g. host‐switching) and habitat divergence. ( vii ) Prezygotic isolation appears to be generally more widespread and important than postzygotic isolation. ( viii ) Rates of diversification (and presumably speciation) are strikingly different across groups, with the fastest rates in plants, and successively slower rates in animals, fungi, and protists, with the slowest rates in prokaryotes. Overall, our study represents an initial step towards understanding general patterns in speciation across all organisms.
Genetic algorithm
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Abstract The process of speciation is the source of biodiversity. The most popularly accepted mode of speciation is allopatric speciation, where geography imposes the initial barrier to gene flow, and then biological barriers come up. On the other hand, sympatric speciation, which was not accepted as a possibility for long, requires that the process of speciation happen in the absence of a geographical barrier, in a well-mixed population. Several attempts have been made to theoretically identify the conditions in which speciation can occur in sympatry, but have several problems associated with them. We propose a model for sympatric speciation based on adaptation for resource utilization. We use this 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. We 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. With this, we provide an insight into the kind of scenarios in which sympatric speciation can be demonstrated in lab.
Disruptive selection
Ecological speciation
Genetic algorithm
Incipient speciation
Assortative mating
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