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13 results for “variation in reproductive isolation”
Data from: Diverse reproductive barriers in hybridising crickets suggests extensive variation in the evolution and maintenance of isolation
Reproductive barriers reduce gene flow between populations and maintain species identities. A diversity of barriers exist, acting before, during and after mating. To understand speciation and coexistence, these barriers need to be quantified and their potential interactions revealed. We use the hybridising field crickets Gryllus bimaculatus and G. campestris as a model to understand the full compliment and relative strength of reproductive barriers. We find that males of both species prefer conspecific females, but the effect is probably too weak to represent a barrier. In contrast, prezygotic barriers caused by females being more attracted to conspecific male song and preferentially mounting and mating with conspecifics are strong and asymmetric. Postzygotic barriers vary in direction; reductions in fecundity and egg viability create selection against hybridisation, but hybrids live longer than pure-bred individuals. Hybrid females show a strong preference for G. bimaculatus songs, which together with a complete lack of hybridisation by G. campestris females, suggests that asymmetric gene flow is likely. For comparison, we review reproductive barriers that have been identified between other Gryllids and conclude that multiple barriers are common. Different species pairs are separated by qualitatively different combinations of barriers, suggesting that reproductive isolation and even the process of speciation itself may vary widely even within closely related groups.
Data from: Wolbachia in the Drosophila yakuba complex: pervasive frequency variation and weak cytoplasmic incompatibility, but no apparent effect on reproductive isolation
Three hybridizing species-the clade ((Drosophila yakuba, D. santomea), D. teissieri) -comprise the yakuba complex in the D. melanogaster subgroup. Their ranges overlap on Bioko and São Tomé, islands off west Africa. All three species are infected with Wolbachia, maternally inherited, endosymbiotic bacteria, best known for manipulating host reproduction to favor infected females. Previous analyses reported no cytoplasmic incompatibility (CI) in these species. However, we discovered that Wolbachia from each species cause intra- and interspecific CI. In D. teissieri, analyses of F1 and backcross genotypes show that both host genotype and Wolbachia variation modulate CI intensity. Wolbachia-infected females seem largely protected from intra- and interspecific CI, irrespective of Wolbachia and host genotypes. Wolbachia do not affect host mating behavior or female fecundity, within or between species. The latter suggests little apparent effect of Wolbachia on premating or gametic RI between host species. In nature, Wolbachia frequencies varied spatially for D. yakuba in 2009, with 76% (N = 155) infected on São Tomé, and only 3% (N = 36) infected on Bioko; frequencies also varied temporally in D. yakuba and D. santomea on São Tomé between 2009 and 2015. These temporal frequency fluctuations could generate asymmetries in interspecific mating success, and contribute to postzygotic RI. However, the fluctuations in Wolbachia frequencies that we observe also suggest that asymmetries are unlikely to persist. Finally, we address theoretical questions that our empirical findings raise about Wolbachia persistence when conditions fluctuate and about the stable coexistence of Wolbachia and host variants that modulate Wolbachia effects.
Data for: Variation in reproductive isolation across a species range
<p>Reproductive isolation is often variable within species, a phenomenon that while largely ignored by speciation studies, can be leveraged to gain insight into the poten- tial mechanisms driving the evolution of genetic incompatibilities. We used experi- mental greenhouse crosses to characterize patterns of reproductive isolation among three divergent genetic lineages of Campanulastrum americanum that occur in close geographic proximity in the Appalachian Mountains. Substantial, asymmetrical repro- ductive isolation for survival due to cytonuclear incompatibility was found among the lineages (up to 94% reduction). Moderate reductions in pollen viability, as well as cytoplasmic male sterility, were also found between some Mountain populations. We then compared these results to previously established patterns of reproductive isola- tion between these Mountain lineages and a fourth, widespread Western lineage to fully characterize reproductive isolation across the complete geographic and genetic range of C. americanum. Reproductive isolation for survival and pollen viability was consistent across studies, indicating the evolution of the underlying genetic incompat- ibilities is primarily determined by intrinsic factors. In contrast, reproductive isolation for germination was only found when crossing Mountain populations with the Western lineage, suggesting the underlying genetic incompatibility is likely influenced by envi- ronmental or demographic differences between the two lineages. Cytoplasmic male sterility was also limited in occurrence, being restricted to a handful of Mountain pop- ulations in a narrow geographic range. These findings illustrate the complexity of spe- ciation by demonstrating multiple, independent genetic incompatibilities that lead to a mosaic of genetic divergence and reproductive isolation across a species range.</p>
Performance tradeoffs and resource availability drive variation in reproductive isolation between sympatrically diverging crossbills
<p>Theoretical models indicate that speciation, especially when the scope for gene flow is great (e.g., sympatric speciation), is most likely when strong performance tradeoffs coincide with reproduction. We tested this classic hypothesis using measures of the strength of three prezygotic reproductive isolating barriers (habitat isolation, reduced immigrant fecundity, and behavioral isolation) between two young (~2,000 yrs) and sympatric red crossbill (<i>Loxia curvirostra</i>) ecotypes. All three isolating barriers increased with increases in performance tradeoffs, with total reproductive isolation varying between 0.72 and 1 (0 represents random mating, 1 represents complete reproductive isolation). Strong tradeoffs led to strong habitat isolation, an inability to breed in the "wrong" habitat, and more assortative flocks, with the latter leading to stronger behavioral isolation. Reproductive isolation decreased as resource availability increased relative to the demands of breeding, with higher resource availabilities eliminating the positive relationship between reproductive isolation and performance tradeoffs. This latter result is consistent with previous work suggesting that increasing resource abundance dampens the effect of strong performance tradeoffs on evolutionary divergence. Because many organisms, with the notable exception of host-specific phytophagous insects, rely on abundant food resources with weak performance tradeoffs while breeding, our results may explain why sympatric speciation is uncommon.</p>
Figure 4 in Reproductive isolation and the causes of speciation rate variation in nature
Figure 4. Some possible relationships between the rate at which lineages evolve reproductive isolation and their rate of speciation. A, direct correspondence, where the evolution of reproductive evolution shows a one-to-one relationship with the macroevolutionary rate of speciation. In this scenario, the evolution of reproductive isolation is the exclusive determinant of macroevolutionary speciation dynamics. B, offset/dampened relationship, where the rate of evolution of reproductive isolation is the dominant control on speciation rates, although speciation rates are lower than predicted by the rate of evolution of reproductive isolation alone. This relationship implies that many populations evolving reproductive isolation fail to persist through deep time. C, decoupled, such that reproductive isolation shows no predictive relationship with macroevolutionary speciation rates. This scenario suggests that reproductive isolation is not the rate-limiting control on the rate of speciation. Adapted from Rabosky (2013).
Figure 3 in Reproductive isolation and the causes of speciation rate variation in nature
Figure 3. Pairwise postzygotic isolation from interspecific crosses of birds as a function of the pairwise genetic distance between them. Results are shown for two major clades (pheasants, Phasianidae; parrots, Psittacidae). For a given level of genetic divergence, pheasants show greater levels of postzygotic isolation than the parrots, indicating that this sort of reproductive isolation accumulates more quickly in pheasants than in parrots. If intrinsic postzygotic isolation (hybrid inviability and sterility) is the dominant control on speciation rates, pheasants should have faster rates of speciation than parrots. Note that relationships are bounded at 0 (all hybrid offspring fully viable and fertile) and 1 (no offspring produced, or all offspring sterile). Lines show fitted linear relationships between reproductive isolation and genetic distance for each clade. Data are from Price & Bouvier (2002) and Gray (1958); analyses are from Rabosky and Matute (2013). For this pair of clades, speciation rates are faster in the clade with faster rates of evolution of reproductive isolation (pheasants: speciation = 0.26 lineages Myr–1; parrots: speciation = 0.22 lineages Myr–1). However, across all birds, these quantities appear to be unrelated.
Figure 1. A in Reproductive isolation and the causes of speciation rate variation in nature
Figure 1. A, per-lineage rates of speciation across a time-calibrated phylogenetic tree of 6670 species of extant birds (67% of the total diversity) as inferred using a statistical model that simultaneously estimates the magnitude of rate variation through time and across lineages. Colours correspond to the instantaneous rate of speciation at each point in the tree. The fastest 5% of rates exceed 0.4 species Myr –1 (dark red). Analysis of evolutionary rates explicitly accounts for extinction, although only speciation rates are shown. B, estimated present-day speciation rates for all 6670 species; these are simply the best estimate of the instantaneous rate of speciation for each tip in the phylogeny. Inset images depict representative birds with fast (western gull, Larus occidentalis) and slow (go-away-bird, Corythaixoides leucogaster) rates of speciation. The phylogenetic dataset is from Jetz et al. (2012) and speciation rate analysis is described in Rabosky & Matute (2013).
Data for: Variation in reproductive isolation across a species range
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Performance tradeoffs and resource availability drive variation in reproductive isolation between sympatrically diverging crossbills
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Data from: Diverse reproductive barriers in hybridising crickets suggests extensive variation in the evolution and maintenance of isolation
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Data from: Wolbachia in the Drosophila yakuba complex: pervasive frequency variation and weak cytoplasmic incompatibility, but no apparent effect on reproductive isolation
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Data from: Male-female genotype interactions maintain variation in traits important for sexual interactions and reproductive isolation
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Figure 2 in Reproductive isolation and the causes of speciation rate variation in nature
Figure 2. Alternative species taxonomies can lead to radically different perspectives on species and speciation. A, time-calibrated phylogeny for stickleback fishes (Gasterosteidae) from Rabosky et al. (2013). Divergence times are broadly congruent with the known fossil history for the genus (Bell, Stewart & Park, 2009). B, hypothetical shape of the stickleback phylogeny if some of the numerous intraspecific morphological variants (open circles) within the threespine stickleback (Gasterosteus aculeatus) are elevated to full species status. Many freshwater stickleback populations are morphologically distinct from the marine ancestral form and potentially warrant recognition as distinct biological species (Moodie & Reimchen, 1976; Mcphail, 1994; Bell, 1995; Nelson, 2006; Reimchen et al., 2013); there are potentially dozens or hundreds of such forms. Bell (1995) suggests that, given the extent of morphological parallelism among divergent forms, '... it is neither practical nor useful to describe the biological species within the G. aculeatus complex as separate taxonomic species'. However, given that many such forms could be recognized as distinct taxonomic species, it is worth considering how their recognition would change our interpretation of speciation rates. Previous studies on speciation rates across fishes (Near et al., 2013; Rabosky et al., 2013) include only information from deep nodes (dark circles). However, recent divergences, almost all of which are no older than the most recent glacial maximum (arrow), are ignored by this approach. For the phylogeny in (A), the maximum likelihood estimate of the speciation rate under a constant-rate birth-death model (Nee et al., 1994) is 0.055 lineages Myr–1. For the phylogeny in (B), which assumes the presence of 30 biological species in 'Gasterosteus aculueatus', all of which have diverged within the past 30 000 years, we infer a speciation rate of 65.4 lineages Myr–1. The number of recently-diverged species spliced into the phylogeny shown in (B) is arbitrary, although it illustrates the profound effects that taxonomic decisions can have on inferences about speciation rate. Understanding the relationship between speciation as studied at population genetic scales (open circles) and macroevolutionary scales (dark circles) is one of the central challenges for the future of speciation research.
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