@article{4305, abstract = {The common shrew (Sorex araneus) is subdivided into several karyotypic races in Britain. Two of these races meet near Oxford to form the "Oxford-Hermitage" hybrid zone. We present a model which describes this system as a "tension zone," i.e., a set of clines maintained by a balance between dispersal and selection against chromosomal heterozygotes. The Oxford and Hermitage races differ by Robertsonian fusions with monobrachial homology (kq, no versus ko), and so F1 hybrids between them would have low fertility. However, the acrocentric karyotype is found at high frequency within the hybrid zone, so that complex Robertsonian heterozygotes (kq no/q ko n) are replaced by more fertile combinations, such as (kq no/k q n o). This suggests that the hybrid zone has been modified so as to increase hybrid fitness. Mathematical analysis and simulation show that, if selection against complex heterozygotes is sufficiently strong relative to selection against simple heterozygotes, acrocentrics increase, and displace the clines for kq and no from the cline for ko. Superimposed on this separation is a tendency for the hybrid zone to move m favor of the Oxford (kq no) race. We compare the model with estimates of linkage disequilibrium and cline shape made from field data.}, author = {Hatfield, Todd and Barton, Nicholas H and Searle, Jeremy}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, number = {4}, pages = {1129 -- 1145}, publisher = {Wiley-Blackwell}, title = {{A model of a hybrid zone between two chromosomal races of the common shrew (Sorex araneus)}}, doi = {10.1111/j.1558-5646.1992.tb00624.x}, volume = {46}, year = {1992}, } @article{4308, author = {Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, number = {2}, pages = {551 -- 557}, publisher = {Wiley-Blackwell}, title = {{On the spread of new gene combinations in the third phase of Wright's shifting balance}}, volume = {46}, year = {1992}, } @article{3646, abstract = {We compare the pattern of morphological and electrophoretic variation in the hybrid zone between Bombina bombina and B. variegata across two transects: one near Cracow and one 200 km away, near Przemysl in southeastern Poland. Morphological variation across the Przemysl transect had been surveyed more than 50 years ago; though we found a significant shift at one site, there is no evidence for gross movement over this period. Morphological and electrophoretic changes coincide, and the average shape of the clines is the same across both transects. At the center, most of the change in frequency of six diagnostic allozymes occurs within w = 6.05 km (2-unit support limits 5.56-6.54 km). These steep gradients are generated not by selection on the allozymes themselves, but by associations with other loci: though these markers are unlinked, they are in strong linkage disequilibrium with each other [R = D/ = 0.22 (0.15-0.29) at the center]. Disequilibria are broken up as alleles diffuse away from the zone and flow into the new genetic background. The net barrier to the flow of genes from bombina into variegata, which is generated by these disequilibria, is B = 51 (22-81) km. The fitness of hybrids must be substantially reduced to produce such a barrier [W̄H/W̄P = 0.58 (0.54-0.68)], and this selection must be spread over many loci [N = 55 (26-88)]. Alleles introgress significantly less far than would be expected from the age of the zone and the estimated dispersal rate [σ = 0.99 (0.82-1.14) km gen.-1/2]: this implies selection of se = 0.37 (0.15-0.58)% on the enzymes themselves. There is weak but significant linkage disequilibrium well away from the center of the zone; this, together with the presence of parental and F1 genotypes, suggests some long-range migration. However, such migration is not likely to cause significant introgression. }, author = {Szymura, Jacek and Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution}, number = {2}, pages = {237 -- 261}, publisher = {Wiley-Blackwell}, title = {{The genetic structure of the hybrid zone between the fire-bellied toads Bombina bombina and B. variegata: comparisons between transects and between loci}}, doi = {10.1111/j.1558-5646.1991.tb04400.x}, volume = {45}, year = {1991}, } @article{3648, abstract = {We investigate the probability of fixation of a chromosome rearrangement in a subdivided population, concentrating on the limit where migration is so large relative to selection (m ≫ s) that the population can be thought of as being continuously distributed. We study two demes, and one- and two-dimensional populations. For two demes, the probability of fixation in the limit of high migration approximates that of a population with twice the size of a single deme: migration therefore greatly reduces the fixation probability. However, this behavior does not extend to a large array of demes. Then, the fixation probability depends primarily on neighborhood size (Nb), and may be appreciable even with strong selection and free gene flow (≈exp(-B·Nb) in one dimension, ≈exp(-B\cdotNb) in two dimensions). Our results are close to those for the more tractable case of a polygenic character under disruptive selection.}, author = {Barton, Nicholas H and Rouhani, Shahin}, issn = {1558-5646}, journal = {Evolution}, number = {3}, pages = {499 -- 517}, publisher = {Wiley-Blackwell}, title = {{The probability of fixation of a new karyotype in a continuous population}}, doi = {10.1111/j.1558-5646.1991.tb04326.x}, volume = {45}, year = {1991}, } @article{3653, abstract = {Frequency-dependent selection on warning color can maintain narrow hybrid zones between unpalatable prey taxa. To measure such selection, we transferred marked Heliconius erato (Lepidoptera: Nymphalidae) in both directions across a 10-km-wide hybrid zone between Peruvian races differing in color pattern. These experimental H. erato were released at four sites, along with control H. erato of the phenotype native to each site. Survival of experimental butterflies was significantly lower than that of controls at two sites and overall. Most selection, measured as differences in survival, occurred soon after release. Selection against foreign morphs was 52% (confidence limits: 25-71%) and was probably due to bird attacks on unusual warning-color morphs (more than 10% of the recaptures had beak marks). Since only three major loci determine the color-pattern differences, this suggests an average selection coefficient of 0.17 per locus, sufficient to maintain the narrow clines in H. erato.}, author = {Mallet, James and Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution}, number = {2}, pages = {421 -- 431}, publisher = {Wiley-Blackwell}, title = {{Strong natural selection in a warning color hybrid zone}}, doi = {10.2307/2409217 }, volume = {43}, year = {1989}, } @article{4309, abstract = {Three methods for estimating the average level of gene flow in natural population are discussed and compared. The three methods are FST, rare alleles, and maximum likelihood. All three methods yield estimates of the combination of parameters (the number of migrants [Nm] in a demic model or the neighborhood size [4πDσ2] in a continuum model) that determines the relative importance of gene flow and genetic drift. We review the theory underlying these methods and derive new analytic results for the expectation of FST in stepping-stone and continuum models when small sets of samples are taken. We also compare the effectiveness of the different methods using a variety of simulated data. We found that the FST and rare-alleles methods yield comparable estimates under a wide variety of conditions when the population being sampled is demographically stable. They are roughly equally sensitive to selection and to variation in population structure, and they approach their equilibrium values at approximately the same rate. We found that two different maximum-likelihood methods tend to yield biased estimates when relatively small numbers of locations are sampled but more accurate estimates when larger numbers are sampled. Our conclusion is that, although FST and rare-alleles methods are expected to be equally effective in analyzing ideal data, practical problems in estimating the frequencies of rare alleles in electrophoretic studies suggest that FST is likely to be more useful under realistic conditions.}, author = {Slatkin, Montgomery and Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, number = {7}, pages = {1349 -- 1368}, publisher = {Wiley-Blackwell}, title = {{A comparison of three methods for estimating average levels of gene flow}}, doi = {10.1111/j.1558-5646.1989.tb02587.x }, volume = {43}, year = {1989}, } @article{4321, abstract = {The fire-bellied toads Bombina bombina and B. variegata differ extensively in biochemistry, morphology, and behavior. We use a survey of five diagnostic enzyme loci across the hybrid zone near Cracow in Southern Poland to estimate the dispersal rate, selection pressures, and numbers of loci which maintain this zone. The enzyme clines coincide closely with each other and with morphological and mitochondrial DNA clines. Although the zone lies on a broad transition between environments suitable for bombina and variegata, the close concordance of diverse characters, together with increased aberrations and mortality in hybrids, suggest that the zone is maintained largely by selection against hybrids. There are strong “linkage disequilibria” between each pair of (unlinked) enzyme loci (R̄ = 0.129 [2-unit support limits: 0.119–0.139]). These are probably caused by gene flow into the zone, and they give an estimate of dispersal (σ = 890 [790–940] m gen−½). The clines are sharply stepped, with most of the change occurring within 6.15 (5.45–6.45) km, but with long tails of introgression on either side. This implies that the effective selection pressure on each enzyme marker (due largely to disequilibrium with other loci) is s* = 0.17 (0.159–0.181) at the center but that the selection acting directly on the enzyme loci is weak or zero (se < 0.0038). The stepped pattern implies a barrier to gene flow of 220 (48–415) km. This would substantially delay neutral introgression but would have little effect on advantageous alleles; the two taxa need not evolve independently. Strong selection is needed to maintain such a barrier: hybrid populations must have their mean fitness reduced by a factor of 0.65 (0.60–0.77). This selection must be spread over a large number of loci to account for the concordant patterns and the observed cline widths (N = 300 [80–2,000]).}, author = {Szymura, Jacek and Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, pages = {1141 -- 1159}, publisher = {Society for the Study of Evolution}, title = {{Genetic analysis of a hybrid zone between the fire-bellied toads Bombina bombina and B. variegata, near Cracow in Southern Poland}}, doi = {10.1111/j.1558-5646.1986.tb05740.x}, volume = {40}, year = {1986}, } @article{3668, abstract = {When two populations which differ at many loci meet, the degree of introgression of alleles across the boundary will depend on the selection acting on each locus (s), the rate of recombination between adjacent loci (r), and the number of loci involved (n). Simple scaling arguments suggest that the behavior of the system should depend on the ratio of selection to recombination (θ = s/r), and on n. This is borne out by mathematical analysis of two demes which exchange individuals at a low rate; when selection is stronger than recombination (θ > 1), the effective selection on each locus is comparable to the total selection over the whole genome (s* ∼ ns). When selection is weaker than recombination (θ < 1), the effective selection is much weaker, but is still stronger than the selection on each locus alone (s* \sim sn20 for small θ). When n is very large, these two regimes are separated by a sharp threshold at θ = 1. The results are extended to two taxa which meet in a continuous habitat; the effective selection pressure, which determines the width of the hybrid zone, behaves in the same way as for the simpler case above. Even when selection is weak compared to recombination, multilocus clines have a sharp step at their center, flanked by tails of introgression in which the alleles behave independently of each other. The set of clines acts as a barrier to gene flow, and it is shown that the barrier is strongest when selection is spread over many loci. The implications of the results for divergence and speciation are discussed.}, author = {Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, number = {3}, pages = {454 -- 471}, publisher = {Society for the Study of Evolution}, title = {{Multilocus clines}}, doi = {10.2307/2408260}, volume = {37}, year = {1983}, } @article{4331, author = {Barton, Nicholas H}, issn = {1558-5646}, journal = {Evolution}, number = {4}, pages = {863 -- 866}, publisher = {Wiley}, title = {{The structure of the hybrid zone in Uroderma bilobatum (Chiroptera: Phyllostomatidae)}}, doi = {10.1111/j.1558-5646.1982.tb05452.x}, volume = {36}, year = {1982}, } @article{3670, abstract = {The grasshopper Podisma pedestris includes two chromosomal races, which differ by a Robertsonian fusion involving the sex chromosome. The two races meet in a cline which runs for 100 km across the Alpes Maritimes in south-eastern France. An intensive study of the easternmost end of this cline shows that it is about 800 m wide; the cline is not smooth, containing substantial spikes in chromosome frequency which might be due to sampling drift. Though the cline seems narrow, it is wide compared with the dispersal rate of the insect; a selective force of only 0.5% would be enough to maintain the cline. It is difficult to determine the nature of this force, but some evidence comes from the position of the cline, and from the presence of coincident clines at other loci. An estimate of the distribution of Podisma has been made, and the cline seems to follow, for the most part, a region of low population density, suggesting that it is maintained by hybrid unfitness. However, in the one region where the cline is relatively free to move, the XY race bulges forwards more than would be expected if hybrids are unfit. The observation of severe inviability in crosses between the races, though it is not associated with the chromosomal difference, also indicates that this cline is the result of some sort of genetic incompatibility.}, author = {Barton, Nicholas H and Hewitt, Godfrey}, issn = {1558-5646}, journal = {Evolution; International Journal of Organic Evolution}, number = {5}, pages = {1008 -- 1018}, publisher = {Wiley-Blackwell}, title = {{A chromosomal cline in the grasshopper Podisma pedestris}}, doi = { 10.1111/j.1558-5646.1981.tb04966.x}, volume = {35}, year = {1981}, }