@article{3631, abstract = {In spatially heterogeneous environments, natural selection for maintenance of adaptation to habitats that contribute little to the population's reproduction is weak. In this paper we model a mechanism that can result in loss of fitness in such marginal habitats, and thus lead to specialisation on the main habitat. It involves accumulation of mutations that are deleterious in the marginal habitat but neutral or nearly so in the main habitat (mutations deleterious in the main habitat and neutral in the marginal habitat have a negligible influence). If the contribution of the marginal habitat to total reproduction in the absence of the mutations is less than a threshold value, selection is too weak to counter accumulation of such mutations. A positive feedback then results in loss of fitness in the marginal habitat. This mechanism does not require antagonistic pleiotropy in adaptation to different habitats, although antagonistic pleiotropy facilitates the mutational collapse of fitness in the marginal habitat. We suggest that deleterious mutations with habitat-specific expression may play a role in the evolution of ecological specialisation and promote evolutionary conservatism of ecological niches.}, author = {Kawecki, Tadeusz and Barton, Nicholas H and Fry, James}, issn = {1010-061X}, journal = {Journal of Evolutionary Biology}, number = {3}, pages = {407 -- 430}, publisher = {Wiley-Blackwell}, title = {{Mutational collapse of fitness in marginal habitats and the evolution of ecological specialisation}}, doi = {10.1046/j.1420-9101.1997.10030407.x}, volume = {10}, year = {1997}, } @article{3632, abstract = {An important but controversial class of hypotheses concerning the evolution of female preferences for extreme male mating displays involves 'indirect selection.' Even in the absence of direct fitness effects, preference for males with high overall fitness can spread via a genetic correlation that develops between preference alleles and high fitness genotypes. Here we develop a quantitative expression for the force of indirect selection that (i) applies to any female mating behavior, (ii) is relatively insensitive to the underlying genetics, and (iii) is based on measurable quantities. In conjunction with the limited data now available, it suggests that the evolutionary force generated by indirect selection on preferences is weak in absolute terms. This finding raises the possibility that direct selection on preference genes may often be more important than indirect selection, but more data on the quantities identified by our model and on direct selection are needed to decide the question.}, author = {Kirkpatrick, Mark and Barton, Nicholas H}, issn = {0027-8424}, journal = {PNAS}, number = {4}, pages = {1282 -- 1286}, publisher = {National Academy of Sciences}, title = {{The strength of indirect selection on female mating preferences}}, doi = {10.1073/pnas.94.4.1282}, volume = {94}, year = {1997}, } @article{3633, abstract = {Gene flow from the center of a species' range can stymie adaptation at the periphery and prevent the range from expanding outward. We study this process using simple models that track both demography and the evolution of a quantitative trait in a population that is continuously distributed in space. Stabilizing selection acts on the trait and favors an optimum phenotype that changes linearly across the habitat. One of three outcomes is possible: the species will become extinct, expand to fill all of the available habitat, or be confined to a limited range in which it is significantly adapted to allow population growth. When the environment changes rapidly in space, increased migration inhibits local adaptation and so decreases the species' total population size. Gene flow can cause enough maladaptation that the peripheral half of a species' range acts as an demographic sink. The trait's genetic variance has little effect on species persistence or the size of the range when gene flow is sufficiently strong to keep population densities far below the carrying capacity throughout the range, but it can increase the range width and population size of an abundant species. Under some conditions, a small parameter change can dramatically shift the balance between gene flow and local adaptation, allowing a species with a limited range to suddenly expand to fill all the available habitat.}, author = {Kirkpatrick, Mark and Barton, Nicholas H}, issn = {0003-0147}, journal = {American Naturalist}, number = {1}, pages = {1 -- 23}, publisher = {University of Chicago Press}, title = {{Evolution of a species' range}}, doi = {10.1086/286054}, volume = {150}, year = {1997}, } @article{4018, abstract = {Given a subspace X subset of or equal to R-d and a finite set S subset of or equal to R-d, we introduce the Delaunay complex, D-X, restricted by X. Its simplices are spanned by subsets T subset of or equal to S for which the common intersection of Voronoi cells meets X in a non-empty set. By the nerve theorem, boolean OR D-X and X are homotopy equivalent if all such sets are contractible. This paper proves a sufficient condition for boolean OR D-X and X be homeomorphic.}, author = {Edelsbrunner, Herbert and Shah, Nimish}, issn = {0925-7721}, journal = {International Journal of Computational Geometry & Applications}, number = {4}, pages = {365 -- 378}, publisher = {World Scientific Publishing}, title = {{Triangulating topological spaces}}, doi = {10.1142/S0218195997000223}, volume = {7}, year = {1997}, } @article{4021, abstract = {A homeomorphism from R-2 to itself distorts metric quantities, such as distance and area. We describe an algorithm that constructs homeomorphisms with prescribed area distortion. Such homeomorphisms can be used to generate cartograms, which are geographic maps purposely distorted so their area distributions reflects a variable different from area, as for example population density. The algorithm generates the homeomorphism through a sequence of local piecewise linear homeomorphic changes. Sample results produced by the preliminary implementation of the method are included.}, author = {Edelsbrunner, Herbert and Waupotitsch, Roman}, issn = {0925-7721}, journal = {Computational Geometry: Theory and Applications}, number = {5-6}, pages = {343 -- 360}, publisher = {Elsevier}, title = {{A combinatorial approach to cartograms}}, doi = {10.1016/S0925-7721(96)00006-5}, volume = {7}, year = {1997}, } @article{4022, abstract = {A halving hyperplane of a set S of n points in R(d) contains d affinely independent points of S so that equally many of the points off the hyperplane lie in each of the two half-spaces. We prove bounds on the number of halving hyperplanes under the condition that the ratio of largest over smallest distance between any two points is at most delta n(1/d), delta some constant. Such a set S is called dense. In d = 2 dimensions the number of halving lines for a dense set can be as much as Omega(n log n), and it cannot exceed O (n(5/4)/log* n). The upper bound improves over the current best bound of O (n(3/2)/log* n) which holds more generally without any density assumption. In d = 3 dimensions we show that O (n(7/3)) is an upper bound on the number of halving planes for a dense set, The proof is based on a metric argument that can be extended to d greater than or equal to 4 dimensions, where it leads to O (n(d-2/d)) as an upper bound for the number of halving hyperplanes.}, author = {Edelsbrunner, Herbert and Valtr, Pavel and Welzl, Emo}, issn = {0179-5376}, journal = {Discrete & Computational Geometry}, number = {3}, pages = {243 -- 255}, publisher = {Springer}, title = {{Cutting dense point sets in half}}, doi = {10.1007/PL00009291}, volume = {17}, year = {1997}, } @article{4023, abstract = {Let B be a finite pseudodisk collection in the plane. By the principle of inclusion-exclusion, the area or any other measure of the union is [GRAPHICS] We show the existence of a two-dimensional abstract simplicial complex, X subset of or equal to 2(B), so the above relation holds even if X is substituted for 2(B). In addition, X can be embedded in R(2) SO its underlying space is homotopy equivalent to int Boolean OR B, and the frontier of X is isomorphic to the nerve of the set of boundary contributions.}, author = {Edelsbrunner, Herbert and Ramos, Edgar}, issn = {0179-5376}, journal = {Discrete & Computational Geometry}, number = {3}, pages = {287 -- 306}, publisher = {Springer}, title = {{Inclusion-exclusion complexes for pseudodisk collections}}, doi = {10.1007/PL00009295}, volume = {17}, year = {1997}, } @article{4174, abstract = {The epiphysial region of the dorsal diencephalon is the first site at which neurogenesis occurs in the roof of the zebrafish forebrain. We show that the homeobox containing gene floating head (flh) is required for neurogenesis to proceed in the epiphysis. In flh(-) embryos, the first few epiphysial neurons are generated, but beyond the 18 somite stage, neuronal production ceases. In contrast, in masterblind(-) (mbl(-)) embryos, epiphysial neurons are generated throughout the dorsal forebrain. We show that mbl is required to prevent the expression of flh in dorsal forebrain cells rostral to the epiphysis. Furthermore, epiphysial neurons are not ectopically induced in mbl(-)/flh(-) embryos, demonstrating that the epiphysial phenotype of mbl(-) embryos is mediated by ectopic Flh activity. We propose a role for Flh in linking the signaling pathways that regulate regional patterning to the signaling pathways that regulate neurogenesis.}, author = {Masai, Ichiro and Heisenberg, Carl-Philipp J and Barth, K Anukampa and Macdonald, Rachel and Adamek, Sylwia and Wilson, Stephen}, issn = {0896-6273}, journal = {Neuron}, number = {1}, pages = {43 -- 57}, publisher = {Elsevier}, title = {{Floating head and masterblind regulate neuronal patterning in the roof of the forebrain}}, doi = {10.1016/S0896-6273(01)80045-3}, volume = {18}, year = {1997}, } @article{4201, abstract = {In zebrafish, as in other vertebrates, an initially singular eye held within the neural plate has to split during morphogenesis to allow the development of two separated eyes. It has been suggested that anterior progression of midline tissue within the neural plate is involved in the bilateralization of the eye held. Mutations in the recently identified silberblick (slb) gene cause an incomplete separation of the eyes. During gastrulation and early somitogenesis, the ventral midline of the central nervous system (CNS) together with the underlying axial mesendoderm is shortened and broadened in slb embryos. While in wild-type embryos the ventral CNS midline extends to the anterior limit of the neural plate at the end of gastrulation, there is a gap between the anterior tip of the ventral CNS midline and the anterior edge of the neural plate in slb. To investigate the cause for the shortening of the ventral CNS midline in slb we determined the fate of labeled ventral CNS midline cells in wild-type and slb embryos at different stages of development. In slb, anterior migration of ventral CNS midline cells is impaired, which indicates that migration of these cells is needed for elongation of the ventral CNS midline. The anterior shortening of the ventral CNS midline in slb leads to medial instead of bilateral induction of optic stalks followed by a partial fusion of the eyes at later developmental stages. The analysis of the sIb phenotype indicates that anterior migration of midline cells within the neural plate is required for proper induction and subsequent bilateralization of an initially singular eye field. These findings may therefore provide a starting point in elucidating the role of neural plate morphogenesis in positioning of the eyes. (C) 1997 Academic Press.}, author = {Heisenberg, Carl-Philipp J and Nüsslein Volhard, Christiane}, issn = {0012-1606}, journal = {Developmental Biology}, number = {1}, pages = {85 -- 94}, publisher = {Elsevier}, title = {{The function of silberblick in the positioning of the eye anlage in the zebrafish embryo}}, doi = {10.1006/dbio.1997.8511}, volume = {184}, year = {1997}, } @inbook{4284, abstract = {The evolutionary processes responsible for adaptation and speciation on islands differ in several ways from those on the mainland. Most attention has been given to the random genetic drift that arises when a population is founded from just a few colonizing genomes. Theoretical obstacles to ‘founder effect speciation’ are discussed, together with recent proposals for avoiding them. It is argued that although certain kinds of epistasis can facilitate the evolution of strong reproductive isolation, this favours divergence by selection as much as by random drift.}, author = {Barton, Nicholas H}, booktitle = {Evolution on islands}, isbn = {9780198501718}, pages = {102 -- 123}, publisher = {Oxford University Press}, title = {{Natural selection and random genetic drift as causes of evolution on islands}}, doi = {10.1098/rstb.1996.0073}, year = {1997}, } @article{4285, abstract = {One of the oldest hypotheses for the advantage of recombination is that recombination allo rvs beneficial mutations that arise in different individuals to be placed together on the same chromosome. Unless recombination occurs, one of the beneficial alleles is doomed to extinction, slowing the rate at which adaptive mutations are incorporated within a population. We model the effects of a modifier of recombination on the fixation probability of beneficial mutations when beneficial alleles are segregating at other loci. We find that modifier alleles that increase recombination do increase the fixation probability of beneficial mutants and subsequently hitchhike along as the mutants rise in frequency. The strength of selection favoring a modifier that increases recombination is proportional to lambda(2)S delta r/r when linkage is tight and lambda(2)S(3) delta r/N when linkage is loose, where lambda is the beneficial mutation rate per genome per generation throughout a population of size N, S is the average mutant effect, r is the average recombination rate, and delta ris the amount that recombination is modified. We conclude that selection for recombination will be substantial only if there is tight linkage within the genome or if many loci are subject to directional selection as during periods of rapid evolutionary change.}, author = {Otto, Sarah and Barton, Nicholas H}, issn = {0016-6731}, journal = {Genetics}, number = {2}, pages = {879 -- 906}, publisher = {Genetics Society of America}, title = {{The evolution of recombination: Removing the limits to natural selection}}, doi = {10.1093/genetics/147.2.879}, volume = {147}, year = {1997}, } @article{4286, abstract = {A local barrier to gene flow will delay the spread of an advantageous allele. Exact calculations for the deterministic case show that an allele that is favorable when rare is delayed very little even by a strong barrier; its spread is allowed by a time proportional to log((B/σ)√2S)/S, where B is the barrier strength, σ the dispersal range, and fitnesses are 1:1 + S:1 + 2S. However, when there is selection against heterozytes, such that the allele cannot increase from low frequency, a barrier can cause a much greater delay. If gene flow is reduced below a critical value, spread is entirely prevented. Stochastic simulations show that with additive selection, random drift slows down the spread of the allele, below the deterministic speed of σ√2S. The delay to the advance of an advantageous allele caused by a strong barrier can be substantially increased by random drift and increases with B/(2Sρσ2) in a one-dimensional habitat of density ρ. However, with selection against heterozygotes, drift can facilitate the spread and can free an allele that would otherwise be trapped indefinitely by a strong barrier. We discuss the implications of these results for the evolution of chromosome rearrangements.}, author = {Piálek, Jaroslav and Barton, Nicholas H}, issn = {0016-6731}, journal = {Genetics}, number = {2}, pages = {493 -- 504}, publisher = {Genetics Society of America}, title = {{The spread of an advantageous allele across a barrier: the effects of random drift and selection against heterozygotes}}, doi = {10.1093/genetics/145.2.493}, volume = {145}, year = {1997}, } @article{4287, abstract = {We evaluate Sewall Wright's three-phase "shifting balance" theory of evolution, examining both the theoretical issues and the relevant data from nature and the laboratory. We conclude that while phases I and II of Wright's theory (the movement of populations from one "adaptive peak" to another via drift and selection) can occur under some conditions, genetic drift is often unnecessary for movement between peaks. Phase III of the shifting balance, in which adaptations spread from particular populations to the entire species, faces two major theoretical obstacles: (1) unlike adaptations favored by simple directional selection, adaptations whose fixation requires some genetic drift are often prevented from spreading by barriers to gene flow; and (2) it is difficult to assemble complex adaptations whose constituent parts arise via peak shifts in different demes. Our review of the data from nature shows that although there is some evidence for individual phases of the shifting balance process, there are few empirical observations explained better by Wright's three-phase mechanism than by simple mass selection. Similarly, artificial selection experiments fail to show that selection in subdivided populations produces greater response than does mass selection in large populations. The complexity of the shifting balance process and the difficulty of establishing that adaptive valleys have been crossed by genetic drift make it impossible to test Wright's claim that adaptations commonly originate by this process. In view of these problems, it seems unreasonable to consider the shifting balance process as an important explanation for the evolution of adaptations. }, author = {Coyne, Jerry and Barton, Nicholas H and Turelli, Michael}, issn = {0014-3820}, journal = {Evolution; International Journal of Organic Evolution}, number = {3}, pages = {643 -- 671}, publisher = {Wiley-Blackwell}, title = {{Perspective: A critique of Sewall Wright's shifting balance theory of evolutionight's shifting balance theory of evolution}}, doi = {10.1111/j.1558-5646.1997.tb03650.x}, volume = {51}, year = {1997}, } @article{4288, abstract = {We measured the heterozygous effects on net fitness of a sample of 12 wild-type third chromosomes in D. melanogaster. Effects on fitness were assessed by competing the wild-type chromosomes against balancer chromosomes, to prevent the production of recombinants. The measurements were carried out in the population cage environment in which the life history had been evolving, in an undisturbed population with overlapping generations, and replicated measurements were made on each chromosome to control for confounding effects such as mutation accumulation. We found significant variation among the wild type chromosomes in their additive genetic effect on net fitness. The system provides an opportunity to obtain an accurate estimate of the distribution of heterozygous effects on net fitness, the contribution of different fitness components including male mating success, and the role of intra-chromosomal epistasis in fitness variation.}, author = {Fowler, Kevin and Semple, Colin and Barton, Nicholas H and Partridge, Linda}, issn = {0962-8452}, journal = {Proceedings of the Royal Society of London Series B Biological Sciences}, number = {1379}, pages = {191 -- 199}, publisher = {The Royal Society}, title = {{Genetic variation for total fitness in Drosophila melanogaster}}, doi = {10.1098/rspb.1997.0027}, volume = {264}, year = {1997}, } @article{4289, abstract = {A worldwide survey of polymorphic molecular markers shows that the human population is genetically homogeneous, in close agreement with evidence from quite different genes and traits.}, author = {Barton, Nicholas H}, issn = {0960-9822}, journal = {Current Biology}, number = {12}, pages = {757 -- 758}, publisher = {Cell Press}, title = {{Population genetics: A new apportionment of human diversity}}, doi = {10.1016/S0960-9822(06)00397-6}, volume = {7}, year = {1997}, } @misc{4290, author = {Barton, Nicholas H}, booktitle = {Genetical Research}, issn = {0016-6723}, number = {2}, pages = {178 -- 180}, publisher = {Cambridge University Press}, title = {{Natural hybridization and evolution}}, volume = {70}, year = {1997}, } @misc{4291, author = {Barton, Nicholas H}, booktitle = {Genetical Research}, issn = {0016-6723}, number = {2}, pages = {180 -- 181}, publisher = {Cambridge University Press}, title = {{The ecological detective: Confronting models with data}}, volume = {70}, year = {1997}, } @inbook{4293, abstract = {Natural populations differ from the simplest models in ways which can significantly affect their evolution. Real populations are rarely all of the same size; the rates of migration into and out of populations vary in space and time; some populations go extinct, and new ones are established, while all populations fluctuate in size. Furthermore, the genetic properties of real species are not like those assumed in simple models. Alleles are exposed to a wide variety of selection mutation rarely creates novel genotypes with each mutation event, generations overlap, and environments vary from place to place. Evolution in a metapopulation can be substantially different from the predictions of single-population models and, indeed, very different from the simplest models of subdivided species.}, author = {Barton, Nicholas H and Whitlock, Michael}, booktitle = {Metapopulation Biology}, editor = {Hanski, Illka and Gilpin, Michael E.}, isbn = {9780123234452}, pages = {183 -- 210}, publisher = {Academic Press}, title = {{The evolution of metapopulations}}, doi = {10.1016/B978-012323445-2/50012-2}, year = {1997}, } @inproceedings{4438, abstract = {In temporal-logic model checking, we verify the correctness of a program with respect to a desired behavior by checking whether a structure that models the program satisfies a temporal-logic formula that specifies the behavior. The model-checking problem for the branching-time temporal logic CTL can be solved in linear running time, and model-checking tools for CTL are used successfully in industrial applications. The development of programs that must meet rigid real-time constraints has brought with it a need for real-time temporal logics that enable quantitative reference to time. Early research on real-time temporal logics uses the discrete domain of the integers to model time. Present research on real-time temporal logics focuses on continuous time and uses the dense domain of the reals to model time. There, model checking becomes significantly more complicated. For example, the model-checking problem for TCTL, a continuous-time extension of the logic CTL, is PSPACE-complete. In this paper we suggest a reduction from TCTL model checking to CTL model checking. The contribution of such a reduction is twofold. Theoretically, while it has long been known that model-checking methods for untimed temporal logics can be extended quite easily to handle discrete time, it was not clear whether and how untimed methods can handle the reset quantifier of TCTL, which resets a realvalued clock. Practically, our reduction enables anyone who has a tool for CTL model checking to use it for TCTL model checking. The TCTL model-checking algorithm that follows from our reduction is in PSPACE, matching the known bound for this problem. In addition, it enjoys the wide distribution of CTL model-checking tools and the extensive and fruitful research efforts and heuristics that have been put into these tools.}, author = {Henzinger, Thomas A and Kupferman, Orna}, booktitle = {Proceedings of the 5th International Workshop on Hybrid and Real-Time Systems}, isbn = {9783540626008}, location = {Grenoble, France}, pages = {48 -- 62}, publisher = {Springer}, title = {{From quantity to quality}}, doi = {10.1007/BFb0014712}, volume = {1201}, year = {1997}, } @inproceedings{4441, abstract = {Rectangular hybrid automata model digital control programs of analog plant environments. We study rectangular hybrid automata where the plant state evolves continuously in real-numbered time, and the controller samples the plant state and changes the control state discretely, only at the integer points in time. We prove that rectangular hybrid automata have finite bisimilarity quotients when all control transitions happen at integer times, even if the constraints on the derivatives of the variables vary between control states. This is sharply in contrast with the conventional model where control transitions may happen at any real time, and already the reachability problem is undecidable. Based on the finite bisimilarity quotients, we give an exponential algorithm for the symbolic sampling-controller synthesis of rectangular automata. We show our algorithm to be optimal by proving the problem to be EXPTIME-hard. We also show that rectangular automata form a maximal class of systems for which the sampling-controller synthesis problem can be solved algorithmically.}, author = {Henzinger, Thomas A and Kopke, Peter}, booktitle = {Proceedings of the 24th International Colloquium on Automata, Languages and Programming}, isbn = {9783540631651}, location = {Bologna, Italy}, pages = {582 -- 593}, publisher = {Springer}, title = {{Discrete-time control for rectangular hybrid automata}}, doi = {10.1007/3-540-63165-8_213}, volume = {1256}, year = {1997}, }