scholarly journals Reproductive isolation via polygenic local adaptation in sub-divided populations: effect of linkage disequilibria and drift

2021 ◽  
Author(s):  
Himani Sachdeva
Keyword(s):  
Reproductive Isolation ◽  
Local Adaptation ◽  
Genetic Drift ◽  
Island Population ◽  
Combined Effects ◽  
Migration Rates ◽  
Linkage Disequilibria ◽  
Deleterious Alleles ◽  
Relative Divergence ◽  
Structured Coalescent

This paper considers how local adaptation and reproductive isolation between hybridizing populations is influenced by linkage disequilibria (LD) between multiple divergently selected loci in scenarios where both gene flow and genetic drift degrade local adaptation. It shows that the combined effects of multi-locus LD and genetic drift on allele frequencies at selected loci and on heterozygosity at neutral loci are predicted accurately by incorporating (deterministic) effective migration rates into the diffusion approximation (for selected loci) and into the structured coalescent (for neutral loci). Theoretical approximations are tested against individual-based simulations and used to investigate the conditions for the maintenance of local adaptation on an island subject to one-way migration from a differently adapted mainland, and in an infinite-island population with two different habitats under divergent selection. The analysis clarifies the conditions under which LD between sets of locally deleterious alleles allows these to be collectively eliminated despite drift, causing sharper and (under certain conditions) shifted migration thresholds for loss of adaptation. Local adaptation also has counter-intuitive effects on neutral (relative) divergence: FST is highest for a pair of subpopulations belonging to the same (rare) habitat, despite the lack of reproductive isolation between them.

scholarly journals Polygenic local adaptation in metapopulations: a stochastic eco-evolutionary model

2020 ◽  
Author(s):  
Enikő Szép ◽  
Himani Sachdeva ◽  
Nick Barton
Keyword(s):  
Population Size ◽  
Local Adaptation ◽  
Genetic Drift ◽  
Diffusion Approximation ◽  
Joint Distribution ◽  
Evolutionary Model ◽  
Ecological Niches ◽  
Demographic Stochasticity ◽  
Linkage Disequilibria ◽  
Population Sizes

AbstractThis paper analyses the conditions for local adaptation in a metapopulation with infinitely many islands under a model of hard selection, where population size depends on local fitness. Each island belongs to one of two distinct ecological niches or habitats. Fitness is influenced by an additive trait which is under habitat-dependent directional selection. Our analysis is based on the diffusion approximation and accounts for both genetic drift and demographic stochasticity. By neglecting linkage disequilibria, it yields the joint distribution of allele frequencies and population size on each island. We find that under hard selection, the conditions for local adaptation in a rare habitat are more restrictive for more polygenic traits: even moderate migration load per locus at very many loci is sufficient for population sizes to decline. This further reduces the efficacy of selection at individual loci due to increased drift and because smaller populations are more prone to swamping due to migration, causing a positive feedback between increasing maladaptation and declining population sizes. Our analysis also highlights the importance of demographic stochasticity, which exacerbates the decline in numbers of maladapted populations, leading to population collapse in the rare habitat at significantly lower migration than predicted by deterministic arguments.

scholarly journals Deleterious mutations in a hybrid zone: can mutational load decrease the barrier to gene flow?

2002 ◽  
Vol 80 (3) ◽  
pp. 197-204 ◽  
Author(s):  
NICOLAS BIERNE ◽  
THOMAS LENORMAND ◽  
FRANÇOIS BONHOMME ◽  
PATRICE DAVID
Keyword(s):  
Gene Flow ◽  
Hybrid Zone ◽  
Small Population ◽  
Structured Populations ◽  
Mutational Load ◽  
Deleterious Mutations ◽  
Migration Rates ◽  
Linkage Disequilibria ◽  
Deleterious Alleles ◽  
Population Sizes

The aim of this paper is to investigate the effect of deleterious mutations in a hybrid zone maintained by selection against hybrids. In such zones, linkage disequilibria among hybrid depression loci, resulting from a balance between migration and selection, are crucial in maintaining the barrier because they allow each locus, in addition to its own selection coefficient, to cumulate indirect selective effects from other loci. Deleterious alleles produce heterosis and increase by this means the effective migration rate in structured populations. In a hybrid zone, they therefore contribute to decrease linkage disequilibria as well as the barrier to gene flow imposed by hybrid depression. However, deleterious mutations have no effect: (i) when selection against hybrids is weak, because linkage disequilibria are small even without heterosis in this case, or (ii) when selection against hybrids is so strong that it overwhelms heterosis. On the other hand, with moderate selection against hybrids, the decrease in the strength of the barrier due to heterosis may reach detectable levels, although it requires relatively small population sizes and/or migration rates. The effect is expected to be small and only within small genomes where loci are tightly linked can it become strong. Nevertheless, neglecting mutational load may to some extent obscure the estimations of selective parameters based either on artificial F1 crosses or on cline characteristics.

scholarly journals Migration pulsedness alters patterns of allele fixation and local adaptation in a mainland-island model

2021 ◽  
Author(s):  
Flora Aubree ◽  
Baptiste Lac ◽  
Vincent Calcagno ◽  
Ludovic Mailleret
Keyword(s):  
Gene Flow ◽  
Local Adaptation ◽  
Island Model ◽  
Stochastic Simulations ◽  
Peripheral Populations ◽  
Migration Rates ◽  
Analytical Approximations ◽  
Deleterious Alleles ◽  
Selection Threshold ◽  
Fixation Probabilities

Gene flow, through allele migration and spread, is critical in determining patterns of population genetic structure, divergence and local adaptation. While evolutionary theory has typically envisioned gene flow as a continuous connection among populations, many processes can render it fluctuating and intermittent. We analyze mathematically a stochastic mainland-island model in continuous time, in which migration occur as recurrent ''pulses''. We derive simple analytical approximations regarding how migration pulsedness affects the effective migration rates across a range of selection and dominance scenarios. Predictions are validated with stochastic simulations and summarized with graphical interpretations in terms of fixation probabilities. We show that migration pulsedness can decrease or increase gene flow, respectively above or below a selection threshold that is s~-1/N for additive alleles and lower for recessive deleterious alleles. We propose that pulsedness may leave a genomic detectable signature, by differentially affecting the fixation rates of loci subjected to different selection regimes. The additional migration created by pulsedness is called a ''pulsedness'' load. Our results indicate that migration pulsedness, and more broadly temporally variable migration, is important to consider for evolutionary and population genetics predictions. Specifically, it would overall be detrimental to the local adaptation and persistence of small peripheral populations.

scholarly journals The collapse of genetic incompatibilities in a hybridizing population

2021 ◽  
Author(s):  
Tianzhu Xiong ◽  
James L MALLET
Keyword(s):  
Gene Regulation ◽  
Reproductive Isolation ◽  
Genetic Drift ◽  
Recombination Rate ◽  
Time Scale ◽  
Genetic Incompatibility ◽  
Genetic Process ◽  
Underlying Mechanisms ◽  
The Stability

Genetic incompatibility has long been considered to be a hallmark of speciation due to its role in reproductive isolation. Previous analyses of the stability of epistatic incompatibility show that it is subject to collapse upon hybridization. In the present work, we derive explicitly the distribution of the lifespan of two-locus incompatibilities, and show that genetic drift, along with recombination, is critical in determining the time scale of collapse. The first class of incompatibilities, where derived alleles separated in parental populations act antagonistically in hybrids, survive longer in smaller populations when incompatible alleles are (co)dominant and tightly linked, but collapse more quickly when they are recessive. The second class of incompatibilities, where fitness is reduced by disrupting co-evolved elements in gene regulation systems, collapse on a time scale proportional to the exponential of effective recombination rate. Overall, our result suggests that the effects of genetic drift and recombination on incompatibility's lifespan depend strongly on the underlying mechanisms of incompatibilities. As the time scale of collapse is usually shorter than the time scale of establishing a new incompatibility, the observed level of genetic incompatibilities in a particular hybridizing population may be shaped more by the collapse than by their initial accumulation. Therefore, a joint theory of accumulation-erosion of incompatibilities is in need to fully understand the genetic process under speciation with hybridization.

Evolution of Flowers, Pollination, and Plant Diversity

2011 ◽  
Author(s):  
Pat Willmer
Keyword(s):  
Reproductive Isolation ◽  
Genetic Drift ◽  
Plant Diversity ◽  
Character Displacement ◽  
Good Evidence ◽  
Early Evolution ◽  
Evolutionary Divergence ◽  
Floral Variation ◽  
Character Correlations ◽  
Animal Pollination

This chapter examines the evolution of flowers, pollination, and plant diversity. There is good evidence for pollinator-mediated selection and appropriate trait heritability in flowers, and there are well-established mechanisms by which this could bring about floral change, reproductive isolation, and evolutionary divergence or specialization. The chapter first considers the origin and early evolution of flowers before discussing the diversification of angiosperms. It then explores the advantages of animal pollination and goes on to discuss the extent to which pollination may have contributed to floral variation, plant speciation, and plant diversification. In particular, it explains whether pollinators select for floral divergence and describes five ways in which floral divergence could arise by selection: adaptation to distinct niches, character displacement, adaptive “wandering,” character correlations, and genetic drift.

scholarly journals The limits to parapatric speciation 3: evolution of strong reproductive isolation in presence of gene flow despite limited ecological differentiation

2020 ◽  
Vol 375 (1806) ◽  
pp. 20190532 ◽  
Author(s):  
Alexandre Blanckaert ◽  
Claudia Bank ◽  
Joachim Hermisson
Keyword(s):  
Gene Flow ◽  
Reproductive Isolation ◽  
Local Adaptation ◽  
Theme Issue ◽  
Genetic Barrier ◽  
Incipient Species ◽  
Postzygotic Reproductive Isolation ◽  
Minimal Configuration ◽  
Parapatric Speciation ◽  
Two Populations

Gene flow tends to impede the accumulation of genetic divergence. Here, we determine the limits for the evolution of postzygotic reproductive isolation in a model of two populations that are connected by gene flow. We consider two selective mechanisms for the creation and maintenance of a genetic barrier: local adaptation leads to divergence among incipient species due to selection against migrants, and Dobzhansky–Muller incompatibilities (DMIs) reinforce the genetic barrier through selection against hybrids. In particular, we are interested in the maximum strength of the barrier under a limited amount of local adaptation, a challenge that many incipient species may initially face. We first confirm that with classical two-locus DMIs, the maximum amount of local adaptation is indeed a limit to the strength of a genetic barrier. However, with three or more loci and cryptic epistasis, this limit holds no longer. In particular, we identify a minimal configuration of three epistatically interacting mutations that is sufficient to confer strong reproductive isolation. This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers’.

scholarly journals Structured Coalescent Processes on Different Time Scales

Genetics ◽  
1997 ◽  
Vol 146 (4) ◽  
pp. 1501-1514 ◽  
Author(s):  
Magnus Nordborg
Keyword(s):  
Local Adaptation ◽  
Time Scales ◽  
Balancing Selection ◽  
Background Selection ◽  
Selfing Rate ◽  
Local Selection ◽  
Structured Coalescent ◽  
High Degree ◽  
Different Time Scales ◽  
Coalescence Times

It is demonstrated that the structured coalescent model can readily be extended to include phenomena such as partial selfing and background selection through the use of an approximation based on separation of time scales. A model that includes these phenomena, as well as geographic subdivision and linkage to a polymorphism maintained either by local adaptation or by balancing selection, is derived, and the expected coalescence time for a pair of genes is calculated. It is found that background selection reduces coalescence times within subpopulations and allelic classes, leading to a high degree of apparent differentiation. Extremely high levels of subpopulation differentiation are also expected for regions of the genome surrounding loci important in local adaptation. These regions will be wider the stronger the local selection, and the higher the selfing rate.

scholarly journals Parental population range expansion before secondary contact promotes heterosis

2020 ◽  
Author(s):  
Ailene MacPherson ◽  
Silu Wang ◽  
Ryo Yamaguchi ◽  
Loren H. Riesesberg ◽  
Sarah P. Otto
Keyword(s):  
Reproductive Isolation ◽  
Range Expansion ◽  
Genomic Analysis ◽  
Population Expansion ◽  
Ancestral Population ◽  
Secondary Contact ◽  
Hybrid Zones ◽  
Deleterious Alleles ◽  
Genomic Divergence ◽  
Slightly Deleterious Mutations

AbstractPopulation genomic analysis of hybrid zones is instrumental to our understanding of the evolution of reproductive isolation. Many temperate hybrid zones are formed by the secondary contact between two parental populations that had undergone post-glacial range expansion. Here we show that explicitly accounting for historical parental isolation followed by range expansion prior to secondary contact is fundamental for explaining genetic and fitness patterns in these hybrid zones. Specifically, ancestral population expansion can result in allele surfing, neutral or slightly deleterious mutations drift high frequency at the front of the expansion. If these surfed deleterious alleles are recessive, they can contribute to substantial heterosis in hybrids produced at secondary contact, counteracting negative-epistatic interactions between BDMI loci and hence can deteriorate reproductive isolation. Similarly, surfing at neutral loci can alter the expected pattern of population ancestry and suggests that accounting for historical population expansion is necessary to develop accurate null genomic models in secondary-contact hybrid zones. Furthermore, this process should be incorporated in macroevolutionary models of divergence as well, since such heterosis facilitated by parental-range expansion could dampen genomic divergence established in the past.

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