scholarly journals The costs and benefits of dispersal in small populations

2021 ◽  
Author(s):  
Jitka Polechova
Keyword(s):  
Gene Flow ◽  
Genetic Drift ◽  
Environmental Gradients ◽  
Strong Increase ◽  
Small Populations ◽  
Marginal Populations ◽  
Neighbourhood Size ◽  
Mean Fitness ◽  
Varying Environments

Dispersal has three major effects on adaptation. First, the gene flow mixes alleles adapted to different environments, potentially hindering (swamping) adaptation. Second, it inflates genetic variance: this aids adaptation to spatially (and temporally) varying environments but if selection is hard, it lowers the mean fitness of the population. Third, neighbourhood size, which determines how weak genetic drift is, increases with dispersal -- when genetic drift is strong, increase of neighbourhood size with dispersal aids adaptation. In this note I focus on the role of dispersal in environments which change smoothly across space, and when local populations are quite small such that genetic drift has a significant effect. Using individual-based simulations, I show that in small populations, even leptokurtic dispersal benefits adaptation, by reducing the power of genetic drift. This has implications for management of small marginal populations: increased gene flow appears beneficial as long as adaptations involves a quantitative, rather than a discrete, trait. However, heavily leptokurtic dispersal will swamp continuous adaptation along steep environmental gradients so that only patches of locally adapted subpopulations remain.

scholarly journals Ecotones: Marginal or central areas of transition?

2006 ◽  
Vol 52 (1) ◽  
pp. 29-53 ◽  
Author(s):  
Salit Kark ◽  
Berndt J. van Rensburg
Keyword(s):  
Environmental Gradients ◽  
Spatial Scales ◽  
Small Scale ◽  
Biodiversity Hotspots ◽  
Ecological Communities ◽  
Marginal Populations ◽  
Multiple Spatial Scales ◽  
Scientific Attention

Areas of environmental transition, where ecological communities coincide, are sometimes termed ecotones. These regions often correspond with sharp environmental gradients. Ecotones occur at multiple spatial scales, ranging from transitions between biomes to local small-scale transitions. In recent years ecotones have received increasing scientific attention after being neglected for years, as studies historically often focused on distinct communities. However, it is still debatable whether these transitional regions are speciation and biodiversity hotspots that deserve special conservation interest or are actually areas that hold marginal populations that depend on other parts of the range for the maintenance of their biodiversity and therefore should not deserve primary investment. This paper discusses some of the recent advancements in our understanding of the role of ecotones in ecology, evolution, and conservation.

scholarly journals Limits to adaptation along environmental gradients

2014 ◽  
Author(s):  
Jitka Polechová ◽  
Nick Barton
Keyword(s):  
Gene Flow ◽  
Genetic Drift ◽  
Environmental Gradients ◽  
Single Species ◽  
Local Conditions ◽  
Deterministic Theory ◽  
Range Margin ◽  
Abrupt Changes ◽  
Analytical Tools ◽  
Range Fragmentation

Why do species not adapt to ever-wider ranges of conditions, gradually expanding their ecological niche and geographic range? Gene flow across environments has two conflicting effects: while it increases genetic variation, which is a prerequisite for adaptation, gene flow may swamp adaptation to local conditions. In 1956, Haldane proposed that when the environment varies across space, ?swamping? by gene flow creates a positive feedback between low population size and maladaptation, leading to a sharp range margin. Yet, current deterministic theory shows that when variance can evolve, there is no such limit. Using simple analytical tools and simulations, we show that genetic drift can generate a sharp margin to a species' range, by reducing genetic variance below the level needed for adaptation to spatially variable conditions. Aided by separation of ecological and evolutionary time scales, the identified effective dimensionless parameters reveal a simple threshold that predicts when adaptation at the range margin fails. Two observable parameters determine the threshold: i) the effective environmental gradient, which can be measured by the loss of fitness due to dispersal to a different environment, and ii) the efficacy of selection relative to genetic drift. The theory predicts sharp range margins even in the absence of abrupt changes in the environment. Furthermore, it implies that gradual worsening of conditions across a species' habitat may lead to a sudden range fragmentation, when adaptation to a wide span of conditions within a single species becomes impossible.

scholarly journals Limits to adaptation along environmental gradients

2015 ◽  
Vol 112 (20) ◽  
pp. 6401-6406 ◽  
Author(s):  
Jitka Polechová ◽  
Nicholas H. Barton
Keyword(s):  
Gene Flow ◽  
Genetic Drift ◽  
Environmental Gradients ◽  
Single Species ◽  
Local Conditions ◽  
Deterministic Theory ◽  
Range Margin ◽  
Abrupt Changes ◽  
Analytical Tools ◽  
Range Fragmentation

Why do species not adapt to ever-wider ranges of conditions, gradually expanding their ecological niche and geographic range? Gene flow across environments has two conflicting effects: although it increases genetic variation, which is a prerequisite for adaptation, gene flow may swamp adaptation to local conditions. In 1956, Haldane proposed that, when the environment varies across space, “swamping” by gene flow creates a positive feedback between low population size and maladaptation, leading to a sharp range margin. However, current deterministic theory shows that, when variance can evolve, there is no such limit. Using simple analytical tools and simulations, we show that genetic drift can generate a sharp margin to a species’ range, by reducing genetic variance below the level needed for adaptation to spatially variable conditions. Aided by separation of ecological and evolutionary timescales, the identified effective dimensionless parameters reveal a simple threshold that predicts when adaptation at the range margin fails. Two observable parameters determine the threshold: (i) the effective environmental gradient, which can be measured by the loss of fitness due to dispersal to a different environment; and (ii) the efficacy of selection relative to genetic drift. The theory predicts sharp range margins even in the absence of abrupt changes in the environment. Furthermore, it implies that gradual worsening of conditions across a species’ habitat may lead to a sudden range fragmentation, when adaptation to a wide span of conditions within a single species becomes impossible.

scholarly journals A steep cline for mitochondrial DNA in Danish mice

1988 ◽  
Vol 52 (3) ◽  
pp. 185-193 ◽  
Author(s):  
F. Vanlerberghe ◽  
P. Boursot ◽  
J. T. Nielsen ◽  
F. Bonhomme
Keyword(s):  
Mitochondrial Dna ◽  
Gene Flow ◽  
Genetic Drift ◽  
Hybrid Zone ◽  
Mus Musculus ◽  
Mitochondrial Gene ◽  
Mus Musculus Domesticus ◽  
Mtdna Variants

SummaryOne hundred and ninety-eight mice trapped along a south–north transect through the Danish hybrid zone between Mus musculus domesticus and M. m. musculus were typed for mitochondrial DNA (mtDNA), the Y chromosome and ten autosomal loci encoding diagnostic proteins. The southern (domesticus) populations display two mtDNA variants (S1 and S2) and the northern (musculus) have a third mtDNA variant (N) of domesticus origin. Across the hybrid zone defined by ten autosomal loci, there is a steep dine between the southern and northern types of mtDNA. As well as confirming an earlier finding that Danish musculus all have a domesticus mtDNA (Ferris et al. 1983a, & b), our results show that this mtDNA takeover is not the result of a persistent mitochondrial gene flow between the two subspecies. While the coincident dines for the ten autosomal loci and the abrupt dine for the Y chromosome can be explained by selection, it is less likely to be the case for the mtDNA exchanges. We discuss the possible role of sex-linked migration and genetic drift to account for the distribution of the mitochondrial variants.

scholarly journals Pervasive Genomic Signatures of Local Adaptation to Altitude Across Highland Specialist Andean Hummingbird Populations

2021 ◽  
Author(s):  
Marisa C W Lim ◽  
Ke Bi ◽  
Christopher C Witt ◽  
Catherine H Graham ◽  
Liliana M Dávalos
Keyword(s):  
Gene Flow ◽  
Local Adaptation ◽  
Genetic Drift ◽  
Population Genetic ◽  
Environmental Gradients ◽  
Functional Differentiation ◽  
High Elevation ◽  
Nucleotide Polymorphisms ◽  
Peruvian Andes ◽  
Tropical Mountains

Abstract Populations along steep environmental gradients are subject to differentiating selection that can result in local adaptation, despite countervailing gene flow, and genetic drift. In montane systems, where species are often restricted to narrow ranges of elevation, it is unclear whether the selection is strong enough to influence functional differentiation of subpopulations differing by a few hundred meters in elevation. We used targeted capture of 12 501 exons from across the genome, including 271 genes previously implicated in altitude adaptation, to test for adaptation to local elevations for 2 highland hummingbird species, Coeligena violifer (n = 62) and Colibri coruscans (n = 101). For each species, we described population genetic structure across the complex geography of the Peruvian Andes and, while accounting for this structure, we tested whether elevational allele frequency clines in single nucleotide polymorphisms (SNPs) showed evidence for local adaptation to elevation. Although the 2 species exhibited contrasting population genetic structures, we found signatures of clinal genetic variation with shifts in elevation in both. The genes with SNP-elevation associations included candidate genes previously discovered for high-elevation adaptation as well as others not previously identified, with cellular functions related to hypoxia response, energy metabolism, and immune function, among others. Despite the homogenizing effects of gene flow and genetic drift, natural selection on parts of the genome evidently optimizes elevation-specific cellular function even within elevation range-restricted montane populations. Consequently, our results suggest local adaptation occurring in narrow elevation bands in tropical mountains, such as the Andes, may effectively make them “taller” biogeographic barriers.

scholarly journals The reduction in fitness from genetic drift at heterotic loci in small populations

1970 ◽  
Vol 15 (2) ◽  
pp. 257-259 ◽  
Author(s):  
Alan Robertson
Keyword(s):  
Population Size ◽  
Effective Population Size ◽  
General Expression ◽  
Genetic Drift ◽  
Small Populations ◽  
Effective Population ◽  
Finite Populations ◽  
Genetic Sampling ◽  
Mean Fitness ◽  
The Mean

SUMMARYIn finite populations, loci maintained segregating by hétérozygote superiority will be disturbed from their equilibrium positions by genetic sampling and the mean fitness of individuals will consequently be reduced. A general expression for this reduction is obtained for the segregation of two alleles. If the probability of continued segregation at the locus is high, the reduction tends to 1/4N, irrespective of the strength of selection, where N is the effective population size. This will always be much less than the segregation load. If n alleles are segregating, so that all heterozygotes have the same fitness, the reduction tends to (n−1)/4N.

scholarly journals Premating barriers in young sympatric snail species

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Arina L. Maltseva ◽  
Marina A. Varfolomeeva ◽  
Arseniy A. Lobov ◽  
Polina O. Tikanova ◽  
Egor A. Repkin ◽  
...  
Keyword(s):  
Gene Flow ◽  
Snail Species ◽  
Sympatric Populations ◽  
Gastropod Species ◽  
Shell Size ◽  
Closely Related Species ◽  
Partial Canonical Correspondence Analysis ◽  
In The Wild ◽  
Isolation Index

AbstractSympatric coexistence of recently diverged species raises the question of barriers restricting the gene flow between them. Reproductive isolation may be implemented at several levels, and the weakening of some, e.g. premating, barriers may require the strengthening of the others, e.g. postcopulatory ones. We analysed mating patterns and shell size of mates in recently diverged closely related species of the subgenus Littorina Neritrema (Littorinidae, Caenogastropoda) in order to assess the role of premating reproductive barriers between them. We compared mating frequencies observed in the wild with those expected based on relative densities using partial canonical correspondence analysis. We introduced the fidelity index (FI) to estimate the relative accuracy of mating with conspecific females and precopulatory isolation index (IPC) to characterize the strength of premating barriers. The species under study, with the exception of L. arcana, clearly demonstrated preferential mating with conspecifics. According to FI and IPC, L. fabalis and L. compressa appeared reliably isolated from their closest relatives within Neritrema. Individuals of these two species tend to be smaller than those of the others, highlighting the importance of shell size changes in gastropod species divergence. L. arcana males were often found in pairs with L. saxatilis females, and no interspecific size differences were revealed in this sibling species pair. We discuss the lack of discriminative mate choice in the sympatric populations of L. arcana and L. saxatilis, and possible additional mechanisms restricting gene flow between them.

scholarly journals Molecular Signatures of Reticulate Evolution within the Complex of European Pine Taxa

Forests ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 489
Author(s):  
Bartosz Łabiszak ◽  
Witold Wachowiak
Keyword(s):  
Gene Flow ◽  
Reticulate Evolution ◽  
Modeling Framework ◽  
Closely Related Species ◽  
Interspecific Gene Flow ◽  
Nucleotide Sequence Variation ◽  
History Of ◽  
Best Fitting ◽  
Species Integrity

Speciation mechanisms, including the role of interspecific gene flow and introgression in the emergence of new species, are the major focus of evolutionary studies. Inference of taxonomic relationship between closely related species may be challenged by past hybridization events, but at the same time, it may provide new knowledge about mechanisms responsible for the maintenance of species integrity despite interspecific gene flow. Here, using nucleotide sequence variation and utilizing a coalescent modeling framework, we tested the role of hybridization and introgression in the evolutionary history of closely related pine taxa from the Pinus mugo complex and P. sylvestris. We compared the patterns of polymorphism and divergence between taxa and found a great overlap of neutral variation within the P. mugo complex. Our phylogeny reconstruction indicated multiple instances of reticulation events in the past, suggesting an important role of interspecific gene flow in the species divergence. The best-fitting model revealed P. mugo and P. uncinata as sister species with basal P. uliginosa and asymmetric migration between all investigated species after their divergence. The magnitude of interspecies gene flow differed greatly, and it was consistently stronger from representatives of P. mugo complex to P. sylvestris than in the opposite direction. The results indicate the prominent role of reticulation evolution in those forest trees and provide a genetic framework to study species integrity maintained by selection and local adaptation.

scholarly journals Recombination Can Evolve in Large Finite Populations Given Selection on Sufficient Loci

Genetics ◽  
2003 ◽  
Vol 165 (4) ◽  
pp. 2249-2258 ◽  
Author(s):  
Mark M Iles ◽  
Kevin Walters ◽  
Chris Cannings
Keyword(s):  
Experimental Evidence ◽  
Genetic Drift ◽  
Finite Population ◽  
Selective Advantage ◽  
Indirect Selection ◽  
Small Populations ◽  
Finite Populations ◽  
Population Sizes ◽  
Population Recombination

AbstractIt is well known that an allele causing increased recombination is expected to proliferate as a result of genetic drift in a finite population undergoing selection, without requiring other mechanisms. This is supported by recent simulations apparently demonstrating that, in small populations, drift is more important than epistasis in increasing recombination, with this effect disappearing in larger finite populations. However, recent experimental evidence finds a greater advantage for recombination in larger populations. These results are reconciled by demonstrating through simulation without epistasis that for m loci recombination has an appreciable selective advantage over a range of population sizes (am, bm). bm increases steadily with m while am remains fairly static. Thus, however large the finite population, if selection acts on sufficiently many loci, an allele that increases recombination is selected for. We show that as selection acts on our finite population, recombination increases the variance in expected log fitness, causing indirect selection on a recombination-modifying locus. This effect is enhanced in those populations with more loci because the variance in phenotypic fitnesses in relation to the possible range will be smaller. Thus fixation of a particular haplotype is less likely to occur, increasing the advantage of recombination.

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