scholarly journals Evolutionary rescue and the limits of adaptation

2013 ◽  
Vol 368 (1610) ◽  
pp. 20120080 ◽  
Author(s):  
Graham Bell
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Relative Fitness ◽  
Population Declines ◽  
Genetic Constraints ◽  
Evolutionary Rescue ◽  
Large Populations ◽  
Effective Selection ◽  
The Cost ◽  
Number Of Individuals

Populations subject to severe stress may be rescued by natural selection, but its operation is restricted by ecological and genetic constraints. The cost of natural selection expresses the limited capacity of a population to sustain the load of mortality or sterility required for effective selection. Genostasis expresses the lack of variation that prevents many populations from adapting to stress. While the role of relative fitness in adaptation is well understood, evolutionary rescue emphasizes the need to recognize explicitly the importance of absolute fitness. Permanent adaptation requires a range of genetic variation in absolute fitness that is broad enough to provide a few extreme types capable of sustained growth under a stress that would cause extinction if they were not present. This principle implies that population size is an important determinant of rescue. The overall number of individuals exposed to selection will be greater when the population declines gradually under a constant stress, or is progressively challenged by gradually increasing stress. In gradually deteriorating environments, survival at lethal stress may be procured by prior adaptation to sublethal stress through genetic correlation. Neither the standing genetic variation of small populations nor the mutation supply of large populations, however, may be sufficient to provide evolutionary rescue for most populations.

Isozyme diversity in large and isolated populations of Luma apiculata (Myrtaceae) in north-western Patagonia, Argentina

2005 ◽  
Vol 53 (8) ◽  
pp. 781 ◽  
Author(s):  
Mayra S. Caldiz ◽  
Andrea C. Premoli
Keyword(s):  
Genetic Variation ◽  
Isolated Populations ◽  
Isozyme Electrophoresis ◽  
Wahlund Effect ◽  
Rivers And Lakes ◽  
Large Populations ◽  
Vegetative Spread ◽  
Number Of Individuals ◽  
North Western ◽  
Isozyme Diversity

We evaluated the amount and distribution of genetic variation in large and small isolated populations of Luma apiculata (DC.) Burret (Myrtaceae) in north-western Patagonia. The hypothesis tested was that isolated smaller populations were more affected by drift and isolation than large stands. Higher genetic diversity was predicted in the latter. Fresh leaf material for isozyme electrophoresis was collected from 30 individuals in four isolated and two large and continuous stands (Quetrihue Peninsula and Punta Norte, Isla Victoria). Five subpopulations were sampled in both large stands, and in addition, three regeneration gaps in Punta Norte. Eleven loci were resolved; 91% were polymorphic in at least one population. Isolated and large populations had similar levels of genetic variation. Reduced observed heterozygosity and elevated inbreeding were measured in subpopulations and regeneration gaps within dense stands. Although small populations consist of a reduced number of individuals they are mostly coastal populations nearby rivers and lakes that may maintain considerable gene flow with other faraway populations counteracting the effects of drift. In addition to potential selfing, increased inbreeding within large populations and regeneration gaps may be due to an intra-population Wahlund effect from local seedling establishment and vegetative spread, resulting in clustered cohorts of similar genotypes.

The adaptive function of sexual reproduction: resampling the pool of genotypic possibilities.

2021 ◽  
Author(s):  
Donal Hickey ◽  
Brian Golding
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Sexual Reproduction ◽  
Finite Population ◽  
Adaptive Function ◽  
Asexual Population ◽  
Sexual Population ◽  
Naturally Occurring ◽  
The Cost ◽  
The Universe

Abstract BackgroundNatural populations harbor significant levels of genetic variability. Because of this standing genetic variation, the number of possible genotypic combinations is many orders of magnitude greater than the population size. This means that any given population contains only a tiny fraction of all possible genotypic combinations.ResultsWe show that recombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. Recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations that did not previously exist in the population. Gradually the selected sexual population approaches a state where the optimum genotype is produced by recombination and where it rises to fixation. In contrast to this, an asexual population is limited to selection among existing lower fitness genotypes.ConclusionsThe significance of the result is two-fold. First, it provides an explanation for the ubiquity of sexual reproduction in evolving populations. Secondly, it shows that recombination serves to remove concerns about the cost of natural selection acting on the naturally occurring standing genetic variation. This means that classic population genetics theory is applicable to ecological studies of natural selection acting on standing genetic variation.

scholarly journals On fitness and the cost of natural selection

1967 ◽  
Vol 9 (1) ◽  
pp. 1-15 ◽  
Author(s):  
William Feller
Keyword(s):  
Steady State ◽  
Natural Selection ◽  
Population Size ◽  
Relative Frequency ◽  
Cumulative Effect ◽  
Relative Fitness ◽  
Individual Species ◽  
Theory Of Evolution ◽  
The Absolute ◽  
The Cost

A Mendelian population without artificial external constraints does in general not increase at a constant rate. Formulas neglecting the changes in population size introduce an error which is negligible under ordinary circumstances but whose cumulative effect over long periods may be disastrous. Questions relating to the cost of natural selection, the nature of an unstable equilibrium, the survival of genes, etc. cannot be treated without regard to absolute population sizes. The limitation of the notion of relative fitnesses is illustrated by the fact that in some typical situations the survival of the a-gene depends only on the absolute fitness of the Aa-heterozygote, but not on the fitnesses of the homozygotes. Furthermore, a decrease of the (absolute or relative) fitness of one genotype may actually increase the viability of the population and its ultimate size.Even when the relative frequency qn of the a-gene tends to zero the absolute number of such genes may increase from generation to generation at a geometric rate. Therefore the circumstance that qn → 0 may be insignificant as compared to the fact that the earth cannot sustain an infinitely increasing population. Ultimately the population size is bound to influence the environment and so the fitnesses will change. Thus we must consider density-dependent fitnesses and then observed fitnesses cannot be used to predict the ultimate fate of a population. It is now known (Dobzhansky, 1965) that relative fitnesses are sometimes very sensitive to small changes in environment and that the same species may occupy a great variety of environmental niches. It is therefore quite likely that at least part of a population will find itself in a modified environment before too many generations have passed. For the evolution of a species and the development of new forms it is then not important that under fixed conditions the relative frequency qn of the a-gene would tend to zero. The problem is whether the actual number of such genes will increase for a period sufficiently long to encounter changed conditions or to establish itself in new combinations. This question is significant because the convergence of the frequencies qn to zero may be extremely slow. Thus even in a population of fixed size a disappearing gene could exist long enough to contribute to evolutionary processes.Speaking generally, the thinking in terms of an assumed steady state and relative fitnesses seems to aggravate the problem of applying the wonderful results of modern genetics to the theory of evolution. For example, various mechanisms which are often considered as eliminating genetic variability may sometimes produce the opposite effect. The theory of evolution should distinguish between what the physicist would call macroscopic and microscopic equilibrium. Even if the world as we see it were in a perfect equilibrium this would not imply an approximate steady state for individual species, not to speak of genes. It is clear that an evolution to higher forms depends on a frequent decrease in fertility rates. If one considers slow changes rather than an unattainable steady state then a loss of fitness may be beneficial in the long run and contribute to genetic variety.

scholarly journals Three types of rescue can avert extinction in a changing environment

2015 ◽  
Vol 112 (33) ◽  
pp. 10557-10562 ◽  
Author(s):  
Ruth A. Hufbauer ◽  
Marianna Szűcs ◽  
Emily Kasyon ◽  
Courtney Youngberg ◽  
Michael J. Koontz ◽  
...  
Keyword(s):  
Genetic Variation ◽  
Genetic Variability ◽  
Inbreeding Depression ◽  
Small Populations ◽  
Genetic Rescue ◽  
Microcosm Experiment ◽  
High Quality ◽  
Evolutionary Rescue ◽  
Population Sizes ◽  
Number Of Individuals

Setting aside high-quality large areas of habitat to protect threatened populations is becoming increasingly difficult as humans fragment and degrade the environment. Biologists and managers therefore must determine the best way to shepherd small populations through the dual challenges of reductions in both the number of individuals and genetic variability. By bringing in additional individuals, threatened populations can be increased in size (demographic rescue) or provided with variation to facilitate adaptation and reduce inbreeding (genetic rescue). The relative strengths of demographic and genetic rescue for reducing extinction and increasing growth of threatened populations are untested, and which type of rescue is effective may vary with population size. Using the flour beetle (Tribolium castaneum) in a microcosm experiment, we disentangled the genetic and demographic components of rescue, and compared them with adaptation from standing genetic variation (evolutionary rescue in the strictest sense) using 244 experimental populations founded at either a smaller (50 individuals) or larger (150 individuals) size. Both types of rescue reduced extinction, and those effects were additive. Over the course of six generations, genetic rescue increased population sizes and intrinsic fitness substantially. Both large and small populations showed evidence of being able to adapt from standing genetic variation. Our results support the practice of genetic rescue in facilitating adaptation and reducing inbreeding depression, and suggest that demographic rescue alone may suffice in larger populations even if only moderately inbred individuals are available for addition.

scholarly journals Sex solves Haldane’s dilemma

Genome ◽  
2019 ◽  
Vol 62 (11) ◽  
pp. 761-768
Author(s):  
Donal A. Hickey ◽  
G. Brian Golding
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Adaptive Evolution ◽  
Limiting Factor ◽  
Reproductive Costs ◽  
Reproductive Cost ◽  
Multiple Loci ◽  
Biological Explanation ◽  
Locus Selection ◽  
The Cost

The cumulative reproductive cost of multi-locus selection has been considered to be a potentially limiting factor on the rate of adaptive evolution. In this paper, we show that Haldane’s arguments for the accumulation of reproductive costs over multiple loci are valid only for a clonally reproducing population of asexual genotypes. We show that a sexually reproducing population avoids this accumulation of costs. Thus, sex removes a perceived reproductive constraint on the rate of adaptive evolution. The significance of our results is twofold. First, the results demonstrate that adaptation based on multiple genes—such as selection acting on the standing genetic variation—does not entail a huge reproductive cost as suggested by Haldane, provided of course that the population is reproducing sexually. Second, this reduction in the cost of natural selection provides a simple biological explanation for the advantage of sex. Specifically, Haldane’s calculations illustrate the evolutionary disadvantage of asexuality; sexual reproduction frees the population from this disadvantage.

scholarly journals Positive interactions within and between populations decrease the likelihood of evolutionary rescue

2020 ◽  
Author(s):  
Yaron Goldberg ◽  
Jonathan Friedman
Keyword(s):  
Human Microbiome ◽  
Positive Interactions ◽  
Population Declines ◽  
Evolutionary Forces ◽  
Selective Pressures ◽  
Multiple Phenotypes ◽  
Evolutionary Rescue ◽  
And Function ◽  
Number Of Individuals ◽  
Theoretical Analyses

AbstractPositive interactions, including intraspecies cooperation and interspecies mutualisms, play crucial roles in shaping the structure and function of many ecosystems, ranging from plant communities to the human microbiome. While the evolutionary forces that form and maintain positive interactions have been investigated extensively, the influence of positive interactions on the ability of species to adapt to new environments is still poorly understood. Here, we use numerical simulations and theoretical analyses to study how positive interactions impact the likelihood that populations survive after an environment deteriorates, such that survival in the new environment requires quick adaptation via the rise of new mutants - a scenario known as evolutionary rescue. We find that the probability of evolutionary rescue in populations engaged in positive interactions is reduced significantly. In cooperating populations, this reduction is largely due to the fact that survival may require at least a minimal number of individuals, meaning that adapted mutants must arise and spread before the population declines below this threshold. In mutualistic populations, the rescue probability is decreased further due to two additional effects - the need for both mutualistic partners to adapt to the new environment, and competition between the two species. Finally, we show that the presence of cheaters reduces the likelihood of evolutionary rescue even further, making it extremely unlikely. These results indicate that while positive interactions may be beneficial in stable environments, they can hinder adaptation to changing environments and thereby elevate the risk of population collapse. Furthermore, these results may hint at the selective pressures that drove co-dependent unicellular species to form more adaptable organisms able to differentiate into multiple phenotypes, including multicellular life.

scholarly journals Sex Solves Haldane’s Dilemma

2019 ◽  
Author(s):  
Donal A. Hickey ◽  
G. Brian Golding
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Adaptive Evolution ◽  
Limiting Factor ◽  
Reproductive Costs ◽  
Reproductive Cost ◽  
Multiple Loci ◽  
Biological Explanation ◽  
Locus Selection ◽  
The Cost

AbstractThe cumulative reproductive cost of multi-locus selection has been seen as a potentially limiting factor on the rate of adaptive evolution. In this paper, we show that Haldane’s arguments for the accumulation of reproductive costs over multiple loci are valid only for a clonally reproducing population of asexual genotypes. We show that a sexually reproducing population avoids this accumulation of costs. Thus, sex removes a perceived reproductive constraint on the rate of adaptive evolution. The significance of our results is twofold. First, the results demonstrate that adaptation based on multiple genes – such as selection acting on the standing genetic variation - does not entail a huge reproductive cost as suggested by Haldane, provided of course that the population is reproducing sexually. Secondly, this reduction in the cost of natural selection provides a simple biological explanation for the advantage of sex. Specifically, Haldane’s calculations illustrate the evolutionary disadvantage of asexuality; sexual reproduction frees the population from this disadvantage.

scholarly journals Coadaptation of mitochondrial and nuclear genes, and the cost of mother's curse

2018 ◽  
Vol 285 (1871) ◽  
pp. 20172257 ◽  
Author(s):  
Tim Connallon ◽  
M. Florencia Camus ◽  
Edward H. Morrow ◽  
Damian K. Dowling
Keyword(s):  
Negative Impact ◽  
Nuclear Genes ◽  
Male Fitness ◽  
Population Genetic Analysis ◽  
Widespread Species ◽  
Genetic Constraints ◽  
Complex Interactions ◽  
Large Populations ◽  
Intermediate Size ◽  
The Cost

Strict maternal inheritance renders the mitochondrial genome susceptible to accumulating mutations that harm males, but are otherwise benign or beneficial for females. This ‘mother's curse’ effect can degrade male survival and fertility if unopposed by counteracting evolutionary processes. Coadaptation between nuclear and mitochondrial genomes—with nuclear genes evolving to compensate for male-harming mitochondrial substitutions—may ultimately resolve mother's curse. However, males are still expected to incur a transient fitness cost during mito-nuclear coevolution, and it remains unclear how severe such costs should be. We present a population genetic analysis of mito-nuclear coadaptation to resolve mother's curse effects, and show that the magnitude of the ‘male mitochondrial load’—the negative impact of mitochondrial substitutions on male fitness components—may be large, even when genetic variation for compensatory evolution is abundant. We also find that the male load is surprisingly sensitive to population size: male fitness costs of mito-nuclear coevolution are particularly pronounced in both small and large populations, and minimized in populations of intermediate size. Our results reveal complex interactions between demography and genetic constraints during the resolution of mother's curse, suggesting potentially widespread species differences in susceptibility to mother's curse effects.

scholarly journals The adaptive function of sexual reproduction: resampling the genotype pool

2020 ◽  
Author(s):  
Donal A. Hickey ◽  
G. Brian Golding
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Finite Population ◽  
Ecological Studies ◽  
Adaptive Function ◽  
Asexual Population ◽  
Sexual Population ◽  
Eukaryotic Species ◽  
Number Of Individuals ◽  
The Universe

AbstractRecombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. This is important in populations that contain abundant genetic variation because, in such populations, the number of potential genotypes is much larger than the number of individuals in the population. Here, we show how recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations. In contrast to this, an asexual population is limited to selection among existing genotypes. Since it has been shown that most eukaryotic species are genetically polymorphic, our model can explain the ubiquity of sex among such species. The model also indicates that classic population genetics theory is applicable to ecological studies of natural selection acting on standing genetic variation.

Theorems of Natural Selection: Results of Price, Fisher, and Robertson

2018 ◽  
Author(s):  
Bruce Walsh ◽  
Michael Lynch
Keyword(s):  
Natural Selection ◽  
Fundamental Theorem ◽  
Selection Response ◽  
Relative Fitness ◽  
Additive Variance ◽  
Change In The Mean ◽  
Important Exception ◽  
The Mean ◽  
The Cost ◽  
Fisher’S Fundamental Theorem

This chapter reviews a number of “theorems” of natural selection. These include exact results (true mathematical theorems): the Robertson-Price identity, Price's general expression for any form of selection response, and the Fisher-Price-Ewens version of Fisher's fundamental theorem. Their generality comes as the cost of usually being very difficult to apply. An important exception is the Robertson-Price identity, which expresses the within-generation change in the mean of a trait as its covariance with relative fitness. This chapter also examines three classic approximations: Fisher's fundamental theorem for the behavior of mean population fitness, and Robertson's secondary theorem and the breeder's equation for the expected response in a trait under selection, showing both how these results are connected and the error given by the various approximations. Finally, the chapter examines the connection between the additive variance of a trait and its correlation with fitness.

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