2. GENETIC VARIATION, DARWINIAN FITNESS, AND GENETIC LOAD

ABSTRACT A model of population structure in heterogeneous environments is described with attention focused on genetic variation at a single locus. The existence of equilibria at which there is no genetic load is examined.—The absolute fitness of any genotype is regarded as a function of location in the niche space and the population density at that location. It is assumed that each organism chooses to live in that habitat in which it is most fit ("optimal habitat selection").—Equilibria at which there is no segregational load ("loadless equilibria") may exist. Necessary and sufficient conditions for the existence of such equilibria are very weak. If there is a sufficient amount of dominance or area in which the alleles are selectively neutral, then there exist equilibria without segregational loads. In the N, p phase plane defined by population size, N, and gene frequency, p, these equilibria generally consist of a line segment which is parallel to the p axis. These equilibria are frequently stable.

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Butterfly survival on an isolated island by improved grip

Biology Letters ◽

10.1098/rsbl.2013.0020 ◽

2013 ◽

Vol 9 (2) ◽

pp. 20130020 ◽

Cited By ~ 7

Author(s):

Anne Duplouy ◽

Ilkka Hanski

Keyword(s):

Genetic Variation ◽

Natural Selection ◽

Local Adaptation ◽

Genetic Load ◽

High Rate ◽

Dispersal Capacity ◽

Flight Capacity ◽

Butterfly Population

On small isolated islands, natural selection is expected to reduce the dispersal capacity of organisms, as short distances do not require a high rate of dispersal, which might lead to accidental emigration from the population. In addition, individuals foregoing the high cost of maintaining flight capacity may instead allocate resources to other functions. However, in butterflies and many other insects, flight is necessary not only for dispersal but also for most other activities. A weakly flying individual would probably do worse and have an elevated rather than reduced probability of accidental emigration. Here, we report results consistent with the hypothesis that a butterfly population on an isolated island, instead of having lost its flight capacity, has evolved better grip to resist the force of wind and to avoid being blown off the island. Our study suggests that local adaptation has occurred in this population in spite of its very small size ( N e ∼ 100), complete isolation, low genetic variation and high genetic load.

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GENETIC VARIATION AND GENETIC LOAD DUE TO THE MALE REPRODUCTIVE COMPONENT OF FITNESS IN DROSOPHILA

Genetics ◽

10.1093/genetics/97.3-4.719 ◽

1981 ◽

Vol 97 (3-4) ◽

pp. 719-730

Author(s):

John G Brittnacher

Keyword(s):

Drosophila Melanogaster ◽

Genetic Variation ◽

Female Choice ◽

Genetic Load ◽

Competition Experiment ◽

Male Mating ◽

Mating Competition ◽

Mating Propensity ◽

The Mean ◽

Male Mating Competition

ABSTRACT The genetic variation and genetic load due to virility, the male reproductive component of fitness, was measured in Drosophila melanogaster and D. pseudoobscura using males homozygous and heterozygous for the second chromosome of each species. Virility was determined in a female-choice, male mating competition experiment where both mating propensity and fertility were taken into account.——The mean virility of the homozygous D. melanogaster males relative to the heterozygous males was 0.50; the relative mean virility of the quasinormal homozygotes was 0.56. The mean virility of the homozygous D. pseudoobscura males relative to the heterozygous males was 0.70; the relative mean virility of the nonsterile homozygotes was 0.72, and of the quasinormal homozygotes, 0.68.——Depending on the species and chromosome sampled, fertile homozygous males had a mean virility 15 to 50% lower than the mean viability of individuals homozygous for a chromosome with quasinormal viability. The genetic load due to virility was also greater than that due to the female reproductive component. This higher level of hidden genetic variation (or genetic load) indicates that the results of PROUT(1971a, b) and BUNDGAARD and CHRISTIANSEN(1972), where the virility component of fitness dominated the dynamics of an artificial polymorphism, may be more general and that virility may dominate the dynamics of natural polymorphisms as well.

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Evolutionary Change in the Eastern Oyster, Crassostrea Virginica, Following Low Salinity Exposure

Integrative and Comparative Biology ◽

10.1093/icb/icab185 ◽

2021 ◽

Author(s):

Joanna S Griffiths ◽

Kevin M Johnson ◽

Morgan W Kelly

Keyword(s):

Genetic Variation ◽

Crassostrea Virginica ◽

Larval Stage ◽

Genetic Load ◽

Early Life History ◽

Eastern Oyster ◽

Evolutionary Change ◽

Low Salinity ◽

Multiple Loci ◽

Breeding Grounds

Synopsis The presence of standing genetic variation will play a role in determining a population's capacity to adapt to environmentally relevant stressors. In the Gulf of Mexico, extreme climatic events and anthropogenic changes to local hydrology will expose productive oyster breeding grounds to stressful low salinity conditions. We identified genetic variation for performance under low salinity (due to the combined effects of low salinity and genetic load) using a single-generation selection experiment on larvae from two populations of the eastern oyster, Crassostrea virginica. We used pool-sequencing to test for allele frequency differences at 152 salinity-associated genes for larval families pre- and post-low salinity exposure. Our results have implications for how evolutionary change occurs during early life history stages at environmentally relevant salinities. Consistent with observations of high genetic load observed in oysters, we demonstrate evidence for purging of deleterious alleles at the larval stage in C. virginica. In addition, we observe increases in allele frequencies at multiple loci, suggesting that natural selection for low salinity performance at the larval stage can act as a filter for genotypes found in adult populations.

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The genetic consequences of dog breed formation—Accumulation of deleterious genetic variation and fixation of mutations associated with myxomatous mitral valve disease in cavalier King Charles spaniels

PLoS Genetics ◽

10.1371/journal.pgen.1009726 ◽

2021 ◽

Vol 17 (9) ◽

pp. e1009726

Author(s):

Erik Axelsson ◽

Ingrid Ljungvall ◽

Priyasma Bhoumik ◽

Laura Bas Conn ◽

Eva Muren ◽

...

Keyword(s):

Genetic Variation ◽

Mitral Valve ◽

Papillary Muscle ◽

Mitral Valve Disease ◽

Genetic Load ◽

Relative Increase ◽

Left Ventricular ◽

Valve Disease ◽

Risk Alleles ◽

Potential Link

Selective breeding for desirable traits in strictly controlled populations has generated an extraordinary diversity in canine morphology and behaviour, but has also led to loss of genetic variation and random entrapment of disease alleles. As a consequence, specific diseases are now prevalent in certain breeds, but whether the recent breeding practice led to an overall increase in genetic load remains unclear. Here we generate whole genome sequencing (WGS) data from 20 dogs per breed from eight breeds and document a ~10% rise in the number of derived alleles per genome at evolutionarily conserved sites in the heavily bottlenecked cavalier King Charles spaniel breed (cKCs) relative to in most breeds studied here. Our finding represents the first clear indication of a relative increase in levels of deleterious genetic variation in a specific breed, arguing that recent breeding practices probably were associated with an accumulation of genetic load in dogs. We then use the WGS data to identify candidate risk alleles for the most common cause for veterinary care in cKCs–the heart disease myxomatous mitral valve disease (MMVD). We verify a potential link to MMVD for candidate variants near the heart specific NEBL gene in a dachshund population and show that two of the NEBL candidate variants have regulatory potential in heart-derived cell lines and are associated with reduced NEBL isoform nebulette expression in papillary muscle (but not in mitral valve, nor in left ventricular wall). Alleles linked to reduced nebulette expression may hence predispose cKCs and other breeds to MMVD via loss of papillary muscle integrity.

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Natural selection as the process of accumulating genetic information in adaptive evolution

Genetics Research ◽

10.1017/s0016672300000616 ◽

1961 ◽

Vol 2 (1) ◽

pp. 127-140 ◽

Cited By ~ 59

Author(s):

Motoo Kimura

Keyword(s):

Natural Selection ◽

Adaptive Evolution ◽

Genetic Information ◽

Genetic Load ◽

Genetic Constitution ◽

Linear Sequence ◽

A Value ◽

Darwinian Fitness ◽

Image Position ◽

Rate Of Accumulation

1. In the course of evolution, complicated organisms have descended from much simpler ones. Since the instructions to form an organism are contained in the nucleus of its fertilized egg, this means that the genetic constitution has become correspondingly more complex in evolution. If we express this complexity in terms of its improbability, defining the amount of genetic information as the negative logarithm of its probability of occurrence by chance, we may say that genetic information is increased in the course of progressive evolution, guided by natural selection of random mutations.2. It was demonstrated that the rate of accumulation of genetic information in adaptive evolution is directly proportional to the substitutional load, i.e. the decrease of Darwinian fitness brought about by substituting for one gene its allelic form which is more fitted to a new environment. The rate of accumulation of genetic information is given bywhere Le is the substitutional load measured in ‘Malthusian parameters’.3. Using Le = 0·199, a value obtained from the application of the ‘principle of minimum genetic load’ (cf. Kimura, 1960 b), we getIt was estimated that the total amount of genetic information accumulated since the beginning of the Cambrian epoch (500 million years) may be of the order of 108 bits, if evolution has proceeded at the standard rate.Since the genetic information is transformed into phenotypic information in ontogeny, this figure (108 bits) must represent the amount of information which corresponds to the improved organization of higher animals as compared to their ancestors 500 million years back.4. Problems involved in storage and transformation of genetic information thus acquired were discussed and it was pointed out that the redundancy of information in the form of repetition in linear sequence of nucleotide pairs within a gene may play an important role in the storage of genetic information.

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Heterozygosity and Fixation Index for ABO Gene in Barak Valley Populations vis-a-vis a Few Exotic Populations

Notulae Scientia Biologicae ◽

10.15835/nsb315461 ◽

2011 ◽

Vol 3 (1) ◽

pp. 07-11 ◽

Cited By ~ 1

Author(s):

Supriyo CHAKRABORTY

Keyword(s):

Genetic Variation ◽

South China ◽

Genetic Diseases ◽

Genetic Load ◽

Fixation Index ◽

Human Populations ◽

Barak Valley ◽

Evolutionary Context ◽

Recessive Genes ◽

The Cost

In a genetic study of 26 human populations including 2 major endogamous populations (Hindus and Muslims) of Barak Valley in Assam and 24 exotic populations, observed heterozygosity (Ho), fixation index (F) and Panmictic index (P) for ABO gene were estimated from gene frequency data to reveal the extent of inbreeding that has taken place in each population during evolution. Observed heterozygosity, a measure of genetic variation, ranged from 0.3254 to 0.6086 in these populations. Expected Hardy-Weinberg heterozygosity of ABO gene was estimated as 0.6666 assuming the occurrence of all the three alleles in equal frequency. Fixation index was the highest in the population of Sudan (51.18%) followed by Australia (48.51%) and Iceland (38.28%) indicating the occurrence of high inbreeding and the presence of more homozygosity in these populations during evolution. But the fixation index was the lowest in the population of South China (8.70%) followed by Central Asia (11.82%) and Russia (12.96%). It suggested the occurrence of low inbreeding and hence more outbreeding in these populations resulting in the existence of more heterozygosity (high genetic variation) in these populations. Panmictic index, a measure of outbreeding, is the opposite of fixation index and it varied from 48.82 (Sudan) to 91.30% (South China). The population showing the highest fixation index recorded the lowest panmictic index and vice-versa. In evolutionary context, outbreeding in human populations would be more desirable to reduce the incidence of genetic diseases caused by recessive genes and to enhance heterozygosity for those loci for better adaptation of future generations, possibly at the cost of gradually increasing genetic load in the population.

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