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Related Concept Videos

Comparing Copy Number Variations and SNPs02:26

Comparing Copy Number Variations and SNPs

Sequencing of the human genome has opened up several best-kept secrets of the genome. Scientists have identified thousands of genome variations that exist within a population. These variations can be a single nucleotide or a larger chromosomal variation.
Copy number variations or CNVs are the structural variations that cover more than 1kb of DNA sequence. The single nucleotide polymorphism (SNP), on the other hand, is a single nucleotide change or a point mutation that is found in more than 1%...
Genome Copying Errors02:46

Genome Copying Errors

DNA replication is a well-evolved process that copies millions of base pairs with high fidelity during each cell division. Occasionally a wrong base or a long stretch of wrong bases may get added to the daughter strands. If the errors are left unchecked, cells might accumulate several mutations that might endanger their  survival. Therefore, the copying errors are checked and repaired at three levels.
Frequency-dependent Selection01:21

Frequency-dependent Selection

When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.Positive Frequency-Dependent SelectionIn positive...
Types of Selection01:46

Types of Selection

Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...
Limits to Natural Selection01:38

Limits to Natural Selection

Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.For one, natural selection can only act upon existing genetic variation. Hypothetically, redtusks may enhance elephant survival by deterring ivory-seeking poachers. However, if there are no gene variants—or alleles—for redtusks, natural selection cannot increase the prevalence of...
Mutation, Gene Flow, and Genetic Drift01:09

Mutation, Gene Flow, and Genetic Drift

In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).Mechanisms of Genetic VariationThe original sources of genetic variation are mutations,...

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Related Experiment Video

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Following the Dynamics of Structural Variants in Experimentally Evolved Populations
04:52

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Published on: February 3, 2023

Reduced purifying selection prevails over positive selection in human copy number variant evolution.

Duc-Quang Nguyen1, Caleb Webber, Jayne Hehir-Kwa

  • 1MRC Functional Genomics Unit, University of Oxford, Department of Physiology, Anatomy and Genetics, Oxford OX1 3QX, United Kingdom.

Genome Research
|August 9, 2008
PubMed
Summary

Copy number variants (CNVs) are not primarily retained for adaptive benefit. Instead, reduced selection efficiency allows slightly deleterious changes and increased CNV allele drift in the human genome.

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Area of Science:

  • Genomics
  • Evolutionary Biology
  • Population Genetics

Background:

  • Copy number variation (CNV) significantly contributes to genomic variation and disease susceptibility.
  • Previous hypotheses suggested CNVs are maintained due to adaptive advantages.

Purpose of the Study:

  • To re-evaluate the evolutionary forces shaping copy number variants (CNVs) in the human population.
  • To investigate whether CNVs are retained due to adaptive benefits or other evolutionary mechanisms.

Main Methods:

  • Analysis of four CNV datasets to assess genic biases and evolutionary rates.
  • Examination of the relationship between CNVs, segmental duplications (SDs), and gene essentiality.
  • Investigation of chromosomal rearrangement, nucleotide substitution, gene density, and G+C content in CNV regions.

Main Results:

  • Genic biases in CNVs are better explained by reduced selection efficiency rather than positive selection.
  • Three out of four CNV datasets showed increased protein evolutionary rates, linked to infrequent recombination in segmental duplications.
  • Essential genes (mouse orthologs with lethal disruption) are depleted in CNVs, supporting reduced purifying selection.
  • CNV regions exhibit higher rates of chromosomal rearrangement, nucleotide substitution, gene density, and G+C content.

Conclusions:

  • CNVs arise and segregate in the human population due to reduced purifying selection (Hill-Robertson interference) in nonessential gene-rich regions.
  • This reduced selection allows fixation of slightly deleterious mutations and increased drift of CNV alleles.
  • High G+C content is associated with sequences prone to CNVs and frequent duplication, while fixed SDs are found in low G+C content regions.