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Related Concept Videos

What is Population Genetics?01:25

What is Population Genetics?

A population is composed of members of the same species that simultaneously live and interact in the same area. When individuals in a population breed, they pass down their genes to their offspring. Many of these genes are polymorphic, meaning that they occur in multiple variants. Such variations of a gene are referred to as alleles. The collective set of all the alleles within a population is known as the gene pool.While some alleles of a given gene might be observed commonly, other variants...
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,...
Hardy-Weinberg Principle01:49

Hardy-Weinberg Principle

Diploid organisms have two alleles of each gene, one from each parent, in their somatic cells. Therefore, each individual contributes two alleles to the gene pool of the population. The gene pool of a population is the sum of every allele of all genes within that population and has some degree of variation. Genetic variation is typically expressed as a relative frequency, which is the percentage of the total population that has a given allele, genotype or phenotype.In the early 20th century,...
Genetic Drift03:33

Genetic Drift

Natural selection—probably the most well-known evolutionary mechanism—increases the prevalence of traits that enhance survival and reproduction. However, evolution does not merely propagate favorable traits, nor does it always benefit populations.Life is not fair. A deer grazing contentedly in a field can have her meal cut tragically short by a bolt of lightning. If the doomed doe is one of only three in the population, 1/3 of the population’s gene pool is lost. Random events like this can...
Genetic Variation01:25

Genetic Variation

Genetic variation is the diversity in DNA sequences found among individuals of the same species. This diversity is crucial for a species' survival because it helps organisms adapt to environmental changes. Genetic variation begins with fertilization, where an egg and sperm cell merge. Each of these cells carries 23 chromosomes, up to 46 in the fertilized egg. Chromosomes are long DNA strands that contain genes, the basic units of heredity.
Genes exist in different versions called alleles, which...
Evolutionary Relationships through Genome Comparisons02:54

Evolutionary Relationships through Genome Comparisons

Genome comparison is one of the excellent ways to interpret the evolutionary relationships between organisms. The basic principle of genome comparison is that if two species share a common feature, it is likely encoded by the DNA sequence conserved between both species. The advent of genome sequencing technologies in the late 20th century enabled scientists to understand the concept of conservation of domains between species and helped them to deduce evolutionary relationships across diverse...

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Application of DNA Fingerprinting using the D1S80 Locus in Lab Classes
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The population genetics of dN/dS.

Sergey Kryazhimskiy1, Joshua B Plotkin

  • 1Biology Department, University of Pennsylvania, Philadelphia, PA, USA.

Plos Genetics
|December 17, 2008
PubMed
Summary

The ratio of non-synonymous to synonymous substitution rates (dN/dS) is often misapplied to population samples. This study finds dN/dS is insensitive to selection within a population, challenging its use for inferring evolutionary pressures.

Area of Science:

  • Evolutionary biology
  • Population genetics
  • Molecular evolution

Background:

  • The non-synonymous to synonymous substitution rate ratio (dN/dS) is a key metric for quantifying evolutionary pressures on proteins.
  • Traditionally, dN/dS is applied to distantly diverged sequences to identify fixed substitutions along independent lineages.
  • However, dN/dS is increasingly used for sequences within a single population, representing segregating polymorphisms.

Purpose of the Study:

  • To investigate the expected dN/dS ratio within a single population under selective pressures.
  • To determine the reliability of dN/dS as an indicator of positive selection in population samples.
  • To re-evaluate the interpretation of dN/dS measurements in population genetics.

Main Methods:

  • Theoretical analysis of the expected dN/dS ratio for samples from a single population.

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  • Modeling the relationship between selection coefficients and dN/dS values within populations.
  • Comparing the behavior of dN/dS under selection in divergent lineages versus within a population.

    • Main Results:

    • Within a single population, dN/dS is largely insensitive to the strength of the selection coefficient.
    • The common indicator of positive selection (dN/dS > 1) is not applicable or is violated for within-population samples.
    • The relationship between selection and dN/dS within populations is non-monotonic, complicating inference.

    Conclusions:

    • The application of dN/dS to population samples requires careful consideration, as it may not accurately reflect selection pressures.
    • The interpretation of dN/dS values derived from population genetic data needs revision.
    • This study highlights the limitations of dN/dS for inferring evolutionary selection within populations.