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The incidental purging of non-deleterious alleles due to such spatial proximity to deleterious alleles is called background selection. [4] This effect increases with lower mutation rate but decreases with higher recombination rate. [5] Purifying selection can be split into purging by non-random mating (assortative mating) and purging by genetic ...
This occurs because natural selection is favoring the purifying selection, and the weeding out of deleterious alleles. [9] Because silent mutations are neutral, a neutrality index lower than 1 (i.e. NI < 1) indicates an excess of nonsilent divergence, which occurs when positive selection is at work in the population.
A ratio greater than 1 implies positive or Darwinian selection (driving change); less than 1 implies purifying or stabilizing selection (acting against change); and a ratio of exactly 1 indicates neutral (i.e. no) selection. However, a combination of positive and purifying selection at different points within the gene or at different times ...
Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone. Balancing selection is rare compared to purifying selection. [1]
Genetic purging is the increased pressure of natural selection against deleterious alleles prompted by inbreeding. [1]Purging occurs because deleterious alleles tend to be recessive, which means that they only express all their harmful effects when they are present in the two copies of the individual (i.e., in homozygosis).
The sequence is experiencing heavy purifying selection, so any new mutation in the sequence is deleterious and is purged off immediately, or; The sequence just experienced a bout of selective sweep (an allele rose to fixation/near fixation), so all alleles became homogenized. The rare polymorphisms you see are very recent, or
Nonsynonymous substitutions have been found to be more common in loci involving pathogen resistance, reproductive loci involving sperm competition or egg-sperm interactions, and genes that have replicated and gained new functions, indicating that positive selection is taking place. [3]
Empirical support for the neutral theory may vary depending on the type of genomic data studied and the statistical tools used to detect positive selection. [44] For example, Bayesian methods for the detection of selected codon sites and McDonald-Kreitman tests have been criticized for their rate of erroneous identification of positive selection.