76
D. Enard
balancing selection only in a small region right next to the balanced site, where
the maintenance of balanced alleles increases the observed diversity. After a long
time, it is possible to detect balancing selection as balanced polymorphisms that
can be detected as cases of trans-specific polymorphisms (Leffler et al. 2013).
The effects of fluctuating selection on patterns of linked neutral diversity have
been less extensively studied and are expected to strongly depend on the type and
pace of fluctuations (Bergland et al. 2014). Fluctuating selection includes seasonal
adaptations where allele frequencies of selected alleles fluctuate across seasons for
extended periods of time, as in the case of Drosophila populations in Northern
America (Bergland et al. 2014).
(C) Background Selection
Unlike hitchhiking and balancing selection, background selection is driven not by
advantageous mutations but by deleterious mutations (Charlesworth et al. 1993).
Deleterious mutations are mutations that decrease an individual’s reproductive
success. They are far more common than advantageous mutations (Boyko et al.
2008). Unlike advantageous mutations that happen very intermittently, there is a
constant input of deleterious mutations at any functionally important locus in the
genome. Because they are negatively selected, deleterious mutations are unlikely to
fix, even the weakly deleterious ones. The important differences between weakly
and strongly deleterious mutations are the time these mutations can segregate in
a population before they are lost and the frequencies they can reach. Strongly
deleterious mutations always segregate at low frequencies and are removed very
quickly from a population. Weakly deleterious mutations can segregate at higher
frequencies and during a greater number of generations before they are removed by
purifying selection.
The effect of background selection on linked neutral variation depends on
whether it is driven by strongly or by weakly deleterious mutations. Strongly
deleterious mutations remain at very low frequencies and disappear as fast as they
appeared. They only take away from the population of the chromosomes where they
initially appeared. By doing so, they reduce the overall diversity but have very little
impact on the SFS (Charlesworth et al. 1993). In this respect, background selection
due to strongly deleterious mutations results in a decrease in genetic diversity.
In other words, the level of neutral genetic variation would look as though the
population size was smaller.
Weakly deleterious mutations also decrease the overall diversity but in addition
also affect the SFS (Charlesworth et al. 1993). Weakly deleterious mutations can
rapidly reach higher frequencies than strongly deleterious ones through genetic
drift. Intermediate frequency, older alleles unlinked to such weakly deleterious
mutations can get fixed faster in the process. Population size is being constant, and
at the same time, there is always the same input of new neutral mutations and the
same number of recent, low-frequency neutral alleles. Background selection due to
weakly deleterious mutations therefore creates an excess of rare alleles relative to
higher-frequency ones in the SFS (Desai et al. 2012). Note however that locally
the effect is not as drastic as in the case of strong hitchhiking where the skew
D. Enard
balancing selection only in a small region right next to the balanced site, where
the maintenance of balanced alleles increases the observed diversity. After a long
time, it is possible to detect balancing selection as balanced polymorphisms that
can be detected as cases of trans-specific polymorphisms (Leffler et al. 2013).
The effects of fluctuating selection on patterns of linked neutral diversity have
been less extensively studied and are expected to strongly depend on the type and
pace of fluctuations (Bergland et al. 2014). Fluctuating selection includes seasonal
adaptations where allele frequencies of selected alleles fluctuate across seasons for
extended periods of time, as in the case of Drosophila populations in Northern
America (Bergland et al. 2014).
(C) Background Selection
Unlike hitchhiking and balancing selection, background selection is driven not by
advantageous mutations but by deleterious mutations (Charlesworth et al. 1993).
Deleterious mutations are mutations that decrease an individual’s reproductive
success. They are far more common than advantageous mutations (Boyko et al.
2008). Unlike advantageous mutations that happen very intermittently, there is a
constant input of deleterious mutations at any functionally important locus in the
genome. Because they are negatively selected, deleterious mutations are unlikely to
fix, even the weakly deleterious ones. The important differences between weakly
and strongly deleterious mutations are the time these mutations can segregate in
a population before they are lost and the frequencies they can reach. Strongly
deleterious mutations always segregate at low frequencies and are removed very
quickly from a population. Weakly deleterious mutations can segregate at higher
frequencies and during a greater number of generations before they are removed by
purifying selection.
The effect of background selection on linked neutral variation depends on
whether it is driven by strongly or by weakly deleterious mutations. Strongly
deleterious mutations remain at very low frequencies and disappear as fast as they
appeared. They only take away from the population of the chromosomes where they
initially appeared. By doing so, they reduce the overall diversity but have very little
impact on the SFS (Charlesworth et al. 1993). In this respect, background selection
due to strongly deleterious mutations results in a decrease in genetic diversity.
In other words, the level of neutral genetic variation would look as though the
population size was smaller.
Weakly deleterious mutations also decrease the overall diversity but in addition
also affect the SFS (Charlesworth et al. 1993). Weakly deleterious mutations can
rapidly reach higher frequencies than strongly deleterious ones through genetic
drift. Intermediate frequency, older alleles unlinked to such weakly deleterious
mutations can get fixed faster in the process. Population size is being constant, and
at the same time, there is always the same input of new neutral mutations and the
same number of recent, low-frequency neutral alleles. Background selection due to
weakly deleterious mutations therefore creates an excess of rare alleles relative to
higher-frequency ones in the SFS (Desai et al. 2012). Note however that locally
the effect is not as drastic as in the case of strong hitchhiking where the skew