12 terms

HIll robinson etc

genetic hitchiking
when a beneficial allele arises in a population and is subject to selection, it increases in frequency. allele frequencies at loci that are physically linked to the locus under selection may also change

because an unselected
or even disadvantageous allele is able to "ride along"
with a nearby favorable allele and thus increase in
over time
recombination will break down the association between the favored allele and those around it. the association will break down quickly for alleles that are far from the selected loci but slowly for near by loci
genetic diversity
at a loci near the selected locus will be reduced relative to what we would expect in a neutral model
deleterious mutations can also cause a reduction ingenetic diversity
deleterious alleles arise in a population, they tend to be eliminated by selection.
Much as beneficial mutations carry nearby alleles to fixation via hitchhiking,
deleterious mutations carry nearby alleles to extinction via a process known as
background selection. As a result of this process, genetic variation is reduced
relative to what we would expect in a neutral model.
the important general points about the evolutionary consequences of selection and physical linkage
Alleles can increase in frequency due to
selection either because they directly code for beneficial traits on their bearers,
or because they are physically linked to other beneficial alleles at other loci.
(2) Natural selection, be it positive or negative, tends to cause a decrease in genetic
variation at loci near the selected allele.
periodic selection
is one
dramatic result of tight linkage across the entire
bacterial genome. Periodic selection refers to
the following process:
1. A new beneficial mutation arises in a
bacterial population.
2. The mutation goes to fixation as a result
of strong natural selection. Because the
other loci on the bacterial chromosome
are tightly linked, alleles at most other
loci are fixed as well. This is known as a
selective sweep.
3. After the previous beneficial allele goes to fixation, a new beneficial allele
arises and the process repeats (
hill robertson
recombination breaks down negative linkae disequilibrium generated by selection and drift threby increasing the rate of adaption (the probability of fixation of a beneficial allele)

the HR effect is often used to describe the consequences of linkage reducing the effectiveness of selection regardles sof the selective reime
HR is caused
by the segregation of mutations under selection. strongly selected mutations either beneficial or deleterious are not expected to segregate within a population for long and hence the HR efect is frequently associated with weak/moderate selection when NE plays a role on the fate of the selective mutations
mullers ratchet
describes the stochastic accumulation of deleterious mutations as a consequence of drift and complete linkage in a model with deleterious mutations (no advantageous or back mutations, ever). The hitchhiking (HH) and pseudohitchhiking (pHH) models describe the spread of strongly favorable mutations (selective sweeps), possibly dragging deleterious mutations to fixation while eliminating extant variability, including other advantageous allele
the background selection
The background selection (BGS) model focuses on negative selection on strongly deleterious mutations and the consequent removal of linked neutral and weakly selected mutations from populations
differences among these models
The differences among these models can be attributed to the particular selective scenarios rather than to their qualitative consequences. All of these models, to a different degree, predict that genetic linkage will cause a reduction in Ne and, consequently, reduced levels of neutral polymorphism, efficacy of selection and rate of adaptation; that is, in one way or another, all predict the general HR effect.
what causes linkage
drift leads to change increase of certain haplotypes generates nonrandom association between alleles at different loci