It was therefore surprising to discover that there is still allelic variation at this locus (e.g. that it has not been fixed by selection) in a breed such as the French Trotter that has been selected for more than a century on racing performances.
As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed.
Both results supported a "selective force" model: indels in the short introns are more likely to be fixed by selection or by genetic drift.
Since α is calculated as the proportion of nonsynonymous substitutions fixed by selection, it is inherently dependant on the number of nonsynonymous substitutions fixed by random genetic drift.
If a new mutation with a frequency-independent effect on fitness occurs in a population, it is either eliminated or fixed by selection or by drift.
HGT is especially a concern as some literature suggests that gene clustering increases the possibility that genes become fixed by selection once they have undergone transfer.
Thus, it is likely that trade-offs resulting from pleiotropy bias the mutations that are ultimately fixed by selection during evolution (Stern and Orgogozo 2009).
It also posits that genetic effects fixed by selection will, at first, be large and then be reduced in a geometric sequence as a population nears optimal fitness.
Patterns consistent with recent positive selection were also detected in TLR21 of L. v. graecus: an excess of divergence, low variation, and six replacement substitutions fixed by selection as inferred by gammamap.
This might be because these two traits are not included in the French LW breeding objectives, so that QTL that affect these traits are less likely to have been fixed by selection than QTL for traits included in selection.
We can suggest that populations carrying the new variant might have spread from a founding clone of B. atticus females, in which the new variant has been fixed by selection and genomic turnover mechanisms allowed by automictic parthenogenesis.
