When a bacterial gene is under low selection pressure, it undergoes a period of frequent nucleotide substitutions because deleterious mutations are not efficiently purged.
It has been argued that the basicranium is a relatively conservative region of the cranium, under relatively low selection pressure [51], and the analyses reported here demonstrate the low disparity of the basicranium in support this view.
If senescence has evolved under equally low selection pressure among all mammals, we expect a slope of zero in the relationship between the ratio of maximum/mean life span and mean life span; that is, maximum life span should represent a more or less constant function of mean life span.
Thus, most pseudogenes, including those that are transcribed, appear to be under very low selection strength.
However, extensive regulatory subfunctionalization might also have occurred under relative low selection constraints, resulting in functional divergence at the expression level.
The gene conversion event possibly responsible for this concerted evolution status in rice resulted in the preservation of crucial expression under high selection constraints; however, extensive regulatory subfunctionalization followed by functional divergence was suggested to have occurred under relative low selection constraints in members of this gene cluster.
The moderate significance of TP184396's alignment to the B. distachyon genome may be due to the fact that the marker is located in an intron, which is usually more likely than exons to be under low selection pressure and diverge substantially across species.
Consequently, traits under strong past selection are less plastic and more robust to environmental stress, whereas traits under low selection pressure have higher plasticities and are more sensitive to environmental changes (Stillwell et al., 2010).
It is therefore possible that many of these short ORFs are eroded pseudogenes or remnants of fragmented genes, as it has been demonstrated in Rickettsia [ 54], where many genes appear to be under low selection coefficients in its intracellular environment.
According to our path models, in all wealth classes female lifespan was under the lowest selection compared to the other maternal key life-history traits studied.
