Johnson K P, Seger J
Department of Biology, University of Utah, Salt Lake City, Utah, USA.
Mol Biol Evol. 2001 May;18(5):874-81. doi: 10.1093/oxfordjournals.molbev.a003869.
Slightly deleterious mutations are expected to fix at relatively higher rates in small populations than in large populations. Support for this prediction of the nearly-neutral theory of molecular evolution comes from many cases in which lineages inferred to differ in long-term average population size have different rates of nonsynonymous substitution. However, in most of these cases, the lineages differ in many other ways as well, leaving open the possibility that some factor other than population size might have caused the difference in substitution rates. We compared synonymous and nonsynonymous substitutions in the mitochondrial cyt b and ND2 genes of nine closely related island and mainland lineages of ducks and doves. We assumed that island taxa had smaller average population sizes than those of their mainland sister taxa for most of the time since they were established. In all nine cases, more nonsynonymous substitutions occurred on the island branch, but synonymous substitutions showed no significant bias. As in previous comparisons of this kind, the lineages with smaller populations might differ in other respects that tend to increase rates of nonsynonymous substitution, but here such differences are expected to be slight owing to the relatively recent origins of the island taxa. An examination of changes to apparently "preferred" and "unpreferred" synonymous codons revealed no consistent difference between island and mainland lineages.
与大种群相比，轻微有害突变在小种群中预计会以相对较高的频率固定下来。对分子进化近中性理论这一预测的支持来自许多案例，在这些案例中，推断长期平均种群大小不同的谱系具有不同的非同义替换率。然而，在大多数这些案例中，谱系在许多其他方面也存在差异，这使得除种群大小之外的某些因素可能导致替换率差异这一可能性仍然存在。我们比较了鸭和鸽的九个密切相关的岛屿和大陆谱系的线粒体细胞色素b和ND2基因中的同义替换和非同义替换。我们假设，自形成以来的大部分时间里，岛屿分类群的平均种群大小比其大陆姐妹分类群的小。在所有九个案例中，岛屿分支上发生了更多的非同义替换，但同义替换没有显示出显著的偏向性。与之前此类比较一样，种群较小的谱系可能在其他方面也存在差异，这些差异往往会增加非同义替换率，但在这里，由于岛屿分类群的起源相对较近，预计此类差异会很小。对明显“偏好”和“非偏好”同义密码子变化的研究表明，岛屿和大陆谱系之间没有一致的差异。
