Departamento de Genética e Biologia Evolutiva, Instituto de Biociências, Universidade de São Paulo, CP 11.461, CEP 05422-970, São Paulo, SP, Brasil.
Evolution. 2010 May;64(5):1470-88. doi: 10.1111/j.1558-5646.2009.00920.x. Epub 2009 Dec 10.
Evolutionary change in New World Monkey (NWM) skulls occurred primarily along the line of least resistance defined by size (including allometric) variation (g(max)). Although the direction of evolution was aligned with this axis, it was not clear whether this macroevolutionary pattern results from the conservation of within population genetic covariance patterns (long-term constraint) or long-term selection along a size dimension, or whether both, constraints and selection, were inextricably involved. Furthermore, G-matrix stability can also be a consequence of selection, which implies that both, constraints embodied in g(max) and evolutionary changes observed on the trait averages, would be influenced by selection. Here, we describe a combination of approaches that allows one to test whether any particular instance of size evolution is a correlated by-product due to constraints (g(max)) or is due to direct selection on size and apply it to NWM lineages as a case study. The approach is based on comparing the direction and amount of evolutionary change produced by two different simulated sets of net-selection gradients (beta), a size (isometric and allometric size) and a nonsize set. Using this approach it is possible to distinguish between the two hypotheses (indirect size evolution due to constraints or direct selection on size), because although both may produce an evolutionary response aligned with g(max), the amount of change produced by random selection operating through the variance/covariance patterns (constraints hypothesis) will be much smaller than that produced by selection on size (selection hypothesis). Furthermore, the alignment of simulated evolutionary changes with g(max) when selection is not on size is not as tight as when selection is actually on size, allowing a statistical test of whether a particular observed case of evolution along the line of least resistance is the result of selection along it or not. Also, with matrix diagonalization (principal components [PC]) it is possible to calculate directly the net-selection gradient on size alone (first PC [PC1]) by dividing the amount of phenotypic difference between any two populations by the amount of variation in PC1, which allows one to benchmark whether selection was on size or not.
新世界猴（NWM）头骨的进化变化主要沿着由大小（包括比例）变化（g(max)）定义的最小阻力线发生。虽然进化的方向与该轴一致，但尚不清楚这种宏观进化模式是源自种群内遗传协方差模式的保守性（长期约束）还是沿着大小维度的长期选择，或者两者都是不可分割的。此外，G 矩阵的稳定性也可能是选择的结果，这意味着 g(max) 中体现的约束和观察到的特征平均值上的进化变化都会受到选择的影响。在这里，我们描述了一种组合方法，该方法可以测试任何特定大小进化实例是否是由于约束（g(max)）而产生的相关副产品，还是由于对大小的直接选择而产生，并将其作为案例研究应用于新世界猴谱系。该方法基于比较两个不同的净选择梯度（β）模拟集产生的进化变化的方向和数量，一个是大小（等距和比例大小），一个是非大小集。使用这种方法，可以区分这两个假设（由于约束而间接的大小进化或直接对大小进行选择），因为虽然两者都可能产生与 g(max)一致的进化反应，但通过方差/协方差模式（约束假设）随机选择产生的变化量要比大小选择（选择假设）产生的变化量小得多。此外，当选择不是大小时，模拟进化变化与 g(max)的对齐不如实际选择大小时那么紧密，这允许对特定观察到的沿最小阻力线进化的情况是选择沿其进行的结果还是不是进行统计检验。此外，通过矩阵对角化（主成分[PC]），可以通过将两个种群之间的表型差异量除以 PC1 中的变异量，直接计算出仅针对大小的净选择梯度（第一主成分[PC1]），这可以使基准化选择是否针对大小。
