Species Selection on Organismal Integration

Selection processes of entities higher than individuals have recently been suggested to play a potential role in macroevolution. In particular, population level traits such as variability seem likely candidates for higher-level selection processes because they interact with population fitness (survival). In this paper, I expand on that theme and argue that a population level trait, such as organismal integration, measured by the genetic variance—covariance matrix, can be subject to interpopulational selection. This is so because a population consisting of individuals with a high degree of integration will respond faster to selection than a less integrated one, and thus more rapidly reach new optima. This idea generates a number of predictions which are supported by data from natural and laboratory populations of a diverse array of organisms. First, the level of genetic integration in morphological characters is generally quite high. Second, there are a number of cases where the within- and among-population correlation matrices are similarly oriented. Third, the main pattern of morphological variation in birds is that species within genera are most exclusively oriented along a size-axis. These results are consistent with the ideas put forward in this paper, though not providing conclusive evidence.