Patterns of phenotypic correlations among morphological traits across plants and animals

Abstract

Despite the long-standing interest of biologists in patterns of correlation and phenotypic integration, little attention has been paid to patterns of correlation across a broad phylogenetic spectrum. We report analyses of mean phenotypic correlations among a variety of linear measurements from a wide diversity of plants and animals, addressing questions about function, development, integration and modularity. These analyses suggest that vertebrates, hemimetabolous insects and vegetative traits in plants have similar mean correlations, around 0.5. Traits of holometabolous insects are much more highly correlated, with a mean correlation of 0.84; this may be due to developmental homeostasis caused by lower spatial and temporal environmental variance during complete metamorphosis. The lowest mean correlations were those between floral and vegetative traits, consistent with Berg's ideas about functional independence between these modules. Within trait groups, the lowest mean correlations were among vertebrate head traits and floral traits (0.38-0.39). The former may be due to independence between skull modules. While there is little evidence for floral integration overall, certain sets of functionally related floral traits are highly integrated. A case study of the latter is described from wild radish flowers.

Keywords: complete metamorphosis; developmental homeostasis; floral integration; modularity; morphological integration; phenotypic correlations.

Figure 1.

Mean phenotypic correlations (±95% CI) for plants, vertebrates and invertebrates. Number of species and total number of correlations are shown.

Figure 2.

Mean phenotypic correlations as in figure 1, but with plant correlations split into within-module (floral–floral and vegetative–vegetative) and across-module correlations.

Figure 3.

Mean phenotypic correlations (±95% CI) for vertebrates only, split into within and among three modules: head, body and limb.

Figure 4.

Mean phenotypic correlations as in figure 2, but with invertebrates split into hemi- and holometabolous insects only.