Cranial integration in the ring-necked parakeet, Psittacula krameri (Psittaciformes: Psittaculidae)

The study of integration and modularity aims to describe the organization of components that make up organisms, and the evolutionary, developmental and functional relationships among them. Both have been studied at the interspecific (evolutionary) and intraspecific (phenotypic and ontogenetic) levels to different degrees across various clades. Although evolutionary modularity and integration are well-characterized across birds, knowledge of intraspecific patterns is lacking. Here, we use a high-density, three-dimensional geometric morphometric approach to investigate patterns of integration and modularity in Psittacula krameri, a highly successful invasive parrot species that exhibits the derived vertical palate and cranio-facial hinge of the Psittaciformes. Showing a pattern of nine distinct cranial modules, our results support findings from recent research that uses similar methods to investigate interspecific integration in birds. Allometry is not a significant influence on cranial shape variation within this species; however, within-module integration is significantly negatively correlated with disparity, with high variation concentrated in the weakly integrated rostrum, palate and vault modules. As previous studies have demonstrated differences in beak shape between invasive and native populations, variation in the weakly integrated palate and rostrum may have facilitated evolutionary change in these parts of the skull, contributing to the ring-necked parakeet’s success as an invasive species.

Evolution of a multifunctional trait: shared effects of foraging ecology and thermoregulation on beak morphology, with consequences for song evolution

While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function not only in foraging but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however, most focus on a single function. Hence, we quantified relative contributions of different functions over an evolutionary timescale. We measured beak shape using geometric morphometrics and compared this trait with foraging behaviour, climatic variables and song characteristics in a phylogenetic comparative study of an Australasian radiation of songbirds (Meliphagidae). We found that both climate and foraging behaviour were significantly correlated with the beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a nearly equal effect on size. We also found that evolutionary changes in beak morphology had significant consequences for vocal performance: species with elongate-shaped beaks sang at higher frequencies, while species with large beaks sang at a slower pace. The evolution of the avian beak exemplifies how morphological traits can be an evolutionary compromise among functions, and suggests that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.

Phylogenetic non-independence in rates of trait evolution

Statistical non-independence of species’ biological traits is recognized in most traits under selection. Yet, whether or not the evolutionary rates of such biological traits are statistically non-independent remains to be tested. Here we test the hypothesis that phenotypic evolutionary rates are non-independent, i.e. contain phylogenetic signal, using empirical rates of evolution in three separate traits: body mass in mammals; beak shape in birds; and bite force in amniotes. Specifically, we test whether rates are non-independent throughout the evolutionary history of each tree. We find evidence for phylogenetic signal in evolutionary rates in all three case studies. While phylogenetic signal diminishes deeper in time, this is reflective of statistical power owing to small sample and effect sizes. When effect size is large, e.g., owing to the presence of fossil tips, we detect high phylogenetic signals even in deeper time slices. Thus, we recommend that rates be treated as being non-independent throughout the evolutionary history of the group of organisms under study, and any summaries or analyses of rates through time – including associations of rates with traits – need account for the undesired effects of shared ancestry.

Phylogenetic non-independence in rates of trait evolution

Statistical non-independence of species’ biological traits is recognized in most traits under selection. Yet, whether or not the evolutionary rates of such biological traits are statistically non-independent remains to be tested. Here, we test the hypothesis that phenotypic evolutionary rates are non-independent, i.e. contain phylogenetic signal, using empirical rates of evolution in three separate traits: body mass in mammals, beak shape in birds and bite force in amniotes. Specifically, we test if evolutionary rates are phylogenetically interdependent. We find evidence for phylogenetic signal in evolutionary rates in all three case studies. While phylogenetic signal diminishes deeper in time, this is reflective of statistical power owing to small sample and effect sizes. When effect size is large, e.g. owing to the presence of fossil tips, we detect high phylogenetic signals even in deeper time slices. Thus, we recommend that rates be treated as being non-independent throughout the evolutionary history of the group of organisms under study, and any summaries or analyses of rates through time—including associations of rates with traits—need to account for the undesired effects of shared ancestry.