Copy-cat evolution: Divergence and convergence within and between cat and dog breeds

Abstract

Many domesticated species exhibit remarkable phenotypic diversity. In nature, selection produces not only divergence but also convergence when organisms experience similar selective pressures. Whether artificial selection during domestication also produces convergence has received little attention. Three-dimensional shape analysis of domestic cat and dog skulls demonstrated convergence at multiple levels. Most broadly, cats and dogs have both diversified greatly: equaling or exceeding the morphological disparity among all modern-day species of their respective families. Moreover, as a result of artificial selection, some breeds of these two phenotypically distinct species, evolutionarily separated for 50 My, have converged to such an extreme extent that they are more similar to each other than they are to many members of their own species or their ancestors, a phenomenon never previously observed in domesticated species. Remarkably, this convergence evolved not only between dogs and cats but also multiple times within each taxon.

Keywords: cat; convergent evolution; dog; domestication; evolution.

Fig. 1.

Principal components analysis (PCA) of three-dimensional geometric morphometric data illustrating skull shape variation across domestic cats and dogs, wildcats, wolves, and broader representatives of Canidae, Felidae, and Carnivora. The morphospace occupied by each group quantifies the amount of skull shape diversity in that group and is outlined by a convex hull; the key to the convex hull lines is shown at the right. The broader groups and skull types within domestic dogs (circles) and cats (squares) are color-coded as indicated in the key. Skull shapes associated with the extreme positive and negative ends of PC1 and PC2 are illustrated by the blue skull models found along each axis.

Fig. 2.

Morphospace as in Fig. 1 but displaying only domestic cats and dogs, wildcats, and wolves to allow examination of the skull variation in each of these groups in more detail. The morphospace occupied by each group, delineated by convex hulls, quantifies skull shape diversity; line styles for convex hulls are keyed at the right. Within domestic dogs (circles) and domestic cats (squares), the skull types are color-coded as indicated. Skull shapes associated with the extreme positive and negative ends of PC1 and PC2 are illustrated by the blue skull models found along each axis.

Fig. 3.

Skull shape variation in wild species of Felidae is greater than shape variation in wild species of Canidae. The morphospace occupied by the specimens sampled within each family is delineated by convex hulls. Genera within Felidae (circles) and Canidae (squares) are color-coded as indicated. Skull shapes associated with the extreme positive and negative ends of PC1 and PC2 are illustrated by the blue skull models found along each axis.

Fig. 4.

Evolutionary convergence of head shape in brachycephalic domestic dogs and cats, as illustrated by photographs and CT scans of canids (A–D) and felids (E–H). Although wolves (A and C) and wildcats (E and G) have very different skull shapes, some of their domestic descendants like Pugs (B and D) and Persians (F and H) have convergently evolved similar skull shapes (D and H) as a result of selection for similar phenotypes.

Fig. 5.

Allometric influences on skull shape variation in domestic cats and dogs. (A) Multivariate regression of cat cranial shape on log centroid size and (B) log average breed weight (kg). (C) Multivariate regression of dog cranial shape on log centroid size and (D) log average breed weight (kg). Gray skull models illustrate average skull shapes associated with small, medium, and large sizes, highlighting size-related morphological trends.