The remarkable diversity within domesticated species, particularly cats and dogs, offers a fascinating lens through which to study evolutionary mechanisms. While artificial selection has sculpted numerous breeds, the extent to which this selection drives not only divergence but also convergence in skull morphology has remained a captivating area of research. A recent study published in the Proceedings of the National Academy of Sciences (PNAS) delves into this phenomenon, revealing striking instances of convergent evolution in skull shape, not only between cats and dogs but also multiple times within each species. This research challenges previous assumptions about the inherent evolvability of different species and highlights the potent influence of selective pressures.
The Unprecedented Diversity of Domesticated Canids and Felids
Domestication has led to an explosion of phenotypic diversity within both dog and cat breeds. Studies have shown that the variation in skull shapes among dog breeds far exceeds that of their wild ancestor, the wolf, and indeed, encompasses a wider range than all modern canid species combined. This research extended this investigation to domestic cats, finding that they too exhibit substantial skull shape variation, comparable to or even exceeding the diversity found within their respective families. This implies that domestication, driven by artificial selection, has unlocked a significant potential for morphological change in both lineages.
The study utilized three-dimensional geometric morphometrics to analyze the skull shapes of over 1,800 specimens, including domestic cats and dogs, their wild ancestors (wildcats and wolves), and other members of the Canidae and Felidae families. By mapping these skull shapes in a multidimensional space, the researchers could quantify and compare the diversity within and between groups.
Convergence: When Cats and Dogs Become More Alike
One of the most surprising findings of the study was the significant convergence in skull shape between certain breeds of domestic cats and dogs. Despite their evolutionary separation dating back approximately 50 million years, some brachycephalic (short-faced) breeds of cats, such as Persians, exhibit skull shapes remarkably similar to those of brachycephalic dog breeds like Pugs and Shih Tzus. This convergence is so pronounced that these breeds are often morphologically closer to each other than they are to their own wild ancestors or even many other breeds within their own species.
This striking similarity is attributed to intense artificial selection by breeders who favor specific aesthetic traits. Breed standards for extreme brachycephalic cats and dogs often emphasize a vertical facial profile, a lack of muzzle protrusion, and the placement of the nose between the eyes – mirroring a “baby schema” that elicits a nurturing response. The study found that this selection pressure has effectively reduced the ancestral differences in skull shape between these two distinct species.
Multilevel Convergence and Divergent Evolutionary Trajectories
The phenomenon of convergence is not limited to the interspecies comparison between cats and dogs. The research also identified instances of convergence occurring multiple times within each species. For example, brachycephaly has evolved independently in different dog breeds, such as Asian toy breeds (like Pekingese) and Western breeds (like Bulldogs). Similarly, within cats, brachycephaly has appeared in breeds like Persians and, more recently, in the distantly related American Burmese.
Interestingly, while the outcome of brachycephaly is convergent, the evolutionary trajectories to achieve it can differ. Dogs, starting from a more elongated skull base (wolves), have undergone significant modifications in muzzle length and cranial flexion to achieve brachycephaly. Cats, with ancestrally shorter muzzles, have also evolved brachycephaly, but the relative contribution of muzzle shortening and palate rotation may differ. This suggests that similar selection pressures can lead to convergent phenotypes through different genetic and developmental pathways.
Wild Relatives: A Contrast in Diversity
The study also highlighted a fascinating contrast between the diversity of wild canids and felids. While domestic dogs exhibit far greater skull shape variation than domestic cats, wild felids, as a group, display substantially more diversity in skull shape than wild canids. This suggests that canids might not be inherently more “evolvable” than felids. Instead, the extreme variation seen in dog breeds may be linked to their wider range of body sizes and potentially different selective pressures acting on their ancestors. The limited diversity in wild canids might reflect natural selection acting to maintain a more consistent set of traits, whereas artificial selection in domestic dogs has pushed boundaries far beyond ancestral norms.
Health Implications of Convergent Evolution
The convergent evolution of brachycephaly in cats and dogs, driven by human aesthetic preferences, unfortunately, comes with significant health implications. Many brachycephalic breeds, in both species, are predisposed to a range of health disorders, including respiratory problems, eye issues, and dental malocclusions. This has led to increasing calls for bans on breeding extreme brachycephalic individuals to improve animal welfare.
Conclusion: Artificial Selection’s Powerful Influence
This PNAS study provides compelling evidence of the power of artificial selection to drive both divergence and convergence in skull morphology. The striking similarities between brachycephalic cats and dogs underscore how intense selection for specific traits can overcome deep evolutionary divides, leading to remarkably parallel evolutionary outcomes. While this phenomenon offers valuable insights into evolutionary mechanisms, it also serves as a stark reminder of the health consequences that can arise when aesthetic desires override the well-being of our animal companions. The research not only enriches our understanding of evolution but also prompts critical reflection on responsible breeding practices for the health and welfare of domestic animals.
