"Intraspecific macroevolution" within domestic dog breeds
Several years ago, I saw a fantastic talk at the Evolution meeting about Intraspecific macroevolution: variation of cranial shape in dog breeds. The talk was by Abby Drake, then a grad student, and reported on a huge digital morphometric comparison of the skulls of dogs and many representatives from the order Carnivora (dogs, cats, bears, sea lions, etc.).
Morphometrics basically consists of taking digital photos of e.g. bones from different angles, and then marking the same landmarks on homologous bones across a big group. Then you can quantitatively compare the differences in shape, independent of things like body size. This is a much more sophisticated analysis than is possible with just calipers, where you can only get length, width, etc.
A previous study had noted that the skull variation in dogs was bigger than the variation in the family Canidae, but the incredible result of Drake’s study was that the variation in shape of dog skulls was bigger than the variation in shape across the entire order Carnivora, which is 60 million years old and includes even mostly-aquatic forms.
Figure 3: Principal component (PC) analysis for skull shape in the complete data set. A-C, Plots of the PC scores. D, Shape changes associated with the PC axes. For each PC, the shapes corresponding to the observed extremes in the positive and negative directions are shown as a warped surface of a wolf skull (Wiley et al. 2005).
And most of this morphological variation took only a few hundred years to produce. It is true that some of these weird skulls would not be favored in the wild – Drake notes that natural selection is reduced when your food comes from a can rather than stuff you hunt – and artificial selection is greatly enhanced by selective breeding. But this is strong evidence that (a) there is no problem on the genetic variability end of the equation for the kinds of variability that we see in a mammalian order like Carnivores; rather the constraint is natural selection for a particular niche. If the selective pressure is there, the morphological change can happen very quickly; and (b) lack of time isn’t the issue; if the conditions are right, hundreds or thousands of years can be plenty of time.
I didn’t even notice when this study came out and got a bunch of press in January, probably because I actually had a girlfriend at the time (see, rare events do happen in geologic time!). But this is a study that should be in the back pocket of any creationism opponent. You can see an example of its usage on Cornelius Hunter here; it’s kind of like a surprise sack of a quarterback.
The other thing I like about the conclusion of “intraspecific macroevolution” is that it tweaks a lot of standard tropes that even we scientists have about what is meant by the word “macroevolution.” The minimal definition of macroevolution is “evolution above the species level”, but it has become a catchall term encompassing everything from speciation to lineage-diversification and extinction dynamics to “evolution of ‘higher taxa’” (ack! go read “down with phyla!”) to vaguely defined “large” amounts of change to evo-devo changes in development. These things then all get mixed together in people’s heads, resulting in the erroneous presumption that “‘large’ amounts of change” = lots of speciation events = the origin of some big Linnaean ‘taxon’ = lots of action at the lineage-counting level. As a very rough approximation it might be true that these different things are often linked, but as this study shows, it ain’t always true. We would probably be better off using specific terms for each of these different topics, and not trying to lump them all together under “macroevolution” as if they were all intrinsically connected. Questions like “is macroevolution just the result of repeated rounds of microevolution” have almost no meaning if “macroevolution” refers to all of these different things at once.
Abby Grace Drake and Christian Peter Klingenberg
Large‐Scale Diversification of Skull Shape in Domestic Dogs: Disparity and Modularity
Am Nat 2010. Vol. 175, pp. 289-301
The variation among domestic dog breeds offers a unique opportunity to study large‐scale diversification by microevolutionary mechanisms. We use geometric morphometrics to quantify the diversity of skull shape in 106 breeds of domestic dog, in three wild canid species, and across the order Carnivora. The amount of shape variation among domestic dogs far exceeds that in wild species, and it is comparable to the disparity throughout the Carnivora. The greatest shape distances between dog breeds clearly surpass the maximum divergence between species in the Carnivora. Moreover, domestic dogs occupy a range of novel shapes outside the domain of wild carnivorans. The disparity among companion dogs substantially exceeds that of other classes of breeds, suggesting that relaxed functional demands facilitated diversification. Much of the diversity of dog skull shapes stems from variation between short and elongate skulls and from modularity of the face versus that of the neurocranium. These patterns of integration and modularity apply to variation among individuals and breeds, but they also apply to fluctuating asymmetry, indicating they have a shared developmental basis. These patterns of variation are also found for the wolf and across the Carnivora, suggesting that they existed before the domestication of dogs and are not a result of selective breeding.
Robert K. Wayne (1986). “Cranial Morphology of Domestic and Wild Canids: The Influence of Development on Morphological Change.” Evolution, Vol. 40, No. 2 (Mar., 1986), pp. 243-261 * Stable URL: http://www.jstor.org/stable/2408805
Abstract l The domestic dog varies remarkably in cranial morphology. In fact, the differences in size and proportion between some dog breeds are as great as those between many genera of wild canids. In this study, I compare patterns of intracranial allometry and morphologic diversity between the domestic dog and wild canid species. The results demonstrate that the domestic dog is morphologically distinct from all other canids except its close relatives, the wolf-like canids. Following this, I compare patterns of static and ontogenetic scaling. Data on growth of domestic dogs are presented and used to investigate the developmental mechanisms underlying breed evolution. Apparently, most small breeds are paedomorphic with respect to certain morphologic characters. In dogs and other domestic animals, morphologic diversity among adults seems to depend on that expressed during development.