Allen’s Rule. One of those things you learn in graduate school along with Bergmann’s Rule and Cope’s Rule. It is all about body size. Cope’s Rule … which is a rule of thumb and not an absolute … says that over time the species in a given lineage tend to be larger and larger. Bergmann’s Rule says that mammals get larger in colder environments. Allen’s Rule has mammals getting rounder in colder climates, by decreasing length of appendages such as limbs, tails and ears.
All three rules seem to be exemplified in human evolution. Modern humans tend to be larger and rounder in cooler environments than in tropical environments. Over time, the human lineage has gotten larger … australopiths of the Miocene and Pliocene were smaller than Homo erectus and modern Homo sapiens. In comparing contemporary African modern humans and European Neanderthals, the latter are rounder and have shorter limbs, especially the distal parts of the limbs (forearms and the leg below the knees). In fact, this difference in body proportion is one of the key features that physical anthropologists use to distinguish between regular modern humans and Neanderthals when faced with that task.
[Revised and Reposted. Teachers, here is a PDF version of this post (slightly revised for better use as a handout). ]
Bunnies demonstrating Allen’s rule.
The usual assumption is that these changes in body form are selected for as a result of various environmental pressures, and that these features of body size and shape become adaptive features seen in particular populations. The body shape story is part of the Darwinian story of adaptation as well as, in some cases, the story of racial differentiation among humans or other organisms.
And of course, it is all wrong, as usual.
Continue reading Allen's Rule, Phenotypic Plasticity, and The Nature of Evolution