Jordan rules, that is for sure, but what about Bergmann? According to a prediction born out of an old biogeographic rule known as Bergmann’s rule, endothermic animals should be adapting to the on going climate warming by getting smaller. Decreased mean body size over time has indeed been documented in several mammal and bird species over the past few decades, but all these studies have been based on entirely phenotypic data. In other words, the genetic basis of the observed changes has not been verified. This means that although the observed trends are consistent with adaptation (=a genetic change) to climate change in accordance of Bergmann’s rule, they can not prove it.
In a study released August 29 in the early edition of PNAS, Red-billed gulls in Kaikoura (NZ) feature as a first genetic test as to whether the observed trends could count as adaptations. As the results show, the gulls are getting smaller with time (1958 -2004), but only at the phenotypic level – there is no corresponding change at the genetic level. This means that the observed change in mean body size over time is likely to be of environmental origin. Possible explanations for this include reduced availability and/or quality of forage over time, as well as increased levels on environmental contaminants in their food.
Read more about the study from the original article in PNAS, as well as from News & Views articles published here in english and finnish. By the way, did you know what the short-hand PNAS is said to stand for? Probably Not Accepted in Science. So true in the case of this particular article.
Teplitsky, C., J.A. Mills, J.S. Alho, J.W. Yarall & J. Merilä (2008) Bergmann’s Rule and climate change: Disentangling environmental and genetic responses in a wild bird population. Proceedings of the National Academy of Sciences U.S.A, in press. doi:10.1073/pnas.0800999105
More about the same topic and concepts can be found from a recent special issue of Molecular Ecology where our contribution entiteled: ‘Climate change and evolution: disentangling environmental and genetic responses‘ was published.