Increase in body size of the Line-cheeked Spinetail along an environmental gradient can be explained by neutral processes. No need to call upon natural selection.
There are only a handful of rules in biology (and each rule has countless exceptions). One of the most common ones is Bergmann’s Rule which states that populations of homeothermic species tend to have larger body sizes in colder climates. The idea is that larger animals have a lower surface to volume ratio, causing them to lose less heat and thus stay warmer in colder environments.
Isolation by Adaptation or Distance?
In many bird species body size is correlated with temperature gradients, suggesting a role for local adaptation. From a genetic point of view, there could be a correlation between genetic differentiation and local adaptation. This pattern has been dubbed isolation by adaptation (or IBA). Alternatively, genetic differentiation could build up by random genetic drift and a decrease in gene flow as populations are geographically farther apart. Geneticists call this pattern isolation by distance (or IBD).
A Cheeky Little Bird
Discriminating between IBA and IBD is challenging, but Glenn Seeholzer and Robb Brumfield attempted just that in a recent Molecular Ecology paper. They studied the Line-cheeked Spinetail (Cranioleuca antisiensis), an arboreal passerine that lives in the Andes from southern Ecuador into Peru. The body mass of this species increases with elevation and decreasing temperature, as predicted by Bergmann’s Rule. In fact, birds in the north are three times as heavy as their southern relatives. But has this relationship been shaped by natural selection?
All You Need is
The genetic analyses (based on more than 5000 SNPs from 172 individuals) provides some support for natural selection, and thus isolation by adaptation. But the pattern can also be explained by other mechanisms, such as phenotypic plasticity. The authors write that ‘our results suggest, but do not prove, that divergent natural selection has driven local adaptation through the body size cline of C. antisiensis.’ In the end, isolation by distance is sufficient to explain the observed patterns.
An important take home message from this study is that one does not always need to invoke natural selection to explain divergent patterns in nature. Often neutral processes are all you need (although the Beatles might argue that all you need is love…)
Seeholzer, G. F., & Brumfield, R. T. (2017). Isolation by distance, not incipient ecological speciation, explains genetic differentiation in an Andean songbird (Aves: Furnariidae: Cranioleuca antisiensis, Line‐cheeked Spinetail) despite near threefold body size change across an environmental gradient. Molecular ecology.