Bergmann’s Rule in the Andes: The Case of the Line-cheeked Spinetail

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?



A Line-cheeked Spinetail (from


All You Need is Love IBD

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.



Wandering Wekas: The Genetic Structure of a Flightless New Zealand Bird

The Weka, a flightless rail on New Zealand, shows clear genetic patterns across a narrow seaway between the two main islands.

New Zealand is comprised of two main islands (conveniently named North Island and South Island), surrounded by about 600 smaller islands. The two islands are separated by Cook Strait, which is 22 kilometers wide at its narrowest point. That doesn’t sound like a big distance if you can fly, but what if you are a flightless rail?



In a recent study, published in Molecular Ecology, Steve Trewick and his colleagues set out to answer this question for the Weka (Gallirallus australis), a flightless land bird that is endemic to New Zealand. The current distribution of this bird on both main islands can be the outcome of several scenarios. Perhaps some birds walked from one island to the other when sea levels where low. Or maybe the populations have always resided on one of the islands, never meeting each other.



A Lousy Choice

In addition to several genetic markers, the researchers also studied two lice species of the Weka populations. These lice are passed on from parent to offspring and provide an independent approach to study evolutionary history. A classic example of this method is the striking concordance between evolutionary trees of seabirds and their parasites. Finally, they compared the Weka results with other (flying) bird species in New Zealand. Do they show similar patterns?


Two Lineages

The findings are clear: all genetic markers and the lice point to two primary lineages corresponding to North and South Island. Moreover, this division existed before the last glacial maximum when it was possible to walk across Cook Strait. So, Wekas from different islands did have the opportunity to meet, but did not interbreed (or very little).

The same pattern holds for some birds species, such as the Toutouwai Robin (Petrcoica australis) and the Whio duck (Hymenolaimus malacorhynchos), but not for others, such as Kereru Pigeon (Hemiphaga novaeseelandiae) and Karearea Falcon (Falco novaeseelandiae). Clearly, the dynamics across the Cook Strait are more complicated and probably species-specific. The authors conclude that “this narrow seaway is unlikely to have been the direct cause of lineage splits. Rather it likely represents an environmental step where spatial and ecological constraints intersect.”



Genetic structure of several New Zealand birds on North (red) and South (blue) island. Notice the clear separation in some species, but not in others (from Trewick et al. 2017)



Trewick, S. A., Pilkington, S., Shepherd, L. D., Gibb, G. C., & Morgan‐Richards, M. (2017). Closing the gap: Avian lineage splits at a young, narrow seaway imply a protracted history of mixed population response. Molecular ecology, 26(20), 5752-5772.