Glimpsing evolutionary instability in mountains

There are many challenging environments on the planet, and the Andes Mountains are no exception. Animals living at height need to overcome a range of extremes – but how do they achieve this and what impact does it have on their evolution? Chauncey Gadek is a Masters student at the University of New Mexico, and his research with Dr Christopher Witt examines the impact of high-altitude adaptation on current bird distributions and diversity. 

The Andes Mountains are an imposing geographical feature by any standards. The elevation gradients can span 5000 meters in less than 100 km. These mountains impose extreme selection pressures upon animal populations. To live high is to cope with extreme temperature fluctuations, rapid water loss, increased UV exposure, and a decreased partial pressure of oxygen (O2). The low O2 availability is perhaps the biggest challenge for warm-blooded vertebrates that have high metabolic rates. Yet, paradoxically, the Andes host some of the most diverse bird communities on Earth. Most of these birds have narrow elevational ranges and are genetically specialized on the particular climates and O2 pressures where they live.

Mountain pass
The Andes Mountains – a challenging environment (Photo: Chauncey Gadek)

But there are a few species, about 5% of Andean songbirds, that conform. These have ranges that span most of the 5000-meter gradient. To see these birds forage along the beaches of the Pacific just as they do on bogs of the high Andean puna defies our expectations. The extreme range of temperatures and pressures should be acting quickly, over evolutionary time, to differentiate the high- and low-elevation populations. Unless, perhaps, they are migrating elevationally, making physiological adjustments to take advantage of seasonally variable resources. Movement across the gradient could swamp out incipient population divergence and potentially maintain these large elevational ranges.

To understand why some species have large elevational ranges, we used a large series of museum specimens of four elevational generalist species — the Rufous-collared Sparrow, House Wren, Hooded Siskin, and Cinereous Conebill — to test for movement across the gradient using hydrogen isotopes ratios, which change predictably with elevation. The isotope data showed signals of movement in the conebill and the siskin, presumably to take advantage of seasonal resources. But it also suggested that the other two species were sedentary. One of the big challenges with the isotope data was to separate the seasonal signal from the elevational one, but I think we were able to do that sufficiently to answer at least the coarse-scale questions of whether each species moves or not.

The isotope study led us to other, related questions about genetic and phenotypic differentiation across the elevational gradient. We sequenced mitochondrial genes and compared morphological measurements between high- and low-elevation specimens. Our analyses revealed distinct population structure between high- and low- elevation sparrows, morphological differentiation between siskins, and signals of recent population expansion among all species except the conebills.

Integrating evidence from these different sources, an overall picture started to emerge that helped us to understand these species with anomalously big elevational ranges. Each species exhibited some combination of recent expansion, ongoing elevational differentiation, or elevational movement. It seems that being an elevational generalist is a rare condition because it requires one of these situations — recent expansion or elevational migration — and because it quickly gives way, either to divergence between high and low subpopulations, or to specialization. The flip side of this conclusion is that it helps us to understand why most tropical species have narrow elevational ranges, building on the hypothetical framework established by Daniel Janzen in 1967 (Janzen 1967).

An unexpected finding from our analyses was that the population expansion of these bird species may have coincided with the onset of irrigation and agricultural expansion across the west slope of the Andes, ~3,500 years ago. This was an intriguing reminder of the potentially pervasive effects of prehistoric humans on species ranges. In a larger sense, this study underscores that populations carry the potential to expand whenever ecological conditions allow it, but that they will constantly facing countervailing pressure to specialize.

Natural history collections
Chauncey’s work is closely tied with the Museum of Southwestern Biology, where he has become a strong student advocate for natural history collections

Conducting this study taught me how important natural history collections can be to studying ecology and evolution. By maintaining, and continuing to grow natural history collections, we create opportunities to test previously unanticipated ideas in an astonishing number of ways. In this study, we used entirely specimens that had been amassed for other purposes, and we put them to good use. By using specimens, we also created a study that can easily be extended in the future, using new technologies. This underscores the importance of building, protecting, and appreciating collections. These irreplaceable objects represent their time and place in the world, and they hold answers that cannot be ascertained elsewhere.

More info:

Gadek, C.R., Newsome, S.D., Beckman, E.J., Chavez, A.N., Galen, S.C., Bautista, E. and Witt, C.C. (2018) Why are tropical mountain passes ‘low’ for some species? Genetic and stable-isotope tests for differentiation, migration and expansion in elevational generalist songbirdsJournal of Animal Ecology. DOI: 10.1111/1365-2656.12779

Janzen, D.H. (1967) Why Mountain Passes are Higher in the Tropics. The American Naturalist, 101: 233–249.

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