1 June 1998

Mile High Club Hummers Have More Staying Power

Remember that movie Deliverance, about hillbillies with unsavoury habits living in the mountains? Genetic throwbacks through inbreeding? Who knows, but a new study suggests that altitude may have something to do with the pace of evolution. A report from the University of Wisconsin published in the Proceedings of the National Academy of Sciences has shown that hummingbirds high in the Andes evolve more slowly than their counterparts at sea level.

The rate at which genes evolve in animals has traditionally been linked to attributes such as size and mass. This report presents some of the first evidence to link an animal's physical environment to the rate at which genes evolve. "This adds elevation to the mix of factors that one might consider as contributors to rates of molecular evolution" says Robert Bleiweiss, professor of zoology and author of the study. Change is governed at the molecular level as genes change or mutate, but the cause of these changes is unknown. Over long periods of time, through many generations, these changes accumulate, manifesting as altered body plans or behaviours.

Comparing the DNA of 26 hummingbirds at a variety of elevations, Bleiweiss charted 'genetic distances' between amino acid base pairs in the birds' genetic material.

He found that these measurements correlated with the altitude of the habitat. This indicates that the higher you go, the slower the molecular clock ticks. This surprise finding was the opposite of that predicted by "the more extensive subdivision and speciation of bird populations living at high elevations."

Elevation is likely to be only one of the factors affecting molecular evolution, cold temperatures can also have an effect - some of these high-living birds enter a state of torpor when they get too cold. Metabolic factors associated with the extraordinary physiology of hummingbirds and the low availability of oxygen at high altitude could also have important influences on genetic mutation rates. Generally, these findings confirm that the pace of evolution depends to some degree on environmental factors. Bleiweiss confirms that it seems "The higher you live, the slower the clock ticks".