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Spring 2014
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ESSAY
 
Keep Your Eye on the Sparrow
You might see natural selection at work.
Professor Hermon Bumpus stared out his office window at Brown University on a wintry afternoon in 1898. House sparrows, soaked by wet snow and shivering in a brisk wind, were suffering from hypothermia.

Sympathy and curiosity drove him to collect the birds and bring them into his laboratory.

House sparrows, passer domesticus, are native to Europe and most of Asia. The Commissioners of Central Park introduced them to New York City in 1852 and as a consequence of numerous introductions they now thrive in North and South America, Africa, Australia, and New Zealand. They are the worldís most widespread bird and one of the most common birds in North America.

Some of the sparrows that Bumpus collected on campus died, and others recovered. He measured nine morphological traits, such as wing length, on each bird, and discovered that surviving birds had intermediate values for eight of the nine traits. Mortality imposed by the harsh storm eliminated the largest and smallest birds and those with unusual shapes. A single storm diminished the flock but also changed its shape.

Years later, University of Kansas professor Richard Johnston and his colleagues, inspired by Bumpusís results, conducted a similar study, but over an entire winter. They found that winter mortality took the smallest males and the largest females. While this experiment differed in detail from its predecessor, it was similar in demonstrating that mortality over a short period of time had measurable effects on a single population. That is, natural selection had significant effects after a single storm, or a prolonged winter, and certainly within a generation.

Johnston and his colleagues, encouraged by their results, went on to compare house sparrows from source populations in England and Germany with introduced populations throughout North America and Hawaii. In North America, they found patterns of color and morphological variation that conformed to common rules of ecogeographic variation (Glogerís rule, Bergmannís rule, Allenís rule). Birds living in warm environments had lighter tones and were more colorful. Birds living in cold environments were considerably larger and also had relatively smaller appendages, to conserve warmth during winter.

That is, house sparrows in southwestern deserts have lighter tones, are smaller and have relatively longer legs and wings than birds living in the moist forests of the Pacific Northwest. House sparrows evolved the same patterns of variation seen in numerous native bird species.

Johnston and his colleagues noted that introduced populations differed significantly from the source populations and that populations from different environments in North America were also quite different. The variation was most apparent between populations in Hawaii and all other populations.

They also noted that the degrees of differences in color and morphology were as large as the differences used to describe subspecies in other bird species. The prevailing belief had been that subspecific differences took about 40,000 generations to evolve, but these differences had evolved in less than 50 years.

House sparrows have taught us about how quickly natural populations can respond to environmental stresses and how quickly evolution occurs.

Jeff Mitton (mitton@colorado.edu) is an evolutionary geneticist at the Univ. of Colorado. While on sabbatical in 2000 he visited the site from which Professor Bumpus rescued the sparrows.

 
 
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