House Sparrows Have Evolved Rapidly in North America
Terry Maxwell, April 27, 2003, San Angelo Standard Times, © 2003
Do you understand the following sentences? "The axes of variation of principal components run orthogonally to one another. Given this, plus the differential character loadings on components I and II, we here assume that we have identified distinct sets of covariant characters." No? Well, don’t worry about it. I often struggle with the jargon as well, and for me, a biologist,
it’s the language of an indispensable tool.
Research develops its own vocabulary, whether in engineering, law, psychology, music, history or biology. But what are you to make of the conclusions drawn by those whose investigations are reported in such a confusing vocabulary?
In your defense, there’s certainly nothing wrong with a healthy skepticism of what you do not understand, but I think you should assume that the results are worth your temporary acceptance when you understand that they have been scrutinized by others who know the vocabulary and are in a position to judge their worth. The words and the ideas they convey could not have been published in a reputable science journal without that peer review.
The particular quote above comes from an important article of 30 years ago by Richard Johnston and Robert Selander - an article entitled "Evolution of the house sparrow. II. Adaptive differentiation in North American populations." Johnston and Selander teamed up for better than a decade to examine what had occurred to this species following its introduction from Europe to our continent.
In their choice of words, house sparrows (Passer domesticus) were "innoculated" into North America between 1852 and 1860. These birds largely came from central England, and that is a very important fact in the subsequent history I relate here today. That they mostly came from one population means that they were similar in their genetic makeup and in the physical
characteristics dictated by those genes.
Johnston and Selander found that in populations of European house sparrows, for which samples existed in the 1850s and in the 1960s, there had been no obvious change. For example, comparison of specimens taken a hundred years apart from Renthendorf, Germany showed no differences in lengths of wings and tails.
So, it is reasonable that house sparrows from central England today are pretty much alike those taken from there and introduced into North America in the 1850s. The central question today is "what changes if any have occurred among these sparrows since they were introduced to our land?" Well, to put it simply, central English sparrows do not look alike or fit the measurements of the same species found today at San Angelo.
House sparrows can be easily shown to have evolved in response to different climates and other environmental features encountered as they have spread. Johnston and Selander measured 16 skeletal features of 1752 specimens from 33 North American localities.
They found, for one example, that North American house sparrows living today in cold climates at high latitude are larger than the original ones introduced. That was predicted by Bergmann’s rule – within a species, smaller body sizes, on average, are found in warmer climates and larger body sizes in colder climates.
And you see the only rational way house sparrows could have those different body size averages in different climates is for them to have evolved. All the introduced 1850s birds – the original stock – were pretty much identical in body size.
Every feature examined, including feather color, demonstrated a change from that seen in the original stock. And those changes were predictable based on what we understand about how organisms adapt to environments.
House sparrows at San Angelo and Austin are today smaller and lighter in overall color than are house sparrows from Seattle and Mexico City, and yet they all trace their origins to a more uniform set of birds from Central England.
That they changed does not surprise me nor any other biologist I know. To us, it would be insensible for them not to have changed. But what did surprise us was its rapidity.
Any house sparrow feeding in your alley cannot be more than 151 generations removed from an English ancestor dumped on our continent’s shore. Sometimes you just have to wade through some difficult language to get to an amazing observation.