Pigeons and Sparrows

Suppose that you could see a park from your window. Suppose that there were lots of pigeons in that park, and a few sparrows, too. Let's imagine that one day it appeared to you that there seemed to be fewer pigeons than there used to be. Eventually, as you continued watching, you realized that you hardly ever saw any pigeons any more, and that most of the birds in the park were sparrows.

Some more time went by, and the pigeons began to reappear in the park. Being a naturally curious person, you decided to investigate to find out what happened. You discovered that a neighborhood boy got a BB gun for Christmas and had been going "on safari" to the park. Pigeons are big, tame, and slow. Sparrows are small, easily frightened, and fly away quickly. The boy was able to shoot the pigeons, but seldom hit the sparrows. Eventually he got tired of shooting birds. He got a real gun, joined a gang, and was killed in a drive-by shooting. With the absence of the predator, the pigeons gradually returned to the park.

Given this story, is it correct to say that you observed evolution occurring today? Yes, at least by one definition of evolution. The demographics of the park (the ratio of different kinds of birds in the park) did evolve in response to the pressure of the predator. Did some of the pigeons evolve into sparrows? Of course not.

In a section titled "Evolution by Natural Selection Occurs Today" in the biology textbook used at Cerro Coso Community College (as well as in numerous other biology books and encyclopedias), we are told the classic story of the evolution of the peppered moth during the Industrial Revolution.

Because the [pale] moths sat quietly on the [pale] lichens during the day, predatory birds could not easily see them (Fig. 16-16a). Occasionally, mutant black individuals appeared. These black moths, extremely conspicuous against the pale lichen, were easily spotted by birds and didn't live long. During the Industrial Revolution, Britain's growing industries began to burn huge quantities of coal for fuel. With no pollution-control technology, soot from the smokestacks soon blanketed the countryside. … Increasingly, pale moths fell prey to birds. Soot-covered trees, however, provided excellent camouflage for black moths. … Black moths, once rare, survived and reproduced, passing on their genes for dark pigmentation for future generations. As the years passed, ever-increasing numbers of black moths appeared. … Fortunately, pollution-control laws have now dramatically reduced emissions of soot and other pollutants, and lichens grow once again on the trees near British cities. As predicted by the principles of evolution, the pale moths are making a comeback in these areas, and the black form is becoming increasingly rare. ¹

In this well-documented example, both black and white moths existed from the beginning. The prejudicial adjective "mutant" may have been unfairly applied to the black moths. Is there any reason not to believe that both black and white varieties of peppered moths are equally normal? But regardless of whether the black moths were mutants or not, they were present at the beginning of the observational period. All that changed were the relative numbers of white and black moths. White moths did not turn into black moths any more than pigeons turned into sparrows in our story.

Some people might mistakenly think that the textbook states as fact that if the evil British industrialists had not been forced to stop polluting the environment by the virtuous British parliament, that moths would have gotten even blacker, and eventually would have turned into ravens or crows. (Some might observe a political bias, as well as an evolutionary bias, in this particular textbook. :-) )

This section of the textbook isn't talking about species changing into other species. It is talking about "directional selection." The textbook says that as a result of directional selection, "a species may evolve in a consistent direction in response" to some environmental pressure. ² The characteristics of a species may change, but it remains the same species.

But at the beginning of the chapter, the textbook says, "species arise from other preexisting species through the process of 'descent with modification' or evolution." ³ Having thus defined "evolution" this way in the introduction, they say in the summary,

Evolution can be observed today. Both natural and human activities drastically change the environment over short periods of time. Significant changes in the characteristics of species have been observed in response to these environmental changes. A well-studied example is the evolution of black coloration among moths in response to the darkening of their environment by industrial pollution. ⁴

In the summary they are talking about directional selection. The evolution of the ratio of existing varieties of species in a geographically-defined population really has been confirmed by observation in modern times. You are likely to think from the concluding paragraph of the chapter, however, that the evolution of species from other preexisting species through the process of descent with modification "can be observed today" because that is how they defined "evolution" at the beginning of the chapter.

They switched their definition of "evolution" in the middle of the chapter without making it perfectly clear to you. We aren't saying that the textbook authors intended to fool you. We just know that it is very confusing and you could be misled if you didn't know better. We are glad to take this opportunity to clear it up for you.

You can use a BB gun to change the ratio of pigeons to sparrows, but you can't use a BB gun to turn a pigeon into a sparrow.

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Footnotes:

¹ Audesirk & Audersirk (1996) Biology 4^th ed. page 317 (Ev)
² Ibid. page 333
³ Ibid. page 303
⁴ Ibid. page 320