The Way of Science

2. Step Two: Natural Selection

Natural selection, or the "struggle for existence" in Darwin's words, is NOT RANDOM. This is the step where an enormous number of individuals/variations is much reduced. NOTE: be very wary of the phrase "survival of the fittest." At this early stage of understanding, it is very likely to mislead you into assuming that "fittest" means biggest, strongest, etc. Let us look at selection first as an abstract concept, and then go to several specific examples (the peppered moth, the Heike crab, and - eventually - Darwin's finches).

A. Adaptation

Selection is basically differential reproduction. If you have a population of 50% blue and 50% pink frobbles, and blue and pink phenotypes are genetically based, and in each generation more blues survive to reproduce than pinks, then each succeeding generation will contain more blues, and fewer pinks, than the previous generation. Why should one variant be more successful than another? Never forget that success is measured by getting more of your genes into the next generation than some other genotype. If the blue form is less likely to be eaten by predators, better at handling the local extreme temperature changes, etc., then it will statistically be more likely to produce more offspring during its lifetime. Note that the two forms could easily have exactly the same lifespan, but the blues are just better at adapting to local conditions. Selection thus results in a gradual shift of genotype ratios, based on how well a genetic variation "fits" a local environment compared to some other variant of that species under the same conditions. One can extend the concept to inter-species competition also.

How does one measure, quantitatively, how "fit" two variants are? The measure of fitness is not size or strength or intelligence, but rather how many of your genes go to the next generation compared to the other variant. Fitness is measured by comparative reproductive success. (Exception: a mule's fitness is zero, no comparison is needed. Why not?)

Let us now consider two examples of selection in action. In the first case, the explanation is given. In the second, it will be up to you to find the solution.

a. Biston betularia: the peppered moth
This widespread, night-flying moth occurs in two genetic variations involving wing and body color: pale, with dark spots, and dark with pale blotches. For simplicity, we will call them "light" and "dark." Before the industrial revolution, most moths in England were light; the dark form was quite rare. These data on early moth populations come from extensive amateur and professional collectors. Soon after industrialization, and the extensive burning of coal, the ratio of dark to light began to change in those parts of England where air pollution was worst. By 1900, the pale form was rarer in these areas than the dark. With the gradual reduction of coal-soot pollution in the 20th century, the ratio reversed again. Clearly, evolution (adaptation) had taken place. We now know that the major factor behind this genetic change was the deposition of dark crud on the normally pale tree-trunks, where the moths spend their daylight hours. During daytime, this species clings to the bark, and camouflages itself against bird predators. Light (unpolluted) bark meant that the pale form was fitter than the dark. Selection by bird predation had been verified experimentally.

b. The Heike crab
The second case of selection is for you to decipher. The bare details are given below; if necessary, your instructor may add to the description.

The Heike crab is a variant of a wide-spread Japanese species. In all individuals of this species, the shell is quite wrinkled. In most, the wrinkles are random, but in one variation, located in the Straits of Shimonoseki, a remarkable pattern of wrinkles occurs. These straits were the site of a momentous and famous battle in 1185. Two powerful clans fought to the death: the Minamoto, who won, and the Taira, also called the Heike. This battle led to the Minamoto becoming the sole military rulers, and began Japan's medieval era. The Shogunate, established here, lasted until the 19th century.

In the late 13th century, a description of the battle was produced, and has become a famous piece of Japanese literary history. It is called the Heike Monogatari, the tale of the Heike. Here is a translation of a bit of the introductory portion of the tale, to set the mood for you.

In the sound of the bell of the Gion temple echoes the impermanence of all things. The pale hue of the flowers of the teak tree shows the truth that they who prosper must fall. The proud do not last long, but vanish like a spring night's dream. And the mighty ones will vanish in the end, like dust before the wind.

"The mighty ones," the Heike, are slaughtered. Women and servants commit suicide by drowning. Bodies of slain samurai litter the beach, and the water is red with blood. The victory is complete and terrible.

After the battle is over, the poor fishermen return, trembling and loath to return to their struggle for existence. In spite of the bodies (and ghosts!) of the dead samurai, they must fish for food, including the abundant crabs. Soon, though, their fear becomes more and more justified: more and more crabs begin to show the shadowy visages of scowling samurai in the wrinkles of their shells. Year after year, the resemblances increase, until no-one would doubt that samurai faces were being displayed. The strong similarity persists to this day. Surely, the ghosts of the slaughtered mighty ones were returning!

Do you have a simpler (non-supernatural) explanation? Think about it for next class!

If there is time, you instructor may wish to consider the extra questions about evolutionary theory in Appendix #3. You should also keep in mind, both in general and for help in answering some of those extra questions, that the environment may produce selective forces "pushing" in opposite directions (complex selection). For example, snail shells are dark in English countryside habitats where warming up early in the day is critical, and the shells of the same species are light-colored where reflection of heat is paramount. In some areas, where selection is based on both factors, the shells are multicolored.