The Darwinian Concept of Natural Selection as Supported by the Microevolution of Biston betularia

Laura Steele

Throughout the eighteenth and nineteenth centuries it was called "the `transmutation' of species" (3). Darwin eventually constructed the phrase "decent with modification" (3). Presently, the dictionary defines his theory as both "a process of change in a certain direction" and "a process in which the whole universe is a progression of interrelated phenomena" (4). No matter the wording, the concept of evolution has survived thousands of years and currently provides the scientific world with the link between all the species of the Earth.

Although the idea of evolution existed for all these years, Charles Robert Darwin
was one of the first individuals to propose and print both the conditions and mechanism
required to support the theory, along with Alfred Russell Wallace (3,5). Darwin's studies
of different organisms spanned over 30 years, ultimately concluding with the publishing
of his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1, 5). Darwin defined natural selection as "the preservation of favorable variations" (4, 5). This essentially states that those individuals possessing traits proving beneficial in their survival would pass their adaptations to their offspring, thus the proportion of the population bearing superior traits would continue while the inferior ones would diminish in each generation (3,5,6). Another common phrase associated with natural selection is "survival of the fittest" (5).

Along with the mechanism for evolution, Darwin presented a number of conditions, or individual postulates, which must happen in order for the process to occur. It is this collection of conditions that provided the backbone for further study of evolutionary change among species. While the exact number of postulates may vary from one resource to another, a general list can be generated through the combination of all the ideas put forward by Darwin. Included in his postulates are the subjects of population growth, individual variation, resource availability, and survival of the few (1,5,6).

In reference to population, Darwin's first postulate resulted from his study of Thomas Malthus's Essay on the Principle of Population (5,6). Malthus described populations as increasing geometrically, where the number of individuals would increase at a constant rate (5). In short, a population could continue to increase its numbers indefinitely if left unchecked (3,5,6). It was seen by Darwin that "every organism has the potential to produce more offspring than can survive" (5). The reality for this idea lies in the fact that populations remain at or near equilibrium year after year, regulated by some force or combination of forces (1,3,5,6).

It was very evident, from studying numerous different captive species and witnessing nature that individuals and their offspring vary. During Darwin's time, he knew that variation occurred but never understood the mechanism behind it, though it was discovered by Gregor Johann Mendel some years before his death (3). Darwin dedicated years of research to observing and breeding animals, specifically pigeons (1). He developed numerous breeds via selection, each varying from the other in often more than one distinct characteristic (1). The fact that individual organisms vary and that these variations are heritable proved essential for Darwin's theory of natural selection.

From these presumptions and observations of populations and variations came the forces that prevented all possible offspring from surviving. As resources become more or less abundant, population numbers will change accordingly (3). The individuals within the populations compete for the available resources and those best able to acquire the essential amounts will survive (3). Ultimately, those individuals that survive will produce offspring carrying the traits of those organisms best adapted to the pressures being applied to their population.

Scientists are able to see evidence of Darwin's postulates through examples of microevolution that can be found everywhere. Microevolution is defined as "evolutionary events examined closely, and usually over a relatively short period of time" (3). One strong example involves the species Biston betularia, a type of moth found in Europe (1,3). They carry two different phenotypes, gray and black, that differ by a single gene (1). The moths in this example have become commonly known as the peppered moths due to their distinguishing colors.

During the time frame before the mid nineteenth century, light or gray colored moths dominated the population because their appearance blended in with the birch and beech trees they inhabited (1,2). The beginning of the industrial revolution occurred just before the black colored moth's numbers began increasing (1,2). It became evident that the dark soot, a result of the factories in industrialized areas, allowed predators of the light gray moths to easily pick them off the lichen covered trees (1,2). The alterations to the environment caused by the industrialization in the areas appeared to be the driving force behind the natural selection of the black, peppered moths over the gray individuals (1,2).

These very visible and recognizable changes lead to many experiments related to the moths. J.W. Tutt presented the hypothesis that "suggested that peppered forms were more visible to predators on sooty trees that have lost their lichens", leading birds to eat them during the day while "the black forms ...were at an advantage because they [are] camouflaged" (2). Bernard Kettlewell went on to test the hypothesis developed by Tutt during the 1950's (2). He went about his testing by first creating populations of both variation and tagging the individuals with paint. Two sets of testing groups, containing both colors of moths, were released in a polluted area near Birmingham and an unpolluted area near Dorset (2).

To obtain quantitative results, traps were set in locations around these woods and the numbers of both types of moths that survived were analyzed (2). In the polluted area, there were a higher percentage of black moths collected. Contrastingly, in the unpolluted area, the number of white moths outweighed the number of black moths. Kettlewell also chose to film the birds selection of their prey and was able to observe the predation of those more visible moths (2). Some results even showed birds "[passing] right over a moth that was the same color as its background" (2). This data collected supported the hypothesis of Tutt and presented a prime example of Darwin's natural selection on a micro scale.

This example and experiment provide the most convincing evidence for microevolution and natural selection for two main reasons. First, it utilizes a visual trait that can be easily explained with only two possible outcomes. Secondly, the experiment could be replicated even today by both scientists and even those with little scientific background and should yield similar results. In the offspring generations of the moths inhabiting areas of high pollution, the trait count for the dark color would be highest. Alternately, when pollution is at its lowest, the reverse trait count would occur with the light or gray colored traits highest. The different assumptions in the hypothesis introduced by Tutt are supported by the outcomes of the experiment and also follow Darwin's ideas of natural selection by illustrating all of the postulates outlined above.

References

1. Weiner, J. 1995. The Beak of the Finch. Vintage Books, NY.

2. Peppered Moths and Industrial Melanism. (Worksheet from Textbook).

3. Kardong, K. 2005. Introduction to Biological Evolution. McGraw Hill.

4. Merriam Webster Online. 2004. Merriam Webster, Incorporated. <http://www.mw.com/>.

5. Johnson, G.B., Raven, P.H. 1999. Biology. Ed. 5. WCB/McGraw Hill.

6. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.

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