The Darwinian Concept of Natural Selection as Supported by the Microevolution of Biston betularia
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
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.
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.
This web site created and maintained by the Coe Writing Center. Copyright 2001.
E-mail Dr. Bob Marrs with any questions, comments or suggestions.