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Fitness and Natural Selection
Different phenotypes interact with the environment in slightly different
ways, leading to variation in fecundity and survival among individuals in
populations. The rate of reproduction of a phenotype – the intrinsic rate of
increase of the life table for the phenotype – is a measure of its fitness. When
differences in fitness among individuals have a genetic basis, those alleles that
have the higher fitness reproduce faster and take over the population. The
phenotypes with the highest finesses are said to be selected, and the change in
genotype frequencies resulting from natural selection is referred to as evolution.
We can now point to many cases of selection producing evolution in
natural populations, often in response to environmental changes brought about
by human activities. Perhaps the most striking case is that of industrial melanism
in the peppered moth in England.
The English have always been avid butterfly and moth collectors, and
such enthusiasts look carefully for rare variant forms. Early in the nineteenth
century, occasional dark (or melanistic) specimens of the common peppered
moth (Biston betularia) were collected. Over the next 100 years, the dark from,
referred to as carbonaria, became increasingly common in industrial areas (today
it makes up nearly 100 percent of some populations.) The phenomenon aroused
considerable interest among geneticists, who showed by cross-mating light and
dark forms that melanism is an inherited trait determined by a single gene.
In the early 1950s H.B.D. Kettlewell, and English physician who had been
practicing medicine for 15 years and was an amateur butterfly and moth
collector, changed the course of his life to pursue the study of industrial
melanism. Several facts about melanism were known before Kettlewell began
his studies. The melanistic trait is an inherited characteristic, so its spread
reflected genetic changes (evolution) in the population. The earliest records of
the carbonaria from were from forests near heavily industrialized regions of
England. In the absence of factories and other heavy industry, the light form of
the moth still prevailed. It was also known that melanism is not unique to the
peppered moth; dark forms have appeared in many other moths and in other
insects.
The peppered moth inhabits dense woods and rests on tree trunks during
the day. Kettlewell reasoned that where melanistic individuals had become
common, the environment must somehow have been altered in such a way as to
give the dark from a survival advantage over the light from. Could natural
selection have led to the replacement of the typical light form by the carbonaria
form? To test this hypothesis, Kettlewell had to find some measure of fitness
other than the relative evolutionary success of the two forms.
Population Genetics and Evolution
Fitness and Natural Selection
To determine whether the carbonaria form had greater fitness than typical
peppered moths in areas where melanism occurred, Kettlewell chose the markrecapture method. He marked adult moths of both forms with a dot of cellulose
paint and released them. The mark was placed on the underside of the wing so
that it would not call the attention of predators to a moth resting on a tree trunk.
Kettlewell recaptured moths by attracting them to a mercury vapor lamp in the
center the woods and to caged virgin females at the edge of the woods. (Only
males could be used in the study because females are attracted to neither lights
nor to virgin females.)
In one experiment, Kettlewell released 201 typical and 601 melanics in a
wooded area near industrial Birmingham, where the tree bark was darkened by
pollution. The results were as follows:
Number of moths released
Number of moths recaptured
Percent recaptured
Typicals
201
34
16
Melanics
601
205
34
These figures indicated that more of the dark form survived over the course of
the experiment.
Although consistent with Kettlewell’s original hypothesis, the results could
be interpreted otherwise: as differential attraction of the two forms to the traps or
as differential dispersion of the two forms away from the point of release.
Variables besides differential mortality had to be accounted for.
To test the hypothesis of natural selection unequivocally, Kettlewell ran a
similar experiment in an unpolluted forest near Dorset, where the tree bark was
lighter. Of 496 marked typicals released, 62 (12.5 percent) were recapture; of
473 marked maleness, only 30 (6.3 percent) were recaptured. Thus in the
unpolluted forest, light adults had a higher recapture rate than dark adults. If
typicals and melanics were differently attracted to light traps or dispersed from a
release point at different rates, the level of pollution would not have influenced
the results. In fact, only differential survival could account for a reversal in the
relative rates of recapture. This confirmed Kettlewell’s hypothesis and
established natural selection as being responsible for the high frequency of the
carbonaria from in industrial areas.
The specific agent of selection was easily identified. Kettlewell reasoned
that in industrial area, pollution had darkened the trunks of trees so much that
“typical” moths stood out against them and were readily found by predators. Any
aberrant dark forms were better camouflaged against the darkened tree trunks,
and their coloration conferred survival value (Figure 17.2). Eventually, differential
survival of dark and light forms would lead to changes in their relative frequency
in a population. To test this idea, Kettlewell placed equal numbers of the light
Population Genetics and Evolution
Fitness and Natural Selection
and dark forms on tree trunks in polluted and unpolluted woods and watched
them carefully at some distance from behind a blind. (A blind is a tent-like
structure intended to conceal observers from their subjects; it is more often called
a hide in England.) He quickly discovered that several species of birds regularly
searched the tree trunks for moths and other insects and that these birds more
readily found a moth that contrasted with its background than one that resembled
the bark it clung to. Kettlewell tabulated the following instances of predation:
Number of moths released
Number of moths recaptured
Percent recaptured
Typicals
201
34
16
Melanics
601
205
34
These figures indicated that more of the dark form survived over the course of
the experiment.
These data were fully consistent with the results of the mark-capture
experiments. Together they clearly demonstrate the operation of natural
selection, which over a long period results in genetic changes in populations of
the peppered moth in polluted areas. Many decades were required for the
replacement of one form by the other. The agents of selection were
insectivorous birds whose ability to find the moths depended on the coloration of
the moth with respect to its background. Kettlewell’s study shows clearly how the
interaction between the organism and its environment determines its fitness.
Light Colored Moth
Typical
Dark Colored Moth
Carbonaria
Population Genetics and Evolution
Fitness and Natural Selection
Fitness and Natural Selection Questions
Directions: Answer each question in a complete paragraph with as much
detail as possible
1.
How is a phenotype selected?
2.
Define evolution according to this article.
3.
What phenomena among peppered moths became a special interest in
England around the beginning of the nineteenth century?
4.
How did Kettlewell determine whether carbonaria peppered moths had
a greater fitness than typical peppered moths?
5.
What was Kettlewell’s hypothesis and was it confirmed?
6.
What agent(s) of selection did Kettlewell identify?
7.
Explain this statement: The interaction between an organism and its
environment determine its fitness.
Population Genetics and Evolution
Fitness and Natural Selection