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1. Industrial melanism--the phenomenon
a. Many moths in Britain come in two different colors, a lighter
"natural" form, and a darker, melanic form.
b. The melanic form is in all ways similar to the natural form except
that it produces a much greater amount of
melanin, the pigment that gives its wings their dark coloration.
c. In Biston betularia, Kettlewell and his group have performed
crosses that have demonstrated that a single
gene controls the difference between natural and melanic morphs.
2. Historical records
a. In the last 150 years, the relative preponderance of the two morphs
has changed dramatically.
b. Old collections from the early 1800's show that the pepper morph
was far and away the most abundant morph.
Melanic morph was collected only very rarely.
c. Beginning in the early to mid 1800's, however, melanic form increased
in abundance dramatically, until today,
in some areas virtually all moths are melanic.
d. There has thus clearly been a dramatic change in gene frequencies,
i.e. evolution has occurred.
3. Observation:
the change in morph frequencies corresponded with the onset of the industrial
revolution in Britain.
One consequence of the industrial revolution was that the smoke and soot
put out by all of the factories that sprung
up across Britain caused a darkening of the tree trunks in many areas,
particularly near urban industrial centers.
Originally, tree trunks had been covered with light-colored lichens, but
after the industrial revolution, these lichens
and the tree trunks they covered became dark in these areas; many likens
were killed.
4. Kettlewell's
hypothesis: Because moths spend a great deal of time during the day
resting on tree trunks, Kettlewell
reasoned that they are probably exposed to a great deal of predation by
birds and other animals. It would therefore
benefit a moth to be cryptically colored, so that it will blend in well
with the background of the tree trunk on which
it rests.
a. Individuals that were more cryptically colored would tend to escape
notice by predators to a greater extent, and
thus would tend to have a survival advantage over non-cryptically colored
individuals.
b. Kettlewell postulated that before the industrial revolution, the
pepper form blended in with the light-colored
background of tree trunks, whereas the black morph was conspicuous, leading
to selection against the melanic
morphs. This genotype would thus be held at very low frequency.
c. With the change in tree trunk color associated with industrialization,
however, the melanic forms became
cryptically colored and the pepper forms became conspicuous. Under
such situations, the rare melanic mutants
would enjoy a survival advantage and increase in frequency until the pepper
forms were eliminated from
the population.
d. Visual inspection of the degree of crypsis on dark and light tree
trunks supports this hypothesis.
5. Experimental test of the hypothesis. I. Compare survival of melanic and pepper moths.
a. Mark-release-recapture experiment. Released several hundred
marked individuals of both morphs into two
types of woodlands:
-- One woodland near a big city; poluted; trunks darkened with soot.
-- One woodland more rural, relatively unpolluted; tree trunks not
darkened.
b. Recaptured using caged females emitting pheromones.
c. Reasoning: if dark morphs have survival advantage in polluted
woods, should recapture more dark morphs than
pepper morphs. By contrast, if pepper morphs have survival advantage
in unpolluted woods, should recapture
more pepper moths there.
d. Results:
%Recaptured
Woodland Melanics Pepper
Urban
27.5 13.0
Rural
6.3 12.5
e. This experiment showed that the melanic form did indeed enjoy
a survival advantage in polluted woodlands, while
the reverse was true in unpolluted areas. However, Kettelwell still
had not shown that this differential survival was
caused by differential susceptibility to predation.
6. Experimental test of the hypothesis. II. Differential predation
a. Placed recently-killed moths of each morph on treetrunks in the
two types of woodland.
b. Sat in blind and observed what happened to moths placed out.
In particular, counted the number of individuals of
each morph eaten by birds.
c. Results
No. moths eaten by birds
Woodland
Melanics Pepper
Urban
15
43
Rural
164
26
d. Interpretation: Kettlewell's hypothesis is confirmed.
In the polluted environment, more pepper morphs are eaten,
while in the unpolluted environment, more melanic morphs are eaten.
7. Summary:
a. Intustrial melanism is genetically controlled by a single locus
in B. betularia.
b. Populations have undergone evolutionary change in color pattern.
c. That change is consistent with the interpretation that it was
due to natural selection, in that there is differential
survival of the genotypes caused by differential predation on a particular
background.
d. Results confirm qualitative prediction of equation for gene frequency
change.
8. Additional points
a. Industrial melanism is seen in more than 70 species of British
moths, and all show patterns similar to that seen
in B. betularia.
b. At least one prediction based on Kettlewell's work has come true:
since the imposition of pollution control
devices on many of Britain's factories in the 1950's, pollution has decreased
markedly in many areas. As a
result, many of the formerly polluted woodlands have returned to their
original condition, in which the color
of the tree trunks is light and mottled. As would be expected from
Kettlewell's work, the melanic forms that
were once so prevalent in these areas have almost disappeared.