I. Experiments
in Nature: Demonstrating Qualitative Agreement with equation for gene
frequency change (Continued
from last lecture).
A.
Industrial melanism in Biston betularia (see H.B.D. Kettelwell. 1973.
Industrial Melanism. Oxford Univ. Press,
Oxford, U.K.)
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
d. Visual inspection of the degree of crypsis on dark and light tree
trunks supports this hypothesis.
(click here for Picture)
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.
9. Properties of evolution by natural selection illustrated by this example.
a. Natural selection is directional.
-- Allele favored by selection starts out at low frequency, gradually
but steadily increases until it eliminates
the original allele.
-- The outcome of selection is more or less predictable. Given
a particular population, we can predict almost
with certainty that the allele favored by selection will eliminate the other
allele. This property is illustrated by
the fact that of the 70 British moth species that exhibit melanic variants,
all species exhibit fixation of the
melanic form in areas of heavy industrial pollution.
b. Which allele is favored depends on the environment.
-- Although in any one environment one allele may be favored over another
by natural seleciton, a change
in the environment may reverse the relative survival values of the two alleles.
-- Melanism was selected for in woodlands near polluted urban centers,
but when pollution was reduced, or
in areas where it was never common, the melanic form lost its survival advantage
and was quickly
eliminated from the population.
-- Thus, we may speak of the relative advantage of one allele over
another only with respect to a
particular environment.
c. Natural selection can be quite rapid.
-- Although one commonly thinks of evolutionary changes occurring over
times measured by geological eras,
the Pepper moth example demonstrates that evolutionary change can occur quite
quickly, in this case over
periods of approximately 100 years.
-- There should be little doubt, then, that there has been ample time
over the last 1 billion years of the earth's
history for natural selection to generate the variety of organisms we see
about us in the world today.
10. Limitations of this study.
-- Not all aspects of fitness were measured. It is conceivable
that if male mating success, survival during the
larval stages, etc., had been measured, the picture obtained would turn out
not to be consistent with theory.
B. Other similar examples
1. Evolution of warfarin resistance in rats (see Explanation)
2. Evolution of DDT resistance in mosquitoes (see
Explanation)