The peppered moth is a temperate species of night-flying moth. Peppered moth
evolution is an example of population genetics and natural selection.
Description The wingspan is 55mm. median It is
relatively stout-bodied, with forewings relatively narrow-elongate. The wings
are white, "peppered" with black, and with more or less distinct cross lines,
also black. The black speckling varies in amount, in some examples it is almost
absent, whilst in others it is so dense that the wings appear to be black
sprinkled with white. The antennae of males are strongly bipectinate.
Distribution Biston betularia is found in China,
Russia, Mongolia, Japan, North Korea, South Korea, Nepal, Kazakhstan,
Kirghizstan, Turkmenistan, Georgia, Azerbaijan, Armenia, Europe and North
America. Ecology and life cycle
In Britain and Ireland, the peppered moth is univoltine, whilst in
south-eastern North America it is bivoltine. The lepidopteran life cycle
consists of four stages: ova, several larval instars, pupae, which overwinter
live in the soil, and imagines. During the day, the moths typically rest on
trees, where they are preyed on by birds.
The caterpillar is a twig mimic, varying in colour between green and brown. On a
historical note, it was one of the first animals to be identified as being
camouflaged with countershading to make it appear flat, in a paper by Edward
Bagnall Poulton in 1887. It goes into the soil late in the
season, where it pupates in order to spend the winter. The imagines emerge
from the pupae between late May and August, the males slightly before the
females. They emerge late in the day and dry their wings before flying that
night. The males fly every night of their lives
in search of females, whereas the females only fly on the first night.
Thereafter, the females release pheromones to attract males. Since the
pheromone is carried by the wind, males tend to travel up the concentration
gradient, i.e., toward the source. During flight, they are subject to
predation by bats. The males guard the female from other males until she lays
the eggs. The female lays about 2,000 pale-green ovoid eggs about 1 mm in
length into crevices in bark with her ovipositor.
= Resting behaviour= A mating pair or a lone individual will
spend the day hiding from predators, particularly birds. In the case of the
former, the male stays with the female to ensure paternity. The best evidence
for resting positions is given by data collected by the peppered moth
researcher Michael Majerus, and it is given in the accompanying charts. These
data were originally published in Howlett and Majerus, and an updated
version published in Majerus, who concluded that the moths rest in the
upper part of the trees. Majerus notes: Creationist critics of the peppered moth
have often pointed to a statement made by Clarke et al.: "... In 25 years we
have only found two betularia on the tree trunks or walls adjacent to our
traps, and none elsewhere". The reason now seems obvious. Few people spend
their time looking for moths up in the trees. That is where peppered moths rest
by day. From their original data, Howlett and
Majerus concluded that peppered moths generally rest in unexposed positions,
using three main types of site. Firstly, a few inches below a branch-trunk joint
on a tree trunk where the moth is in shadow; secondly, on the underside of
branches and thirdly on foliate twigs. The above data would appear to support
this. Further support for these resting
positions is given from experiments watching captive moths taking up resting
positions in both males and females. Majerus, et al., have shown that
peppered moths are cryptically camouflaged against their backgrounds
when they rest in the boughs of trees. It is clear that in human visible
wavelengths, typica are camouflaged against lichens and carbonaria against
plain bark. However, birds are capable of seeing ultraviolet light that humans
cannot see. Using an ultraviolet-sensitive video camera,
Majerus et al. showed that typica reflect ultraviolet light in a speckled
fashion and are camouflaged against crustose lichens common on branches,
both in ultraviolet and human-visible wavelengths. However, typica are not as
well camouflaged against foliose lichens common on tree trunks; though they are
camouflaged in human wavelengths, in ultraviolet wavelengths, foliose lichens
do not reflect ultraviolet light. During an experiment in Cambridge over
the seven years 2001–2007 Majerus noted the natural resting positions of
peppered moths, and of the 135 moths examined over half were on tree
branches, mostly on the lower half of the branch, 37% were on tree trunks,
mostly on the north side, and only 12.6% were resting on or under twigs.
Polymorphism = Introduction on Forms=
There are several melanic and non-melanic morphs of the peppered moth.
These are controlled genetically. A particular colour morph can be indicated
in a standard way by following the species name in the form "morpha morph
name". The use of "form" in the method of Biston betularia f. formname in
detailing these variations is also a widespread practice.
These forms are often accidentally elevated to subspecies status when they
appear in literature. Not adding the "f." or morpha infers that the taxon is
a subspecies instead of a form, as in Biston betularia carbonaria instead of
Biston betularia f. carbonaria. Rarely forms have been elevated to species
status as in Biston carbonaria. Either of these two circumstances might lead to
the erroneous belief that speciation was involved in the observed evolution of
the peppered moth. This is not the case; individuals of each morph interbreed and
produce fertile offspring with individuals of all other morphs; hence
there is only one peppered moth species. By contrast, different subspecies of the
same species can theoretically interbreed with one another and will
produce fully fertile and healthy offspring but in practice do not, as
they live in different regions or reproduce in different seasons.
Full-fledged species are either unable to produce fertile and healthy
offspring, or do not recognize each other's courtship signals, or both.
European breeding experiments have shown that in Biston betularia betularia, the
allele for melanism producing morpha carbonaria is controlled by a single
locus. The melanic allele is dominant to the non-melanic allele. This situation
is, however, somewhat complicated by the presence of three other alleles that
produce indistinguishable morphs of morpha medionigra. These are of
intermediate dominance, but this is not complete.
= Form Names= In continental Europe, there are three
morphs: the white morph typica, the dark melanistic morph carbonaria, and an
intermediate form medionigra. In Britain, the typical white morph is
known as typica, the melanic morph is carbonaria, and the intermediate
phenotype is named insularia. In North America, the melanic black
morph is morpha swettaria. In Biston betularia cognataria, the melanic allele
is similarly dominant to the non-melanic allele. There are also some intermediate
morphs. In Japan, no melanic morphs have been recorded; they are all morpha
typica. At present, the precise molecular
genetics and biochemistry of the melanism in this species remains
unknown. True has reviewed this and suggests work based on candidate genes
from other insects such as the fruit fly Drosophila melanogaster. In any case, it
is rather likely that the underlying mechanism is not overly complex and, as
indicated above, does not involve very many genes and alleles: Unlike for
example the variation seen in human skin color, Peppered Moth morphs are not
clinal and can generally be readily distinguished from another.
Evolution The evolution of the peppered moth over
the last two hundred years has been studied in detail. At the start of this
period, the vast majority of peppered moths had light coloured wing patterns
which effectively camouflaged them against the light-coloured trees and
lichens upon which they rested. However, due to widespread pollution during the
Industrial Revolution in England, many of the lichens died out, and the trees
which peppered moths rested on became blackened by soot, causing most of the
light-coloured moths, or typica, to die off due to predation. At the same time,
the dark-coloured, or melanic, moths, carbonaria, flourished because they
could hide on the darkened trees. Since then, with improved environmental
standards, light-coloured peppered moths have again become common, and the
dramatic change in the peppered moth's population has remained a subject of
much interest and study. This has led to the coining of the term "industrial
melanism" to refer to the genetic darkening of species in response to
pollutants. As a result of the relatively simple and easy-to-understand
circumstances of the adaptation, the peppered moth has become a common
example used in explaining or demonstrating natural selection to
laypeople and classroom students through simulations.
The first carbonaria morph was recorded by Edleston in Manchester in 1848, and
over the subsequent years it increased in frequency. Predation experiments,
particularly by Bernard Kettlewell, established that the agent of selection
was birds who preyed on the carbonaria morph.
Jonathan Wells is one of a number of creationists who have criticized the use
of peppered moth melanism as an example of evolution in action. In his book
Icons of Evolution, Wells alleges that peppered moth studies, and in particular
Kettlewell's experiments, were erroneous. Similarly, in 2002 Judith
Hooper repeatedly implied fraud and error in Kettlewell's experiments in her
book titled Of moths and men. Despite some valid criticisms of the early
experiments, there has been no evidence of fraud. Subsequent experiments and
observations have supported the initial evolutionary explanation of the
phenomenon. Gallery
References External links
Bruce Grant has written several papers on melanism in the peppered moth which
are listed on his home page. Online lecture: "The rise and fall of
the melanic Peppered Moth" presented by Laurence Cook.
Matt Young. "Moonshine: Why the Peppered Moth remains an Icon of Evolution".
Department of Physics, Colorado School of Mines. Retrieved 2009-01-03.
The Peppered Moth: Decline of a Darwinian Disciple. This is the
transcript of Michael Majerus' lecture delivered to the British Humanist
Association on Darwin Day 2004. The Peppered Moth: The Proof of
Darwinian Evolution. This is the transcript of Majerus' lecture given at
the European Society for Evolutionary Biology meeting on 23 August 2007. The
accompanying powerpoint presentation is also available as a pdf file.
Majerus, Michael E. N.. "Industrial Melanism in the Peppered Moth, Biston
betularia: An Excellent Teaching Example of Darwinian Evolution in Action".
Evolution: Education and Outreach 2: 63–74. doi:10.1007/s12052-008-0107-y.
Accusations of data fudging and scientific fraud in the case are found
to be vacuous. An interactive game to simulate how
evolution works with Biston betularia