evolucion y biologia

1. Parallel Rise and Fall of Melanic Peppered
Moths in America and Britain
B. S. Grant, D. F. Owen, and C. A. Clarke
From the Department of Biology, College of William &
Mary, Williamsburg, Virginia 23187-8795 (Grant),
School of Biological and Molecular Sciences, Oxford
Brookes University, Oxford 0X3 OBP, UK (Owen), and
the Department of Genetics and Microbiology, Univer-
sity of Liverpool, Liverpool L69 3BX, UK (Clarke). We
thank H. Crum, M. Nielsen, M. Seaward, R. Shannon, W.
Westrate for discussion, D. Harren for air quality data,
K. Moody for autocorrelation analysis, and Derek
Whiteley for illustrating the phenotypes of Biston be-
lularia. We are indebted to the University of Michigan
for hosting our work at the E. S. George Reserve. This
study was supported in part by a grant from the Nuf-
field Foundation to C.A.C.
Journal of Heredity 1996;87:351-357; 0022-1503/96/55.00
The peppered moth (Biston betularia) is well known for the rapid rise in the fre-
quency of alleles producing melanic phenotypes correlated with a general black-
ening of the environment following the nineteenth-century industrial revolution. In
recent years the frequency of melanics has been dropping steadily in Britain in
apparent response to improved air quality. Some regional American populations of
this same species also experienced significant increases in melanics, by 1959 ex-
ceeding 90% in southeastern Michigan, but Michigan populations were not reex-
amined for over 30 years. In 1994 and 1995 we trapped moths in southeastern
Michigan and here report that a parallel decline in melanism has occurred in Amer-
ican peppered moths. Furthermore, we document that changes in Michigan's air
quality as measured by atmospheric sulfur dioxide (SO2) and suspended particu-
lates also parallel the changes recorded in Britain. The traditional interpretation is
that pale phenotypes of peppered moths at rest by day on lichen-encrusted trees
are camouflaged from bird predators; industrial fallout kills lichens and darkens
resting surfaces, favoring darker colored moths. However, the changes in allele
frequencies in the moth populations we sampled have occurred in the absence of
perceptible changes in the local lichen floras. We suggest that the role of lichens
has been inappropriately emphasized in chronicles about the evolution of melanism
in peppered moths.
Industrial melanism in the Lepidoptera
ranks among the best examples of observ-
able evolution by natural selection. The
most thoroughly documented case is that
of the geometrid moth Biston betularia,
with the fundamental details of its recent
evolutionary history described in virtually
every biology textbook. Its common name,
"peppered" moth, describes the appear-
ance of the so-called "typical" form or
phenotype which is covered with white
scales "peppered" with black over the
wings and body. Several other pigmenta-
tion phenotypes are known. A melanic
form, named carbonaria, is nearly solid
black. Intermediates between the pale and
fully melanic phenotypes are called insu-
laria. The range in variation from lightest
to darkest is continuous, but the pheno-
typic differences result from multiple al-
leles at a single locus which exhibit an ap-
proximate dominance hierarchy: the car-
bonaria allele shows complete dominance
over the insularia and typical alleles, and
the several insularia alleles show incom-
plete dominance over the typical allele
(Lees and Creed 1977). Most attention has
focused on the typicals and carbonaria
melanics in general accounts of industrial
melanism in this species, partly because
the difference between the extreme phe-
notypes is qualitative, and because insu-
laria intermediates have remained rela-
tively uncommon in many regions.
Melanic phenotypes were not known un-
til the middle of the last century when the
first specimen was caught near Manches-
ter, England; by the turn of the century the
formerly rare melanic phenotypes had
reached frequencies above 90% in popu-
lations surrounding British industrial cen-
ters because the original paler phenotype
had become conspicuous to predators in
habitats blackened by industrial soot. By
contrast, moth populations well away
from industrial regions remained unchan-
ged, with melanics either rare or absent in
unpolluted habitats [for review see Berry
(1990)].
B. betularia is widely distributed across
the higher latitudes of the Northern Hemi-
sphere. Its several geographic races, once
regarded by taxonomists as separate spe-
cies, have been reclassified as subspecies
by Rindge (1975). One such subspecies, B.
betularia cognataria, is common in North
351
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2. America. The American and British sub-
species are fully interfertile (Kettlewell
1961) and their mating pheromones are in-
distinguishable (Clarke et al. 1993).
The American subspecies also shows
wing-pattern polymorphism over parts of
its range. The "typical" phenotype, dubbed
the "pepper-and-salt geometer," is gener-
ally darker than the British typical and ex-
hibits continuous lines marking the wings.
The lines marking British typicals are in-
terrupted, producing mottled phenotypes.
A melanic phenotype, named swettaria in
America, is identical in appearance to Brit-
ish carbonaria. These two "form" names
were introduced to the literature before
American and British peppered moths
were recognized as members of the same
species. Rather than continue the practice
of using different names to refer to a single
phenotype, we will refer to the fully pig-
mented phenotype simply as "melanic."
As in Britain, intermediate phenotypes
also occur in American populations, and
the degree of pigmentation also appears
to be determined by multiple alleles at a
single locus which show a dominance hi-
erarchy with the darkest alleles dominant
to alleles producing less pigment (West
1977) (see journal cover).
Compared to the considerable attention
this species has received in Britain, pub-
lished information about American B. be-
tularia is meager. Melanics have been re-
corded at low frequencies in populations
in rural Massachusetts (Sargent 1974;
Treat 1979) and in the Virginia mountains
(West 1977), and at intermediate frequen-
cies in central Pennsylvania (Manley 1988).
The only American region where melanics
have been documented at frequencies ex-
ceeding 90% is in southeastern Michigan;
yet, prior to 1929, melanics were unknown
in that region (Owen 1961, 1962a). Owen
(1962b) interpreted the rapid rise in the
frequency of melanics in southeastern
Michigan as an evolutionary parallel to
the industrial melanism documented in
British peppered moths.
The situation has now changed in Brit-
ain. The melanic phenotype of the pep-
pered moth has been declining rapidly
throughout its range (Cook et al. 1986;
Lees 1981; West 1994) and nowhere has
this been more extensively recorded than
at Caldy Common, 18 km west of Liverpool
(Clarke et al. 1994). The drop in frequency
of melanics has been attributed to habitat
modifications associated with improved
air quality as measured by reductions in
atmospheric sulfur dioxide (SOj) and sus-
pended particulate matter following the
Clean Air Acts begun in 1956 (Clarke et al.
1985, 1990).
Similar clean air legislation was also in-
augurated in the United States in 1963, just
1 year after the last sample of the B. be-
tularia population in southeastern Michi-
gan was taken. Since then no further re-
ports about the frequencies of melanics in
that region have been published. Our ob-
jective was to determine whether the
Michigan population has changed since
the last census was taken there over 30
years ago, and to compare any changes to
those recorded in Britain to determine if
parallel evolution continues. During the
summers of 1994 (Grant et al. 1995) and
1995, we ran moth traps at the E. S. George
Reserve, a biological field station 65 km
west of Detroit, to assess the frequencies
of melanic phenotypes in the 6. betularia
population. In addition we analyzed air
quality data recorded for that region of
Michigan to determine if reductions in at-
mospheric pollutants also parallel the de-
clines reported in Britain.
Materials and Methods
Sampling Locations
We selected two locations to run moth
traps in 1994 and 1995: Caldy Common in
England and the George Reserve in Mich-
igan. Caldy Common was selected because
it has been sampled each year since 1959;
to date, nearly 18,000 specimens of pep-
pered moths have been taken. At no other
location has the decline in melanic fre-
quency been so thoroughly documented
(Clarke et al. 1994). The George Reserve
was selected because it is the only Amer-
ican site for which published data docu-
ment melanic frequencies in excess of 90%
(comparable to moth populations near
British industrial centers before the Clean
Air Acts). Between 1959 and 1961 a com-
bined total of 576 B. betularia were col-
lected at the George Reserve, of which 515
were melanic (Owen 1962a). Thefinalsam-
ple taken there was in 1962; it included 18
melanics of 22 moths (Owen DF, unpub-
lished data). Unfortunately, no additional
population samples were taken on the
George Reserve until we returned in 1994.
That sampling at both Caldy Common and
the George Reserve began independently
the same year was a coincidence.
Trapping Procedures
There are several widely used methods to
collect moths from the wild, and descrip-
tions of these are available in most field
guides (e.g., Covell 1984). Methods of
choice depend on the targeted species. B.
betularia, for example, does not feed in the
adult stage; therefore, "sugaring" methods
cannot be employed. The standard meth-
ods used for this species are light traps
and assembling traps. We used light traps
of two basic designs: (1) Robinson funnel
traps (Robinson 1952) fitted with mercury
vapor (MV) lamps, and (2) stretched bed
sheets illuminated either by MV or fluo-
rescent black lights. In addition to the
light traps we often used assembling
traps, which are cages containing virgin fe-
males; local males are attracted to the
cage by airborne pheromones released by
the females held inside the cage. Assem-
bling traps are highly species specific, that
is, the species of the males attracted is de-
termined by the species of the females
used as lures. Light traps, on the other
hand, attract enormous numbers of spe-
cies of moths as well as many other kinds
of night-flying insects. On warm, overcast
summer evenings a single light trap may
catch thousands of moths, but the species
composition varies greatly over the
course of the summer as different species
fly at different times of the year. A differ-
ence of a few weeks or even days can net
very different results when trapping for
particular species. Timing is critical.
At Caldy Common the same large MV
Robinson trap was used in 1994 and 1995
that had been used every year since sam-
pling began at that locality in 1959. Traps
were run nightly during the months of
June and July. Assembling traps were also
used most years (Clarke et al. 1994). The
original collections on the George Reserve
were made with a stretched sheet illumi-
nated by MV lamps (Owen 1962a). In 1994
we used the same method and trapping
sites, plus three MV Robinson traps and
an assembling trap. In an effort to increase
sample sizes in 1995, we ran eight moth
traps on the George Reserve [a stretched
sheet, six Robinson traps (four with MV
lamps and two with black lights), and an
assembling trap]. We operated the traps
for 7 weeks each summer but the majority
of the B. betularia were caught during Au-
gust. Throughout the sampling periods all
light traps in operation were switched on
every evening at dusk and remained on all
night regardless of weather conditions.
Each morning the contents of the Robin-
son traps were emptied and all other spe-
cies were released except for the B. betu-
laria, which were kept and scored by phe-
notype as either typical, intermediate, or
melanic. The illuminated stretched sheet
was examined repeatedly throughout the
3 5 2 The Journal of Heredity 1996:87(5)
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3. night to remove specimens as they set-
tled. Assembling traps were operated
whenever we had virgin females available
from bred stock.
Pollution Assessment
Quantitative assessments of environmen-
tal changes in Britain show that declines
in frequencies of melanic phenotypes in
peppered moth populations are correlated
with improvements in air quality, especial-
ly the gradual reduction of atmospheric
S02 and suspended particulate matter
(Clarke et al. 1985, 1990). We therefore
sought information about air quality in the
vicinity of the George Reserve using the
same assessment criteria as have been ap-
plied in Britain. On request we were gen-
erously provided annual reports issued by
the Air Monitoring Unit of the State of Mi-
chigan's Department of Natural Resources.
Specifically we requested data about at-
mospheric SO2 and suspended particu-
lates measured by sampling stations near-
est the George Reserve that operated con-
tinuously from as long ago in years as re-
cords are available to the present. For
many single locations the records are frag-
mentary as sampling stations were peri-
odically relocated. The sampling station
closest to the George Reserve for which
continuous annual SO2 records are avail-
able is at Detroit, 65 km east of the re-
serve, and the nearest station with contin-
uous annual records for suspended partic-
ulates is at Lansing, 60 km northwest of
the reserve. The George Reserve, in Liv-
ingston County, is about midway between
the two sampling stations. Sampling pro-
cedures are detailed in the annual reports
and are available on request.
Results
Melanic Frequencies
We recorded the gender and wing-pattern
phenotype of each individual B. betularia
collected at the two locations. To ensure
consistency in scoring phenotypes we did
not rank the intermediates into grades; in-
stead we assigned each moth to one of
three phenotypes at each location: typical,
intermediate (= insularid), or melanic (=
carbonaria = swettaria). The major shifts
in phenotype frequencies at both loca-
tions involve reversals of typicals and me-
lanics. The intermediates have shown a
slight but statistically significant (by re-
gression) increase in frequency at Caldy
Common, but have never reached 5% dur-
ing the 37 years samples have been taken
at that location. The 1994 and 1995 sam-
O
'c
0
100
90
80
70
60
50
40
30
20
10
0
V_
A
V
I 1
• • 1 ' ' • 1
• ' I '
59 63 67 71 75 79 83 87 91 95
Year
Figure 1. The frequency of melanic B betulana at Caldy Common in England (solid circles) and at the George
Reserve in Michigan (solid diamonds) from 1959-1995. Sampling was continuous at the British site; the American
site was sampled from 1959-1962 and again in 1994 and 1995. The percentages of melanics at the two sites in 1994
and 1995 were 18.7% (N = 348) and 17.6% (N = 261) at Caldy, and 16% (N = 25) and 20% (N = 35) at George
Reserve, respectively. The open circles mark the theoretical expectations based on a constant selection coefficient
of s = 0.153 against the dominant allele.
pies there included 14 intermediates of
348 (4%) and 6 of 261 (2%), respectively.
Likewise, intermediates were rare among
the early samples at the George Reserve
(Owen 1962a) and have changed little; the
1994 and 1995 samples included 1 of 25
(4%) and 2 of 35 (6%), respectively. Essen-
tially there are only two predominant phe-
notypes present at each location: typicals
and melanics.
Figure 1 shows the decline in frequency
of melanic phenotypes at both Caldy Com-
mon and at the George Reserve from
1959-1995. Despite the 32 year gap in sam-
pling at the George Reserve, it is clear that
both locations started with nearly equal
frequencies of melanics when sampling
began and reached nearly identical fre-
quencies by the time sampling resumed.
The similarity in frequencies of melanics
at the two locations is striking. The com-
bined 1994 and 1995 samples of 60 moths
from the George Reserve contained only
11 (18.33%) melanics; this frequency is vir-
tually identical to the frequency of melan-
ics over the same time period at Caldy
Common, but with N = 609 of which 111
(18.23%) were melanic (G, = 0.0004, ns). It
is also obvious that the changes at the
George Reserve between the interval of
the early samples (1959-1962, N = 598
with 533 melanics) and the 1994-1995
samples are significantly different (G, =
135.354, P « .001).
Sample Sizes
The purpose of this study was to compare
changes in allele frequencies at the two lo-
cations; however, the tenfold difference in
sample sizes over the past 2 years is cause
for concern. The large differences in the
numbers of B. betularia captured do not
reflect differences in the trapping efforts
taken. In fact, considerably greater effort
was invested in the Michigan site, partic-
ularly in 1995 when we doubled the num-
ber of traps put in the field in a concerted
effort to increase sample size over the 1994
results. Literally hundreds of thousands of
moths were caught each summer, yet the
numbers of B. betularia remained low. Avo-
cational moth collectors elsewhere in
Michigan also have noticed a recent de-
cline in the numbers of B. betularia coming
to their traps (Nielsen MC and Westrate
WP, personal communications). One might
argue that populations subjected to strong
selection experience reductions in popu-
lation size; however, it appears on exami-
nation that wide fluctuations in local den-
sity are regular occurrences in this spe-
cies regardless of allele frequencies at the
melanism locus. Published reports by oth-
er workers (e.g., Treat 1979; West 1977)
Grant et al • Parallel Evolution 353
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4. 800
sz
o
V)
Jumber
650
500
350
200
50
59 63 67 71 75 79 83 87 91 95
Year
Figure 2. Yearly fluctuations in population size as reflected by the numbers of moths caught at Caldy Common
by use of a single mercury vapor light trap operated throughout June and July each year from 1959-1995.
show extremely wide variation in sample
sizes taken at single locations during spec-
ified time periods over several years, and
Manley (1988) reported that B. betularia in
central Pennsylvania undergo large fluc-
tuations in population density every 4-5
years. The Caldy Common records cover
the longest span of years at any single lo-
cation. In Figure 2 we have plotted the
numbers of specimens of B. betularia
taken at Caldy each year by use of a single
MV light trap. Moths caught by assem-
bling traps have been excluded to adjust
for unequal trapping efforts. [For com-
plete data through 1993, see Clarke et al.
(1994)]. If sample size reflects local pop-
ulation density, it is clear that the B. be-
tularia population size fluctuates widely,
and that these fluctuations are indepen-
dent of the steady decline in melanic fre-
quencies illustrated in Figure 1. A runs test
against median sample size indicates non-
random fluctuations in sample size (P <
.05), and autocorrelation analysis suggests
size fluctuations are cyclic (P < .05). Re-
gression of sample size over years indi-
cates no significant long-term change in
population size (b = -3.6, / = —1.55, P =
.13). Owen's (1962a, unpublished data)
sample sizes over six generations at the
George Reserve varied from 22 to 173.
Based on this history, the differences be-
tween the 1994-1995 sample sizes at the
George Reserve and those at Caldy Com-
mon are most likely explained by normal
fluctuations in local population density.
Wide fluctuations in population densities
are apparently common occurrences
among other lepidopteran species (Ford
1975).
Sex Ratio
Only males are attracted to assembling
traps. For many species most of the moths
caught by light traps are males as well.
This is certainly true of B. betularia even
though females are winged and fully ca-
pable of flight. Of the 11,049 B. betularia
light-trapped at Caldy Common from 1959—
1993, only 91 were females (0.8%) (Clarke
et al. 1994). Others report similarly
skewed sex ratios in light-trapped samples
of the American subspecies (Manley 1981;
West 1977). Yet, the primary sex ratio
among reared broods is 1:1 (West 1977).
The likely explanation for the extreme gen-
der bias among adult moths coming to
light traps is that males are considerably
more active fliers than are females. An in-
terpretation based on observations of cap-
tive moths (Grant and Howlett 1988) is
that females disperse shortly after eclo-
sion but soon become sedentary once
they begin broadcasting pheromone to at-
tract mates. After mating, they begin lay-
ing eggs without ever taking flight again.
Males are capable of mating more than
once and in captivity can remain active for
up to 2 weeks. That there were no females
among the 60 B. betularia caught in 1994—
1995 at the George Reserve conforms to
expectations. The paucity of females in
our samples does not bias our estimation
of the decline in the frequency of the allele
for melanism in natural populations. The
locus for melanism is autosomal (West
1977) and allele frequencies are expected
to be identical in males and females. The
decline in the frequency of melanics
among those few females that have been
trapped at Caldy Common is consistent
with expectations (Clarke et al. 1994).
Pollution
Atmospheric SO2 and suspended particu-
lates have declined significantly in south-
eastern Michigan following clean air leg-
islation (Figure 3). The annual arithmetic
mean concentration of atmospheric SO2
(in jJLg/m3
) at Detroit shows a significant
decrease by regression (b = —0.705, t =
-3.142, P < .004, with 51% of the variation
accounted for by years). The trend paral-
lels those recorded in England (Clarke et
al. 1985, 1990), with current levels now
well below those exacerbating human re-
spiratory ailments (Lawther et al. 1970).
The annual geometric mean concentration
of suspended particulates (in jjig/m3
) at
Lansing also shows a significant decrease
by regression (b = -1.989, t = -6.359, P
« .001, with 82% of the variation in levels
of atmospheric particulate matter account-
ed for by years).
Discussion
The melanic phenotype of the peppered
moth has had an up and down history.
Once rare, it became common, and now it
is becoming rare again throughout much
of the species' range in the United King-
dom. We conclude that the changes in me-
lanic frequencies in the Michigan popula-
tion have paralleled in both directions the
well-documented changes in England. We
know of no other study that shows paral-
lel evolution of alleles at a single locus in
widely separated populations of the same
species going first in one direction, then
in the other.
Are the parallel genetic changes the re-
sult of parallel causes? Of the four evolu-
tionary forces of classical population ge-
netics (genetic drift, mutation, migration,
and natural selection), we can rule out
three. While all finite populations experi-
ence drift, the long-term directional trajec-
tory of the allele frequency changes re-
corded at Caldy Common argues against
drift as a relevant factor. Mutation, of
course, accounts for the origin of variation
at the locus for melanism, but the rapid
changes in allele frequency vastly exceed
354 The Journal of Heredity 1996:87(5)
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5. A
CO
O)
CM
O
CO
CO
75-
60-
45-
30-
I 15H
64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94
Year
B
CO
E
O)
c
w
o
JO
ticiPar>nded
o»
Q.
(0
110-
100-
90-
80-
70-
60-
50-
40-
30
• •
• •
66 68 70 72 74 76 78 80 82 84 86 88 90
Year
Figure 3. Changes in air quality in southeastern Michigan. (A) The annual arithmetic mean concentration of
atmospheric SO2 (in jig/m3
) at Detroit, east of the George Reserve. (B) The annual geometric mean concentration
of suspended particulates (in (ig/m3
) at Lansing, west of the George Reserve. The single square symbol represents
a measurement at another nearby location and was not used in regression calculations.
any expectations generated by known mu-
tation rates. That such rapid parallel
changes in allele frequencies in Michigan
and England have resulted from migration
between peppered moth populations liv-
ing on separate continents seems extreme-
ly remote; this argument is supported by
significant genetic divergence in the fre-
quencies of mtDNA haplotypes in Ameri-
can and British B. betularia (Sekula JE and
Graves JE, unpublished data). We cannot
eliminate natural selection as the evolu-
tionary force driving the parallel changes
in allele frequencies at the locus for mel-
anism at Caldy Common and the George
Reserve.
As with most moth species B. betularia
is active at night and remains motionless
by day, concealed from its predators. In
polluted habitats, with potential resting
surfaces blackened by soot, melanic moths
are apparently less conspicuous than pale
moths; on lighter backgrounds the reverse
is true. The traditional interpretation is
that differential predation by birds is chief-
ly responsible for the changes in the fre-
quencies of melanic phenotypes in moth
populations (Kettlewell 1973). Differential
predation as the sole selective force has
been questioned (Lees and Creed 1975;
Steward 1977), and preadult viability dif-
ferences between the phenotypes argue
for nonvisual selective components
(Creed et al. 1980); nevertheless, preda-
tion continues to receive the greatest at-
tention because experimental work has re-
peatedly demonstrated that conspicuous
moths are more likely to be discovered
and eaten by predators than are moths
that are harder to find (Clarke and Shep-
pard 1966; Kettlewell 1955, 1956, 1973;
Whittle et al. 1976).
Tutt (1896) suggested the resemblance
of peppered moths to lichens, and he ad-
vanced the theory that pale phenotypes
were cryptic when at rest on lichen-cov-
ered tree trunks, but where the lichens
have been killed and replaced by soot
from industrial fallout the melanic pheno-
types were better hidden from predators.
Since then other authors have also advo-
cated that local extinctions of lichen flora
following industrial development triggered
the rise of melanic phenotypes (Bishop et
al. 1975; Cook et al. 1990; Liebert and Brake-
field 1987).
It is clear however that pale phenotypes
of peppered moths do not depend on the
presence of lichens to thrive, because the
return of pale peppered moths at Caldy
Common began long before lichens start-
ed to reinvade. Even now coverage on
trees by lichens is sparse in that region. If
pale peppered moths require the local
presence of lichens for concealment from
predators, it logically follows that the
reestablishment of a lichen flora in for-
merly polluted habitats must precede the
rise in frequency of pale moth pheno-
types, that is, the hiding places should re-
cover before the hiders can return. Fur-
thermore, there has been no perceptible
change in an abundant lichen flora on the
George Reserve or in that region of south-
Grant et al • Parallel Evolution 355
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6. eastern Michigan over the last 30 years
(Crum H, personal communication) during
which time the melanic phenotype of B.
betularia fell from above 90% to below
20%. That substantial changes in melanic
frequencies in peppered moth populations
have occurred without obvious lichen suc-
cession suggests that the role of lichens in
the evolutionary history of peppered
moths has been exaggerated.
Other changes in the habitats have been
considered. Clarke et al. (1985) comment-
ed on the gradual lightening of tree sur-
faces at Caldy Common in the absence of
industrial soot, and Grant and Howlett
(1988) suggested that the regional spread
of silver birch trees (Betula penduld) dur-
ing the last several decades expanded po-
tential hiding places for peppered moths.
Birch trees had been felled for use as fire-
wood before the creation of smokeless
zones mandated by the Clean Air Acts.
While it may be relevant in Britain that the
wing pattern of pale peppered moths
bears a striking resemblance to the mot-
tled bark of silver birch trees, no similar
succession of birch trees has occurred on
the George Reserve or in its vicinity. Light-
barked birch species [gray (Betula populi-
folid) and paper birch (B. papyriferd)] are
indeed plentiful in northern Michigan but
they are scarce in the southern half of the
state. The George Reserve has remained a
mosaic habitat of fields, swamps, and
woodlands of mixed deciduous trees with
an abundance of dark-barked oaks (Quer-
cus spp.), black walnut (Juglans nigrd),
weeping willow (Salix babylonica), white
mulberry (Morus alba), and some quaking
aspen (Populus tremuloides). Most of these
tree species, and many more besides, are
suitable food plants for B. betularia cater-
pillars (Tietz 1972), but just where B. be-
tularia adults might hide among such trees
is unknown. There is little direct evidence
from nature (Howlett and Majerus 1987)
and considerable published conjecture
about the daytime resting place of this
nocturnally active moth based on the be-
havior of moths observed in captivity
(e.g., Grant and Howlett 1988; Mikkola
1984). A key to understanding which hab-
itat modifications promote shifts in phe-
notype frequencies in peppered moth pop-
ulations will require learning where the
adult moths hide during the day and
which predators they must elude.
The environmental change common to
both Caldy Common and the George Re-
serve is improved air quality. Much atten-
tion has been paid to atmospheric SO2 in
studies of British peppered moth popula-
tions because it is an index of pollution, it
is toxic to lichens, and the decrease in me-
lanic phenotypes is correlated with a de-
crease in SO2 levels (Clarke et al. 1985,
1990). The level of suspended particulates
also reflects industrial activity and it too
has declined following the Clean Air Acts
in both America and Britain. Larger air-
borne particles ultimately settle onto sur-
faces as soot. Changes in the levels of air-
borne particles produce changes in the re-
flectance of surfaces. The various pheno-
types of peppered moths are evidently
differentially concealed from predators on
backgrounds of different reflectance. Pre-
dation experiments (see above) confirm
this. The common denominator at Caldy
Common and the George Reserve is nei-
ther lichens nor birch trees, but is the re-
duction in atmospheric pollutants. Corre-
lations taken alone do not establish causal
relationships, but similar changes in air
quality and in moth populations in geo-
graphically separated regions argue against
sheer coincidence as the explanation for
the changes in melanic frequencies we
have recorded.
The tempo of the allele frequency changes,
however, must have been different at the
two locations. In southern Michigan there
are two generations each summer (bivol-
tine); in England there is only one (uni-
voltine). From 1959-1995 the Caldy popu-
lation completed 37 generations while the
George Reserve population completed 74.
Yet both populations have experienced
virtually identical changes in allele fre-
quencies. Accepting that the evolution at
both locations is driven by selection,
three different variables can produce the
same end result: (1) The selective disad-
vantage of melanic versus nonmelanic
phenotypes at the George Reserve and
Caldy Common are equivalent but the
shift in phenotype frequency at the George
Reserve began more recently. Unfortu-
nately there were no interim population
samples taken in Michigan, so it is impos-
sible to know when selection against me-
lanic phenotypes began there. (2) Selec-
tion against the melanic phenotypes on
the George Reserve is weaker than selec-
tion against the melanics at Caldy Com-
mon, so the year-to-year changes are
roughly equivalent at each location. Al-
though the melanics of both subspecies
are indistinguishable, American "typicals"
are much darker than their British coun-
terparts, and because of this American
typicals are not so conspicuously different
from American melanics. If differential pre-
dation by birds is a major cause for the
shifts in frequencies of melanics in B. be-
tularia populations, the advantage of one
American phenotype over another may re-
quire less dramatic environmental alter-
ation than has been observed in pre- and
postindustrial Britain. Other than the
quantitative improvements in air quality
indicated in Figure 3, the George Reserve
has not changed conspicuously, at least to
the human eye, since Owen (1962a) initi-
ated this study. (3) Selection coefficients
need not be constant each generation to
produce the same net change. Environ-
mental conditions experienced by the
moths not only differ from year to year,
but are also likely to differ between first
and second generations where the species
is bivoltine, for example, spring versus
late summer rainfall, temperature, vegeta-
tion, and predators. Changes in selection
coefficients need only be in the same di-
rection, on average, to produce concor-
dant shifts in allele frequencies at the two
locations. The selection coefficients we es-
timated from the Caldy Common data
from 1959-1995 have increased gradually
but not significantly (by regression). A
simulation using a constant fitness model
with selection acting against the dominant
allele shows that a single selection coeffi-
cient (s = 0.153) generates a theoretical
curve not significantly different (Kolmo-
gorov-Smirnov two-sample test, P = .32)
from the observed decline of melanic phe-
notypes at Caldy Common (Figure 1). If
the current rate of descent continues, the
Caldy population will be effectively mono-
morphic by the year 2010.
Clegg and Epperson (1988) point out
that two kinds of information are required
for a mechanistic analysis of natural selec-
tion: (1) a causal relationship between an
environment and the differential selection
of phenotypes must be established, and
(2) the genetic basis of the phenotypic dif-
ferences must be determined. Their own
analysis of flower color polymorphisms in
morning glory populations serves as a
good example. There are others, too, span-
ning a wide range of selection pressures
including differential predation by birds
on land snails polymorphic for shell col-
ors and banding patterns (Cain and Shep-
pard 1950), sexual selection in polymor-
phic African butterflies (Smith 1981), in-
secticide resistance and DNA polymor-
phism in tobacco budworm moths (Taylor
et al. 1995), and no doubt best known
among these is sickle-cell anemia in hu-
man populations and resistance to malaria
(Allison 1954). Yet nature has provided
few replicate "experiments" of natural se-
356 The Journal of Heredity 1996:87(5)
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7. lection affecting allele frequencies at sin-
gle loci in geographically isolated popula-
tions of the same species. The decline of
melanic phenotypes in populations of B.
betularia presents us with a rare opportu-
nity to observe parallel evolution as it
takes place. If current trends continue,
however, the melanic form may disappear
before we fully understand its saga.
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Received February 5, 1996
Accepted March 11, 1996
Corresponding Editor: Stephen J. O'Brien
Grant et al • Parallel Evolution 357
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