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Natural selection at work: a case study

We can make a strong case that natural selection is operating, even if the details of that selection are not immediately apparent.

For example, on rocky shores, animals have ranges that form clear spatial patterns. Some species live only in deep water, and some only live much higher up the shore. A snail common on California shores ( Tegula funebralis , at right) can be found in both ranges. In Southern California, Tegula live high up on the shore, while in Northern California, they live in deeper water.

Could natural selection explain this pattern? Researcher Michael Fawcett thought so and formulated a hypothesis to test. He found that predators, such as octopuses, starfish, and crabs, were more abundant in southern California than in northern California. Perhaps intense predation in the south selected for snails that lived higher up the shore, out of reach of many predators. In the north, selection might not have been as strong, so the snails were not selected to live high on the shore.

Fawcett tested this hypothesis by transplanting snails. He took northern and southern snails, released them in deep water and watched what happened. If predators were around, all the snails high-tailed it towards higher ground (snails can probably sense the chemicals exuded by predators). But southern snails moved further up the shore faster than northern snails. Because the northern snails were slower and didn’t move high enough, they were more likely to be eaten by predators.

What did this experiment show?

• There is an innate difference between southern and northern snails (i.e., some difference that is not merely a function of being on a southern or northern shore). This difference is probably genetic (but we would need to do more experiments to be absolutely sure).
• This difference can lead to differential survival. If predation is intense, snails that move higher faster are more likely to survive.

These results suggest that natural selection has occurred, altering the predator escape trait. Remember, all you need is

• Variation : There is variation in a trait between and within populations.
• Heredity : The variation probably has a genetic basis.
• Differential reproduction : The variants of the trait have different probabilities of surviving to reproduction.

These three features define natural selection. Without them, natural selection does not happen.

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Get more examples of how biologists study natural selection:

• Sex, speciation, and fishy physics , a news brief with discussion questions.
• Musseling in on evolution , a news brief with discussion questions.

Teach your students about natural selection:

• Clipbirds , a classroom activity for grades 6-12.
• Breeding bunnies , a classroom activity for grades 9-12.

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September 23, 2016

How natural selection acted on one penguin species over the past quarter century

Biologists of all stripes attest to evolution, but have debated its details since Darwin’s day. Since changes arise and take hold slowly over many generations, it is daunting to track this process in real time for long-lived creatures.

“We know that evolution occurs — that species change,” said Dee Boersma , a University of Washington professor of biology. “But to see this process in long-lived animals you have to look at generations of individuals, track how traits are inherited and detect selection at work.”

The scene at Punta Tombo in Dec. 2012. Dee Boersma

Boersma studies one particularly intriguing long-lived species, the Magellanic penguins of South America. She has spent 34 years gathering information about their lifespan, reproduction and behavior at Punta Tombo , a stretch of Argentine coast that serves as their largest breeding site. Boersma and her colleagues combed through 28 years’ worth of penguin data to search for signs that natural selection — one of the main drivers of evolution — may be acting on certain penguin traits. As they report in a paper published Sept. 21 in The Auk: Ornithological Advances, selection is indeed at work at Punta Tombo.

“This is the first decades-long study to measure selection in penguins, and only the second one for birds overall,” said lead author Laura Koehn , a graduate student in the UW School of Aquatic and Fishery Sciences who worked with Boersma as an undergraduate.

Heads held high at Punta Tombo. Dee Boersma

Like all penguins, the Magellanic variety are natural swimmers, where they feed on the bounty of the oceans. But once a year they return to the Argentine and Chilean coasts to mate and molt. Their “serial monogamy” — fidelity to one partner per breeding season — as well as a nagging preference to breed in the same general location each year make it possible to track individual birds over time, Koehn said. Boersma began the project, which is ongoing, in 1982.

To keep track of individuals amid a colony that at its height held 500,000 birds, Boersma and her team attached unique metal bands to the flipper of each penguin they studied. Each breeding season, the scientists would search for tagged penguins that made it back to Punta Tombo, measure basic physical characteristics and tag new chicks to add to their tracking duties.

“We chose characteristics that might be important to the success of individual penguins, like body size and bill depth,” said Boersma. “And once we had generations of trackable data for individuals and their descendants, we could ask: do these traits change over time?”

Through natural selection, individuals with traits that allow them to adapt and thrive in their environment can pass their favorable traits to their offspring.

Adult Magellanic penguin and two chicks, begging for food. Dee Boersma

“Our question was simple: for these traits, do offspring resemble their parents?” said Koehn.

By measuring an entire population — like the Magellanic penguins at Punta Tombo — Boersma’s team could see if individuals with certain characteristics, for example a large body, were more successful at breeding over the years.

Koehn and co-authors searched for signs of selection across 28 years of Boersma’s data.

She could detect selection in seven of the 28 years for both males and females. Selection is likely acting on these traits every year, but the highly variable conditions at Punta Tombo mean that the “direction” of selection on each trait may fluctuate too much to see over just 28 years, said Boersma. As the study continues, researchers may divine signatures of natural selection over additional years.

For the seven years the researchers could detect natural selection in males, there was a clear trend. Larger males held an edge in lean years when resources are fewer. In females, they detected selection acting on traits such as foot size, bill depth and body size. But unlike males, they saw no clear trend on how selection was shaping the females of the species over time.

Magellanic penguins at Punta Tombo. Dee Boersma

“Those traits appear to be important for survival,” said Boersma. “But if environmental conditions change rapidly then selection also constantly changes, and it’s harder to see a clear trend over time.”

This is only the second time natural selection has been observed over 20 years or more for a bird species. Evolutionary biologists Peter and Rosemary Grant spent more than 20 years cataloguing traits in seed-eating finches on the Galapagos island of Daphne Major. These are some of the same Galapagos finches that inspired a young Darwin to come up with his theory of evolution by natural selection in the 19 th century. The Grants detected signatures of natural selection at work on this island, correlating the changes with the influence of El Niño conditions.

“Now we’ve been able to track natural selection in a second bird species thanks to these decades of observations at Punta Tombo,” said Boersma.

Encouraged by her team’s findings with Magellanic penguins, Boersma intends to continue collecting data — tracking traits and survival for even more generations and repeating this analysis.

“This is only the beginning,” she concluded.

Co-authors include Jeffrey Hard with the Northwest Fisheries Science Center and Elaine Akst with Montgomery College.

The study is funded by the Wildlife Conservation Society, the Pew Fellows Program in Marine Conservation, the ExxonMobil Foundation, the Disney Worldwide Conservation Fund, the National Geographic Society, the Wadsworth Endowed Chair in Conservation Science at the University of Washington and Friends of the Penguins — as well as the Chase, Cunningham, CGMK, Offield, Peach, Thorne, Tortuga and Kellogg foundations.

For more information, contact Koehn at 206-616-2791 or [email protected] . Boersma is currently abroad and unreachable.

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• Published: 05 December 2012

The peppered moth and industrial melanism: evolution of a natural selection case study

• L M Cook 1 &
• I J Saccheri 2  

Heredity volume  110 ,  pages 207–212 ( 2013 ) Cite this article

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From the outset multiple causes have been suggested for changes in melanic gene frequency in the peppered moth Biston betularia and other industrial melanic moths. These have included higher intrinsic fitness of melanic forms and selective predation for camouflage. The possible existence and origin of heterozygote advantage has been debated. From the 1950s, as a result of experimental evidence, selective predation became the favoured explanation and is undoubtedly the major factor driving the frequency change. However, modelling and monitoring of declining melanic frequencies since the 1970s indicate either that migration rates are much higher than existing direct estimates suggested or else, or in addition, non-visual selection has a role. Recent molecular work on genetics has revealed that the melanic ( carbonaria ) allele had a single origin in Britain, and that the locus is orthologous to a major wing patterning locus in Heliconius butterflies. New methods of analysis should supply further information on the melanic system and on migration that will complete our understanding of this important example of rapid evolution.

Introduction

The peppered moth Biston betularia (L.) and its melanic mutant will be familiar to readers of Heredity as an example of rapid evolutionary change brought about by natural selection in a changing environment, even if the details of the story are not. In fact, the details are less simple than usually presented; they have accrued and undergone changes in emphasis during the century and a half of study. In this review, we outline the way patterns, techniques and ideas have evolved and discuss the problems that remain to be solved. (Industrial melanism in Biston betularia (L.) involves complex historical and geographical changes in habitat and ecology ( Kettlewell, 1973 ; Berry, 1990 ; Majerus, 1998 ). The original phenotype was speckled black and white (typical). Melanic forms are the almost uniform black carbonaria and several intermediate phenotypes ( insularia ). In the north English industrial region, insularia alleles played little part in change of melanic frequency, but in south Wales and adjacent parts of England, they reached high frequencies and had a different history. Similarly, industrialisation developed in different ways in different places. For hundreds of years until the eighteenth century, London was the most industrialised and polluted city. After that, industry developed, and sometimes declined at an early date, in places we now think of as rural. In areas that remained industrial, the extent of continuous conurbation varied considerably. Atmospheric pollution, darkening of surfaces, food plant abundance and predators therefore also vary. To keep the description simple, just two pairs of categories have been used where possible in this account: melanic and typical moths, and industrial and rural habitats. For the same reason, parameter estimates are given to the nearest order of magnitude, although original calculations are more precise and more qualified. There is no attempt to review the full contributions of the various authors cited.)

Early evidence of change

The peppered moth was the most diagrammatic example of the phenomenon of industrial melanism that came to be recognised in industrial and smoke-blackened parts of England in the mid-nineteenth century. The typical individual has a sprinkling of black marks on a white background. Melanic forms (almost uniform black carbonaria and intermediate insularia ) were shown to be determined by a series of dominant alleles at a single locus ( Bowater, 1914 and many later studies. The early controversy involving induction of melanic variants is reviewed by Rudge, 2009 ). They, and melanic variants in numerous other moth species, increased in frequency during the nineteenth century. In the peppered moth, the increase was so great that in some industrial parts of England the original wild type was almost lost by the end of the century. Records were made by amateur lepidopterists and discussed in the entomological literature, particularly as regards why melanic forms arise in nature ( Rudge, 2010 ). Tutt (1891 considered the possible factors implicated—humidity, environment, heredity, disease, temperature or protection (meaning camouflage)—without clearly favouring a single cause; he credits the idea of protection and natural selection to White (1877) and Chapman (1888) . Shortly after, he used the peppered moth as the example in a vivid description of selection for crypsis by predators in a changing environment ( Tutt, 1894 ), later repeated even more emphatically ( Tutt, 1896 ; see Owen, 1997 ). There were no records of predation at the time, and other writers such as Prest (1877) and, in an important review, Porritt (1907) disagreed. At the instigation of Bateson (1900) , who wished to assemble examples of discontinuous variation, the records were brought together ( Barrett, 1901 ; Doncaster, 1906 ) to provide a starting point for wider discussion.

Punnett (1915) summarised the view that birds did not act as selective agents. He also pointed out that the increase in melanic peppered moths implied appreciable selection, noting breeding results that suggested melanics were hardier. Haldane (1924) estimated the average selection required, which was much greater than the level hitherto associated with evolutionary change. Since then, strong selection has been an accepted part of the peppered moth story. Just how strong is more difficult to settle, as the change was from one extreme frequency to the other, but one of Haldane’s calculations suggested an advantage as great as 30 per cent.

The phenomenon of industrial melanism was discussed by Ford (1937 , 1953 , 1955 ) in relation to the prevalence of polymorphism. Ford defined polymorphism as ‘the occurrence together in the same locality of two or more discontinuous forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation’ ( Ford, 1964 ). The unstated assumptions were that mutations are deleterious and selection rules out drift. Current definitions include drift and allelic, as distinct from phenotypic, variability, so that allelic polymorphism is coexistence with the most common allele at no more than 99 per cent. Ford distinguished stable from transient polymorphisms, the best example of the latter being the peppered moth. Why were the melanic mutants dominant and advantageous when most mutants are deleterious and recessive? Ford offered some possible answers.

Ford’s ecological genetics

There are over 200 species of British moths with naturally occurring melanic variants ( Bowater, 1914 ). Ford had a wide knowledge of them and combined with it ideas about the nature of dominance. These were developed by Wright (1929) , Haldane (1930) and others, but came to him particularly from Fisher (1928 , 1930 , 1931 ). Other things being equal, a newly arisen mutant heterozygote would be disadvantageous and distinguishable from the wild-type homozygote, but repeated selection against it would favour individuals with genotypes in which the mutant phenotype was poorly expressed. The genome becomes adjusted to the normal requirements of the organism. Ford considered the case of an advantageous mutant. Increasing dominance would enhance its fitness. If it was pleiotropic, advantageous traits would tend to dominance and disadvantageous ones to recessiveness. Heterozygote advantage (HA) would develop, and we should expect that many of the observed polymorphisms resulted from HA. This is the ostinato that runs through Ecological Genetics ( Ford, 1964 ), and has had repeated echoes with respect to interpretation of changes in the peppered moth.

Moth species often have melanic variants at low frequency outside of industrial regions. They may have been favoured in the forests that formerly clothed much of northern Europe, in the course of time becoming visually dominant and fittest as heterozygotes. That would explain both their occurrence in rural parts and their sudden rise in frequency when again favoured by changes consequent on industrialisation ( Ford, 1937 , 1955 ).

This speculation received a boost when Haldane (1956) noted that in Manchester the peppered moth reached a high melanic frequency but not complete fixation. Perhaps, (1) selection was less intense than previously calculated, (2) immigrants came from unpolluted areas (both considered unlikely) or (3) there was HA. Kettlewell (1958) and Sheppard favoured the last option ( Clarke and Sheppard, 1963 ). Sheppard worked closely with Ford at Oxford. Later, in Liverpool, he collaborated with Clarke to analyse mimetic polymorphisms in swallowtail butterflies (for example, Sheppard, 1959 ). These were not maintained by HA, but illustrated the development of supergenes consisting of loci operating together to perfect the phenotype. Sheppard also studied peppered moths, identifying a cline of melanic morph frequency running from Liverpool into north Wales ( Clarke and Sheppard, 1966 ). At the industrial end of this transect, data from three successive dates were fitted better if HA were assumed than if it were not.

Kettlewell’s surveys and experiments

That was the intellectual environment in which Kettlewell began work on melanism in Lepidoptera in general and the peppered moth in particular ( Kettlewell, 1956 , 1973 ). He had a formidable knowledge of Lepidoptera ( Berry, 1990 ) and made his own independent research plans ( Rudge, 2006 , 2010 ), but many of his experiments and observations echo Ford’s theoretical preoccupations. He located early melanic specimens in museums and considered whether expression had evolved. He looked for dominance breakdown in crosses of individuals from widely separated places. Some of his work involved species with polymorphisms in Scottish pine forests and northern isles ( Kettlewell, 1961 ; Kettlewell and Berry, 1961 ; Kettlewell and Cadbury, 1963 ), where local habitats might shed light on the origin of dominant melanics. He was content with the assumption of HA ( Kettlewell, 1958 ). But above all, he took the peppered moth studies forward in two important ways. He coordinated country-wide surveys of morph frequency (1958 and 1965), which became the standard to compare earlier and later work, and produced the first convincing evidence that birds eat the moths and could do so selectively ( Kettlewell, 1955 ). Following his example, other authors tested for selective predation with moths of different phenotypes exposed on trees to be eaten ( Clarke and Sheppard, 1966 ; Bishop, 1972 ; Lees and Creed, 1975 ; Whittle et al., 1976 ; Steward, 1977a ; Bishop et al., 1978 ; Murray et al., 1980 ; Howlett and Majerus, 1987 ). Overall, these experiments suggested that the melanic peppered moth had an advantage of up to 2 to 1 over the typical form in industrial locations, where melanic frequency was 80 per cent or more, but a disadvantage, sometimes large, where the frequency was lower. In SW England, where melanics were almost absent, Kettlewell’s result suggested a fitness difference of the order of 1 to 2.

As Kettlewell was making a quantitative study of melanism in the peppered moth, Ford’s ecological genetics programme began to come apart. Instead of being commonplace, very few cases of HA were actually made to stick ( Lewontin, 1974 ). Researchers took to studying enzymes and base sequences rather than distinct phenotypes. The neutral model of evolution was an increasingly attractive way to interpret them ( Kimura, 1983 ), and the idea of evolution of dominance through modifiers was criticised (for example, Charlesworth, 1979 ).

Balance of visual selection and migration

The industrial environment was also radically altered. In the 1970s, old grimy building stock was removed, and smoke control introduced. Observations from Liverpool and north Wales were extended eastwards to Manchester and, using data of Sutton, as far as Leeds in Yorkshire, to produce a well-defined transect on which changes could be monitored ( Bishop et al., 1978 ). The first slight indication of increase of typicals in smoky locations was noted ( Askew et al., 1970 ; Lees and Creed, 1975 ). Transects were compared in south Wales, Birmingham and adjacent more rural regions ( Lees, 1981 ).

It was always clear that migration might influence frequency changes, but little information existed ( Kettlewell, 1958 ). Early dispersion of melanic forms, as they spread progressively southward, suggested that migration rates could be high ( Steward, 1977b ). Bishop (1972) was the first to provide an estimate. Using marked adults released at a central point and recaptured in traps progressively distant from it, he showed that male moths could move an average of about 2 km per night. The small amount of information available suggests an effective density of 40–50 moths per km 2 ( Bishop, 1972 ; Saccheri et al., 2008 ).

With these results it became possible to examine the basic tenets of the Ford model. Using estimates of visual selection along it, Bishop fitted a curve to the frequency cline in north Wales. There were too many melanics to the west, which could indicate that heterozygotes had a non-visual selective advantage (agreeing with the suggestions of Haldane and Sheppard). Must it therefore be assumed that HA had developed, or had always been present, in this system? Sheppard considered that it was ( Whittle et al., 1976 ) supported to some extent by Bishop, but perhaps the country-wide pattern could be reproduced assuming directional selection and migration alone ( Bishop and Cook, 1975 ; Bishop et al., 1978 ). Modelling was attempted with selection values based on the predation experiments, and migration rate and density based on Bishop’s estimates ( Cook and Mani, 1980 ). This showed that if selection estimates were about right, migration had to be some 10 times greater than assumed. If not, melanics must have a non-visual advantage. There was, however, no evidence that HA would help.

Non-visual selection

From their experiments on predation and conspicuousness, Lees and Creed (1975) concluded that other factors besides predation influenced melanic frequency. Creed et al. (1980) examined all available breeding records in which melanics and typicals segregated—83 crosses and over 12 000 progeny. Although not formally significant, the results suggested that survival to adult was higher for homozygous melanics than for other genotypes. Mani (1980) reanalysed the north Wales cline and found he could best describe it if he accepted the advantage indicated by this analysis. Weak frequency-dependent protection of rare morphs is also possible ( Whittle et al., 1976 ; Bishop and Cook, 1980 ). Using this assumption, too, Mani produced further theoretical analyses and satisfactory fits to field records ( Mani, 1982 , 1990 ; Mani and Majerus, 1993 ; Cook et al., 1998 ).

In some insects, diet affects availability of melanin precursors, which in turn influences coloration ( Talloen et al., 2004 ). Their synthesis is involved not only in melanin production but also immune defence and sclerotin production, so that competitive trade-offs are possible ( Windig, 1999 ; Stoehr, 2006 ). Pre-adult fitness differences between melanic and non-melanic morphs are reported for the noctuid species Mythimna separata ( Jiang et al., 2007 ). In some moths and other insects, individuals with darker cuticles tend also to have stronger immune defence ( True, 2003 ; Mikkola and Rantala, 2010 ) and immune defence and wing melanization can interact ( Freitak et al., 2005 ). Immune encapsulation of foreign bodies can be triggered by heavy metal pollution ( van Ooik et al., 2008 ). None of these investigations involve peppered moths, but they are evidence that change in visible melanic phenotype may be accompanied by other adjustments.

Moth settling position

Another variable about which too little was known was the pattern and strength of visual selection. In Kettlewell’s experiments, melanic and typical moths were at relatively high densities. There was a massive difference in relative visibility of the forms to the human eye between industrial and rural locations, and a corresponding difference in selective removal by birds. Experiments by others gave comparable results. But what matters is avian rather than human vision. To improve the estimates, it is critical to know how birds react to moths settled at the density and in the locations they adopt under their own volition. These aspects were investigated by Mikkola (1979 , 1984 , Howlett and Majerus (1987) and Liebert and Brakefield (1987) . Although some rest on trunks (36 per cent; Majerus, 2007 ; Cook et al., 2012 ), most moths are higher in trees under horizontal branches or in the crooks of smaller branches, where they are better protected from predation ( Mikkola, 1979 ). Howlett and Majerus (1987) compared individuals in the two types of resting position and found the disadvantage of melanics was halved by being in the better place. Under UV, which is visible to birds, carbonaria individuals are inconspicuous on foliose lichens, which may give them added protection in rural regions ( Majerus et al., 2000 ). These may be reasons why Bishop failed to predict the spread of melanics into north Wales ( Majerus, 1998 ).

The actual selective values remained uncertain. Moths are more abundant in rural than industrial regions, which have fewer suitable trees, more disorienting lights and different associations of birds. Industry and climate also affect epiphyte cover on many of the surfaces where moths can settle, suggesting possible interaction with changes in another biotic system ( Lees et al., 1973 ; Lees, 1981 ; Howlett and Majerus, 1987 ; Cook et al., 1990 ). Direct and indirect approaches can be used to learn more about these factors.

Further estimates of selection

Majerus (2007) adopted the direct approach in a properly controlled experiment. Over 6 years he allowed a total of 4864 moths at the melanic frequency for the location to settle naturally at low density in a wooded area near Cambridge. The results, including direct observation of predation, showed that birds exerted a selective pressure of about 10 per cent against melanics compared with typicals ( Majerus, 2007 ; Cook et al., 2012 ). The actual selection may be larger depending on the length of life of the moths and the extent of continuing selective predation. The melanic frequency in the population from which the experimental animals came was also declining. Taken together, the data on changing frequency in Cambridge ( Lees and Creed, 1975 ; Majerus, 1998 , Cook and Turner, 2008 ) suggest a disadvantage of the same order.

Similar selective values emerged from other surveys of changing frequency. An Open University survey in 1983/1984 showed the plateau of high frequency to have contracted compared with the picture produced by Kettlewell ( Cook et al., 1986 ), consistent with an average melanic disadvantage of about 12 per cent. The Rothamsted Insect Survey from 1974 to 1999 indicated further contraction and an average disadvantage of about 10 per cent ( Cook et al., 2002 ). Clarke and his co-workers ( Clarke et al., 1985 , 1990 , 1994 ; Grant et al., 1996 ) sampled every year from 1959 to 2002 at a location near Liverpool. Series that are less complete, but better than any from the initial rise, are available for Leeds, York, Manchester, Nottingham, Cambridge and west Kent (listed in Cook and Turner, 2008 ). These provide figures from 8 to 35 per cent for different locations and periods. Of course, such estimates include a component due to migration, which will vary depending on how close the site is to a frequency cline.

Grant et al. (1998) made pairwise comparisons of early and late records separated by several generations, with comparable results. One interesting feature, noticeable because a wide range of locations was used, was that the rate of decline was lower where the initial frequency was lower ( Cook, 2003 ). The estimated difference, with low migration and no non-visual advantage, is about 15 per cent selection against melanics in industrial areas, dropping to 2–3 per cent in rural areas ( Cook et al., 1990 ). An elementary interpretation, tenable if migration is low, is that selection is less intense in rural locations than in industrialised regions. This also accords with the experimental findings of Howlett and Majerus (1987) .

Change in melanic frequency was not only a British phenomenon. Sequential records from the Netherlands ( Brakefield and Liebert, 2000 ) and of the American subspecies in the United States ( Grant et al., 1996 ; Grant and Wiseman, 2002 ) show that decline occurred in industrialised regions parallel to reduction in atmospheric pollution at rates comparable to those observed in Britain. These common correlations suggest a common cause.

Migration again

The issue of migration is critical; change in morph frequency does not give a good measure of selection if there is high migration. Male adults move much further on the first than on subsequent nights ( Brakefield and Liebert, 1990 ). Females tend to be sessile, but perhaps only if quickly discovered by a male ( Cook, 2003 ). Newly emerged larvae also have a strong tendency to disperse, spinning threads that can carry them away from the oviposition site. Kettlewell noted that this spread them over a variety of food plants; it may reduce risk of predation, parasitisation and viral disease. If the larvae enter the air stream, however, they could go much further ( Liebert and Brakefield, 1987 ), the distance depending on how high they get. Studies using radar tracking show that British noctuid moths can be displaced 400 km or more in 8 h by prevailing winds ( Wood et al., 2009 ; Chapman et al., 2010 ).

In 2002 Saccheri collected a series of samples along the north Wales, north-west England cline to compare with the earlier records. The sensitive analytic procedure used by François Rousset ( Saccheri et al., 2008 ) to relate the new pattern to the earlier cline suggests that migration is indeed much greater than hitherto assumed, implying that visual selection is strong along its full length and does not fall off to the west. A microsatellite-based isolation-by-distance estimate of gene flow along this cline was also consistent with high dispersal, but had a wide confidence interval ( Saccheri et al., 2008 ).

Molecular advances

In contrast to the analysis of morph frequency and predation data, molecular genetics have been applied to this system only recently. Informed by results from Drosophila ( Wittkopp et al., 2003 ) and Papilio xuthus ( Koch et al., 2000 ), it was initially thought likely that the developmental switch for melanism would be found within the canonical melanisation pathway ( True, 2003 ). A survey of all the candidate genes revealed that this is not the case ( Van’t Hof and Saccheri, 2010 ), paving the way for a genome-wide segregation analysis of amplified fragment length polymorphisms, which localised the carbonaria locus to a chromosomal region with no known function in melanisation ( Van’t Hof et al., 2011 ). Curiously, and quite unexpectedly, the carbonaria region is orthologous to a major wing patterning locus controlling mimicry forms in Heliconius butterflies ( Papa et al., 2008 ; Joron et al., 2011 ).

The specific identity and mode of action of the functional sequence polymorphism remains to be discovered. Progress in understanding the developmental mechanism may shed light on longstanding questions about pleiotropy (which could give rise to non-visual fitness effects) and what determines the degree of dominance of the various melanic morph alleles (the dominance rank of typical, insularia and carbonaria alleles follows the degree of melanisation). In this context, the availability of a linkage map encompassing all 31 chromosomes ( Van’t Hof et al., 2012 ), featuring melanisation and colour patterning genes, facilitates future genetic investigations.

Patterns of linkage disequilibrium around the melanism locus indicate that, throughout the United Kingdom, the melanic morph carbonaria is descended from a single mutational event in the recent past ( Van’t Hof et al., 2011 ), thus settling the question whether industrial melanism arose several times in Britain and showing that the initial extension of range was a result of migration. Ongoing work aims to document the genetic consequences of the rise and fall of carbonaria , principally by analysing changes in linkage disequilibrium and nucleotide diversity along the north Wales, north-west England cline through time, using archival material. These data will also provide an opportunity to detect any deviations in the expected frequencies of carbonaria homozygotes and heterozygotes.

Conclusions

The peppered moth remains the type example of rapid response to human-induced environmental change, driven by selective predation ( Grant, 2012 ). Some of the issues with which it was once associated now receive less attention. Kettlewell’s (1958) ‘ancient carbonaria ’ remain to be properly examined at the genetic level, but there is evidence that modification of expression at the carbonaria locus may occur (see Grant (2004) ). The question why melanic forms in so many species are dominant still needs an agreed answer. Dominance modification continues to be of interest in connection with the evolution of genetic systems ( Mayo and Bürger, 1997 ; Bagheri, 2006 ). HA is not necessary to explain the changing gene frequency in the peppered moth, but there are other species where it could be important.

Breeding results suggest that homozygous melanic peppered moths may have a non-visual advantage, which, if true in the wild, would rule out HA. Creed et al. (1980) noted that the data are uncontrolled, and only part of the total data set is critical in establishing fitness difference. Further work is certainly needed, but if there really is a difference, melanics would have increased rapidly in polluted industrial regions with even a quite small visual advantage once sufficient melanic homozygotes had accumulated. At the end of the industrial period, there must correspondingly be strong visual disadvantage to overcome the non-visual fitness effect unless there is high migration. The following possibilities then exist to explain why rural areas continued to have low melanic frequencies:

With an estimated displacement of about 2 km per day there could be a small visual disadvantage to melanics and little non-visual difference (option 1) or stronger visual selection coupled with substantial non-visual advantage (option 2). The analysis of decay of the north Wales cline suggests that migration is indeed greater by an order of magnitude than previously assumed, giving option 3. If a non-visual advantage exists, then the visual disadvantage must have been even larger (option 4). At present, none of these options can certainly be eliminated.

Migration and selection can be used interchangeably in models. It is therefore important to make better estimates of migration and to explore the question of non-visual fitness difference. As the melanics disappear, field studies on morph frequency change become increasingly difficult. With respect to migration, it would be possible to conduct further release and recapture trials. Tighter indirect estimates of gene flow, encompassing adult and larval stages, using a large panel of markers would also be useful. Adult females fly less than males. Comparison of the pattern of genetic isolation by distance for unlinked autosomal loci (for example, Daly et al., 2004 ) versus mitochondrial or W-chromosome-linked loci could in principle detect dispersal differences between adult males and females.

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Acknowledgements

Many people have read and commented on earlier drafts of this paper. We are grateful to them all, with particular thanks to Bruce Grant and John Turner. We thank three anonymous referees whose observations have improved the presentation.

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Cook, L., Saccheri, I. The peppered moth and industrial melanism: evolution of a natural selection case study. Heredity 110 , 207–212 (2013). https://doi.org/10.1038/hdy.2012.92

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293 Natural Selection

• Published: October 2001
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The theory that organisms become adapted to their environment through the process of natural selection has become so ingrained in modern biological thought, and more generally in Western culture of the late 20th century, that it is surely one of the great scientific paradigms of the present era. Evolution and adaptation were both well-accepted concepts by the mid-19th century, at least among French and British natural philosophers. The theory of natural selection, developed by Wallace (1858) and Darwin (1859), provided a functional connection between the two processes. However, despite its logical consistency, natural selection was not accepted as a necessary or sufficient explanation for adaptation until the “evolutionary synthesis” of the mid-20th century, when knowledge from population and quantitative genetics, natural history (e.g., biogeography, ecology, behavior), systematics, and paleontology merged to form the unified theory of adaptive evolution known as neo-Darwinism (see Futuyma 1998 for a concise review of this history). Since that time, natural selection has been accepted as the universal mechanism leading to adaptation, and the two terms have become so closely associated as to be almost tautological. Adaptationist hypotheses are now fundamental to much of modern biology and are becoming increasingly apparent in more disparate fields, such as anthropology, medicine, biochemistry, and psychology (Futuyma 1999). Nevertheless, there is much that natural selection cannot explain. For example, chance events may strongly influence macroevolutionary trends (i.e., the origin and extinction of species and higher taxa), some aspects of molecular evolution, and evolution within small or subdivided populations (Mazer and Damuth, this volume, chapter 2; Nunny, this volume). For this reason, adaptationist hypotheses should be viewed with skepticism until adequately tested (Reznick and Travis, this volume). In this chapter, we carefully define natural selection and discuss methods of measuring selection in natural populations as a means of testing adaptationist hypotheses. These methods are most appropriate for testing hypotheses concerning the adaptive significance of contemporary trait distributions within and among populations (“microevolutionary” hypotheses) and thus have particular relevance for evolutionary ecologists. Readers will find many additional examples of these and other methods of testing microevolutionary adaptationist hypotheses throughout this volume, such tests being an essential component of most research programs in evolutionary ecology.

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London's Peppered Moths

A Case Study in Natural Selection

Ian Redding/Getty Images

• Habitat Profiles
• Marine Life
• M.S., Applied Ecology, Indiana University Bloomington
• B.S., Biology and Chemistry, University of Illinois at Urbana-Champaign

In the early 1950s, H.B.D. Kettlewell, an English physician with an interest in butterfly and moth collecting, decided to study the unexplained color variations of the peppered moth.

Kettlewell wanted to understand a trend that had been noted by scientists and naturalists since the early nineteenth century. This trend, observed in the industrialized areas of Britain, revealed a peppered moth population—once primarily made up of light, gray-colored individuals—that now consisted primarily of dark gray individuals. H.B.D. Kettlewell was intrigued: why had this color variation taken place in the moth population? Why were dark gray moths more common only in industrial areas while light gray moths were still predominant in rural areas? What do these observations mean?

Why Did This Color Variation Occur?

To answer this first question, Kettlewell set about designing several experiments. He hypothesized that something in Britain's industrial regions had enabled the dark gray moths to be more successful than the light gray individuals. Through his investigations, Kettlewell established that dark gray moths had greater fitness (meaning they produced, on average, more surviving offspring) in the industrial areas than light gray moths (who, on average, produced fewer surviving offspring). H.B.D. Kettlewell's experiments revealed that by better blending into their habitat, the dark gray moths were more able to avoid predation by birds. The light gray moths, on the other hand, were easier for birds to see and capture.

Dark Gray Moths Adapted to Industrial Habitat

Once H.B.D. Kettlewell had completed his experiments, the question remained: what was it that had changed the moth's habitat in industrial regions that enabled the darker-colored individuals to blend into their surroundings better? To answer this question, we can look back into Britain's history. In the early 1700s, the city of London—with its well-developed property rights, patent laws, and stable government—became the birthplace of the Industrial Revolution .

Advancements in iron production, steam engine manufacturing, and textile production catalyzed many social and economic changes that reached far beyond London's city limits. These changes altered the nature of what had been predominantly an agricultural workforce. Great Britain's plentiful coal supplies provided the energy resources needed to fuel the fast-growing metalworking, glass, ceramics, and brewing industries. Because coal is not a clean energy source, its burning released vast quantities of soot into London's air . The soot settled as a black film on buildings, homes, and even trees.

In the midst of London's newly industrialized environment, the peppered moth found itself in a difficult struggle to survive. Soot coated and blackened the trunks of trees throughout the city, killing lichen that grew on the bark and turning tree trunks from a light gray-flecked pattern to a dull, black film. The light gray, pepper-patterned moths that once blended into the lichen-covered bark, now stood out as easy targets for birds and other hungry predators.

A Case of Natural Selection

The theory of natural selection suggests a mechanism for evolution and gives us a way to explain the variations we see in living organisms and the changes evident in the fossil record. Natural selection processes can act on a population either to reduce genetic diversity or increase it. The types of natural selection (also known as selection strategies) that reduce genetic diversity include: stabilizing selection and directional selection.

The selection strategies that increase genetic diversity include diversifying selection, frequency-dependent selection, and balancing selection. The peppered moth case study described above is an example of directional selection: the frequency of color varieties changes dramatically in one direction or another (lighter or darker) in response to the predominating habitat conditions.

• Directional Selection in Evolutionary Biology
• Luna Moth, Actias luna
• Is Natural Selection Random?
• Types of Natural Selection: Disruptive Selection
• The 5 Types of Selection
• Painted Lady (Vanessa Cardui)
• Stabilizing Selection in Evolution
• Insects: The Most Diverse Animal Group in the Planet
• 7 Insect Pollinators That Aren't Bees or Butterflies
• Life Cycle of Butterflies and Moths
• Habits and Traits of Owlet Moths
• Nature Study Themes for Spring
• How to Keep Fall Caterpillars Alive Until Spring
• Natural Selection Hands on Lesson Plan
• Science Worksheets
• 12 Important Animals of North America

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The Case Study: I'm Looking Over a White Striped Clover: A Case of Natural Selection

Journal of College Science Teaching – July/August 2007

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9.1: Case Study: Everyday Evolution

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• Page ID 16767

• Suzanne Wakim & Mandeep Grewal
• Butte College

Case Study: Flu, from Pigs to You

One night in April 2009, Mateo woke up soaked in sweat. He had a fever of 102.4 °F, chills, an intense headache, and body aches. He soon develops a sore throat and a bad cough. The next day he felt so sick and exhausted that he could hardly get out of bed, and his fever and other symptoms lasted for days. Clearly, this was not just a mild cold virus — Mateo most likely had influenza, commonly known as the flu.

While watching TV as he recovered in bed, Mateo saw a news report about a new “swine flu” strain of the influenza virus that was spreading in people throughout North America, particularly in Mexico. It was called the swine flu because scientists thought it most likely originated in pigs, based on similarities in its genetic sequence with viruses that infect pigs. However, contact with pigs was not necessary for people to catch swine flu. This version seemed to spread directly between people, similar to the typical seasonal flu virus.

Mateo’s symptoms were similar to those described in the news report on swine flu. Although he was beginning to recover, he saw that others were not so lucky. Many people with swine flu developed severe pneumonia, and some even died. Because this was a new strain of flu virus that was significantly different than the previous seasonal flu viruses, the existing flu vaccine was largely ineffective against swine flu. Therefore, the only way to try to prevent infection by the swine flu virus was to limit exposure to it, including avoiding contact with people with the flu and using good hand washing practices. The news report showed people in Mexico wearing masks as they went about their daily lives, to try to prevent exposure to the virus.

By June 2009, Mateo was back to normal, but many other people worldwide were not. Within just a few months, the swine flu had spread from North America to over 70 countries and territories throughout the world. The World Health Organization declared the spread of swine flu to be a pandemic, meaning that a significant portion of the world’s population was infected. In September 2009, over 99% of the influenza viruses circulating in the U.S. were the swine flu strain, which is also known as the 2009 H1N1 virus. If you had the flu in the U.S. during this time period, chances are high that it was the swine flu.

How could a new viral strain like this emerge so suddenly? And how could it change from infecting pigs to infecting humans? This is an example of evolution in action. You may think of evolution as something that occurred in the distant past, for instance, how humans evolved from earlier primates. But evolution is occurring all the time. As you will learn in this chapter, evolution is the process by which characteristics of biological entities, such as living organisms or viruses, change over time. Evolution can occur very slowly or more quickly, but it is particularly rapid in viruses and bacteria. In the Case Study Conclusion for this chapter, you will learn specifically about how the 2009 H1N1 virus evolved from a virus that infects pigs to one that infects humans.

Chapter Overview: Biological Evolution

In this chapter, you will learn about the theory of evolution, evidence for evolution, how evolution works, and the evolution of living organisms on Earth. Specifically, you will learn about:

• Darwin’s theory of evolution by natural selection and how he developed this theory.
• Evidence for the theory of evolution from fossils, DNA, and observations of living organisms.
• Microevolution, which is an evolution that occurs over a relatively short period of time within a population.
• How allele frequencies in a population change due to the forces of evolution, which include mutation, gene flow, genetic drift, and natural selection.
• Macroevolution, which is an evolution that occurs at or above the species level. This includes the generation of new species and coevolution between species.
• Influences on the timing of macroevolution.
• The tools used by scientists to study evolution including the fossil record, methods of establishing the age of fossils, and molecular clocks based on DNA or amino acid sequences.

As you read this chapter and learn more about evolution, think about the following questions about the swine flu virus.

• Viruses can replicate quickly. Why does this contribute to their rapid rate of evolution?
• Mutation plays an important role in the evolution of viruses. How does mutation relate to evolution?
• One of the reasons why the 2009 H1N1 swine flu virus evolved is that different types of influenza viruses can exchange genetic material with each other if they infect the same host, in a process called reassortment. Why might this lead to a new strain of influenza virus with different characteristics? How is this similar to the genetic variation produced by sexual reproduction?
• It is thought that contact between North American and Eurasian pigs, possibly through international trade, may have contributed to the evolution of the swine flu virus. What are some other examples in which the movement of organisms or contact between organisms has contributed to evolutionary changes?

Attributions

• Sow and five piglets by Scott Bauer, U.S. Department of Agriculture, public domain via Wikimedia Commons
• Masked Train Passengers by Eneas De Troya from Mexico City, México, CC BY 2.0 via Wikimedia Commons
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0