The most frequently cited example of natural selection concerns changes in the coloration of “peppered” moths around Manchester, England. In recent years, the moth story has come under some criticism; but the basic premise remains valid, so we use it to illustrate how natural selection works. Before the nineteenth century, the most common variety of the peppered moth was a mottled gray color. During the day, as moths rested on lichen covered tree trunks, their coloration provided camouflage . There was also a dark gray variety of the same species, but since the dark moths weren’t camouflaged, they were eaten by birds more frequently and so they were less common. (In this example, the birds are the selective agent, and they apply selective pressure on the moths.) Therefore, the dark moths produced fewer offspring than the camouflaged moths. Yet, by the end of the nineteenth century, the common gray form had been almost completely replaced by the darker one.
The cause of this change was the changing environment of industrialized nineteenth-century England. Coal dust from factories and fireplaces settled on trees, turning them dark gray and killing the lichen. The moths continued to rest on the trees, but the light gray ones became more conspicuous as the trees became darker, and they were increasingly targeted by birds. Since fewer of the light gray moths were living long enough to reproduce, they contributed fewer genes to the next generation than the darker moths did, and the proportion of lighter moths decreased while the dark moths became more common. A similar color shift had also occurred in North America. But when the advent of clean air acts in both Britain and the United States reduced the amount of air pollution (at least from coal), the predominant color of the peppered moth once again became the light mottled gray. This kind of evolutionary shift in response to environmental change is called adaptation.
Another example of natural selection is provided by the medium ground finch of the Galápagos Islands. In 1977, drought killed many of the plants that produced the smaller, softer seeds favored by these birds. This forced a population of finches on one of the islands to feed on larger, harder seeds. Even before 1977, some birds had smaller, less robust beaks than others (that is, there was variation); and during the drought, because they were less able to process the larger seeds, more smaller beaked birds died than larger-beaked birds. Therefore, although overall population size declined, average beak thickness in the survivors and their offspring increased, simply because thicker beaked individuals were surviving in greater numbers and producing more offspring. In other words, they had greater reproductive success. But during heavy rains in 1982–1983, smaller seeds became more plentiful again and the pattern in beak size reversed itself, demonstrating how reproductive success is related to environmental conditions (Grant, 1986; Ridley, 1993).
The best illustration of natural selection, however, and certainly one with potentially grave consequences for humans, is the recent increase in resistant strains of disease-causing microorganisms. When antibiotics were first introduced in the 1940s, they were hailed as the cure for bacterial disease. But that optimistic view didn’t take into account the fact that bacteria, like other organisms, possess genetic variability. Although an antibiotic will kill most bacteria in an infected person, any bacterium with an inherited resistance to that particular therapy will survive. Subsequently, the survivors reproduce and pass their drug resistance to future generations, so that eventually, the population is mostly made up of bacteria that don’t respond to treatment. What’s more, because bacteria produce new generations every few hours, antibiotic-resistant strains are continuously being produced. As a result, many types of infection no longer respond to treatment. For example, tuberculosis was once thought to be well controlled, but it has seen a resurgence in recent years because the bacterium that causes it is now resistant to many antibiotics.
These three examples (moths, finches, and bacteria) provide the following insights into the fundamentals of evolutionary change produced by natural selection:
- 1. A trait must be inherited if natural selection is to act on it. A characteristic that isn’t hereditary (such as a temporary change in hair color produced by the hairdresser) won’t be passed on to succeeding generations. In finches, for example, beak size is a hereditary trait.
- 2. Natural selection can’t occur without population variation in inherited characteristics. If, for example, all the peppered moths had initially been gray (you will recall that some dark forms were always present) and the trees had become darker, the survival and reproduction of all moths could have been so low that the population might have become extinct. Selection can work only with variation that already exists.
- 3. Fitness is a relative measure that changes as the environment changes. Fitness is simply differential reproductive success. In the initial stage, the lighter moths were more fit because they produced more offspring. But as the environment changed, the dark gray moths became more fit, and a further change reversed the adaptive pattern. Likewise, the majority of Galápagos finches will have larger or smaller beaks, depending on external conditions. So it should be obvious that statements regarding the “most fit” mean nothing without reference to specific environments.
- 4. Natural selection can act only on traits that affect reproduction. If a characteristic isn’t expressed until later in life, after organisms have reproduced, then natural selection can’t influence it. This is because the inherited components of the trait have already been passed on to offspring. Many forms of cancer and cardiovascular disease are influenced by hereditary factors, but because these diseases usually affect people after they’ve had children, natural selection can’t act against them. By the same token, if a condition usually kills or compromises the individual before he or she reproduces, natural selection acts against it because the trait won’t be passed on.
So far, our examples have shown how different death rates influence natural selection (for example, moths or finches that die early leave fewer offspring). But mortality isn’t the complete picture. Another important aspect of natural selection is fertility, because an animal that gives birth to more young passes its genes on at a faster rate than one that bears fewer offspring.
However, fertility isn’t the entire story either, because the crucial element is the number of young raised success fully to the point at which they themselves reproduce. We call this differential net reproductive success. The way this mechanism works can be demonstrated through yet another example. In swifts (small birds that resemble swallows), data show that producing more offspring doesn’t necessarily guarantee that more young will be successfully raised. The number of eggs hatched in a breeding season is a measure of fertility. The number of birds that mature and are eventually able to leave the nest is a measure of net reproductive success, or offspring successfully raised. The following table shows the correlation between the number of eggs hatched (fertility) and the number of young that leave the nest (reproductive success), averaged over four breeding seasons (Lack, 1966):
| Number of eggs hatched (fertility) | 2 eggs | 3 eggs | 4 eggs |
| Average number of young raised (reproductive success) | 1.92 | 2.54 | 1.76 |
| Sample size (number of nests) | 72 | 20 | 16 |
As you can see, the most efficient number of eggs is three, because that number yields the highest reproductive success. Raising two offspring is less beneficial to the parents, since the end result isn’t as successful as with three eggs. Trying to raise more than three is actually detrimental, since the parents may not be able to provide enough nourishment for any of the offspring. In evolutionary terms, offspring that die before reaching reproductive age are equivalent to never being born. Actually, death of an offspring can be a minus to the parents, because before it dies, it drains parental resources. It may even inhibit their ability to raise other offspring, thereby reducing their reproductive success even further. Selection favors those genetic traits that yield the maximum net reproductive success. If the number of eggs laid is a genetic trait in birds (and it seems to be), natural selection in swifts should act to favor the laying of three eggs as opposed to two or four.
Reference
The classic example of evolutionary change is demonstrated by the response of peppered moth species to the pressure caused by the atmospheric pollution during industrial revolution in United.Kingdom. Within the span of last 100 years darkened forms of peppered moths have appeared in varying frequencies in U.K., a phenomenon which is known as industrial melanism. Up to 1848, all reported forms of peppered moth (Biston betularia) were creamy white with black dots with darkly shaded areas . In 1848, a black form of moth was reported from Manchester and by 1895,98% of the peppered moth population was black in Manchester. The black ‘melanic’ variety formed by a recurring random mutation, which has a strong selective advantage in the industrial area. Its phenotypic appearance (black colour) was suitable for its survival.
The moths rest during the day on tree trunks, well camouflaged as the coloration merges with the lichens growing on the trunks and they fly by night. With the SO2 pollution caused by the burning of coal as industrial revolution advanced, lichens growing on trees got killed in the industrial areas because they were susceptible to air poIlution. As a result bark got covered and darkened by black soot deposits.
In 1950s Kettlewell released known number of moths marked light and dark forms of moth in two areas, one a polluted area near Birmingham and other an unpolluted area in Dorset. Kettlewell demonstrated through film that robins and thrushes (types birds feed on the moths. The results show that melanic (dark) carbonica form of the moth Biston betularia carbonica has a selective advantage in industrial area over the lighter forms, Biston betularia typica whereas lighter form has the selective advantage in non-polluted areas.
Evidence of Positive Selection in Humans
Within the last decade, our ability to probe our own species for evidence of selection has increased dramatically due to the flood of genetic data that have been generated. Starting with the complete sequence of the human genome (Lander et al., 2001), which provides a framework and standard reference for all human genetics, key data sets include the completed or near-completed genomes of several related species (e.g., chimpanzee, macaque, gorilla, and orangutan), a public database of known genetic variants in humans, and surveys of genetic variation in hundreds of individuals in multiple populations (Chimpanzee Sequencing and Analysis Consortium, 2005; Gibbs et al., 2007; Sherry et al., 2001; International HapMap Consortium, 2007). With these new data, it is now possible to scan the entire human genome in search of signals of natural selection.
Although the study of natural selection in humans is still in an early stage, the new data, building on decades of earlier work, are beginning to reveal some of the landscape of selection in our species. In fact, researchers have identified many genetic loci at which selection has likely occurred, and some of the selective pressures involved have been elucidated. Three significant forces that have been identified thus far include changes in diet, changes in climate, and infectious disease.
Lactose Tolerance
The domestication of plants and animals roughly 10,000 years ago profoundly changed human diets, and it gave those individuals who could best digest the new foods a selective advantage. The best understood of these adaptations is lactose tolerance (Sabeti et al., 2006; Bersaglieri et al., 2004). The ability to digest lactose, a sugar found in milk, usually disappears before adulthood in mammals, and the same is true in most human populations. However, for some people, including a large fraction of individuals of European descent, the ability to break down lactose persists because of a mutation in the lactase gene (LCT). This suggests that the allele became common in Europe because of increased nutrition from cow’s milk, which became available after the domestication of cattle. This hypothesis was eventually confirmed by Todd Bersaglieri and his colleagues, who demonstrated that the lactase persistence allele is common in Europeans (nearly 80% of people of European descent carry this allele), and it has evidence of a selective sweep spanning roughly 1 million base pairs (1 megabase). Indeed, lactose tolerance is one of the strongest signals of selection seen anywhere in the genome. Sarah Tishkoff and colleagues subsequently found a distinct LCT mutation also conferring lactose tolerance, in this case in African pastoralist populations, suggesting the action of convergent evolution (Tishkoff et al., 2007).
Malaria Resistance
The development of agriculture also changed the selective pressures on humans in another way: Increased population density made the transmission of infectious diseases easier, and it probably expanded the already substantial role of pathogens as agents of natural selection. That role is reflected in the traces left by selection in human genetic diversity; multiple loci associated with disease resistance have been identified as probable sites of selection. In most cases, the resistance is to the same disease—malaria (Kwiatkowski, 2005).
Malaria’s power to drive selection is not surprising, as it is one of the human population’s oldest diseases and remains one of the greatest causes of morbidity and mortality in the world today, infecting hundreds of millions of people and killing 1 to 2 million children in Africa each year. In fact, malaria was responsible for the first case of positive selection demonstrated genetically in humans. In the 1940s and 1950s, J. B. S. Haldane and A. C. Allison demonstrated that the geographical distribution of the sickle-cell mutation (Glu6Val) in the beta hemoglobin gene (HBB) was limited to Africa and correlated with malaria endemicity, and that individuals who carry the sickle-cell trait are resistant to malaria (Allison, 1954). Since then, many more alleles for malaria resistance have shown evidence of selection, including more mutations in HBB, as well as mutations causing other red blood cell disorders (e.g., a-thalassemia, G6PD deficiency, and ovalocytosis) (Kwiatkowski, 2005).
Malaria also drove one of the most striking genetic differences between populations. This difference involves the Duffy antigen gene (FY), which encodes a membrane protein used by the Plasmodium vivax malaria parasite to enter red blood cells, a critical first step in its life cycle. A mutation in FY that disrupts the protein, thus conferring protection against P. vivax malaria, is at a frequency of 100% throughout most of sub-Saharan Africa and virtually absent elsewhere; such an extreme difference in allele frequency is very rare for humans.
Pigmentation
As proto-Europeans and Asians moved northward out of Africa, they experienced less sunlight and colder temperature, new environmental forces that exerted selective pressure on the migrants. Exactly why reduced sunlight should be a potent selective force is still debated, but it has become clear that humans have experienced positive selection at numerous genes to finely tune the amount of skin pigment they produce, depending on the amount of sunlight exposure.
The role of selection in controlling human pigmentation is not a new idea; in fact, it was first advanced by William Wells in 1813, long before Darwin’s formulation of natural selection (Wells, 1818). In recent years, signals of positive selection have been identified in many genes, with some signals solely in Europeans, some solely in Asians, and some shared across both continents (Lao et al., 2007; McEvoy et al., 2006; Williamson et al., 2007). Evidence for purifying selection has also been found to maintain dark skin color in Africa, where sunlight exposure is great.
A good example of selection for lighter pigmentation is the gene SLC24A5, which was one of the first to be characterized. Rebecca Lamason and her colleagues identified a mutation in the zebrafish homologue of this gene that is responsible for pigmentation phenotype. The investigators then demonstrated that a human variant in the gene explains roughly one-third of the variation in pigmentation between Europeans and West Africans, and that the European variant had likely been a target of selection (Lamason et al., 2005). In related work, Angela Hancock and her colleagues examined many genes involved in metabolism, and they showed that alleles of these genes show evidence of positive selection and correlate strongly with climate, suggesting that humans adapted to cooler climates by changing their metabolic rates (Hancock et al., 2008).
Signals for Positive Selection Mark the First Step to Understanding the Story of These Loci
While these instances of selection illustrate the power this line of research has to answer important biological and historical questions, in most cases, little or nothing of the underlying story is understood. For the great majority of selective sweeps, the pressure that drove selection, the trait selected for, and even the specific gene involved are unknown. Understanding these will require case-by-case study, identifying the possible causal mutations within each region based on strength of signal and function (e.g., mutations that alter amino acids or gene regulatory regions), and then finding the biological effects of each.
Such detailed investigations are underway, and they are intriguing. For example, a strong signal of selection in Asia localizes to amino acid substitution in the gene EDAR Sabeti et al., 2007). Mutations in EDAR cause defects in the development of hair, teeth, and exocrine glands in both mice and humans. Meanwhile, there is also evidence for selection at other genes in the same pathway in humans, as well as in stickleback fish (Colosimo et al., 2005), where the pathway regulates scale development. The phenotypic variation for this mutation is only just being elucidated, but it has already been linked to thicker head hair in Asia and has been shown to affect gene activity in the molecular pathway (Bryk et al., 2008; Fujimoto et al., 2008), although what trait was actually under selection is not yet clear. In another case, Scott Williamson and his colleagues found the strongest signal of selection in Europe and Asia at the gene DTNA, a component of the dystrophin complex (Williamson et al., 2007). While the target polymorphism and genetic variation have yet to be elucidated, the dystrophin complex is known to be important in the architecture of muscle tissue, as well as in the pathogenesis of many infectious agents, including arenaviruses and mycobacterium leprae. Another candidate gene for selection, LARGE, is also important for dystrophin function, and it has been shown to be critical for entry of various arenaviruses, including Lassa virus (Sabeti et al., 2007).
Understanding the biology behind these cases, and the many others like them, will not be easy, and it will require contributions from diverse fields, including genetics, molecular biology, developmental biology, and the study of model organisms (Figure 2). Nevertheless, the potential rewards are high. Through the study of natural selection in humans, researchers hope to learn more about how our species has changed over time, about the challenges the species has faced and how it has overcome them, and about past and present causes of disease.