Evolution and The Hardy–Weinberg principle

Evolution

The general theory of evolution is that organisms have changed over time. Usually, natural selection keeps things the way they are. This is stabilising selection (Figure a and b, and Figure 17.10). Agouti rabbits are the best adapted rabbits to survive predation, so the agouti allele remains the most common coat colour allele in rabbit populations. Unless something changes, then natural selection will ensure that this continues to be the case.?

However, if a new environmental factor or a new allele appears, then allele frequencies may also change. This is called directional selection (Figure c).?

A third type of selection, called disruptive selection, can occur when conditions favour both extremes of a population. This type of selection maintains different phenotypes (polymorphism) in a population (Figure d)?

Figure? If a characteristic in a population, such as body mass, shows wide variation, selection pressures often act against the two extremes (graph a). Very small or very large individuals are less likely to survive and reproduce than those whose size lies nearer the centre of the range. This results in a population with a narrower range of body size (graph b). This type of selection, which tends to keep the variation in a characteristic centred around the same mean value, is called stabilising selection.(?Graph c)?shows what would happen if selection acted against smaller individuals but not larger ones. In this case, the range of variation shifts towards larger size. This type of selection, which results in a change in a characteristic in a particular direction, is called directional selection. (Graph d) shows the result of selection that favours both large and small individuals, but acts against those whose size is in the middle of the range. This is disruptive selection.

Figure? The tuatara, Sphenodon punctatus, is a lizard like reptile that lives in New Zealand. Fossils of a virtually identical animal have been found in rocks 200 million years old. Natural selection has acted to keep the features of this organism the same over all this time.

A new environmental factor?

Imagine that we are plunged into a new Ice Age. The climate becomes much colder, so that snow covers the ground for almost all of the year. Assuming that rabbits can cope with these conditions, white rabbits now have a selective advantage during seasons when snow lies on the ground, as they are better camouflaged (like the hare in Figure ). Rabbits with white fur are more likely to survive and reproduce, passing on their alleles for white fur to their offspring. The frequency of the allele for white coat increases at the expense of the allele for agouti. Over many generations, almost all rabbits will come to have white coats rather than agouti.

Figure? The white winter coat of a mountain hare provides excellent camouflage from predators when viewed against snow.

A new allele?

Because they are random events, most mutations that occur produce features that are harmful. That is, they produce organisms that are less well adapted to their environment than ‘normal’ organisms. Other mutations may be neutral, conferring neither an advantage nor a disadvantage on the organisms within which they occur. Occasionally, mutations may produce useful features.

?Imagine that a mutation occurs in the coat colour gene of a rabbit, producing a new allele which gives a better camouflaged coat colour than agouti. Rabbits possessing this new allele will have a selective advantage. They will be more likely to survive and reproduce than agouti rabbits, so the new allele will become more common in the population. Over many generations, almost all rabbits will come to have the new allele.

Such changes in allele frequency in a population are the basis of evolution. Evolution occurs because natural selection gives some alleles a better chance of survival than others. Over many generations, populations may gradually change, becoming better adapted to their environments. Examples of such change are the development of antibiotic resistance in bacteria (described in the next section) and industrial melanism in the peppered moth, Biston betularia.?

In contrast, the role of malaria in the global distribution of sickle cell anaemia is an example of how the interaction of two strong selection pressures can maintain two alleles within certain populations.

Antibiotic resistance?

Antibiotics are chemicals produced by living organisms, which inhibit or kill bacteria but do not normally harm human tissue. Most antibiotics are produced by fungi. The first antibiotic to be discovered was penicillin, which was first used during the Second World War to treat a wide range of diseases caused by bacteria. Penicillin stops cell wall formation in bacteria, so preventing cell reproduction.

?When someone takes penicillin to treat a bacterial infection, bacteria that are sensitive to penicillin die. In most cases, this is the entire population of the disease causing bacteria. However, by chance, there may be among them one or more individual bacteria with an allele giving resistance to penicillin. One example of such an allele occurs in some populations of the bacterium Staphylococcus, where some individual bacteria produce an enzyme, penicillinase, which inactivates penicillin.

As bacteria have only a single loop of DNA, they have only one copy of each gene, so the mutant allele will have an immediate effect on the phenotype of any bacterium possessing it. These individuals have a tremendous selective advantage. The bacteria without this allele will be killed, while those bacteria with resistance to penicillin can survive and reproduce. Bacteria reproduce very rapidly in ideal conditions, and even if there was initially only one resistant bacterium, it might produce ten thousand million descendants within 24 hours. A large population of a penicillin-resistant strain of Staphylococcus would result.?

Such antibiotic-resistant strains of bacteria are continually appearing (Figure ). By using antibiotics, we change the environmental factors which exert selection pressures on bacteria. A constant race is on to find new antibiotics against new resistant strains of bacteria.?

Alleles for antibiotic resistance often occur on plasmids. Plasmids are quite frequently transferred from one bacterium to another, even between different species. Thus it is even possible for resistance to a particular antibiotic to arise in one species of bacterium, and be passed on to another. The more we use antibiotics, the greater the selection pressure we exert on bacteria to evolve resistance to them.

Figure The red gel in each of these Petri dishes has been inoculated with bacteria. The small light blue circles are discs impregnated with antibiotics. Bacteria that are resistant to an antibiotic are able to grow right up to the disc containing it.?

Industrial melanism?

One well-documented case of the way in which changing environmental factors may produce changes in allele frequencies is that of the peppered moth, Biston betularia (Figure a), in the UK and Ireland. This is a night flying moth which spends the day resting underneath the branches of trees. It relies on camouflage to protect it from insect-eating birds that hunt by sight. Until 1849, all specimens of this moth in collections had pale wings with dark markings, giving a speckled appearance.?

In 1849, however, a black (melanic) individual was caught near Manchester (Figure b). During the rest of the 19th century, the numbers of black Biston betularia increased dramatically in some areas, whereas in other parts of the country the speckled form remained the more common.?

Figure a Dark form of peppered moth on dark and pale tree bark.

Figure b The distribution of the pale and dark forms of the peppered moth, Biston betularia, in the UK and Ireland during the early 1960s. The ratio of dark to pale areas in each circle shows the ratio of dark to pale moths in that part of the country.

The difference in the black and speckled forms of the moth is caused by a single gene. The normal speckled colouring is produced by a recessive allele of this gene,?c, while the black colour is produced by a dominant allele,?C. Up until the late 1960s, the frequency of the allele C increased in areas near to industrial cities. In non-industrial areas, the allele c remained the more common allele.?

The selection pressure causing the change of allele frequency in industrial areas was predation by birds. In areas with unpolluted air, tree branches are often covered with grey, brown and green lichen. On such tree branches, speckled moths are superbly camouflaged.?

However, lichens are very sensitive to pollutants such as sulfur dioxide, and do not grow on trees near to or downwind of industries releasing pollutants into the air. Trees in these areas therefore have much darker bark, against which the dark moths are better camouflaged. Experiments have shown that pale moths have a much??higher chance of survival in unpolluted areas than dark moths, while in polluted areas the dark moths have the selective advantage. As air pollution from industry is reduced, the selective advantage swings back in favour of the speckled variety. So we would expect the proportion of speckled moths to increase if we succeeded in reducing the output of certain pollutants. This is, in fact, what has happened since the 1970s.?

?It is important to realise that mutations to the C allele have probably always been happening in B. betularia populations. The mutation was not caused by pollution. Until the 19th century there was such a strong selection pressure against the C allele that it remained exceedingly rare. Mutations of the c allele to the C allele may have occurred quite frequently, but moths with this allele would almost certainly have been eaten by birds before they could reproduce. Changes in environmental factors only affect the likelihood of an allele surviving in a population; they do not affect the likelihood of such an allele arising by mutation.

Sickle cell anaemia?

HbS, of the gene that codes for the production of the β-globin polypeptide can produce sickling of red blood cells. People who are homozygous for this allele have sickle cell anaemia. This is a severe form of anaemia that is often lethal.?

The possession of two copies of this allele obviously puts a person at a great selective disadvantage. People who are homozygous for the sickle cell allele are less likely to survive and reproduce. Until recently, almost everyone with sickle cell anaemia died before reaching reproductive age. Yet the frequency of the sickle cell allele is very high in some parts of the world. In some parts of East Africa, almost 50% of babies born are carriers for this allele, and 14% are homozygous, suffering from sickle cell anaemia. How can this be explained??

The parts of the world where the sickle cell allele is most common are also the parts of the world where malaria is found (Figure a). Malaria is caused by a protoctist parasite, Plasmodium, which can be introduced into a person’s blood when an infected mosquito bites (Figure b). The parasites enter the red blood cells and multiply inside them. Malaria is the major source of illness and death in many parts of the world.

Figure a The distribution of people with at least one copy of the sickle cell allele, and the distribution of malaria, in Africa.

Figure b Red blood cells infected with malarial parasite. Some cells have multiple parasites.

In studies carried out in some African states, it has been?found that people who are heterozygous for the sickle?cell allele (HbS ) are much less likely to suff er from a serious attack of malaria than people who are homozygous?for the HbA allele. Heterozygous people with malaria only have about one-third the number of Plasmodium in their blood as do HbAHbA homozygotes. In one?study, of a sample of 100 children who died from malaria, all except one were HbAHbA homozygotes, although within the population as a whole, 20% of people were heterozygotes. Th ere are, therefore, two strong selection pressures acting on these two alleles.

■ Selection against people who are homozygous for the sickle cell allele, HbSHbS, is very strong, because they become seriously anaemic.?

■ Selection against people who are homozygous HbAHbA, is also very strong, because they are more likely to die from malaria.

In areas where malaria is common, heterozygotes, HbAHbS, have a strong selective advantage; they do not suff er from sickle cell anaemia and are much less likely to suff er badly from malaria.?

So both alleles remain in populations where malaria is an important environmental factor. In places where malaria was never present, selection against people with the genotype HbSHbS has almost completely removed the HbS allele from the population. Th e examples of natural selection given above show the eff ect of a non-random process on the allele frequencies of a population of organisms. These allele frequencies may also change thanks to a random process called genetic drift .

Genetic drift?

Genetic drift is a change in allele frequency that occurs by chance, because only some of the organisms of each generation reproduce. It is most noticeable when a small number of individuals are separated from the rest of a large population. They form only a small sample of the original population and so are unlikely to have the same allele frequencies as the large population. Further genetic drift in the small population will alter the allele frequencies still more and evolution of this population may take a different direction from that of the larger parent population. Th is process, occurring in a recently isolated small population, is called the founder effect.

The Hardy–Weinberg principle?

When a particular phenotypic trait is controlled by two alleles of a single gene, A/a, the population will be made up of three genotypes: AA, Aa and aa. Calculations based on the Hardy–Weinberg principle allow the proportions of each of these genotypes in a large, randomly mating population to be calculated.?

The frequency of a genotype is its proportion of the total population. Th e total is the whole population (that is 1) and the frequencies are given as decimals (e.g. 0.25) of the total.?

?We use the letter p to represent the frequency of the dominant allele, A, in the population and the letter q to represent the frequency of the recessive allele, a. Th en, since there are only two alleles of this gene:?

■ the chance of an offspring inheriting a dominant allele from both parents = p × p = p2

■ the chance of an offspring inheriting a recessive allele from both parents = q × q = q2

■ the chance of an offspring inheriting a dominant allele from the father and a recessive allele from the mother = p × q = pq?

■ the chance of an offspring inheriting a dominant allele from the mother and a recessive allele from the father = p × q = pq

These Hardy–Weinberg calculations do not apply when the population is small or when there is:?

■■ significant selective pressure against one of the genotypes

?

■■ migration of individuals carrying one of the two alleles into, or out of, the population

■■ non-random mating.?

What is the use of these calculations? When the ratios of the different genotypes in a population have been determined, their predicted ratios in the next generation can be compared with the observed values. Any differences can be tested for significance using the χ2 test. If the differences are significant and migration and non-random mating can be discounted, then there is evidence that directional selection is occurring in the population.

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