Selective Breeding and Evolution PDF

# 17.4 Natural Selection - Explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the 'struggle for existence' only the individuals that are best adapted survive to breed and pass on their alleles to the next generation - Stat...

# 17.4 Natural Selection - Explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the 'struggle for existence' only the individuals that are best adapted survive to breed and pass on their alleles to the next generation - State the general theory of evolution that organisms have changed over time - Discuss the molecular evidence that reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data Evidence from fossils shows that organisms have changed over time. Natural selection is the process by which organisms that are better adapted to their environment tend to survive and reproduce while those less well adapted tend not to. Those that are adapted and so survive to reproductive age will be the ones that pass on their favourable alleles to the next generation. ## Survival of the fittest Charles Darwin and Alfred Wallace in 1865 independently developed the theory of evolution by natural selection based on the following principles: - All organisms produce more offspring than can be supported by the supply of food, light, space, etc. - Despite the over-production of offspring, most populations remain relatively constant in size. - There must hence be competition between members of a species to be the ones that survive = intraspecific competition, with individuals competing for resources such as food, breeding sites, space, light and water. - Within any population of a species there will be a wide variety of genetically different organisms (Topic 17.1). - Some of these individuals will possess alleles that make them better adapted to survive (fitter) and so more likely to breed. - Only those individuals that do survive and breed will pass on their alleles to the next generation. - The advantageous alleles that gave these individuals the edge in the struggle to survive and breed are therefore likely to be passed on to the next generation. - Over many generations, the individuals with beneficial alleles are more likely to survive to breed and therefore increase in number at the expense of the individuals with less favourable alleles. - The frequency of favourable alleles in the population will increase over time. ## Specific examples of how natural selection produces changes within a species include: - antibiotic resistance in bacteria - industrial melanism. ## Antibiotic resistance in bacteria It was not long after the discovery of antibiotics that it was realised after treating people with bacterial infections that some antibiotics no longer killed bacteria as effectively as before. It was found that these populations of bacteria had acquired resistance, as the result not of a cumulative tolerance to the antibiotic, but rather a chance mutation within the bacteria. The bacteria with the mutation could produce an enzyme, penicillinase, which broke down the antibiotic penicillin before it was able to kill them. When penicillin is used to treat the same disease, only the susceptible (non-resistant) forms of the bacteria are killed. There is therefore a selection pressure favouring the resistant form when exposed to penicillin. These penicillin-resistant bacteria therefore gradually form the greatest proportion of the population. The frequency of the allele for penicillin resistance increases in the population. This type of selection is called directional selection and is described in Topic 17.6. It is also important that the allele for antibiotic resistance is carried on plasmids and these circular DNA molecules can be transferred from cell to cell by natural as well as artificial means. Resistance can therefore find its way into other bacterial species. Overuse of antibiotics, e.g. for minor infections that present no danger, increases the likelihood of selection of resistant strains over ones that are more susceptible to the antibiotic. More details of how antibiotics work and antibiotic resistance are given in Topic 10.9. ## Industrial melanism Some species of organisms have two or more distinct forms or morphs. These different forms are genetically distinct but exist within the same interbreeding population. This situation is called polymorphism ('poly' = many; 'morph' = form). One example is the peppered moth (Biston betularia in England). It existed only in its natural light form until the middle of the nineteenth century. Around this time a melanic (black) form occurred as the result of a mutation. These mutants had undoubtedly occurred before (one existed in a collection made before 1819) but they were highly conspicuous (very easily seen) against the light background of lichen-covered trees and rocks on which they normally rest. Insect-eating birds such as robins and hedge sparrows are predators of the peppered moth. The melanic form of moths were more easily seen and eaten by these birds than the better camouflaged, normal light forms. When in 1848 a melanic form of the peppered moth was captured in Manchester, a large city, most buildings, walls and trees were blackened by the soot of 50 years of industrial development. The sulfur dioxide in smoke emissions killed the lichens that previously covered trees and walls. Against this black background the melanic form was less, not more, conspicuous than the light natural form. As a result, the light form was taken by birds more frequently than the melanic form and, by 1895, 98% of Manchester's population of the moth was of the melanic type (Figure 1). This is an example of how a change in the environment can rapidly alter the allele frequency in populations. Here the variation that exists is the light and dark forms and the selection pressure exerted is predation by birds. The moths are still members of the same species and can interbreed. To become two distinct species, the two populations would need to become reproductively isolated from one another (Topic 17.9). ## Molecular evidence for similarities between organisms We have seen how natural selection leads to changes in a species. We shall see in Topic 17.9 how this can lead to new species. As these changes occurred over millions of years, how can we determine which organisms are closely related? One method is to compare the mitochondrial DNA of different species. The DNA found in mitochondria is made up of relatively few genes. The nucleotide bases of these genes can be sequenced to reveal patterns that are recognisable in different species. Mitochondrial DNA is only inherited along the female line and so remains relatively unchanged from generation to generation, as no meiosis occurs to introduce variety. The only change to mitochondrial DNA is by mutations, which are very rare. However, assuming very occasional mutations, the patterns of nucleotide bases will change over time, although very slowly. If we compare the mitochondrial DNA of two species, then the more similar their nucleotide base patterns are, the more closely they are related. Another molecular device to reveal the evolutionary relationships between species is to look at the sequences of amino acids in their proteins. The sequence of amino acids in proteins is determined by DNA. The degree of similarity in the amino acid sequence of the same protein in two species will therefore reflect how closely related the two species are. Once the amino acid sequence for a chosen protein has been determined for two species, the two sequences are compared. This can be done by counting either the number of similarities or the number of differences in each sequence. Figure 2 shows a short sequence of seven amino acids of the same protein in six different species, highlighting the number of differences and the number of similarities. ## SUMMARY TEST 17.4 The theory of natural selection by survival of the fittest was developed independently by Charles Darwin and (1). It is based on the principles that all organisms produce (2) offspring than can be supported by the food, light and space available for them. However the size of most populations is (3) as a result of competition between members of each species for the limited resources available. This type of competition is called (4). Within any population there will be many types of (5) different individuals. Those individuals with (6) that better suit them to the prevailing conditions are more likely to survive and hence more likely to produce (7). As only those that survive can pass their characteristics to the next generation, there is an increased chance that the next generation will have a greater proportion of these advantageous characteristics. An example of natural selection in practice is (8) resistance in bacteria. Another example of natural selection has occurred in the peppered moth. This moth has two forms, a natural light coloured one and a dark coloured one called the (9) form. The general term for the situation where a species has two or more distinct forms is (10). # 17.5 The roles of over-production and variation in natural selection - Explain why genetic variation is important in selection - Explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the 'struggle for existence' only the individuals that are best adapted survive to breed and pass on their alleles to the next generation - Explain why organisms become extinct, with reference to climate change, competition, habitat loss and killing by humans Remember: If an environmental change is great enough, there may be no phenotype suited to the new conditions, in which case the population will die out. The process of evolution by means of natural selection depends upon a number of factors. Two of the most important are that: - organisms produce more offspring than can be supported by the available supply of food, light, space, etc. - there is genetic variety within the populations of all species. ## Over-production of offspring Darwin appreciated that all species have the potential to increase their numbers exponentially. He realised that, in nature, populations rarely, if ever, increased in size at such a rate (Figure 1). He rightly concluded that the death rate of even the most slow-breeding species must be extremely high. For most species, the rate of reproduction and the production of offspring is high, but only a very small proportion survive. The reason why reproductive rates are high is because a species cannot control the climate, rate of predation, availability of food, etc. Therefore to ensure a sufficiently large population survives to breed and produce the next generation, each species must produce vast numbers of offspring. This is to compensate for considerable death rates from predation, lack of food (including light in plants) and water, extremes of temperature, natural disasters such as earthquake and fire, disease, etc. How organisms over-produce depends on the species in question and its means of reproduction. Some examples include: - A bacterium can divide by binary fission about every 20 minutes when conditions are favourable. A single bacterium could theoretically give rise to 4 x 1021 cells in just 24 hours. - Some fungi can produce over 500000 spores each minute at the peak of production. Each spore has the potential to develop a new fungal mycelium. - Higher plants can spread rapidly by vegetative propagation, e.g. the production of bulbs, rhizomes, runners, etc. - Flowering plants produce vast amounts of pollen from their anthers. These can fertilise the many ovules in plants of the same species, leading to the production, in some cases, of millions of seeds from a single plant. - Animals produce vast numbers of sperm, and sometimes large numbers of eggs also. A female oyster, for example, can produce 100 million eggs in a year and the male oyster produces many more times this number of sperm. - Many organisms, e.g. birds such as blue tits and mammals like the rabbit, produce several clutches/litters every year, each of which comprises several offspring. The importance of over-production to natural selection lies in the fact that, where there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available. The greater the numbers, the greater this competition and the more individuals will die in the struggle to survive. These deaths are, however, not random. Those individuals best suited to prevailing conditions (have adaptations that make them better able to hide from or escape predators, better able to obtain light or catch prey or better able to resist disease) will be more likely to survive than those less well adapted. These individuals will be more likely to breed and so pass on these favourable characteristics, via their alleles, to the next generation, which will therefore be slightly different from the previous one - i.e. the species will have changed over time to be better adapted to the prevailing conditions. This selection process, however, depends on individuals of a species being genetically different from one another. ## Variation and natural selection If an organism can survive in the conditions in which it lives, you may wonder why it doesn't produce offspring that are identical to itself. These will, after all, be equally capable of survival in these conditions, whereas variation may produce individuals that are less suited. However, conditions change over time and having a wide range of different individuals in the population means that some will have the combination of alleles needed to survive in almost any set of new circumstances. Populations showing little individual variation are vulnerable to new diseases and climate changes. It is also important that a species adapts to changes resulting from changes to the allele frequencies within other species. If, for example, rabbits in a particular region have alleles as a result of mutation that allow them to run faster, then foxes and other predators will be less able to catch them. The foxes will therefore have less food, unless they in turn develop greater speed as a result of new adaptations occurring from mutation. Mutations occur in populations that are neutral in their effect. These may become beneficial in a changing environment. A species cannot predict future changes; it does not know whether the climate will become wetter/drier, warmer/colder or how its prey or predator populations will change, or what new disease agent may occur. However, the larger a population is, and the more genetically varied the organisms within it, the greater the chance that one or more individuals will have the genetic characteristics that give them an advantage in the struggle for survival. These individuals will therefore be more likely to breed and pass their more advantageous alleles on to future generations. Variation therefore provides the potential for a species to evolve and so adapt to new circumstances. The influence of variation on natural selection is best summarised by Darwin himself who, nearly a hundred and fifty years ago, wrote: 'How can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits or instinct, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would have a better chance'. ## Why organisms become extinct Extinction is a normal and natural process; indeed it is essential to the process of evolution. More than 99% of species known from fossil records are extinct. However, the current rate of extinction is alarmingly high, with some scientists estimating that up to 20% of current species could become extinct in the next 30 years. There is a number of possible reasons for this increase. - Climate change: Global warming is changing vegetation patterns throughout the world, leading to a redistribution of species. In some cases, the members of a species are unable to migrate, because of geographical barriers, and the pace of the warming is too rapid for them to adapt. The death of the last golden toad in Central America in 1999 is an example of a species probably made extinct by climate change. Increased tropical sea temperatures are causing the extinction of certain algal species on coral reefs. Rises in sea level as a result of global warming could result in the loss of nesting sites for turtles and their possible extinction. - Habitat loss: In exploiting natural habitats, humans often destroy them and so cause the extinction of species in those habitats. Examples include timber extraction, which destroys forests and endangers species such as the orang-utan, as well as industrial and agricultural developments that threaten many plant species of the rainforests. In addition, modern farming methods drain ponds as well as remove trees and hedgerows. These practices endanger the species that live and breed there. Pollution has led to the loss of some pond and river habitats and overgrazing has led to the loss of some terrestrial ones. More information on habitat loss is provided in Topic 18.10. - Competition: Interspecific competition has always led to extinctions. Species compete with each other for food, territory and nesting sites, but the competition other species face from humans has accelerated the process. As more and more land is taken up for building, industry and farming, there is more intense competition among wildlife for food, shelter and breeding sites in the remaining natural habitats. Humans often introduce new species that outcompete natural ones. Many marsupials in Australia have been exterminated by competition from introduced rabbits and predation by introduced foxes and cats. - Hunting and fishing: Humans hunt for sport, for animal skins, for trophies, for the pet trade and for research purposes. These are in addition to the numerous species hunted purely as food or for use in traditional medicine. These activities have led to the extinction of species such as the passenger pigeon. Other species, such as the black rhinoceros, have been brought to the brink (edge) of extinction. More details are given in Topic 18.10. # 17.6 How environmental factors act as forces of natural selection - Explain, with examples, how environmental factors can act as stabilising, disruptive and directional forces of natural selection - Explain how selection may affect allele frequencies in populations Environmental factors help to contribute to variation within a population. These environmental factors may be an agent for constancy or an agent for change according to the type of selection pressure they exert. ## Selection pressure Every organism faces a process of selection, based upon the organism's suitability for survival under the conditions that exist at the time. The environmental factors that act on and limit a population of a species are called selection pressures or environmental resistances. These selection pressures include: - competition for food - competition for a space in which to live, breed and rear young - need for light, water, oxygen, etc. - climate changes, e.g. temperature, rainfall, wind/water currents - predation - disease. The extent and direction of selection pressures varies from time to time and place to place. These selection pressures determine the frequency of an allele within the gene pool. A gene pool is the total of all the alleles of all the genes of all individuals within a particular population at a given time. There are three main types of selection: - Selection that preserves the characteristics of a population by favouring average individuals (those at or near the mean of the population) = stabilising selection. - Selection that changes the characteristics of a population by favouring individuals that vary in one direction from the mean of the population = directional selection. - Selection that changes the characteristics of a population by favouring individuals at the extremes rather than those around the mean of the population = disruptive selection. ## Directional selection Within a population there will be a range of individuals in respect of any one characteristic. The continuous variation amongst these individuals forms a normal distribution curve which has a mean that represents the optimum value for the characteristic under the existing conditions. If the environmental conditions change, so will the optimum value needed for survival. Some individuals, either to the left or the right of the mean, will possess a phenotype ## SUMMARY TEST 17.6 The theory of natural selection by survival of the fittest was developed independently by Charles Darwin and (1). It is based on the principles that all organisms produce (2) offspring than can be supported by the food, light and space available for them. However the size of most populations is (3) as a result of competition between members of each species for the limited resources available. This type of competition is called (4). Within any population there will be many types of (5) different individuals. Those individuals with (6) that better suit them to the prevailing conditions are more likely to survive and hence more likely to produce (7). As only those that survive can pass their characteristics to the next generation, there is an increased chance that the next generation will have a greater proportion of these advantageous characteristics. An example of natural selection in practice is (8) resistance in bacteria. Another example of natural selection has occurred in the peppered moth. This moth has two forms, a natural light coloured one and a dark coloured one called the (9) form. The general term for the situation where a species has two or more distinct forms is (10). # 17.7 Allelic frequencies - Use the Hardy-Weinberg principle to calculate allele, genotype and phenotype frequencies in populations and explain situations when this principle does not apply Remember: Recessive and dominant has nothing to do with whether an allele is harmful or beneficial. People with type O blood group have two recessive alleles for the ABO gene. As it is the most common blood group it can hardly be harmful. Also, Huntington's disease is a fatal condition due to a dominant allele. In theory, any sexually mature individual in a population is capable of breeding with any other. This means that the alleles of any individual organism may combine with the alleles of any other individual in the population. Before looking at allelic frequencies, we need to understand what is meant by a 'population'. A population is a group of organisms of the same species that occupies a particular space at a particular time and may potentially interbreed. Any species may exist as one or more populations. More information on populations is given in Topic 18.4. All the alleles of all the genes of all the individuals in a population at any one time is known as the gene pool. Sometimes the term is used to refer to all the alleles of one particular gene in a population, rather than all the genes. The number of times an allele occurs within the gene pool is referred to as the allelic frequency. Let us look at this more closely by considering just one gene that has two alleles, one of which is dominant and the other recessive. An example is the gene responsible for cystic fibrosis, a disease in humans in which the mucus produced by affected individuals is thicker than usual. The gene has a dominant allele (F) that leads to normal mucus production and a recessive allele (f) that leads to the production of thicker mucus and hence cystic fibrosis. Any individual human has two of these alleles in every one of their cells, one on each of the pair of homologous chromosomes on which the gene is found. As these alleles are the same in every cell, we only count one pair of alleles, per gene, per individual when considering a gene pool. If there are 10000 people in a population, there will be twice as many (20000) alleles in the gene pool of this gene. The pair of alleles of the cystic fibrosis gene has three different possible combinations, namely homozygous dominant (FF), homozygous recessive (ff) and heterozygous (Ff). When we look at genotype frequencies, however, it is important to appreciate that the heterozygous combination can exist in two different arrangements, namely Ff and fF (it is just convention that we put the dominant allele first in all cases). This is because the male parent may contribute either F or f as gametes and the female parent also F or f. In any population the total number of alleles is taken to be 1.0. In our population of 10000 people, if everyone had the genotype FF, then the frequency of the dominant allele (F) would be 1.0 and the frequency of the recessive allele (f) would be 0.0. If everyone was heterozygous Ff, the frequency of the dominant allele (F) would be 0.5 and the frequency of the recessive allele (f) would be 0.5. Of course, in practice, the population is not made up of one genotype, but a mixture of all three, the proportions of which vary from population to population. How then can we work out the allele frequency of these mixed populations? ## The Hardy-Weinberg principle The Hardy-Weinberg principle provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population. The principle predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next provided that five conditions are met: - no mutations arise - the population is isolated, i.e. there is no flow of alleles into or out of the population - there is no selection, i.e. all alleles are equally likely to passed to the next generation - the population is large - mating within the population is random. Although these conditions are probably never totally met in a natural population, the Hardy-Weinberg principle is still useful when studying allele and genotype frequencies. To help us understand the principle let us consider a gene that has two alleles, a dominant allele A and a recessive allele a. Let the frequency of allele A = p. Let the frequency of allele a = q. The first equation we can write is: p + q = 1.0 because there are only two alleles and so the frequency of one plus the other must be 1.0 (100%). A genotype results from the fertilisation of the male and female gamete. The probability of 'taking' an A gamete from the pool = p The probability of 'taking' an a gamete from the pool = q (male A female A) = p × p = p² (male a female a) = q q = q² (male A female a) = p x q = pq (male a female A) = q x p =pq Therefore we can state that: AA + Aa + aA + aa = 1.0 or, expressing this as genotype frequencies: p² + 2pq + q2 = 1.0 (Hardy-Weinberg equation) We can now use these equations to determine the frequency of any allele in a population and to calculate the frequency of the heterozygous genotype in a population. For example, suppose that a particular characteristic is the result of a recessive allele a, and we know that one person in 25000 displays the characteristic. - The characteristic, being recessive, will only be observed in individuals who have two recessive alleles aa. - The frequency of aa must be 1/25000 or 0.00004. - The frequency of aa is q². - If q² = 0.00004, then q = 0.00004 or 0.0063 approx. - We know that the frequency of both alleles A and a is p + q and is equal to 1.0. If p + q = 1.0, and q = 0.0063 then, p = 1.00.0063 = 0.9937, i.e. the frequency of allele A = 0.9937. - We can now calculate the frequency of the heterozygous individuals in the population. - From the Hardy-Weinberg equation we know that the frequency of the heterozygotes is 2pq. - In this case 2pq = (2 × 0.9937 × 0.0063) = 0.0125. - In other words, 125 individuals in 10000 carry the recessive allele for the characteristic. This is the equivalent of 313 in our population of 25 000. - These individuals act as a reservoir of recessive alleles in the population, although they do not express the allele in their phenotype. Remember: Unless an allele leads to a phenotype with an advantage or a disadvantage compared with other phenotypes, its allele frequency in a population will stay the same from one generation to the next. ## SUMMARY TEST 17.7 All the alleles of all the genes of all the individuals in a population at any one time is known as the (1), and the number of times an allele occurs within it is referred to as the (2). A mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population is known as the (3) principle. It requires that five conditions are met: the population must be both (4) and (5), no (6) or (7) should occur, and mating within the population should be (8). If the frequency p of a dominant allele is 0.942, then the frequency of the heterozygous genotype in the population will be (9). You will need to a calculator to help with this last question. # 17.8 The processes affecting allelic frequencies - Explain how selection and the founder effect may affect allele frequencies in populations Environmental changes affect the probability of an allele surviving in a population and therefore the number of times it occurs within the gene pool. It must be emphasised that environmental factors do not affect the probability of a particular mutant allele occurring, they simply affect the frequency of a mutant allele that is already present in the gene pool. It should also be remembered that some environmental factors may influence the overall mutation rate (Topic 16.12), but that this is a general and random process rather than one that affects a specific allele in a specific way. To illustrate the effect of an environmental factor on the frequency of an allele, we shall look at the condition in humans known as sickle cell anaemia. ## Sickle cell anaemia We saw in Topic 16.12 that sickle cell anaemia is the result of a gene mutation in which a single base substitution in DNA causes the wrong amino acid to be incorporated into two polypeptides in haemoglobin molecules. The result is red blood cells with a sickle (crescent) shape. Sickle cell anaemia is the result of a single gene for the B-globin polypeptide chain of haemoglobin which has two codominant (Topic 16.7) alleles, HbA (normal) and Hbs (sickled). The malarial parasite, Plasmodium, is unable to exist in red blood cells with Hbs. Table 1 shows the three possible genotype combinations of these two alleles and the corresponding phenotypes. The selection pressures on each genotype differ as follows: - Homozygous for haemoglobin-S (HbHbs) - individuals with sickle cell anaemia and are considerably disadvantaged without medical attention. They rarely live long enough to pass their alleles on to the next generation. Their anaemia is so severe it outweighs being resistant to one form of malaria and so individuals are always selected against. - Homozygous for haemoglobin-A (HbAHb^) - individuals lead normal healthy lives, but are susceptible to malaria in areas of the world where the disease is endemic and they are therefore selected against in these regions only. - Heterozygous for haemoglobin (HbAHbs) - individuals are said to have sickle cell trait, but are not badly affected except when the oxygen concentration of their blood is low, e.g. in exercising muscles, when the abnormal haemoglobin makes the red blood cells sickle shaped and less able to carry oxygen. In these cases the person may therefore become tired more easily, but in general the condition is symptomless. They do, however, have protection against the serious forms of malaria and this advantage outweighs the disadvantage of tiredness in areas of the world where malaria occurs. This situation is called heterozygote superiority or heterozygote advantage. To summarise, in parts of the world where malaria is prevalent, the heterozygous state (HbAHbs) will be selected for at the expense of both homozygous states. This is a form of stabilising selection (Topic 17.6). In areas where malaria does not occur, the homozygous state for haemoglobin-A (HbAHb^) (and the heterozygous state, to a large extent) is selected for a form of directional selection (Topic 17.6). The homozygous state for haemoglobin-S (HbHbs) is so debilitating that it is always selected against. (Figure 2 shows the proportion of these three genotypes in malarial and non-malarial regions.) Individuals homozygous for HbS remain in the population because an HbS allele is always present in each heterozygous individual of the population. When two heterozygous individuals produce offspring, there is a one in four chance that one will be homozygous HbSHbs. Because a greater proportion of the population in a malarial region are heterozygous than in a non-malarial region, it follows that the frequency of the sickle cell allele (Hbs) is greater in areas where malaria is present than in ones where it is absent (Figure 3). ## The founder effect The founder effect occurs when just a few individuals from a population colonise a new region. These few individuals will carry with them only a small fraction of the alleles of the population as a whole. This means that the gene pool of the founder population may not be representative of the gene pool of the larger population. The new population that develops from the few colonisers will therefore show less genetic diversity than the population from which they came. The founder effect often takes place when new volcanic islands rise up out of the sea. The few individuals that colonise these barren islands give rise to populations that are genetically distinct from the populations they left behind. The new population may, in time, develop into a separate species. As these species have lower genetic diversity they are less able to adapt to changing conditions. ## SUMMARY TEST 17.8 The total number of all individual alleles of all the genes in a population is called the (1) and the number of times an allele occurs within a population is called the (2). The occurrence of an allele in a population is affected by environmental factors through the process of (3). An example of such an environmental factor occurs in the condition known as sickle cell anaemia that results from a single base in DNA being (4) by another base. As a result, affected individuals have (5) cells that are sickle (crescent) shaped. The gene for the beta-globin chain of haemoglobin has two distinct alleles, HbA and Hbs, that are (6). Individuals with sickle cell anaemia have the genotype (7) and are so disadvantaged that they rarely survive long enough to breed. Individuals with normal haemoglobin lead normal lives but are susceptible to malaria. Heterozygous individuals are affected when the (8) concentration of their blood is low. These individuals are, however, more protected against malaria, which puts them at an advantage over both homozygous states in areas where this disease is common. This is an example of (9) selection. In areas where malaria is absent, individuals with normal haemoglobin are selected in favour of those with the HbS HbS genotype - an example of (10) selection. # 17.9 Isolation mechanisms in the evolution of new species - Explain how genetic drift may affect allele frequencies in populations - Explain how speciation may occur as a result of geographical separation (allopatric speciation), and ecological and behavioural separation (sympatric speciation) - Explain the role of pre-zygotic and post-zygotic isolating mechanisms in the evolution of new species Speciation is the evolution of new species from existing ones. A species is a group of individuals that have a common ancestry and so share the same set of genes and are capable of breeding with one another to produce fertile offspring. In other words, members of a species are reproductively isolated. It is through the process of speciation that evolutionary change has taken place over millions of years. This has resulted in great diversity of forms amongst organisms, past and present. ## How new species are formed The formation of new species can occur in two different ways: - Cross fertilisation between individuals of two different species that leads to the formation of a hybrid. This is thought to have occurred in the production of modern wheat plants from a chance hybridisation of emmer wheat and goat grass followed by chromosome doubling (polyploidy) (Topic 17.11). - Reproductive isolation followed by genetic change due to natural selection. Within a population of any species there are groups of individuals that breed with one another. These breeding subpopulations are called demes. Although individuals tend to breed only with others in the same deme, they are capable of breeding with individuals in other demes. In other words, the population has a single gene pool. Suppose, however, that the demes become isolated in some way and each undergoes different mutations and becomes genetically different. Each deme will then adapt to the different environmental influences it is subjected to. This is known as adaptive radiation and results in changes to the allele frequencies known as genetic drift in each population and in the various phenotypes present. As a result of these genetic differences it may be that, even if the species were no longer physically isolated from one another, they would be unable to interbreed successfully. Each group would now be a different species, each with its own gene pool. This type of speciation has two main forms, allopatric speciation and sympatric speciation. ## Allopatric speciation Allopatric means 'different countries' and describes the form of speciation where two populations become geographically isolated. Geographical isolation may be the result of any physical barrier between two populations which prevents them interbreeding. These barriers include oceans, rivers, mountain ranges and deserts. What proves a barrier to one species may be no problem to another. While an ocean may isolate populations of goats, it can be crossed by many birds and for marine fish it is their mode of getting from place to place. A tiny stream may be a barrier to snails, whereas the whole of the Pacific Ocean fails to separate populations of certain birds. If environmental conditions either side of the barrier vary, then natural selection will influence the two populations differently and each will adapt in order to survive in their local conditions. These changes take many hundreds or even thousands of generations, but in the end lead to reproductive isolation and the formation of separate species. Figure 1 shows how speciation might occur when two populations of a forest-living species become geographically isolated by a region of arid grassland. ## Sympatric speciation Sympatric means 'same country' and describes the form of speciation that results from populations living together becoming reproductively isolated. There are various forms of isolation that lead to sympatric speciation and two examples of these are ecological isolation and behavioural isolation. In ecological isolation, members of the population living in one area form subpopulations that may live in different microhabitats, experiencing different microclimates and differences in available food resources. Individuals rarely move out of their small area and over time changes occur so that interbreeding becomes no longer possible. In behavioural isolation, changes to behaviour, usually in mating rituals and in courtship behaviour, can bring about reproductive isolation. Mutations occurring in subpopulations may result in differences in morphology, for example changes in colours that are associated with attracting mates, or differences in courtship behaviour. Only members within the subpopulation respond to these changes during courtship and mating, so that eventually groups are so different that interbreeding no longer occurs. ## The role of pre-zygotic and post-zygotic isolation mechanisms in the evolution of new species The formation of new species requires time for the gene pools of the reproductively isolated populations to become so different that interbreeding is no longer possible (or if it is possible, no fertile offspring are produced). Two types of mechanism may operate to ensure that groups remain reproductively isolated: prezygotic