Chapter 17: Selection & Evolution - Variation

# Selection and evolution ## 17.1 Variation - Describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive genes control a characteristic) and discontinuous variation (one or few genes control a characteristic) Every one of th...

# Selection and evolution ## 17.1 Variation - Describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive genes control a characteristic) and discontinuous variation (one or few genes control a characteristic) Every one of the billions of organisms on planet Earth is unique. Even monozygotic twins, although genetically identical, vary as a result of their different environmental experiences. The phenotypic variation shown within a species for a particular characteristic can be quantified. There are two main types of variation: continuous variation and discontinuous variation. ### Continuous variation Some characteristics of organisms appear to have a graded effect and the phenotypes do not appear to fall into distinct classes. In humans, two examples are height and mass. Characteristics that display this type of variation are not controlled by a single gene, but by many genes (polygenes). These genes have an additive effect, each contributing in some way, and to a different extent, to the overall phenotype produced. ### Discontinuous variation Some characteristics in organisms fit into a few distinct forms; there are no intermediate types, or at least there is very little overlap between groups. This is called discontinuous variation. In the ABO blood grouping system (Topic 16.7), for example, there are four distinct groups: A, B, AB and O (Figure 2). A characteristic displaying discontinuous variation is usually controlled by a few genes or often just a single gene - in the case of blood groups the ABO gene. Discontinuous variation can be represented on bar charts or pie graphs. Environmental factors have little influence on discontinuous variation. ## 17.2 Environment and phenotype - Explain, with examples, how the environment may affect the phenotype of plants and animals You may recall that in Topic 16.4 we saw how the final appearance of an organism - its phenotype - is the result of the genotype and the effect of the environment upon it. If organisms of identical genotype are exposed to different environmental influences, they show considerable variety. Because environmental influences, e.g. temperature and light intensity, are themselves very various and because they form gradations, they are largely responsible for continuous variation within a population. ### How the environment may affect the phenotype The alleles that make up the genotype of an organism provide a blueprint that determines the limits within which the organism will develop. The degree to which an allele is expressed often depends on the environment. Examples include: - The recessive c^s allele in Siamese cats and the equivalent c^h allele in Himalayan rabbits code for a heat sensitive form of the enzyme tyrosinase (Topic 16.7). This enzyme is involved in the production of the dark pigment, melanin. The c^s/c^h form of the enzyme does not function at temperatures above 33°C. Over much of the body surface of Siamese cats and Himalayan rabbits the temperature is above 33°C and so the enzyme is inactive and no melanin is produced during development. The fur in these regions is therefore light in colour. At the extremities such as the tips of the tail, ears, feet and nose, the temperature is usually below 33°C and so the heat sensitive form of tyrosinase is active and melanin is produced. These regions are therefore much darker in colour (Figure 1). - A small Californian plant, Potentilla glandulosa, has a number of genetic forms, each adapted to growing at different altitudes. Experiments were carried out as follows: - plants of Potentilla were collected from three altitudes - high, medium and low - one plant from each location was split into three cuttings, each of which therefore had an identical genotype - one of the cuttings from each location was grown at each altitude (high, medium and low) - three separate sets of genetically identical plants were therefore grown in three different environments. The results are illustrated in Figure 3 and show that plants with identical genotypes differ in phenotype (height, number of leaves, overall size and shape) and even survival rate, according to the environment in which they live. - Arctic foxes (Figure 2) have the alleles to make fur pigments and so produce dark coats. These pigments are, however, produced only in warm temperatures. They are therefore not produced as the colder temperatures of winter approach and the surface hairs are slowly replaced by white ones. By the time the winter snows cover the ground, the Arctic fox is completely white and better camouflaged and therefore more able to capture its prey. - The height of humans is determined by the range of alleles for height that each of us inherits from our parents. However, even if our alleles allow us to grow tall, our diet will influence whether we do so. For example, a lack of calcium, phosphate or poor overall nutrition especially at critical growth periods (early years and adolescence) may prevent maximum bone and body growth and so we fail to realise our full potential height. ## 17.3 The t-test - Use the t-test to compare the variation of two different populations The t-test is used to find out if the difference between the mean of two sets of continuous data is significant or if it is purely due to chance. The t-test is used if: - the data that have been collected are continuous - the data are from a population that is normally distributed - the standard deviations are approximately the same - each of the two samples has fewer than 30 values. 'The equation for the t-test is in two parts that are expressed as: x₁-x₂ / √(s₁²/n₁ + s₂²/n₂) and v = n₁ + n₂-2 Where: x = mean value s = standard deviation n = sample size (number of observations) v = degrees of freedom The t-test makes use of the mean and standard deviation, so let us begin by looking at these. ### Mean and standard deviation A normal distribution curve always has the same basic shape (Figure 1). It differs in two measurements: its maximum height and its width. - The mean is the measurement at the maximum height of the curve. The mean of a sample of data provides an average value and useful information when comparing one sample with another. It does not, however, provide any information about the range of values within that sample. For example, the mean number of children in a sample of eight families may be 2. However, this could be made up of eight families each with two children or six families with no children and two families with eight children each. - The standard deviation (s) is a measure of the width of the curve. It gives an indication of the range of values either side of the mean. A standard deviation is the distance from the mean to the point where the curve changes from being convex to concave (the point of inflexion). Of all the measurements, 68% lie within this range. Increasing this width to almost two (actually 1.96) standard deviations takes in 95% of all measurements. These measurements are shown in Figure 1. ### Calculating the standard deviation At first sight, the formula for standard deviation can look complex: standard deviation = √Σ(x – x)² / (n-1) Where: Σ = the sum of x = measured value (from the sample) x = mean value n = total number of values in the sample. However, it is straightforward to calculate and less daunting if you take it step by step. The following very simple example, using the six measured values (x) 4, 1, 2, 3, 5 and 0, illustrates each step in the process. 1. Calculate the mean value (X), i.e. (4+1+2+3+5+0) / 6 = 2.5. 2. Subtract the mean value (2.5) from each of the measured values (x - X). This gives: +1.5, -1.5, -0.5, +0.5, +2.5, -2.5. 3. As some of these numbers are negative, we need to make them positive. To do this, square all the numbers (x-x)². Remember to square all the numbers and not just the negative ones. This gives: 2.25, 2.25, 0.25, 0.25 6.25, 6.25. 4. Add all these squared numbers together: Σ(x – x)² = 17.5 5. Divide this number the original number of measurements less one, i.e. 5: Σ(x – x)² / (n-1) = 17.5 / 5 = 3.5 6. As all the numbers have been squared, the final step is to take the square root in order to get back to the same units as the mean: √Σ(x-x)² / (n-1) = √3.5 = 1.87 ### Significant figures You will need to use a calculator to find the value of standard deviations, as it will considerably speed up your calculation. In doing so you will often find the calculator gives a long figure running to many decimal places. In our calculation, for example, the calculation √3.5 produces the answer 1.870828693. Clearly the significance of the latter digits is less than the earlier ones. It is normal to reduce these figures to a certain number of significant figures. In our case we have rounded down the answer to three significant figures, namely 1.87. We did this because we calculated our square values to be 2.25, 0.25, 6.25, etc. As these had three significant figures, we used the same number in our final calculation. ## 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. ## 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 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 10^21 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. ## 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 with the new optimum for the characteristic and so there will be a selection pressure moving the mean to either the left or the right of its original position. Directional selection therefore results in one extreme of a range of variation being selected against in favour of the other extreme. Figure 1 illustrates a theoretical example of directional selection. A specific example is antibiotic resistance in bacteria (Topic 17.4). ### Stabilising selection Stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the opportunity for evolutionary change. It arises where the environmental conditions are constant. One example is fur length in a particular mammalian species. In years when the environmental temperatures are hotter than usual, the individuals with shorter fur length will be at an advantage because they can lose body heat more rapidly. In colder years the opposite is true and those with longer fur length will survive better as they are better insulated. Therefore, if the environment fluctuates from year to year, both extremes will survive because each will have some years when it can thrive at the expense of the other. If, however, the environmental temperature is constantly 10°C, individuals at the extremes will never be at an advantage and will therefore be selected against in favour of those with average fur length. The mean will remain the same, but there will be fewer individuals at either extreme (Figure 2). An actual example of stabilising selection is the body mass of human children at birth. Babies born with a body mass greater or less than the optimum of 3.2 kg have a higher mortality rate. ### Disruptive selection Disruptive selection is the opposite of stabilising selection. It favours the two extreme phenotypes at the expense of the intermediate phenotype. Although the least common form of selection, it is the most important in bringing about evolutionary change. Disruptive selection occurs when an environmental factor, such as temperature, takes two or more distinct forms. In our example this might occur if the temperature alternated between 5°C in winter (favouring long fur length) and 15°C in summer (favouring short fur length). This could lead at some time in the future to two separate species of the mammal - one with long fur and active in winter, the other with short fur and active in summer (Figure 3). An example is coho salmon, where large males and small males have a selective advantage over intermediate-sized males in passing on their alleles to the next generation. The small males are able to sneak up to the females in the spawning grounds. The large males are fierce competitors. This leaves intermediate-sized males at a disadvantage. ## 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 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 × q = pq (male a female A) = q × 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. ## 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 (HbSHbS)** - 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 (HbAHbA)** - 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 (Hb^Hbs) 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 (HbSHbS) 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. ## 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 specics 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