Selection and Evolution PDF - Past Paper with Exam Board and Questions

Here's the converted markdown format of the textual content in the images, including LaTeX for mathematical formulas where applicable: ```markdown ## 17 Selection and evolution ### 17.1 Variation **On these pages you will learn to:** * Describe the differences between continuous and discontinu...

Here's the converted markdown format of the textual content in the images, including LaTeX for mathematical formulas where applicable: ```markdown ## 17 Selection and evolution ### 17.1 Variation **On these pages you will learn to:** * 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. Environmental factors play a major role in determining where on the continuum an organism lies. For example, individuals who are genetically predetermined to be the same height grow to different heights as a result of variations in environmental factors, such as diet. This type of variation is the product of polygenes and the environment. Table 1 shows the number of people in a particular sample (frequency) with various heights. If we take these data and plot them on a graph, we obtain a bell-shaped curve known as a normal distribution curve (Figure 1). **Table 1 Frequency of heights in a sample of humans (measured to the nearest 2 cm)** | Height/cm | Frequency | | :---------- | :---------- | | 140 | 0 | | 144 | 1 | | 148 | 23 | | 152 | 90 | | 156 | 261 | | 160 | 393 | | 164 | 440 | | 168 | 413 | | 172 | 177 | | 176 | 63 | | 180 | 17 | | 184 | 4 | | 188 | 1 | | 190 | 0 | | 192 | 0 | **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. *The first image shows a graph titled "Graph of frequency against height for a sample of humans" with a caption describing the x and y axis scales. The x-axis represents height/cm (Independent variable), and the y-axis represents Frequency (Dependent variable)* --- As features showing discontinuous variation are often controlled by a single gene and gene mutations normally affect a single gene, it follows that many human diseases that result from gene mutations show discontinuous variation. Examples include albinism, sickle cell anaemia and Huntington's disease, which were discussed in Topic 16.3, and haemophilia (Topic 16.9). *The second image shows a bar graph titled "Discontinuous variation illustrated by the percentage of the UK population with blood groups A, B, AB and O". The x-axis represents Blood group and the y-axis represents % of UK population* **Causes of variation - genetic differences** Genetic variation is the result of the genotype of each individual, in other words the genes they possess and the forms of allele of each gene that are present. This genetic make-up varies from generation to generation as a result of: * **Mutations** - a change in sequence of bases in DNA is called a gene mutation (Topic 16.12). While mutations in body cells are not passed on to the next generation, those in reproductive tissues (testes and ovaries in animals and anthers and ovaries in flowering plants) may be inherited. In organisms that are asexually reproducing, mutations are the only source of genetic variation. * **Crossing over** and the consequent formation of recombinants during prophase I of meiosis (Topic 16.2) leads to equivalent sections of non-sister chromatids being exchanged and therefore separates alleles of linked genes that would otherwise be inherited together. * **Random assortment** of pairs of homologous chromosomes at metaphase I of meiosis (Topic 16.2) results in daughter cells that are genetically different. This is because the chromosome set they have received is the result of a random alignment of homologous pairs at the spindle equator. The greater the haploid number, the greater the number of different possible combinations. * **Random mating** between individuals within a species - which pair of organisms of a species that mate, and hence which two genotypes combine, is largely a matter of chance, although in some animals (especially humans) there may be an element of choice involved. * **Random fusion** of gametes at fertilisation. When mating takes place, which gamete fuses with which at fertilisation is random. **Causes of variation - environmental influences** Environmental influences affect the way a genotype is expressed and result in different phenotypes (Topic 17.2). Climatic factors (e.g. temperature, availability of water and sunlight) lead to variation, as do pH and quantity and type of nutrients. The environment may also influence genetic variation by, for example, affecting the rate of mutation. It may also switch genes on and off. The contribution of the environment to variation can be studied when organisms that are genetically identical are used. In the case of humans, monozygotic (identical) twin studies can be carried out. **SUMMARY TEST 17.1** There are two main types of variation in organisms. In continuous variation, the various forms of a characteristic grade into one another. Examples in humans include (1) and (2). In a large sample of humans, if we plot the number of people at each point within the range of either of these characteristics, we obtain a bell-shaped graph called a (3). Continuous variation results from the interaction of many genes rather than a single gene. These genes are collectively called (4). Continuous variation is not only the result of a group of genes, but is also affected by (5). In discontinuous variation, organisms fit into a number of distinct groups with no intermediate types. It is usually the result of (6) gene and an example in humans is (7). Discontinuous variation is hardly, if ever, influenced by the environment. Variation has many causes. A change in the sequence of bases of DNA, known as a (8), can cause variation, as can crossing over during (9) stage of meiosis and the consequent formation of (10). Further variation results from the (11) of pairs of homologous chromosomes during the (12) stage of meiosis as well as random (13) between individuals of a (14) and random (15). --- ### 17.2 Environment and phenotype **On these pages you will learn to:** * 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 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. --- *The third image shows a Siamese cat, which has dark fur extremities.* *The fourth image shows the artic fox which has has gray-brown summer coat and white winter coat* --- *The next image describes the "Effect of environment on phenotype", it contains images of the glandulosa plants at different altitudes.* **SUMMARY TEST 17.2** The phenotype of an organism is the result of the effect of the (1) and the organism's (2). For example, in Siamese cats there is a heat sensitive form of the (3) called tyrosinase that is involved in the production of the pigment known as (4). The production of this heat sensitive form of tyrosinase is controlled by an (5) represented by c³. This form of the enzyme is inactive at temperatures (6) 33°C. Body temperatures below this normally occur at the (7) of the body and so in these regions the pigment is produced and the fur is coloured (8). If genetically identical forms of the plant Potentilla glandulosa are grown at different (9) their phenotype differs in (10) and (11) due to the different environments they experience. It is clear therefore that the (12) of an organism determines the (13) of possible phenotypes, but it is the (14) that influences its final appearance. **EXTENSION** **Further proof!** How can we be sure that the dark extremities of Siamese cats are the result of the temperature at which fur develops, rather than simply being genetically determined? A couple of simple experiments prove the case: * Some dark fur is removed from the tail of a Siamese cat which is then kept in a warmer than usual environment. The new fur that develops is light in colour. * Some light fur is removed from the back of a Siamese cat and the shaved area kept at a lower than normal temperature. The new growth of fur is black. If fur colour is determined only by genes, with no environmental influence, then in both cases the new fur would have matched the original colour. --- ### 17.3 The t-test **On these pages you will learn to:** * Use the t-test to compare the variation of two different populations **Remember** A large standard deviation means a lot of variety; a small standard deviation means little variety. 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: $t = \frac{\overline{x_1} - \overline{x_2}}{\sqrt{\frac{s_1^2}{n_1} + \frac{s_2^2}{n_2}}}$ and $v = n_1 + n_2 - 2$ Where: $\overline{x}$ = 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. 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 *The next image shows the normal distribuition curve with mean indicated and the standard deviation values. with inflexion points.* --- **Calculating the standard deviation** At first sight, the formula for standard deviation can look complex: $standard \ deviation=\sqrt{\frac{\sum(x - \overline{x})^2}{n-1}}$ Where: $\sum$ = the sum of $x$ = measured value (from the sample) $\overline{x}$ = mean value $n$ = total number of values in the sample. However, it is straighforward to calculate and less daunting you take it step by step. The following very simple example, using the six measured vaues $(x)$ illustrating each step in the process. **Step 1** Calculate the mean value $(\overline{x})$, i.e $4+1+2+3+5+0 = 15$ $15 \div 6 = 2.5$ **Step 2** Subtract the mean vlaue from wach of measured values $(x-\overline{x})$ This gives: $1.5, -1.5, -0.5, +0.5, +2.5, -2.5$ **Step 3** As some of these numbers are negative, we need to make them positive. To do this square all the numbers $(x-\overline{x})^2$. 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 **Step 4** Add all these squared numberas together: $\sum(x - {x})^2 = 17.5$ **Step 5** Divide the original nunber of measurements less one, $i.e.$ 5. $\frac{\sum(x - {x})^2}{n-1} = \frac{17.5}{5} = 3.5$ **Step 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: $\sqrt{\frac{\sum(x - {x})^2}{n-1}}=\sqrt{3.5}=1.87$ **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. For example the calculation √3.5 produces the answer 1.870828693 Clearly the significance of the latter digits is less than the earlier one. It is normal produce these fuguresto certain number of significant figures. In our case we have rounded down the answer to three significant figures name 1.87. We did this because calculated our square 2.25, 0.25, 6.25, etc as had three significant figures we used the number in final calculation. --- **Table 1 Yield of wheat for fertilisers A and B** | Number of tonnes of wheat per plot | Fertilizer A | Fertilizer B | | :----------------------------------- | :----------- | :----------- | | | 5 | 4 | | | 9 | 3 | | | 11 | 6 | | | 9 | 7 | | | 10 | 5 | | | 7 | 3 | | | 5 | 3 | | | 8 | 5 | | Total | 64 | 36 | | Number of plots | 8 | 8 | | Mean | 8 | 4.5 | To demonstrate how the t-test world let us consider our imagninary exampla farmer want to file out if one fertiliseris better than another in her of yield to the did divide our fields into 16 plots and plots which treat number of tonnes of plots in Table 1 The first stage of the t-test is to calculate standard devisation for the same .to do must calclate mean of deviation leading the mean quare deviation and deviation All these calculated and showin in table 2 **Table 2 Calculating the standard deviation for each sample** | | | Fertilizer 1 | | Fertilizer 2 | | | :----------- | :------------ | :------------------------------ | :--------------------------- | :------------------------------- | :--------------------------- | | | | | | | | | Observation | Devikation | Square of deviation | Observation | Deviation from the mean $(x - X_2)$ | Square of deviation | | ( x) | from the $( x - X_1)$ | $( X- X_1 )^2$ | ( x) | | $( X- X_2 )^2$ | | 8 | 4 | .5 | 8 | .5 | 8 | | 5 | 4 | .5 | 5 | .5 | 5 | | | 7 | .5 | 7 | .5 | | | 4 .5 .5 We substitute calculated in equation as follow: $\frac{\overline{x_1} - \overline{x_2}}{\sqrt{\frac{s_1^2}{n_1}} + \sqrt{\frac{s_2^2}{n_2}}}$ $t = \frac{8-4.5}{\sqrt{\frac{2.20^{2}}{8} + \sqrt{\frac{1.51^{2}}{8}}}}$ $t = \frac{3.5}{\sqrt{\frac{4.84}{8} + \frac{2.28}{8}} }$ $t = \frac{3.5}{\sqrt{0.61+0.28}}$ $t = \frac{3.5}{\sqrt{0.90}}$ $t = \frac{3.5}{0.95}$ $t = 3.68$ --- **Table 3 The t-table** | Degrees of Freedom | Value of t | | :----------------- | :------------ | | | *Probability that chance Produced this Value of t* | | 1 | 6.31 | | 2 | 2.92 | | 3 | 2.35 | | 4 | 2.13 | | 5 | 2.02 | | 6 | 1.94 | | 7 | 1.90 | | 8 | 1.86 | | 9 | 1.83 | | 10 | 1.81 | | 11 | 1.80 | | 12 | 1.78 | | 13 | 1.77 | | 14 | 1.76 | | 15 | 1.75 | | 16 | 1.75 | | 17 | 1.74 | | 18 | 1.73 | | 19 | 1.73 | | 20 | 1.73 | | 21 | 1.72 | | 22 | 1.72 | | 23 | 1.71 | | 24 | 1.71 | | 25 | 1.71 | | 26 | 1.71 | | 27 | 1.70 | | 28 | 1.70 | | 29 | 1.70 | | 30 | 1.70 | | 40 | 1.68 | | 60 | 1.67 | --- ### 17.4 Natural selection **On these pages you will learn to:** * 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 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 tees and rocks on which they normally test Insect-eating birds such as robins and heage sparrows. The melanic form and so eaten by these forms. --- ***The Image after that shows Industrial melanism in the peppered moth Biston betularia. Against a natural background, is for more visible one and more readlity presented by The population is predorminating* --- *The next image shows the Comparison of amino acid sequence in part of the same protein in six species* **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 there is a to passing to the there than these characterisitc of example ### 17.5 The roles of over-production and variation in natural selection **On these pages you will learn to:** * 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 depends by food, light, spaca ete