Biology Teach Yourself Series Topic 14: Population genetics

Transcription

1 Biology Teach Yourself Series Topic 14: Population genetics A: Level 14, 474 Flinders Street Melbourne VIC 3000 T: W: tssm.com.au E: TSSM 2011 Page 1 of 24

2 Contents Population genetics... 3 The gene pool Allele frequency Review Questions... 4 Selection pressures Darwin s theory of evolution by natural selection Answering questions about Natural selection Review Questions... 7 Types of natural selection Sources of variation Population bottlenecks Review Questions The founder effect Review Questions Genetic drift Review Questions Genetic equilibrium Review Question Speciation Reproductive isolation Heterozygous advantage Solutions to Review Questions TSSM 2011 Page 2 of 24

3 Population genetics Population genetics is the study of allele frequencies and the changes that occur to allele frequencies under the influence of evolutionary processes. The gene pool The gene pool is the total aggregate of genes and all of their alleles in a population at any given time. The Hardy-Weinberg theory states that if a population is stable and non-evolving then the composition of that population's gene pool will not change. It is said to be in equilibrium. However, for this to be true, five conditions must be met: The population must be very large The population must be reproductively isolated. There are no net changes to alleles due to mutations. Mating is random. All genotypes are equal in their reproductive success. There are a range of factors that can affect whether a gene pool changes or remains stable. These include: Population size Mate selection Mutations Gene flow Natural selection Genetic drift Population bottlenecks Founder effect TSSM 2011 Page 3 of 24

4 Allele frequency Allele frequency is the proportion of each allele in a specific population. Allele frequency is always expressed as a decimal and is calculated by dividing the number of each type of allele by the total number of alleles. So: Allele frequency = number of an allele / total number of alleles. It is important to remember that the number of alleles will be double the number of individuals, as each individual has 2 alleles. Example: A population contains individuals with one of three genotypes: AA, Aa and aa. If the population has 20 individuals with the AA genotype, 30 individuals with the Aa genotype and 10 individuals with the aa genotype the allele frequency would be calculated as follows: There are 20 individuals with the AA genotype so there are 40 A alleles. There are 30 individuals with the Aa genotype so there are 30 A alleles and 30 a alleles There are 10 individuals with the aa genotype so there are 20 a alleles. In this case there are 60 individuals so there will be 120 alleles in total. There are 70 A alleles and a 50 a alleles. Frequency of A allele = 70/120 = 0.58 Frequency of a allele = 50/120 = 0.42 Review Questions 1. A geneticist is calculating the allele frequency for a single gene. How many alleles are there in a population of 100 individuals? 2. In a population of plants 40 individuals have white flowers, 30 individuals have pink flowers and 20 individuals have red flowers. Calculate the frequency of each allele. TSSM 2011 Page 4 of 24

5 Selection pressures A selection pressure is any factor that influences the survival of an individual, a segment of the population or the species as a whole. Selection pressures act on the phenotype of the individual. Selection pressures only become relevant when there is variation in a population, with some of the population being fitter than others. For example, a group of giraffes have necks of various lengths. This does not matter until a drought occurs and all of the low lying vegetation is killed off. The availability of food then becomes a selection pressure and the long necked giraffes now have a selective advantage. Selection pressures can be due to abiotic factors or they can come from competition with other organisms. Competition falls into two categories: Intraspecies competition- Within-species competition, for example completion for mates. A female bower bird will select the male who builds the best bower, whereas a lioness will select a mate who is large and strong. Interspecies competition- Competition between species, for example, competition for food sources, nesting sites etc. If a magpie and a sparrow wanted the same nesting space it is likely that the magpie would have the advantage. TSSM 2011 Page 5 of 24

6 Darwin s theory of evolution by natural selection The central idea of natural selection is that better adapted individuals are more likely to survive and produce larger numbers of viable offspring than other individuals that are less well adapted. The concepts included in this theory include: Populations produce more young than are necessary to replace the parents. Many of these young will die or fail to reproduce. There are a variety of phenotypes for each trait and each population contains a range of varied individuals. Generally there will be limited resources and the struggle for survival will favour the individuals which have the most favourable phenotypes (this doesn t mean at least some of the others will not survive they are just less likely to do so). The phenotypes of the parents are passed on to the offspring. Each subsequent generation should have more individuals with the favourable phenotype and less with the unfavourable phenotypes. Natural selection acts on the phenotype, changing the allele frequencies in a population. Favourable mutations are selected for and their frequency increases over time TSSM 2011 Page 6 of 24

7 Answering questions about Natural selection Questions about natural selection are quite common in exams. The information is usually presented as a description, a table or a graph. To completely answer a question about natural selection you need to include the following points: 1) Variation exists in a population (this part is generally forgotten, but is essential because natural selection cannot occur if there is no variation). 2) A selection pressure occurs (specify what it is). 3) Those with a favourable phenotype (specify what it is) are more biologically fit and will be selected for, whilst unfavourable phenotypes (specify what it is) are selected against. 4) Those with the favourable phenotype are more likely to survive long enough to reproduce, so the next generation will be genetically more like them. The incidence of the favourable trait will increase in subsequent generations. Review Questions 3. Explain why natural selection does not operate in populations where the organisms are all genetically identical. TSSM 2011 Page 7 of 24

8 4. The graph below shows the effect of using pesticides on a group of insects over a period of 10 years. PESTICIDE RESISTANCE IN INSECTS Percentage of insects killed Time (Years) Use the data from the graph to explain the process which caused the percentage of insects killed by the pesticide to alter over the period of 10 years. TSSM 2011 Page 8 of 24

9 Types of natural selection Natural selection can influence the frequency of phenotypes in several different ways. Stabilising selection: the extreme variations are selected against and the middle range phenotypes are selected for. The result of this type of selection is a decrease in genetic variation. Selected for Selected against Directional selection: This type of selection favours the phenoypes at one end of the range of phenotypes. The phenotype of the species is shifted in 1 direction. Selected for Selected against Disruptive selection: the extreme variations are selected for and the middle range phenotypes are selected against. Selected against Selected for TSSM 2011 Page 9 of 24

10 Sources of variation There are a variety of sources of variation in a sexually reproducing population. These include: Mutations to germ line cells. Mutations that occur to somatic cells will affect the phenotype of the individual during their life, but will not be passed onto their offspring. Crossing over during meiosis Independent assortment of chromosomes during meiosis. The particular combination of parents. In a varied population different parents may have different alleles. The particular combination of gametes that fuse together at the time of fertilisation. In humans there are over 8.3 million possible combinations for gametes just from one parent (2 to the power of 23). Population bottlenecks Population bottlenecks occur when a random non-selective event such as a fire or a volcanic eruption drastically reduces the numbers of a population. The allele frequencies in the surviving population are not reflective of the original population, as alleles may be lost or they may be at a much lower frequencies. The population number may increase in the future, but the genetic diversity will be decreased compared to the original population. Poulation bottlenecks tend to have an adverse effect on the population, as it usually leads to a lack of variation and inbreeding. However, it is useful to remember that a lack of genetic variation that is due to the results of natural selection may mean that the whole population is extremely well adapted to their surroundings, and some species survive quite well despite a low level of diversity. However, an adverse selection pressure can have a dramatic effect on such populations. TSSM 2011 Page 10 of 24

11 Review Questions 5. Explain what the term non-selective means. 6. Endangered species such as cheetahs tend to be subject to population bottlenecks. The lack of variation in such a population tends to cause a number of problems. Explain why a small endangered population is often unable to recover. TSSM 2011 Page 11 of 24

12 The founder effect This is a type of genetic drift that occurs when a small section of a population leaves a larger population and colonises another area. The small group is not representative of the larger population, so the allele frequencies will be different. The small population may be exposed to different selection pressures and may evolve differently to the parent population. AA AA Aa aa Aa Aa AA aa aa AA AA AA Aa aa AA Aa Aa aa AA Individuals that leave the original population Original population Review Questions 7. Calculate the allele frequency for the original population 8. After 10 generations the population that left has built up to 150 individuals. State whether the allele frequency of this population is likely to be the same as the original population. Provide a reason to support your answer. TSSM 2011 Page 12 of 24

13 Genetic drift Genetic drift refers to random changes in allele frequencies. Small populations are particularly susceptible to genetic drift since the loss of a small number of individuals will have a larger proportional effect on the genes passed onto the next generation than the loss of the same number of individuals in a large population. Rare alleles are susceptible to loss in small populations. As time progresses it is possible that alleles becomes fixed (it's frequency becomes 100%) or they are lost (frequency becomes 0%). AA AA aa AA AA Aa Aa AA AA aa AA AA aa AA AA Aa Aa AA AA AA Aa aa AA AA AA Aa Aa Aa AA AA Generation 1 A = 0.70 a = 0.30 Generation 2 A = 0.50 a = 0.50 Generation 3 A = 1.00 a = 0.00 Review Questions 9. Explain why genetic drift has a greater impact on small population compared to larger populations. 10. What terms can be used to describe what has happened to allele A and allele a in generation 3? TSSM 2011 Page 13 of 24

14 Genetic equilibrium This is also called gene pool stability. For a gene pool to remain stable the frequency of dominant and recessive alleles remains constant from one generation to the next. Natural selection, population bottlenecks, the founder effect and genetic drift will all have an effect on allele frequency but there are other factors which also have an effect. Population size: small populations favour change because each individual makes a proportionately large contribution to the gene pool e.g. if a gene pool consists of 10 individuals and 1 dies then 10% of the gene pool has been lost. However, if a gene pool consists of 1000 individuals and 1 dies then only 0.1% of the gene pool has been lost. Mate selection: mating is either random (which means that mates are not selected based upon their phenotype and all individuals are equal in reproductive success) or mating is assortive which means that mates are selected on the basis of having a trait perceived as being desirable. The diagram below shows both types of mating, the arrows indicate the maes selected. AA AA AA Aa Aa aa aa aa AA Aa Aa AA aa AA Aa aa Aa AA Aa aa AA Assortive mating Random mating Gene flow: this is the transfer of genes between different populations of the same species due to emigration and immigration. Individuals are able to migrate between populations so gene flow can occur. A barrier has been placed between the 2 populations so that gene flow cannot occur. TSSM 2011 Page 14 of 24

15 Mutations: if a new allele is produced it will be subjected to natural selection and either remain in the population or be removed. Review Question 11. Choose which of the following pairs favours gene pool stability Large Population Mutations Genetic drift No natural selection Gene flow Random mating Population bottleneck No founder effect Small Population No mutations No genetic drift Natural selection No gene flow Assortive mating No population bottleneck Founder effect TSSM 2011 Page 15 of 24

16 Speciation Members of a species possess similar anatomical characteristics and have the ability to interbreed and produce fertile offspring that are able to produce more of the same species. When a population of a species splits they may be subjected to different selection pressures. This tends to lead to changes in the gene pool of each sub-group over many generations. As these changes progress, significant phenotypic differences may develop in each group. For a while, breeding between groups will remain theoretically possible. Eventually, however, the accumulated genetic changes in each group's gene pool would prevent the production of viable offspring between the two groups. Allopatric speciation occurs when a population is split by a physical barrier. As a result of geographic isolation and different selection pressures the two populations evolve into different subspecies. As time passes genetic changes continue until there are two species that are reproductively isolated. Sympatric speciation occurs when new species from a single ancestral population develop alongside one another. For example, some members of a population may feed on one area of a tree and some on another. They may experience different selection pressures and over a long period of time they may become reproductively isolated. Reproductive isolation This refers to any factor that prevents two species from interbreeding and producing viable offspring. It limits the chances of gene flow occurring. Factors that influence reproduction isolation include: Geographical factors: physical barriers isolate populations Temporal factors: different species may have different breeding times or seasons Ecological factors: different species may inhabit different microhabitats and do not interbreed. Behavioural factors: different species may have different mating rituals e.g. calls, dances or displays. Chemical factors: different species may produce specific pheromones, which only attract others of the same species. Mechanical factors: members of different populations may be physically unable to mate. Gamete mortality: sperm and ova may not be able to unite or the blastocyst may fail to develop All of these factors with the exception of gamete mortality are pre zygotic; gamete mortality is post zygotic. TSSM 2011 Page 16 of 24

17 Review Questions Use this information to answer the following questions. Kangaroos and wallabies are marsupials that belong to a small group of animals called macropods. They are only found naturally in Australia and Papua New Guinea. Most macropods have hind legs larger than their forelimbs, large hind feet, and long muscular tails which they use for balance. All kangaroos and wallabies have forward-opening pouches. Other marsupials such as wombats and koalas have pouches that open backwards. The kangaroo super family consists of two smaller family groups. One group includes kangaroos, wallabies, pademelons and tree kangaroos. The other group includes rat-kangaroos, bettongs and potoroos. Currently there are 45 known species of kangaroos and wallabies. The Western Grey Kangaroo (Macropus fulginosus) is a found extensively across the southern part of Australia including lower Queensland, the Murray Darling basin in NSW, Coastal South Australia and Western Victoria. A subspecies of the western grey kangaroo is located on Kangaroo Island just off the coast of South Australia. This subspecies is known as the Kangaroo Island Kangaroo. It is believed that the population of Kangaroo Island Kangaroos was isolated from the mainland population approximately 10,000 years ago as a result of sea levels rising at the end of the last ice age. When comparisons were made between the mainland western grey kangaroos and the Kangaroo Island Kangaroos it was found that there were both morphological and genetic differences between the two sub species. 12. Assuming that the Western Grey Kangaroo and the Kangaroo Island Kangaroo have become 2 different species, identify the technical term used describe the formation of 2 different species and outline the sequence of events that results in the formation of the 2 different species. TSSM 2011 Page 17 of 24

18 13. Kangaroo Island has its own unique kangaroo, a smaller, longer-haired, darker coloured sub-species of the Western Grey Kangaroo that can be found on the mainland. Identify the process involved and explain how the population of Kangaroo Island Kangaroos has evolved to have thicker coats than their mainland relatives. 14. It has also been found that the Kangaroo Island Kangaroo is the slowest moving of all of the Macropus family. On the mainland this would be a disadvantage. Explain why this trait does not seem to affect the biological fitness of the Kangaroo Island Kangaroos. TSSM 2011 Page 18 of 24

19 Microsatellites, also known as Simple Sequence Repeats (SSR s) are short sequences of DNA consisting of 1 to 6 nucleotides. These sequences are repeated many times and are used as molecular markers to determine the extent of relatedness between individuals and species. Scientists obtained blood samples from a number of Kangaroo Island Kangaroos and found that there were very few differences between them, indicating a low level of genetic diversity. 15. It is believed that this similarity is partially due to the effect of a population bottleneck. Define the term population bottleneck and explain how a bottleneck can result in a population with a low level of genetic diversity. 16. Kangaroo Island Kangaroos are very common and the population has remained stable for the last 50 years. Provide an explanation for the population numbers remaining stable despite the lack of genetic variation. 17. It has been suggested that a population of Western Grey Kangaroos from mainland South Australia should be released onto Kangaroo Island. What would be the purpose of performing this operation? TSSM 2011 Page 19 of 24

20 18. Studies have proved that the Kangaroo Island Kangaroo is actually a subspecies of the Western Grey Kangaroo rather than being a separate species. Provide a reason to explain why these populations have not evolved into 2 separate species. How could it be proved that these 2 sub-species have not developed into 2 separate species at this point in time? 19. When the genome of the Kangaroo Island Kangaroo was compared to that of the Mainland Western Grey Kangaroo population if was found that the frequency of a specific allele was 0.74 compared to the frequency of 0.22 in the mainland population. Identify the term used to describe this phenomenon and explain how this process occurs. TSSM 2011 Page 20 of 24

21 Heterozygous advantage Heterzozygous advantage occurs when the heterozygous genotype confers a greater fitness advantage than either of the homozygous genotypes. This occurs when a trait is controlled by alleles that are codominant or incompletely dominant. An example of this is the inheritance of the sickle cell anaemia trait in areas where malaria is common. Sickle cell Normal red blood cell Individuals who are homozygous either have normal blood cells or sickle cells, whilst heterozygous individuals have both types of cell. Individuals who are homozygous for normal blood cells have the advanatage of normal cell functioning. However, the parasite responsible for malaria is easily able to inviade these cells. This parasite is not able to invade the sickle shaped cells, which is an advantage for individuals who only have sickle cells. These people, however, suffer severe illness or death as a result of the condition. Heterozygous individuals have sufficient normal shaped cells for normal functioning and sufficient sickle cells to limit the ability of the parasite to invade their cells and are therefore less likely to develop malaria. Heterozygous individuals have an advantage over both of the homozygotes. TSSM 2011 Page 21 of 24

22 Solutions to Review Questions alleles 2. White allele = 110/180 = 0.61 Red allele = 70/180 = The central idea behind natural selection is that some phenotypes are fitter than others and hence they are selected for. If there is no variation then there is nothing to select for or against and all members of the population are equally vulnerable to a specific selection pressure. 4. Initially some of the population of insects were susceptible to the pesticide and others were not. A selection pressure, the insecticide was applied. Those insects that were resistant had the favourable phenotype and were selected for. Those insects that were sensitive were selected against. The resistant insects were more likely to survive and reproduce so the incidence of the resistant trait increased in subsequent generations. This is reflected in the fact that fewer insects are killed by the pesticide as time passes. 5. Non-selective means that no phenotype is any more advantageous than any other. For example, if a beach is flooded then traits such as size, strength and speed will have no effect on which individuals survive and which don t. 6. There is insufficient variation so all individuals are equally susceptible to selection pressures. Also inbreeding occurs and mutations accumulate. 7. Frequency of A allele = 15/30 = 0.5 and a allele = 15/30 = It is very unlikely that the allele frequency in the new population will ever be the same as that of the original population. This is an example of the founder effect. The allele frequency of the population that left the original population is not the same as the original population being 0.88 for the A allele and 0.12 for the a allele. Since the founding population is small it is likely that the frequency of the A allele will increase. 9. Genetic drift has a greater impact on small populations because each individual has a greater effect on the gene pool than occurs in larger populations. 10. Allele A has become fixed and allele a has been lost. 11. The following factors favour gene pool stability: Large population No mutations No genetic drift No natural selection No gene flow Random mating TSSM 2011 Page 22 of 24

23 No population bottleneck No founder effect 12. This would be an example of speciation. The sequence of events would be as follows: Originally both sub-species are part of the same population. A geographic barrier, in this case the rising sea levels separate the original population into 2 smaller sub-populations. The isolated populations are subjected to different selection pressures; different phenotypes are favoured, leading to changes in the gene pool accumulating over time. When members of the two populations are brought together again they are no longer able to interbreed producing viable offspring. 13. This is an example of natural selection. The sequence of events would be as follows: Phenotypic variation exists in the original population; some kangaroos have thicker, darker coats than others. The selection pressure is that there is colder weather on Kangaroo Island than on the mainland. Those with the favourable phenotype, the a thicker coat, are selected for and those with the thinner coats are selected against Those with the favourable phenotype, the thick woolly coat are more likely to survive and reproduce than those with the thinner coat. Following generations will be phenotypically and genetically more like those that are selected for than those selected against. Eventually the entire population has the trait of thicker, darker coats. 14. Kangaroos on the mainland have to contend with predators, but there are no predators on Kangaroo Island so their slowness does not affect their biological fitness. 15. A population bottleneck occurs when a random non-selective event drastically reduces the numbers of a species. The allele frequencies in the surviving population are not reflective of the original population. The smaller population size and change in allele frequencies means that the population is more likely to experience genetic drift, further reducing the genetic diversity of the population. 16. If no selection pressures occur population numbers should remain stable. 17. Although the 2 populations have been reproductively isolated for approximately 10,000 years this has not been sufficient time for speciation to have occurred. The purpose of introducing Western Grey Kangaroos onto Kangaroo Island would be to introduce new alleles into the Kangaroo Island Population to increase variation and pevent inbreeding. 18. The means of proving whether speciation has occurred or not would be to interbreed the kangaroo island kangaroo with the western grey kangaroo. If they can be successfully interbred and produce viable offspring then the two types of kangaroos remain members of the same species. 19. This is an example of the founder effect. When sea levels rose the ancestors of the Kangaroo Island Kangaroos were geographically and reproductively isolated from the mainland population of Western Grey Kangaroos. The smaller founding population on Kangaroo Island does not have the same allele frequencies as the larger mainland population. TSSM 2011 Page 23 of 24

24 When the population of the Kangaroo Island Kangaroos increases over time the allele frequencies will still not be the same as that of the mainland population. TSSM 2011 Page 24 of 24