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Transcript
Heritability: The evolution of quantitative traits by artificial selection After studying Drosophila Fly genetics we learned that some traits can be controlled by simple Mendelian processes. But, what if the F1 generation in the initial cross completed in the study was not all wildtype and the F2 generations was not 3:1? Does it mean that the trait under consideration is not heritable? Are there other factors controlling heritable traits besides Mendelian processes? What about traits that are not all or nothing in their expression? What about traits that are thought to have a large environmental component like alcoholism? Is there a way to determine how much of a given trait is under genetic control versus how much a trait is influenced by the environment? And how do these traits evolve on a micro and macro-scale? In the following activity, we will analyze some data produced over the last 4 years by Bryn Mawr Intro Bio students. The study selectively bred Brassica rapa plants in order to determine whether or not plant “hairiness” was a heritable trait and if so, what fraction of the variation in “hairiness” is explained by genetic variation and how much is due to environment. Assignment: Read through the activity and answer the italicized questions in a new document you create and submit to your “Portfolio” within 24hrs. Quantitative trait variation Unlike Mendelian traits that are controlled by only one gene, quantitative traits most often results from many genes of relatively small affect whose exact number is unknown. Description and analysis of variation and selection on such traits is based on statistical measures and relations. Statistical variance quantifies variation around the average value of the trait. Such phenotypic variance (VP) can be divided into a genetic component, the genetic variance (VG) an environmental component, the environmental variance (VE) and a genotype by environment interaction (VGE), thus; VP = VG + VE + VGE. Offspring will tend to resemble their parents both because of common environment and because they share a common genetic background for the trait. However, selection operates only on the genetic variance, thus a trait can only evolve if variation has a genetic component. Thus assessing the genetic contribution to traits, VG is critical to understanding how traits might evolve. The genotype by environment interaction term accounts for the fact that differences in the environment do not have a uniform effect among all genotypes in the population. For example, one genotype might grow better at 20°C but another better at 30°C. The environment does not have a uniform effect among genotypes. Natural selection and artificial selection Natural selection stands as one corner stone of evolutionary biology and explanation for the diversity of life on earth. Some have called natural selection a deceptively simple concept, or instilled it with purposeful intent. However, natural selection is in essence a mathematical process. It is simply differential survival and reproduction. Natural selection does not lead to differential survival and reproduction, nor is it brought about by differential survival and reproduction, natural selection is differential survival and reproduction. In this exercise with artificial selection you will become familiar with the processes surrounding evolution and natural selection. Fundamentally, artificial selection and natural selection are the same. Artificial selection differs from natural selection primarily in that the reproductive success of the organism hinges on a single character or small set of characters chosen by the human investigator rather than by the organism’s overall survival and reproduction. Natural selection also lacks the purposeful “directedness” of artificial selection. Heritability and Selection The rate of evolution by artificial or natural selection depends on the level of genetic variation in the trait and the strength of the selection applied to the population. The process is best illustrated with an example. Figure 1 shows the frequency distribution of a given trait, let’s say beak size, in a population of finches. The distribution of individuals is bell-shaped (although this need not be the case) and XPo indicates the mean value of the trait for the whole population. Suppose that in the population, birds that have larger beaks are able to crack open a wider variety of seeds and therefore store more fat and survive cold winters more often than birds that have smaller beaks. During one winter, only finches with beaks of larger than a certain size survive, so that the average beak size among the survivors is XS (Fig 1a). The differential survival resulting from larger beak size is directional selection. If beak size is heritable, that is if the variance in size of a bird’s beak is at least partially determined by genes, a) Selection differential frequency of individuals XPo XS 40 30 20 10 0 S frequency of individuals b) Selection response R 40 30 20 10 0 c) realized heritability h2 = R/S Fig. 1 Illustration of selection differential and response for a population with normally distributed trait values. then the directional selection will produce a shift in average beak size in the next generation. The difference between the average beak size among survivors and that in the original population (which includes the survivors) is known as the selection differential (S: Fig. 1a). We measure strength of selection as the selection differential (S). The selection differential is simply the difference between the mean of the trait in the selected group of parents (XS) and that in the entire base population (XP, Fig. 1a). Selection response (R) is the change in the average value of the trait in the offspring generation compared to that of the entire parental generation (not only the selected parents, Fig. 1b). A response to selection thus provides evidence for a genetic basis to the trait. Such a response also represents an evolutionary change. The relation between S and R can be used to provide an estimate of the genetic component of the trait of interest. Selection response can be expressed as a proportion of the selection differential, to yield h2 the realized heritability (Fig. 1c). h2 = R/S h2 = XP1-XPo / XS-XPo (1) (2) Thus, if the realized heritability of a trait in a population = 1, then all the variation of that trait in the population is due to genetic factors. If h2 = 0, then, R had to be zero and therefore, there was no response to the selection. If the next generation shows no response to a selection force, then that trait does not have a genetic component. That is, all the variation comes from the environment or factors other than genetic make-up. Selective Breeding The plants you will be working with are the same species as cultivated turnip, pak choi, and Chinese cabbage, each a product of selective breeding for specific phenotypic traits. Brassica rapa forms part of a complex of related species that comprise many of the most important vegetable crops world-wide, for example, B. rapa: pak choi, Chinese cabbage, turnip, saichin B. oleracea: kale, cauliflower, broccoli, head cabbage, Brussel sprouts, kohlrabi, collard greens B. juncea: brown mustard, mustard greens B. napus: canola 1. If you were trying to breed Brassica rapa to increase trichome number (hairiness), describe in as much detail as possible the methods. Results of First Round of Selective Breeding In an attempt to increase trichome number or hairiness on Brassica rapa (thought to increase resistance to drought as well as protect against herbivory), one hundred individual plants were scored for trichome number on the first true leaf petiole (see Fig 2). Table 1 shows the distribution of trichome number for the first original population (po) of 100 individuals. The mean trichome number for the entire population was calculated as XPo = 3. Then the 10 most hairy plants were selected and bred to generate a new offspring population 1 (p1). The mean trichome number of the selected parents was calculated as XS = 16. Petiole is between main stem and leaf blade. Figure 2. Photograph of trichomes on the first true leaf of B. rapa Table 1: Trichome # of Original Populations Trichomes Gen 1 '05 0-4 5-9. 10-14. 15-19. 20-24. 25-29. 30-34. 35-39. 40-44. 45-49 >50 63 23 8 5 1 0 0 0 0 0 0 2. Given the properties of the original population and the mean trichome number of the 10 selected parents, predict and explain what the next generation will look like with regards to trichome number. Hint: Think about what the distribution of the offspring generation would be if h2 = 1.0 and h2 = 0.0. Also, what would the XP1 be for both cases? You may want to open MS Excel and use the spreadsheet functions to graph and make calculations. Table 2 shows the distribution of the second generation (offspring of the selected parents) and three additional rounds of selective breeding carried out. Table 3 gives the mean of each entire generations and mean trichome number of the selected parents in each round of breeding. Table 2: Distribution of Trichomes for the all rounds of selective breeding. Gen 1-5 are the different generations/populations used in the breeding. Each subsequent generation is the offspring of the previous generation. Trichomes 0-4 5-9. 10-14. 15-19. 20-24. 25-29. 30-34. 35-39. 40-44. 45-49 >50 Gen 1 Gen 2 Gen 3 Gen 4 Gen 5 63 23 8 5 1 0 0 0 0 0 0 21 27 26 13 10 2 1 0 0 0 0 5 11 30 28 15 5 3 1 2 0 0 0 2 15 30 28 14 8 2 0 0 0 0 1 3 13 25 22 14 14 5 4 0 Table 3: Mean trichome # of entire populations and selected parent populations. XPo is mean of entire original population. X P1 is mean of entire offspring population. Xs is the mean of only the top 10 selected parents Iteration of Selective Breeding Means G1-G2 G2-G3 G3-G4 G4-G5 Xp0 3 10 16 21 Xp1 11 15 23 28 Xs 16 18 29 33 Analyze the reported results from Table 2 and 3 and answer the following questions. 3. Is trichome number a heritable trait in Brassica rapa and if so, what portions of the variation in trichome number is explained by genes? Show calculations. 4. Why does realized heritability of trichome number vary from generations to generation if it is the estimated genetic contribution of a given trait? That is to say these data seem to suggest that the genetic contribution to a trait can change from one generation to the next. Explain. 5. If you were to do another round of selective breeding starting from Generation 5 population (G5-G6), exactly what would you expect the mean trichome number to be of the next offspring generation (XP1 of G6) if the top 10 hairiest plants of G5 had a mean trichome number of 51 (XS= 51)?Make a specific predication. Explain your assumptions and show your calculations. 6. If microevolution is defined as change in heritable phenotypes within the original range of variation of a trait and macroevolution is a change of phenotypes outside the original range of variation, how would you classify this Brassica rapa breeding study? Explain and support with evidence (Make a figure showing the distribution of trichome number for the different populations and then explain).