Artificial Selection. BIOL 101L: Principles of Biology Lab

Transcription

1 BIOL 101L: Principles of Biology Lab Artificial Selection Two weeks ago, you were introduced to methods for sampling and describing populations. Specifically, you measured the cold tolerance of fruit flies sampled from a large population. First, you measured ten flies and selected the three flies that tolerated cold the best (i.e., they recovered from freezing the fastest). Then, you measured ten more flies and selected the three flies that tolerated cold the worst (i.e., they recovered from freezing the slowest). Finally, you measured ten flies and selected three of them randomly with respect to their cold tolerance. You probably did not realize it at the time, but you were performing artificial selection. Your experiment was analogous to the process used by breeders of dogs, horses, cattle, crops, and other organisms that have been domesticated by humans. Usually, such breeders select for changes in a trait in one particular direction (e.g., bigger or smaller). In your case, you selected flies for changes in both directions and randomly selected flies as a control group. Because each lab section performed the same experiment, these selection regimes were replicated throughout the week. This experimental design will enable us to observe evolution in action. This week, you will quantify the evolutionary response to selection by measuring the cold tolerance of the offspring of your selected flies. You lab instructor will give you a Petri dish containing these offspring. You will likely have fewer flies to measure than you did on the first week because (i) the selected populations were initially small and (ii) only two weeks have passed since selection occurred. Don t worry about your sample size. We will pool data from all of the lab groups to increase the power of our experiment. By the end of the period, you should accomplish the following: 1) Measure the cold tolerance of offspring from your three selected populations 2) Graph the data in Excel to visualize the variation within and among populations. 3) Compare the cold tolerance among these populations using simple statistical analyses. 4) Determine whether heritable variation in cold tolerance existed in the original population. Evolutionary Response to Selection Evidence for natural selection abounds, and humans have shrewdly manipulated this process to alter the traits of organisms in desirable directions. When humans play the role of the selective agent, biologists refer to the process as artificial selection. Recall that Darwin himself dabbled in the artificial selection of pigeons to convince himself of the power of natural selection. Whether selection stems from the stresses of nature or the whims of a human, one should expect to observe an evolutionary response. The magnitude of this response (R) depends on two factors: the intensity of selection (i) and the heritability of a trait (h 2 ):

2 R = i h 2 The intensity of selection equals the difference between the mean of the trait in the original population and the selected sample of the population (see Fig. 1 below). Nevertheless, only genes can be passed on from parents to offspring. Consequently, selection only leads to change in the trait if the variation among individuals results from genetic factors. The heritability of a trait varies from 0 to 1; a value of zero means that the trait does not depend on genetic factors, whereas a value of one means that the trait depends entirely on genetic factors. If one performs artificial selection, the magnitude of the response can be used to calculate the heritability of the selected trait. For example, if no response occurs, one can conclude that the variation in the trait was not heritable. Figure 1. The intensity of selection equals the difference between the mean phenotypes of two groups: the original population and the selected population. The plot on the left shows a linear relationship between a trait s values and the fitness of the organism. This relationship causes a difference in the mean and variance of the trait when measured in the original and selected populations. The plot on the right highlights the difference between the means for these two populations (i). Methods for Data Collection 1. Obtain a sample of flies from your lab instructor. These flies are the offspring of your selected flies from the first lab (Sampling Populations). Depending on the number of flies that have developed, you may receive one to three groups of flies. Regardless of how many flies you receive, you will pool your data with everyone else in your lab section before beginning your analyses.

3 2. Measure the cold tolerance of your flies as you did in the first week of lab. Recall that cold tolerance was defined as the time required to recover from a brief exposure to 0 C. If you forget how you measured this trait, see the handout for the pervious lab (Sampling Populations). 3. Import you data into Excel. Do not bother to calculate means and variances yet. Instead, give your data to your lab instructor by using a flash drive or by ing the data. Your lab instructor will then pool the data for your lab section and give you the pooled data set. Methods for Data Analysis 1. Once you have the pooled data, start by describing these data with simple statistics such as the mean, variance, and standard deviation. Practice using the function in Excel that you were shown in lecture (average, var, stdev). If you don t remember how to use these functions, try looking them up using the Help feature of Excel. You will find that these functions make the calculations much easier than they were when you performed them during the first lab. When you have finished these calculations, consider the patterns in your data. Which group had the lowest mean and which group had the highest mean? Which group had the greatest variance? 2. Use Excel to create histograms of the data for each group. Do the means a variances that you calculated in Excel seem to correspond to the distributions of data shown in the histograms? In other words, do both suggest similar differences among the means and variances of the three groups? 2. Plot the data for the three groups (fast, slow, and random flies) using a standard bar graph. Add error bars to show the standard deviations of the means. Do these means appear different when viewed graphically? If so, do the differences among groups match your expectations? 3. Recall that differences among groups can arise from sampling error, even when no biological process has created real differences. This sampling error makes it impossible to know for sure whether the mean cold tolerances of your selected and random flies were different. Fortunately, we can use statistical tests to conclude whether the means were probably different. To test for whether the means were different, we can state a null hypothesis. The null hypothesis claims that there is no real difference between the true population means estimated by the sample means. Write down the null hypothesis for your flies: This is not a statement that we can ever know with certainty. Instead, the best we can do is estimate the probabilities associated with rejecting the null hypothesis when it is really

4 true. For example, we would commit an error if we conclude that the two generations of flies tolerated cold differently, when in reality they had the same population mean. If we make some assumptions about the distributions of cold tolerances (e.g., we assume normal distributions or bell curves), we can estimate the probability of making this error. Specifically, the P-value tells us the probability that we would record the observed difference between sample means (or a greater difference) when there really was not difference between the population means. Biologists, commonly accept a 5% chance of committing this error, which means we should only conclude that the means were different if P < To determine the P-value, we will use a t-test. Use the Data Analysis package of Excel to conduct t-tests, as you did in the previous lab on natural selection (Evolution Simulations). Note that Excel will give you two different P-values: a one-tailed value and a two-tailed value. In this case, you can use the one-tailed P-value because you expect the cold tolerance of each selected population to differ from the cold tolerance of the control population in a specific way (e.g., fast flies < control flies). Conduct three t-tests to draw conclusions about your experiment: 1) Fast selected flies vs. control flies 2) Slow selected flies vs. control flies 3) Fast selected flies vs. slow selected flies For each test, note the following pieces of information: 1) the t-value 2) the P value 3) whether or not you should reject the null hypothesis (i.e., whether you would conclude the population means were different) 4) the biological interpretation of this result as it pertains to your experiment Recall from lecture that a professional biologist would usually use a different kind of statistical test to compare means among three or more groups (an F test). Nevertheless, these the t-tests that you performed should lead to a similar conclusion without the need for you to learn a more advanced statistical test at this early stage in your training. Conclusion about the Heritability of Cold Tolerance As described above, we would ideally quantify the heritability of the selected trait from the intensity of selection and the evolutionary response. Unfortunately, we lack the ability to precisely control environmental factors that influence cold tolerance over the period of three weeks. For example, we know from our previous data that the temperature of the lab bench affects the time to recover from cold exposure (recall our discussion of the lab data in lecture). Very likely, your lab bench was a different temperature in week one than it was today; there s just no practical way we could control that variable. Therefore, we might not be able to accurately measure the evolutionary response to selection by comparing the means of the flies between weeks. However, we can infer an evolutionary response if the cold tolerance of the selected flies and control flies differed significantly.

5 Furthermore, the magnitude of the difference between the means reflects the magnitude of the heritability. Based on your analyses, was cold tolerance a heritable trait?