Lab 2: Investigating Variation Across Spatial Scales

Transcription

1 Lab 2: Investigating Variation Across Spatial Scales What are scales and variation in a biological context? The world around us displays incredible diversity across many scales. Today s lab investigates variation at the microscopic and macroscopic scales, while providing you with practice using and becoming competent with tools scientists use to investigate variation. In biology, we can study variation (differences among or between organisms) through a variety of levels: cellular, organismal, population, community and ecosystems. These levels biologists term scales. We can study things at the macroscopic scale, the microscopic scale, or even any points along a time scale (temporal scales). Furthermore, we can sub divide any object we are observing into multiple scales. For example, we may look at the entire scale of a lake, the sub scale of a shoreline riparian zone, or the benthic zone along the bottom of the lake. Scales become the context for our questions in biology, allowing us to orient to the system that we are studying always being mindful of the scale from which we approach the problem. Scientific tools used to measure variation Variation in the world around us is often plainly visible to us (e.g., some trait being displayed), but may also require the use of tools and techniques to elicit variation. Modern light microscopes enable biologists to easily view variation in the micro world, cellular and tissue structures. These instruments allow biologists to make measurements of objects in microns (=micrometers; 10 3 millimeters). Compound microscopes make use of two glass lenses, the objective and ocular lenses. The objective lenses form a magnified intermediate image of the specimen illuminated below by the lamp. The image is further magnified by the ocular lens. The total magnification perceived is the product of the magnification of the objective and ocular lenses. Dissecting microscopes use two separate optical paths to produce a 3 dimensional magnification rather than a flat, 2 dimensional view through the compound scope. The light source for this microscope is external rather than internal, compared to the internal light source of the compound scope. The objective lenses on this microscope have a fixed degree of magnification (usually 4x), and additional magnification may be achieved through zoom magnification. Click on this link to watch a video about using the compound microscope: (Please note that depending on the application you are using to read the file, clicking on the link may not work. You may have to type the url into your browser window.) w98ka8uqu In your review of this video, focus on how the students use the microscope, how they prepare a slide and how they clean the microscope. These are universal skills, applicable to this biology lab and beyond. Variation can also be measured at the molecular level through computers, and genome level techniques. At a larger scale, variation can be measured within and between populations, communities and ecosystems by taking quantitative or qualitative measure specific to the variation you are interested in studying. Science is about understanding patterns in variation across scales, and therefore learning to recognize and measure variation is something we focus on in BioSci. Qualitative and Quantitative Methods One approach to understanding variation is through qualitative methods. Qualitative methods are rich, written descriptions of field observations. These written observations provide insight into scales of Page 1 of 7

2 variation that may or may not be easily quantified. Observations of variation often spark the development of research questions or approaches for field scientists. These methods can be descriptions of study locations, weather conditions, field notes about the study site and procedures, and sketches or pictures of specimens. Darwin and other naturalists, like Thoreau, relied heavily upon their field notes coupled with their quantitative values to help make sense of the variation they observed. Quantitative data can also provide information about study locations, weather conditions, and specimen descriptions, but offer numerical data which can be statistically analyzed. Statistics provide a helpful way of making sense of our data with some certainty that what we are seeing is not due to chance alone, but might actually be meaningful. Coupling qualitative descriptions with quantitative methods of data collection and statistical analyses is often used in scientific study. What you will do Today, we will use microscopic specimens to investigate variation in body size, and use simple statistics to make inferences about variation at a small scale. Along the way, you will make decisions about how to prepare your specimen for viewing under a microscope (i.e., the mount technique), and which microscope to use to properly see your specimen. You will record your qualitative and quantitative observations in your guidebook and you will create a simple data table to organize the information you will be collecting. Laboratory Objectives As a result of participating in this laboratory activity, you will: 1. Develop and practice observational skills across spatial scales 2. Proficiently use the tools of science to observe variation by: a. Selecting appropriate microscopes and slide techniques b. Focusing the microscope at all magnifications c. Measuring and calculating size using the ocular micrometer 3. Use both qualitative and quantitative methods to document variation a. Create tables to effectively record and communicate data b. Organize and link qualitative data with quantitative data 4. Use statistics to analyze variation 5. Pose questions about variation 6. Evaluate appropriate methods of data collection and analysis Methods Part 1: Microscopic Variation A. Viewing Specimens and Collecting Data Choose any 10 specimens you are interested in viewing, and complete the following for each specimen: 1. What type of slide preparation would best allow you to view this specimen? Why did you choose this technique? 2. Which microscope will you use to examine your specimen? Why did you choose this particular scope? Will measuring the size be possible with this chosen scope? Page 2 of 7

3 3. Complete the slide preparation technique for your specimen. A detailed description of making a wet mount is described at the end of this lab (Appendix A). 4. View your slide and make careful observations in your lab guidebook. a) Draw/sketch your specimen (noting movement, color and texture) 5. Measure the size of your organism*, if possible, and record data in a table. A detailed guide to using the ocular micrometer is found in Appendix B. 6. Organize your data, and submit relevant parts to the class data set. *Make sure that you have the size of at least 7 specimens. You may have replication within a genus/species B. Data Analysis and Interpretation To make sense of our data, we will use statistics (see Appendix C) that help us determine the mean size of our specimens, and also the variation within the sizes of our organisms: 1. Calculate the mean of your data 2. Find the mean, median and the range of your data; are all three the same? Why might this be the case? 3. Calculate the standard deviation of your observations 4. Calculate the standard error of your observations 5. What do these numbers tell you about the variability in the specimens you sampled today? 6. Using the class data, determine the mean, standard deviation and standard error for each specimen sampled. 7. What do these results tell you about the variability within a single species? Part 2: Macroscopic Variation A. Observations and Measuring Variation Spend a few minutes outside (maybe at the Beal Botanical Garden): 1. Observe the ecological variation you see a. Make sketches b. Write descriptions (include color/texture/other descriptive factors) 2. Pose a question you would like to investigate about variation. This question may or may not be prompted by the observational field notes you completed for number 1 above. 3. How would you quantitatively measure the variation you are interested in? Create a simple method for how you would measure this variation. Page 3 of 7

4 After completing this observation, reflect on the similarities and differences between qualitative and quantitative ways of capturing variation and synthesize what you have learned today through a table/model/reflective statement in your guidebook. Appendix A. Slide preparation techniques There are two different techniques you will use to view microorganisms. The first is the wet mount technique (this technique was demonstrated in the microscope video). This technique allows you to see the organism s true color and shape, because the organism remains alive, floating and moving in the liquid. Organisms remain alive in a wet mount, so you can also see the organism move. This technique is very easy to perform and does not require any special reagents or chemicals. There are a few disadvantages to viewing an organism by the wet mount technique, though. One problem is that the organism is moving, making it difficult to locate and observe fine details. Because many microbes are translucent and blend into the surrounding medium, it can be difficult to see the organism unless you have very good microscopy skills. A second approach involves staining techniques, which were developed to overcome the limitations of the wet mount technique. There are many different staining techniques, and the slides we will be observing today are pre stained for you. Simple staining techniques use one dye and stain everything the color of the dye. This type of staining is useful since it is quick, requires few reagents and makes organisms visible. Even though staining can be used to see the organism and visualize differences between organisms (by using differential stains), it has disadvantages. One is that organisms are killed by the staining process, which means that motility cannot be observed. A second disadvantage is that staining masks an organism s true colors and their true shape. Staining also requires more time, more reagents and supplies than the wet mount technique. Take a moment and reflect: You are interested in locomotion of a Paramecium. What technique would you use to mount a slide of Paramecium and why? Page 4 of 7

5 Appendix B. Using the ocular micrometer In the right ocular eyepiece of the compound microscope, there is an ocular micrometer, which appears like a ruler in the eyepiece. This scale can be used to measure the size of anything in the microscope s field of view. To calculate the size of an organism: a. Move the specimen of interest under the micrometer scale b. Measure the length (size) of the specimen by counting the number of small increments (0 1 = 10 small increments) c. Multiply the number of small increments by a correction factor for the objective lens you observed your specimen under to obtain a size in microns: CORRECTION FACTORS: 4x = multiply by 25 40x = multiply by x = multiply by x = multiply by 1 Page 5 of 7

6 Appendix C. Statistical Formulas Today, we will be using simple statistics to measure central tendency (i.e., what is the center point of our data), and variation (i.e., how much variability is present in our data). Measures of Central Tendency Mean:, where x is the individual responses in the sample, and n is the sample size. Median: When all the samples are ordered from least to greatest, the median is the number in the center spot. For example, if you have five numbers in your data set, the third number is your median. If you have an even number (e.g., 10 numbers in your dataset), then the median value is the average of the two center numbers (e.g., numbers in the 5 th and 6 th spots). Range: This is the range, from highest to lowest, of the numbers in your dataset. Ask, what is my smallest number? What is my largest number? Measures of Dispersion Standard Deviation: This measure gives us an understanding of how spread out the data points are from the mean value. If the data are all relatively close to the mean, the standard deviation is small; however, if the data are very spread out (i.e., may have a large range of values), the standard deviation will be bigger., where x i is the individual item in the dataset, is the mean, and n is the sample size. While the equation looks complicated, the steps are easy: (1) compute the mean, (2) subtract each item in your data set from the mean, (3) square each individual deviation Page 6 of 7

7 (result of step 2), and (4) add up the squared deviations. Then, divide that sum by (n 1), and take the square root of that result. Standard Error: This measure takes the spread of the data points, and makes them relative to the sample size, enabling comparisons across different sample sizes (n). where σ = standard deviation, and n is the sample size., Page 7 of 7