Biology Lab 7: Macroevolution PDF

BIOLOGY 1101 LAB 7: MACROEVOLUTION READING: Please read chapter 14 in your text prior to the lab. LABORATORY OBJECTIVES: The purpose of this lab is to introduce you to some basic methodology for studying macroevolution. In this lab, and from your readings, you will le...

BIOLOGY 1101 LAB 7: MACROEVOLUTION READING: Please read chapter 14 in your text prior to the lab. LABORATORY OBJECTIVES: The purpose of this lab is to introduce you to some basic methodology for studying macroevolution. In this lab, and from your readings, you will learn to: recognize macroevolutionary patterns of punctuated equilibrium and gradualism determine the relative age of fossils by their location in rock layers build phylogenetic trees based on shared derived characteristics use phylogenetic trees to formulate and evaluate hypotheses about evolutionary relationships investigate how changing habitats drive large-scale patterns of evolutionary change INTRODUCTION: Macroevolution is evolution on a grand scale, and it is what we see when we look at the history of life over large timescales. Macroevolutionary biology examines major events, like the origin and extinction of taxonomic groups (above the species level). Scientists reconstruct the past using evidence from sources such as the fossil record, biogeography, and living organisms. Once we determine what speciation and extinction events have occurred, we can test hypotheses about the processes involved. The same mechanisms that drive microevolution – mutation, gene flow, genetic drift, and natural selection – are also responsible for macroevolution. Life on earth has been accumulating genetic mutations for the past 3.8 billion years, which is more than enough time to see large-scale evolutionary change. Charles Darwin viewed evolution as a gradual, stepwise process, as populations of species accumulated mutational changes in small increments over a long time (Fig 1a). It’s difficult to find evidence of gradualism in the fossil record, but Darwin attributed the lack of evidence to gaps in the fossil record – a good assumption considering it is unlikely each small evolutionary change would be preserved. In the 1970’s, evolutionary scientists suggested the gaps in the fossil record were real and represented periods of morphological stasis. They hypothesized that species undergo long periods – millions of years – with little morphological change, followed by a relatively “quick” burst of evolutionary change that leads to speciation. They called this mode of macroevolution punctuated equilibrium. Species evolving in a pattern of punctuated equilibrium won’t experience slow, gradual changes in mainstream populations. Instead, isolated populations that exist on the periphery may experience rapid evolutionary change that eventually leads to speciation (Fig 1b). It’s important to note that these hypotheses are not mutually exclusive, which means one does not preclude the other from occurring. 67 Figure 1. Phylogenetic trees showing (a) the slow accumulation of changes that lead to speciation (gradualism) and (b) periods of stasis followed by bursts of change that lead to speciation (punctuated equilibrium). Adaptive radiation refers to periods of elevated speciation rates that are hypothesized to coincide with the evolution of an adaptation to a new environment or new way of life. This is, in some ways, the opposite of mass extinctions, in which many species die off because of a rapid change in the environment. Adaptive radiation allows multiple speciation events of one lineage or organisms, while mass extinctions affect many groups of organisms simultaneously. A cladogram is a branching diagram that illustrates relationships among groups of organisms. The cladogram to the right shows a period of adaptive radiation (circled), with many forks (or branch points) representing speciation events over a short period of time. The lines are drawn to show a mix of gradualism and punctuated equilibrium. Can you tell which parts of the tree reflect gradual change and which parts reflect punctuated change? Adaptive radiation occurs because populations respond differently to environmental conditions. When the environment changes, populations with traits favorable to the new conditions will have a higher survival and reproduction rate than those that are poorly adapted to the new environment. Evolutionary biologists use cladograms showing phylogenies to reconstruct the history of speciation and extinction. A phylogeny is a hypothesis about the evolutionary history of organisms. A phylogenetic tree is a cladogram that is scaled relative to time. Much of the evidence for macroevolution comes from fossils. The number and placement of fossils within certain types and layers of rock is known as the fossil record. This record gives us an indication of what types of organisms existed at different times in the past, many millions of years ago. By studying fossils, we can piece together the story of how life has evolved over geological time. 68  AU21 EXERCISES: In Part 1 of this lab, you will use a cladistic approach to track changes in the fossil record of related species of gastropods. Then in Part 2, you will continue to gain experience in analyzing traits to construct phylogenetic trees, as you work through a series of evolutionary puzzles online. Part 1. Imagine that an outcropping of rock has been recently exposed. Geologists find shells embedded in different layers of rock. The layer closest to the surface is the youngest in geologic time, and layers below it are successively older. Assume these shell types have all evolved from a common ancestor (not present in the sample). Scientists notice some interesting traits in the shells: older shells have smooth texture and little to no colored banding pattern. Older shells are also thinner with larger apertures (openings). You will examine a group of “fossil” shells and collect data on traits of interest. For the purpose of this exercise, you will assume these are all adaptive traits. You will use the data to place the shell types in order from oldest to youngest. You will draw a phylogenetic tree that shows divergence between each group with each new derived trait. Finally, you will formulate a hypothesis about why each trait may have evolved. Evolutionary traits of interest include: Shell texture – rough texture evolved from smoother texture Banding pattern – banding (banded coloration) evolved from lack of banding Shell thickness – thick shells evolved from thinner shells Relative aperture size – small openings evolved from larger openings Materials: Desktop fossil set – 5 shell groups with 5 shells in each group o Please be careful when handling the shells, as they are difficult to replace. Shell morphology layout sheet (page 78 of this manual) Caliper Procedure: 1. Carefully remove the shells from the container and place them on the lab table. There are five shell groups that represent five different geologic times. The first step is to decide which specimens belong to the same group. Working with your group, carefully examine the 25 specimens and arrange them into five groups such that variation among individuals within each group is minimal compared to variation among groups. Assign each group a unique name or number. 69  AU21 2. Working with your group, choose two of the four characteristics to examine: a. Choose shell texture or banding. b. Choose shell thickness or aperture size. c. Record the trait values for each individual shell in Table 1 on page 71 as described below. After all data have been recorded, calculate the mean (average) value for each trait. 3. Using what you know about the evolution of shell traits (given above), place each group on the shell morphology sheet in order of oldest (bottom row) to youngest (top row). 4. Construct a cladogram with the group names (or numbers) and place the adaptive trait in the appropriate spot. See Figure 3 for an example. *Note: While we have tried to provide shells that are similar in size within each geologic layer, be careful not to let shell size influence your judgment, as we are not taking absolute size into account in this lab. For texture and banding, grade the shells using the following relative scales: Texture Scale Texture Very Smooth Somewhat Smooth Somewhat Rough Very Rough Score 1 2 3 4 Banding Scale Banding No Banding Light Medium Heavy Score 1 2 3 4 Measure shell thickness: Insert the outer lip of the shell into the caliper and record the thickness in millimeters in the data table. Measure aperture size: Measure the shell aperture by measuring from the inner to the outer lip, and from the posterior canal to the anterior canal (see figure 2). For shells with a long anterior canal, measure from the posterior canal to the start of the anterior canal. Divide the first value by the second value and record the ratio in the table on the next page. We are using this ratio as a relative measure of aperture size to avoid comparing the absolute size of the shells. Figure 2. Shells with apex different morphologies, including aperture shape. posterior canal Crossed lines are a guide to measuring the aperture. aperture inner lip outer lip anterior canal 70  AU21 Table 1. Data table for shell characteristics. Characteristic #1 Characteristic #2 (texture OR (thickness OR Group Shell no. pattern) aperture size) 1 1 2 3 4 5 Mean 1 2 2 3 4 5 Mean 1 3 2 3 4 5 Mean 1 4 2 3 4 5 Mean 1 5 2 3 4 5 Mean 71  AU21 Group name Group name Group name Group name Group name Group name Trait names should be placed next to Figure 3. Example of a each dash, indicating phylogenetic tree that shows when/where that trait the divergence of each shell first appeared type with the adaptive trait that distinguishes it from the others. Questions 1. Assuming these traits are products of natural selection, what selective pressures do you think these gastropods experienced? (Hint: what sort of environment would induce these evolutionary changes?) Write a separate hypothesis for what may have caused the appearance of each trait. Texture: 72  AU21 Banding: Shell thickness: Aperture size: 2. Although you have seen a general evolutionary trend in the shell traits as you move from older to younger fossils, there was likely some variation among the individuals of each group. What explanation can you provide for variation among individuals within a group? 73  AU21 Part 2. Through this series of online activities, you will gain more experience in analyzing traits of organisms and using those traits to build phylogenetic trees. Building phylogenetic trees is a bit like solving puzzles. For these puzzles, you will be grouping organisms together based on shared traits. You will find it helpful to have a pencil and paper handy, because as you work through each puzzle, you will need to make a list of the characteristics of each species found in the phylogenetic tree. This will help you identify characteristics that two or more species have in common – traits they share because they inherited them from a common ancestor. Procedure: Go to the NOVA Labs Evolution Lab web site at: https://www.pbs.org/wgbh/nova/labs/lab/evolution/ First, play the video intro (your instructor may play it for the entire class). Take note of the following: Who discovered the theory of evolution by natural selection? What is natural selection? What are two main factors necessary for natural selection to occur? What is a phylogenetic tree? 74  AU21 Next, click on “Play Game” and build some phylogenetic trees! Answer the following questions as you work. Some of them come from the brief films before each mission, some come from examining your completed phylogenetic trees, and others come from the comprehension questions asked at the end of each mission. 1. “Mission 1: Training Trees.” Begin by watching the video, and then complete the activities. a. Is an animal or a plant more closely related to fungus? b. Is a banana more closely related to an onion or a lemon? 2. “Mission 2: Fossils, Rocking the Earth.” Begin by watching the video, and then complete the activities. a. What skeletal characteristic do birds (like chickens and ostriches) have in common with dinosaurs? b. According to the tree you built in “One Small Step,” what was the first trait that helped aquatic species evolve into creatures that lived on land? c. In the “Origin of Whales” activity, you learned that which species lacked a tail fluke? 3. “Mission 3: DNA Spells Evolution.” Begin by watching the video, and then complete the activities. a. The DNA sequence of the West Indian Ocean coelacanth is closest to which species? b. In 2013, scientists found that coelacanths are not the closest relatives of four-footed amphibians and other animals. Which species is? c. The “Where the tiny wild things are” activity explores relationships among certain Archaea and Bacteria. Why is examining DNA often better than examining physical traits when creating phylogenetic trees? 75  AU21 4. “Mission 4: Where Life Lives.” Begin by watching the video, and then complete the activities. a. A few hundred million years ago, all land was part of a supercontinent named. Starting about 170 million years ago, the continents drifted apart, and populations of species along the edges of those continents were separated. Over time, they evolved into separate species. b. In the activity “Saving Hawaiian Treasure” you explored the relationships among birds called honeycreepers. If a new species of honeycreeper were discovered, and it had a short, straight beak, which bird in the puzzle would likely be its closest relative? c. In the “Cone Rangers” activity, you examined relationships among several species of trees that reproduce with cones. In your completed phylogenetic tree, you should find that the monkey puzzle tree is more closely related to the paraná pine tree than the other trees in the puzzle, because they both share which trait?. d. Thanks to DNA testing, scientists have discovered that a tree in South America is genetically similar to one in Australia. What is one possible evolutionary inference they could make from this discovery? e. In the “Kangas, gliders, and snakes, oh my!” activity, your completed phylogenetic tree should show that sugar gliders are more closely related to kangaroos than the other animals in the puzzle, because they both share what trait? f. Despite living oceans apart, the North American kangaroo rat and the Australian hopping mouse look similar. Both are nocturnal and burrow underground. What can you infer? 76  AU21 Comprehension Questions Make sure you understand and can answer the following questions, as the concepts may appear on a lab practical exam. 1. What is the difference between microevolution and macroevolution? 2. What are the mechanisms that drive evolution (common to both micro- and macroevolution)? 3. How does gradualism differ from punctuated equilibrium? How are the two hypotheses similar? 4. The shells you examined came from gastropod species that evolved certain characteristics in response to environmental pressures, driven by the non- random process called. 5. A branching diagram that illustrates relationships among organisms is called a. A branching diagram that adds a time component to the relationships is called a. 77  AU21 SHELL MORPHOLOGY LAYOUT SHEET Youngest 1 2 TIME 3 4 Oldest 5 Place the five shells from each group in the rows. Each row represents a geologic layer, from oldest on the bottom to youngest at the top. Trace the evolution of each trait from layer 5 (oldest fossils) to layer 1 (youngest fossils). 78  AU21 This page intentionally left blank. 79