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Unformatted text preview: Stickleback Evolution Three-spine sticklebacks are small fish that live in oceans, streams, and lakes across the northern hemisphere. Sticklebacks that live in freshwater lakes often look quite different from their oceandwelling cousins. One difference is the amount of protective armor that covers their bodies: while oceanic sticklebacks have about 30 armor plates extending from head to tail, most freshwater sticklebacks have just a handful of plates that sit closer to the front of the body. A fully armored stickleback from the ocean near Loberg Lake (top), and a low-armored stickleback taken from Loberg Lake in 1994 (bottom). The fish have been stained with a dye called Alizarin Red S, which stains bones, in order to highlight their differences. Photos courtesy Michael A. Bell, Professor of Ecology and Evolution, Stony Brook University. Disclaimer: This animation shows a highly simplified view of actual events that occurred at Loberg Lake. Michael Bell and members of his laboratory at Stony Brook University have tracked and documented these events in much more detail. You can learn more by visiting the Bell Lab website or by reading the publications listed in the References section at the bottom of the page. Less Armor is an Advantage in Freshwater Environments Low-armored forms of sticklebacks evolve in freshwater environments again and again. Given how quickly these shifts occur, the freshwater environment is most likely selecting for lowarmored gene variants that are already present at a low frequency in ocean populations. When a group of fish moves from the ocean to fresh water, the low-armored variants survive and reproduce at a higher rate than the fully armored individuals. Here's why: Predation by saltwater fish favors more armor. Predation by insect larvae that live in fresh water favors faster-moving fish with less armor. Low-armor forms grow faster, making them (1) too big for predators, (2) reach sexual maturity more quickly, and (3) able to store more energy reserves, which increases their chance of overwinter survival. Studying Sticklebacks to Understand Evolution MISCONCEPTION Evolution may have happened in the past, but it's no longer happening today. REALITY Scientists observe evolution in progress today. Sticklebacks inhabit hundreds if not thousands of lakes and streams throughout the northern hemisphere. In addition to changes in body armor, these isolated stickleback populations have evolved a variety of changes that set them apart from their ocean-dwelling ancestors. In order to figure out what genes are driving these differences, researchers crossbreed marine and freshwater sticklebacks. By studying the resulting hybrid offspring, geneticists can see what specific changes to which genes are causing the differences. In sticklebacks, as in other organisms, small changes to single genes can have major effects. The Ectodysplasin gene appears to be responsible for changes in body armor in many freshwater stickleback populations. Recessive low-armored gene variations are found in about 1% of marine sticklebacks. Evidence suggests that it is this variant that is repeatedly being selected for in freshwater environments. No 5 Summary After the end of the last ice age 10,000 years ago, populations of marine stickleback fish became stranded in freshwater lakes dotted throughout the Northern Hemisphere in places like Alaska and British Columbia. These fish have adapted to a freshwater environment drastically different than the ocean. tickleback bodies have undergone a dramatic transformation, some populations completely losing long projecting body spines that defend them from large predators. Various scientists, including David Kingsley and Michael Bell, have studied living populations of threespine sticklebacks, identified key genes and genetic switches in the evolution of body transformation, and even documented the evolutionary change over thousands of years by studying a remarkable fossil record from the site of an ancient lake ten million years ago. Watch this film to learn about a species where we can study evolution in action, identify key genes, and peer deep into the evolutionary past. Tutorial 1: How to Score a Pelvis in Living Fish In this tutorial, you will learn how to "score" stickleback pelvic structures—the parts colored red in the drawing below. You will need to know how to correctly identify different pelvic structures to collect and analyze data in Experiment 1. Depending on how many fish you score correctly, you will be prompted to proceed to Experiment 1 after scoring either 10 or 20 specimens; however, you can exit the tutorial and move on to Experiment 1 at any time by clicking on the appropriate link in the lab window. In this Tutorial and in Experiment 1, you will categorize pelvic structures as Complete, Reduced, or Absent. Complete: a pelvis with a full pelvic girdle and two pelvic spines. (Note that only the fish with a complete pelvis have pelvic spines.) Reduced: a range of structures from a simplified girdle with no pelvic spines to an oval nub for a girdle. Absent: no pelvic girdle and no spines. Tutorial 2: How to Score a Pelvis in Fossil Fish We learned about the stickleback pelvic structures in the previous tutorial. These structures are also visible in fossil stickleback. Click here to learn more about how this fossil formed. As in Experiment 1, we will use the following scoring matrix: Complete, Reduced, and Absent. A score of Complete refers to a pelvis with a full pelvic girdle and two pelvic spines: Sometimes you cannot see both spines in a fossil fish specimen. That might be because the rock containing the original fossil was split in such a way that each slab ended up with one spine. As long as you see a spine in your fossil specimen, you should score it as complete! A score of Reduced refers to a range of structures from a simplified girdle with no pelvic spines to an oval nub for a girdle. Here is one example: Click here to see more examples of reduced pelvises. A score of Absent means there is no pelvic girdle and no spines at all: There is no pelvic structure visible in the fish above (circle), although the rest of the fish skeleton seems intact; we would therefore score this pelvis as "absent." Be sure not to confuse the ectocoracoid (arrows) with the pelvis! But, does a score of absent really mean absent? Click here to find out. Experiment 1: Analyze Stickleback Fish from Lakes Background | Part 1 | Part 2 | Part 3 | Quiz | Analysis Objective: In this experiment, you will analyze the pelvic structures of stickleback fish collected from two lakes around Cook Inlet, Alaska, to determine whether there are significant differences between the two populations. You will then use your data and information about the lakes to draw conclusions about the possible environmental factors affecting the evolution of pelvis morphology. View a map of these lakes. Watch a video about how postglacial lakes formed. Individuals that are better adapted to a particular environment live longer and reproduce more. As a result, their genetic variants are passed from one generation to the next and the associated traits increase in frequency in the population. In the ocean, a threespine stickleback with a complete pelvis is less likely to be eaten by larger fish that prey on it. Thus, the presence of predatory fish in the ocean environment is most likely selecting for the complete pelvis trait in sticklebacks. Almost all marine and sea-run threespine stickleback have a full pelvic girdle and pelvic spines. The freshwater threespine stickleback populations in lakes in Alaska have evolved recently from sea-run stickleback populations. These freshwater lakes provide a wide range of habitats for the fish, which are different from their ancestral ocean environment. Watch a video of the scenery around Cook Inlet. What features in these environments might be selecting for a complete or reduced pelvis? 7 analysis A more readily observable measure of selection strength is the selection response, R, which quantifies the amount of evolutionary (i.e., genetic) change in a phenotypic trait value between generations and is therefore defined as the change in a population's mean phenotypic value during one generation.Dec 1, 2012 ...
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