How the Guppy Got Its Spots PDF

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An examination of the history and mechanisms of evolutionary change. Topics include molecular evolution, natural and sexual selection, adaptation, speciation, biogeography, and macroevolution....

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Krishna Patel Evolution Bioe 109 Assignment #4 Living in a Crossfire Between Your Enemies And Your Potential Lover How the Guppy Got Its Spots

Part A The goal of this lab was to design an experiment that documents evolution by natural selection in the wild by simulating work done by Endler in 1980 on a natural population of guppies. The first exercise begins with a simulation of four wild populations of guppies that are geographically isolated. Variations in spot brightness or number of spots were observed in male guppies, while the females had neutral colors. Three of the four pools consisted of different predatory fish, either Rivulus, cichlids, or both. This first exercise permitted us to explore and hypothesize the causes of color variation in male guppies: Hypothesis 1: Predators are causing guppy populations to become more drab by preying on the most brightly colored individuals and eliminating them from the gene pool. Hypothesis 2: Guppy populations are evolving to more closely match (or stand out from) their environment. Hypothesis 3: Female guppies are choosing to mate with the most brightly colored males, giving those males a higher probability of passing their genes on to the next generation. For the first experiment, we are testing the spot brightness and how it will be effected by the presence of predators. Two tanks will be used, each with 20 male guppies. The first tank will have the 20 male guppies alone and no predators and will thus serve as the control group. The second tank will have the 20 guppies along with one of the Cichlid A predators. Data will be collected in intervals of 20 for a total of 100 days, and will be displayed in a histogram. This information will represent the changes in number of spots or spot brightness in males guppies. If the hypothesis is true, then by the

end of the experiment the guppies should have a higher spot brightness than it did at the start of the experiment. If our hypotheses are not supported, then the two histograms will be the same or the histogram that represents the data taken at the end of the experiment will have a lower spot brightness in males than it had at the start of the experiment. We will use the same environment for both tanks to make sure the habitat will not play a role in this experiment (tank 2 will be used for both). In addition, another experiment in which the two tanks have varying environments will be performed in order to determine the effects of different habitats on the number, size, and brightness of the spots in males. After maintaining our simulation pool, we found that the guppy numbers increased in absence of predators and decreased in the presence of Cichlid A predators, supporting the first hypothesis. In addition, on coarse gravel, the guppies with the larger spots are more successful in predator avoidance because their spots more closely resemble their background. The guppies will appear very similar to the background gravel when they are in front of the gravel if the guppies’ spots are smaller than the background. For example, on coarse gravel, guppies with larger spots are at an advantage to guppies with smaller spots, because the larger spots closely resemble the background gravel much more closely than do the smaller ones

Part B We have seen in our simulations that the more brightly colored a male guppy is, the more likely he will be seen and, thus eaten by a predator. In a simulation or in the wild, where predators are plentiful, male guppies become increasingly dull over generations, pushed by predation pressure toward greater camouflage. So if camouflage confers such an obvious survival benefit to prey species like guppies when it comes to predator avoidance, what possible advantage could there be to sporting colors and patterns that make an individual more conspicuous? The answer lies in the fact that guppies have to do more than just survive. They also have to reproduce– and to do that, they have to attract

mates. The brighter colors a male guppy has, the more likely a female guppy will choose him as a mate, giving him the opportunity to pass his genes along to the next generation. This is sexual selection at work, and it is the force that pushes guppy coloration toward conspicuousness just as hard as predation pushes coloration toward drabness. The results from the experimental simulations support my hypothesis that female choice affects the spot brightness of male guppies. The male guppies spot brightness range at the beginning of the procedure varied from 2 to14, and after the 500-day trial, the proportions shifted greatly to a range of 12 to 20. The female guppies choose to mate with either brightly colored males, or less colorful males, and this positive shift suggests that showier male guppies are selected as mates by females, and therefore become more frequent within the population over generations as their genes are passed on. To further test this hypothesis one may choose to alter the sex ratios of the males and females, as throughout these experiments, male guppies were always in abundance over the female guppies. Adding predatory fish to the population at different stages of the trial may also aid in future research. There may be several evolutionary reasons why female guppies prefer colorful males. On the most basic level, the male with the biggest, brightest spots will stand out the most and catch females attention quicker. Bright colors may also indicate good genes, in the way the strong physique of a human athlete is a direct indicator of that individual's health and vitality. Whatever the reasons, it is clear from this SimBio lab that male guppies live in the crossfire between their enemies and their potential mates, with the opposing forces of predation and sexual selection forever pushing the guppy coloration in opposite directions....