Anth 5_ Problem Set 4

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Jan 9, 2024

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Anthropology 005 Problem Set # 4: Fall 2023 1. As we have seen, in species with short enough generation times, we can use actual experiments to test hypotheses about the functions of adaptations. But we evolutionists have additional methods for figuring out why certain species have, and other species lack, a particular trait. Those additional methods (comparative methods to be precise) involve using the actual outcomes of evolution as evidence. A. What is evolutionary convergence, and why is it useful to think of a case of convergence as an experiment performed by natural selection? Evolutionary convergence is when two different species that are exposed to similar selection pressures, develop similar adaptations. As discussed in the textbook, thinking about convergence as an experiment is a good means of testing the function of an adaptation. For example, looking at melanin, if we think that melanin blocks UVb radiation, where there’s less UVb radiation, there will be less melanin in the population. We can look for convergence of species (paler species) on either side of the equator, to help prove the function of melanin. B. If species just happen to resemble each other by chance, is that convergence? If two species were to resemble each other by mere chance it would not be considered convergence. Convergence has specific requirements, those being that species that have come to resemble each other through convergent evolution must have experienced similar selection pressures. If a species didn’t go through similar selection pressures and just happens to phenotypically resemble another species you couldn’t consider that convergence. C. What is evolutionary divergence, and why is it useful to think of a case of divergence as an experiment performed by natural selection? Evolutionary divergence is when members of a population are exposed to different selection pressures and thus develop different adaptations. Thinking about divergence as an experiment is useful in order to find out if different conditions lead to different adaptations; natural selection will select different useful mutations in different parts of the population that are experiencing different conditions. D. Would selection ever delete a trait after having built it? If so, under what conditions would that happen? Trait deletion would likely be possible if a species were to undergo significant change in selection pressure. For example, if a population of cave dwelling fish were to change environments and now instead reside in an area that has light, it would be probable that selection would delete the eyeless trait and instead begin to select traits that build eyes. It would no longer be a waste of energy for the population of fish living in a lit area to have eyes, as it formerly was in a lightless area. In order for a trait to be deleted by selection it would have to no longer be

useful, and even possibly be harmful. E. How are your answers to 1C and 1D related? Divergence and trait deletion go hand and hand. If a population were to be divided and experience different selection pressures–exactly what happens in evolutionary divergence–it’s more than possible that a once necessary trait could be selected out because it is no longer useful, or the trade off between benefit and energy usage is too high (or vice versa). In short, I believe that divergence is the exact process that could lead to a trait being deleted from a population after already being built upon. 2. I want to suggest an idea about the adaptive function of sleep that you probably have never heard before: Sleep did not originally evolve for rest and repair (though those functions may have been added later—remember that selection acts cumulatively, layering new adaptation on older ones.). Instead, I’ll hypothesize that sleep was originally designed by natural selection to keep an organism out of harm’s way during periods to which it is poorly adapted. Note that day and night can differ dramatically in lighting, temperature, humidity, abundance and type of predators and prey, etc., so an organism that was well adapted to one set of conditions would be poorly adapted to the other. (This same hypothesis could explain hibernation, estivation, etc., not just day/night cycles.) To summarize, according to this hypothesis, sleep helps organisms avoid temporal regions to which they are poorly adapted. A. Taking a reverse-engineering perspective, does this hypothesis make any predictions about the expected mechanisms involved in sleep? (To do this, try answering what would trigger sleep and waking; where would organisms sleep; what would sleep look like?) First, considering what would trigger sleep, it would be the opposite environmental conditions that a species is adapted to (e.g. the sun setting and onset of nighttime for humans because our eyesight is better used in daylight). As for the environment in which species would sleep, it would likely be an environment that provides additional protection to the at risk species/individuals (e.g. a species sleeping during the nighttime might try and find a dark or “off the beaten path” place to sleep in order best avoid harm from either predators or weather). Sleep would also be something that is easily awakened from, so if any predator or harmful environmental factors could be easily warded off. If animals weren’t able to easily awaken from sleep then they could be considered more at risk. Food availability and environmental familiarity could also be other factors that play into the duration or sleep (e.g. if there’s less food or less familiarity with the environment an individual would sleep for longer). There’s a variety of predictions that the hypothesis makes about the mechanisms involved in the sleep of an individual. B. Of course nocturnal animals sleep during the day, and diurnal animals sleep at night, and that make sense in terms of our hypothesis; but what are the circumstances or environments where animals would evolve to sleep very little?

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