Anth 5_ Problem Set 4

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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?

If an animal was very well adapted to its environment, and there is minimal daily change (light, temperature) occurring in the environment, this could possibly cause the animal to sleep less. This would be a scenario in which it’s almost perfectly adapted to its minimally changing environment, so there’s very little reason for it to sleep. An organism could also have a protective adaptation such as camouflage, meaning that there is a much smaller selection of variables that would put it in harm's way, the result being that they sleep less. It could also be that predators high up, or at the top of the chain of commandment would possibly sleep very little because predatory concerns aren’t applicable to them. My secondary hypothesis is that the better adapted and topmost predators need to devote less time to sleep. C. Given your answer to B, show that you can employ both convergence and divergence to formulate comparative tests of this hypothesis about sleep’s original function? Looking first at convergence as a means of formulating a comparative test, we could compare very two very distant species that are both well adapted to daylight and see if they both have a preference towards diurnal sleeping. Eagles and humans which both have good daytime vision, and see if these two species–while vastly different–both sleep during the night. As for a comparative test using divergence, we could compare a mouse species that has recently dispersed amongst two different environments. Let’s say that the mouse species originated in a desert and some individuals have recently moved into a neighboring tropical forest biome. We could look to see if the mice in the forest sleep more (they are more poorly adapted to their environment) and to see if the desert mice sleep less (they are better adapted to their environment). These two experiments would help us see how species either converge or diverge on their sleeping habits based on the adaptations. 3. Altruistic traits are surprising from an evolutionary perspective. A. Why are they surprising? Why would natural selection, as understood by Darwin and as explained through Chapter 8 of your textbook, not be expected to produce altruistic traits? Altruistic traits are surprising because they are opposite of the exact thing that natural selection fights to preserve. They decrease the reproductive success of the individual or species that processes the trait. Natural selection is focused on sorting through traits and passing on those that enhance the reproductive success of a species and according to the Darwinian theory of evolution altruistic traits wouldn’t be passed on because they reduce the reproductive success of the possessor (although altruistic traits provide a reproductive benefit to its neighbors, that is not the way in which we identify selection to work). B. What is the key “neoDarwinian” insight of kin selection theory, and how does that insight offer one route for the evolution of altruistic traits? The key “neoDarwinian” insight of kin selection theory is the idea that alleles use one individual as a sacrifice in order to give individuals that also possess that same allele benefits. This will only happen if in fact sacrificing the one individual will result in an overall net profit for the

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allele or when rb > c. In kin selection individuals are agents of allele benefit; through fitness swaps alleles for altruism work by looking for other individuals carrying the same gene and aiming the altruistic effects at them. C. Are there any other ways that altruistic traits might evolve? Explain, at least briefly. There is another theory of altruistic trait evolution called reciprocity. Reciprocity disregards the variable r (relatedness) and hones in on the variables c and b. In this scenario b > c and each participant must exchange roles as both the altruist and the recipients, and if there is no exchange of roles then individuals must withhold the benefits from a non-reciprocator. Reciprocity also requires mental adaptations that aren’t required in kin selection (e.g. recognizing and remembering past actions).

Anth 5_ Problem Set 4

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