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Dec 6, 2023

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Brooke Uehlinger Anthropology 5 Problem Set # 1 Fall 2023 1. To answer these questions you will first need to watch the short video called Evolution of coat color in pocket mice (in Week 2, on the course website). A. As a result of geologic events, what was once a single population of pocket mice now find themselves living in two adjacent but different habitats. Mice in both habitats are often eaten by visually hunting predators. In the face of that predation, what role does coat color (fur color) play in a mouse’s survival? Explain. Mouce’s coat color plays a role in its survival because it helps them camouflage with the background color in their habitat. For example, mice that live in lava rock habitats evolved to have dark fur so predators from above cannot spot them as easily as they would with light fur. B. Can you interpret your answer to 1A as an example of natural selection? Explain. Over time, natural selection may begin to favor mice with a specific coat color. The favored mice will likely be the ones that show higher survival rates in their environment. This natural selection could lead to genetic variations of different fur colors to have a higher chance of avoiding predators. C. Do genes affect coat color in these mice, and does that matter to your answer to 1B? Explain. Fur color is a trait controlled by several genes. Light and dark mice have almost identical genes other than four chemical letters (Mc1r). This does matter to my previous answer because a mouse with these mutations grows dark fur, causing their chances of survival to increase on a dark background. D. Traits change over evolutionary time and thus it is useful to be able to talk about what came first and what (changes) came later. For any given case, the ancestral condition (also called the primitive condition) is what came first. What do you think the ancestral (primitive) coat color was in these mice, and what change came later? What makes you think that was the order? I think the primitive coat color was light for several reasons. I assume their original habitat was sandy and light, which would relate to why their fur started off as light. As geological events happened, like lava flow, the mice had to adapt from light to dark fur to increase their chances of survival on that new background. E. This change (from one coat color to another) happened “convergently” across the desert southwest. “Convergently” means that the same change happened multiple times, in different populations. What is the evidence that it happened multiple times? There are actually two separate kinds of evidence; please mention both.

Once beneficial mutations appear, natural selection can under similar conditions favor almost identical adaptations. Also, the rock pocket mouse is a real life example that evolution does repeat itself and why it’s changes are never ending. Although the mice look almost identical, they were found to have different genes F. All of the mice, regardless of which habitat they live in, have white bellies. Why did they evolve white bellies? A full answer would mention two ideas. The mice evolved white bellies because predators are coming from above meaning the belly has no reason to evolve to be dark. Their dark fur everywhere but on their bellies provides camouflage for them. Also, Mutations causing dark bellies do not occur. 2. Selection changes traits until they reach the optimum (ideal) value. For example, if trait X is too small (given the prevailing environment), selection disfavors any mutations that decrease the size of trait X and favors any mutations that increase the size of trait X, until trait X reaches the optimum. I’m going to suggest that virtually every trait in every species has an optimum and that selection is always pushing towards that optimum; or stabilizing that optimum, once it is reached. A. We said how selection will work when trait X is below the optimum (too small). How will selection work if trait X is above the optimum (too large)? Selection will work if trait X is above the optimum because they will have a higher chance of fitness compared to other individuals. Just like individuals with traits below the optimum, ones above will be more likely to have higher chances of survival and reproduction. Overtime, selection can begin to be disrupted because there will be abnormal amounts of trait distribution of trait values in the population. B. And how will selection work once trait X reaches the optimum? Once trait X reaches the optimum, stabilizing selection will maintain that trait by favoring individuals with traits near the optimum and not with extreme deviations. This helps to keep the traits adaptive fit and not make some individuals over powered. 3. Let’s consider a human trait that could be too big or too small, birth weight (the amount a woman grows her baby before she gives birth to it). Assume that birth weight is influenced by genes (it is, e.g., by genes that affect the flow of nutrients to the fetus during gestation). For questions 3A-C try to answer in terms of conditions during the long phase of human evolution before modern medicine (99.95% of the time our species has existed!). A. What might have been harmful to fitness about being too small at birth? If a human is too small at birth, this can be harmful to fitness because infants born with a low weight have a higher chance of mortality after being born. Also, these babies can

have more health issues and developmental challenges like weak immune systems and nutritional deficiencies. B. What might have been harmful to fitness about being too big at birth? If a human is too big at birth, multiple complications can arise. First, this can harm the mother by making it necessary to get a c-section because the baby is too large to pass through the birth canal. Additionally, being too large at birth can lead to health issues later in life like obesity. C. What would have been the joint effect of these two selection pressures? The joint effects of these two selection pressures would have led to stabilizing selections so that an infant's birth weight would maximize the chances of survival and its future reproductive success. Not only would the selection adapt to make the infant healthy, but to also maintain the mothers well being during birth. D. Now let’s think about the present. Two medical services have become much more widely available in the last 100 years: Cesarean section and Neonatal Intensive Care Units. What does each service do? Given that, how might Cesarean section and Neonatal Intensive Care Units be changing how selection is operating on human birth weight. Cesarean sections are a medical procedure where an infant is removed from the mother’s abdomen and uterus through an incision. The Neonatal Intensive Care Units provide medical care to newborn infants born prematurely or with health issues. These two medical services are changing the way selection is operating on human birth weight because c-sections have allowed the delivery of infants who would not have made it with a natural childbirth, and the NICU is able to provide intense care to babies who would not have survived on their own due to their vulnerability to infections and other health problems. 4. Let’s consider another example, this one from Chapter 2 in your Human Evolution textbook. In the second lizard study (pages 21-22 of your textbook) you learned that some lizards belonging to the species Anolis carolinensis have lately had to deal with new competitors belonging to a related species, Anolis sagrei . A. Populations of A. carolinensis that were invaded by A. sagrei (but not those that were not invaded) show certain phenotypic changes: What are those changes? Populations of A. carolinensis show changes of an increase in toe pad size and an increase in the number of lamellae on the toe pads. Both of these changes happened within 20 generations. B. What were the selection pressures arising out of competing with A. sagrei that caused these phenotypic changes in A. carolinensis ?

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The selection pressures that arose from the competition with the A. sagrei that caused phenotypic changes in A. carolinensis was resource competition and dietary shifts. These changes caused adaptations that would raise their chances of competitive ability and survival. These species often shared food sources, so the A. carolinensis needed to adapt to be able to consume different types of food in their shared environment. These dietary adaptations helped to reduce the amount of competition on the island. C. What is the difference between evolution and development? Development is “the differential expression of a single, fixed genotype over the lifetime of one individual” and evolution is “the change in the frequency of genes in a population across generations”. In simpler terms, development just refers to the process where an individual grows and matures, but evolution is a whole process where a population changes over generations, which eventually leads to new species and life. D. All traits have both an evolutionary basis and a developmental basis; but a particular change can either be the result of evolution or the result of development. Consider the critical experiment described on page 22 of your textbook. Does this experiment suggest that the change you described in your answer to 4A was a developmental change or an evolutionary change. How can you tell? This experiment suggests that the change I described in my answer to 4A was an evolutionary change because these changes came about through 20 generations, not just in one lifetime. Also, the increase of size and number of lamellae was a response to a competition for resources with the A. sagrei, meaning the A. carolinensis needed to evolve to enhance their fitness and survival.

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