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BIO 1100 CSU How Homology Is Different From Convergent Evolution Biology Questions


Question 1

Pine trees that are too tall or too short do not do as well as pine trees that are average in height. The short trees do not get as much light as tall or average trees. The tall trees are more likely to break off in storms. Tell how this is an example of stabilizing selection. Be sure to define stabilizing selection in your answer.

Your response must be at least 75 words in length.

Question 2

There is a moth in England called the peppered moth. Before Britain’s industrial revolution, these moths were usually salt and pepper colored. Because of their coloring, they blended in well with the tree trunks on which they tended to rest. The coloring helped them hide from the birds that ate them. During the British industrial revolution, industry expelled a lot of soot from the burning of coal into the environment. This soot darkened the tree trunks, and it was noted that black-colored moths were becoming predominant. The idea is that with soot in the environment, black-colored moths fared better than light-colored moths. There is some debate as to whether this is actually the case or not, but for the sake of this question, let’s assume it is.

the observations were in the text book i sent you under 12.2. I copied the text here but its not really reader friendly in this format. Can you please adjust question #2 appropiately? Thanks in advance

12.2 Natural Selection Causes Evolution In the Origin of Species, Charles Darwin put forth two major ideas: the theory of common descent (Chapter 11) and the theory of natural selection. Darwin’s presentation of the theory of common descent—that all species living today appear to have descended from a single ancestor—was thorough and convincing. Within 20 years of the publication of his book, the theory of common descent had been accepted by most scientists. However, it was another 60 years before the scientific community accepted Darwin’s theory of natural selection, which explains in large part how organisms evolved from a common ancestor to become the great variety we see today. Darwin proposed that through the process of natural selection, the physical or behavioral traits of organisms that lead to increased survival or reproduction become common within their population, while less favorable traits are lost. The changes accumulating within populations via natural selection can lead to the development of new species. Darwin reasoned that the process of natural selection is an inevitable consequence of the competition for survival among variable individuals in a population. Today, natural selection is considered one of the most important causes of evolution (although others, such as the processes of genetic drift and sexual selection as described in Chapter 13, also cause populations to change over time). Darwin’s Observations The theory of natural selection is elegantly simple. It is an inference based on four general observations: 1. Individuals within Populations Vary Observations of groups of humans support this statement—people do come in an enormous variety of shapes, sizes, colors, and facial features. It may be less obvious that there is variation in nonhuman populations as well. For example, in a litter of gray wolves born to a single female, individuals may vary in coat color, while in a field of flowers, one plant may bloom earlier than others (Figure 12.6). We can add all kinds of less obvious differences to this visible variation; for example, the amount of caffeine produced in the seeds of a coffee plant varies among individuals in a wild population. Each different type of individual in a population is called a variant. Visualize This Under what conditions might it be an advantage for an individual plant to bloom earlier than other nearby flowers? Figure 12.6 Observation 1: Individuals within populations vary. (a) Gray wolves vary in coat color, even within a single litter of animals. (b) Flowers may vary in blooming time, with some individual plants blooming much earlier than others of the same species. Figure 12.6 Full Alternative Text 2. Some of the Variation among Individuals Can Be Passed on to Their Offspring Although Darwin did not understand how it occurred, he observed many examples of the general resemblance between parents and offspring. He also noticed that people took advantage of the inheritance of variation in other species. Pigeon fanciers in Darwin’s time clearly recognized the inheritance of variation; they could see, for instance, that pigeons with neck ruffs were more likely to produce offspring with neck ruffs than were pigeons without ruffs. Thus, when enthusiasts wanted to produce a ruffed variety of pigeon, they encouraged breeding among the birds with this trait (Figure 12.7). Darwin hypothesized that offspring tend to have the same characteristics as their parents in natural populations as well. Figure 12.7 Observation 2: Some of the variation among individuals can be passed on to their offspring. Darwin noted that breeders could create flocks of pigeons with fantastic traits by using as parents of the next generation only those individuals that displayed these traits. For several decades after the Origin of Species was published, the observation that some variations were inherited was the most controversial part of the theory of natural selection. Because scientists could not adequately explain the origin and inheritance of variation, many were unwilling to accept that natural selection could be a mechanism for evolutionary change. When Gregor Mendel’s work on inheritance in pea plants (Chapter 8) was rediscovered in the 1900s, the mechanism for this observation became clear—natural selection operates on genetic variation that is passed from one generation to the next. 3. Populations of Organisms Produce More Offspring than Will Survive This observation is clear to most of us—the trees in the local park make literally millions of seeds every summer, but only a small fraction of these survive to germinate, and only a few of the seedlings live for more than a year or two. In the Origin of Species, Darwin gave a graphic illustration of the difference between offspring production and survival. In his example, he used elephants, animals that live long lives and are very slow breeders. A female elephant does not begin breeding until age 30, and she produces about 1 calf every 10 years until around age 90. Darwin calculated that even at this very low rate of reproduction, if all the descendants of a single pair of African elephants survived and lived full, fertile lives, after about 500 years their family would have more than 15 million members (Figure 12.8)—many more than can be supported by all the available food resources on the African continent! Visualize This Predict what will happen to the elephant population as a result of limited resource availability when Generation 4 is produced. Figure 12.8 Observation 3: Populations of organisms produce more offspring than will survive. Even slow-breeding animals like elephants are capable of producing huge populations relatively quickly. Figure 12.8 Full Alternative Text 4. Survival and Reproduction Are Not Random In other words, the subset of individuals that survives long enough to reproduce is not an arbitrary group. Some variants in a population have a higher likelihood of survival and reproduction than other variants do; that is, there is differential survival and reproduction among individuals in the population. The survival and reproduction of one variant compared with others in the same population is referred to as its relative fitness. Traits that increase an individual’s relative fitness in a particular environment are called adaptations. Individuals with adaptations to a particular environment are more likely to survive and reproduce than are individuals lacking such adaptations; in other words, these individuals have higher relative fitness. Darwin referred to the results of differential survival and reproduction as natural selection. Adaptations are “naturally selected” in the sense that individuals possessing them survive and contribute offspring to the next generation. Although Darwin used the word selection, which implies some active choice, natural selection is a passive process that is simply determined by differences among individuals and their success in their particular environment. For example, among the birds called medium ground finches living on an island in the Galápagos archipelago, scientists have observed that when rainfall is scarce, a large bill is an adaptation that can be observed. The large bill can be explained because birds with this attribute are able to crack open large, tough seeds—the only food available during severe droughts. As shown in Figure 12.9, the 90 survivors of a 1977 drought had an average bill depth that was 6% greater than the average bill depth of the original population of 751 birds. In these environmental conditions, a large bill increases survival. Working with Data Use the graph to determine how the total population size of ground finches changed between 1976 and 1978. Figure 12.9 Observation 4: Survival and reproduction are not random. The pale purple curve summarizes bill depth in ground finches on Daphne Island in the Galápagos in 1976. The dark purple curve below it represents the population in 1978, after the drought of 1977. These data indicate that survivors of the drought had a larger average bill depth than the predrought population. The change in the population’s average bill size occurred because finches with larger-than-average bills had higher fitness than did small-billed birds during the drought. Figure 12.9 Full Alternative Text Adaptations are not only traits that increase survival. Any trait possessed by an individual that increases the number of offspring it produces relative to other individuals in a population is also an adaptation. For example, flowers in a meadow may have a relatively limited number of potential insect pollinators. More pollinator visits generally result in more seeds being produced by a single flower, so any trait that increases a flower’s attractiveness to pollinators, such as a brighter color or greater nectar production, should be favored by natural selection (Figure 12.10). Figure 12.10 Adaptations are not about survival only. Variations that increase a flower’s attractiveness to a pollinator can increase its reproductive success by increasing the number of seeds it produces. Darwin’s Inference: Natural Selection Causes Evolution Based on his observations, Darwin reasoned that the result of natural selection is that inherited variations that are favorable within a given environment tend to increase in frequency in a population over time, while variations that are unfavorable within a given environment tend to be lost within the population. In other words, adaptations become more common in a population as the individuals who possess them contribute larger numbers of their offspring to the succeeding generation. Natural selection results in a change in the traits of individuals in a population over the course of generations—that is, evolution. Although there are other factors, such as genetic drift and migration of individuals, that can cause populations to evolve over time, natural selection is the only force that can lead to the adaption of a population to its environment. It is a testament to the power of the theory of natural selection that today it seems self-evident to us. But the theory of natural selection only became so powerful after it was tested and shown to work—in nature—in the manner Darwin described. Natural selection proved such a powerful idea that it has influenced how we think about many phenomena, from the success of particular brands of soft drinks to the relationships among nations. Natural selection also explains the emergence of XDR-TB. Testing Natural Selection Darwin proposed a scientific explanation of how evolution occurs, and like all good hypotheses, it needed to be tested. All of the tests described next illustrate that natural selection is an effective mechanism for evolutionary change. Artificial Selection Selection imposed by human choice is called artificial selection. It is artificial in the sense that humans deliberately control the survival and reproduction of individual plants and animals to change the characteristics of the population. Individuals with preferred traits are permitted to breed, whereas those that lack preferred traits are not allowed to breed. The fancy pigeons that Darwin studied arose by artificial selection, and the great variety of domestic dogs we see today also resulted from this process. In each case, different breeds evolved through selection by breeders for various traits (Figure 12.11). These examples demonstrate that differential survival and reproduction change the characteristics of populations. However, because of the direct intervention of humans on the survival and reproduction of these organisms, artificial selection is not exactly equivalent to natural selection. Can change in populations occur without direct human intervention? Visualize This What would this sequence of changes look like if the selection was for dogs with a particular behavioral trait, for instance, pointing at a prey animal? Figure 12.11 Artificial selection can cause evolution. When breeders select dogs with certain traits to produce the next generation of animals, they increase the frequency of that trait in the population. Over generations, the trait can become quite exaggerated. Dachshunds are descendants of dogs that were selected for the production of very short legs. Figure 12.11 Full Alternative Text Natural Selection in the Lab Another test of the effectiveness of natural selection is to examine whether populations living in artificially manipulated laboratory environments change over time. An example of this kind of experiment is one performed on fruit flies placed in environments containing different concentrations of alcohol. High concentrations of alcohol cause cell death in fruit flies. Many organisms, including fruit flies and humans, produce enzymes that metabolize alcohol—that is, they break it down, extract energy from it, and modify it into less-toxic chemicals. There is variation among fruit flies in the rate at which they metabolize alcohol. In a typical laboratory environment, most flies process alcohol relatively slowly, but about 10% of the population possesses an enzyme variant that allows those flies to metabolize alcohol twice as rapidly as the more common variant. In an experiment (Figure 12.12), scientists divided a population of fruit flies into two randomized groups. Initially, these two groups had the same percentage of fast and slow alcohol metabolizers. One group of flies was placed in an environment containing typical food sources; the other group was placed in an environment containing the same food spiked with alcohol. After 57 generations, or about two years in the laboratory, the percentage of fast alcohol-metabolizing flies in the environment with only typical food sources was the same as at the beginning of the experiment—10%. But after the same number of generations, the percentage of fast alcohol-metabolizing flies in the alcohol-spiked environment was 100%. Because all of the flies in this environment were now of the fast alcohol-metabolizing variety, the average rate of alcohol metabolism in the population in this environment was much higher in generation 57 than in generation 1. The population had evolved. Working with Data This data can be represented as a line graph as well as a bar graph. Sketch the line graph. Figure 12.12 Natural selection in laboratory conditions. When fruit flies are placed in a high-alcohol environment, the percentage of flies that can rapidly metabolize alcohol increases over many generations because of natural selection. In the normal laboratory environment, there is no selection for faster alcohol processing, so the average rate of alcohol metabolism does not change. Figure 12.12 Full Alternative Text The evolution of the fruit flies in this experiment was a result of natural selection. In an environment where alcohol concentrations were high, individuals that were able to metabolize alcohol relatively rapidly had higher fitness. Because they lived longer and were less affected by alcohol, the fast alcohol-metabolizing flies left more offspring than the slow alcohol- metabolizing flies did. Thus, each generation had a higher frequency of fast alcohol-metabolizing individuals than the previous generation did. After many generations, flies that could rapidly metabolize alcohol predominated in the population. Selection can change populations in highly regulated laboratory environments. But does it have an effect in natural wild populations? Natural Selection in Wild Populations The evolution of M. tuberculosis from being susceptible to antibiotics to being resistant is one example of natural selection in a wild population; clearly, a change in the environment (that is, the introduction of antibiotics) caused a change in the bacteria population. Dozens of other pathogens, organisms that cause disease, have become resistant to drugs and pesticides in the past 50 years as well. But even these changes may seem less convincing to some readers because the adaptation is to a human-imposed environmental change. Although studying adaptation to natural environmental changes in the field is a significant challenge, the effects of natural selection have also been observed in dozens of wild populations. A classic example of natural selection in a natural setting is the evolution of bill size in Galápagos finches in response to drought (review Figure 12.9). The survivors of the drought tended to be those with the largest bills, which could more easily handle the tough seeds that were available in the dry environment. The survival of this nonrandom subset of birds resulted in a dramatic change in the next generation. The population of birds that hatched from eggs in 1978—the descendants of the drought survivors—had an average bill depth 4 to 5% larger than that of the predrought population. A more recent example of natural selection causing evolution has occurred in the past few decades on the eastern coast of the United States, where an invasive Asian crab species is wreaking havoc on native mussels. But one species, the blue mussel, has quickly evolved the ability to thicken its shell when it grows in the presence of the Asian crab, thwarting their attacks (Figure 12.13). Scientists at the University of New Hampshire were able to demonstrate that this was an evolutionary change by comparing blue mussel populations in regions invaded by the Asian crab with those in more northerly waters, where the Asian crab cannot survive. These researchers demonstrated that while both populations of mussels thicken their shells in response to the presence of native crabs, only the mussels that had been living with the Asian crabs responded to the presence of this species. Clearly, natural selection for individual mussels that could recognize this new species of crab as a predator had caused a change in the mussel population. Figure 12.13 Natural selection in the wild. (a) Asian shore crabs are recent invaders to the east coast; because they were unfamiliar predators to the native mussels, they initially devastated mussel beds. (b) Some populations of blue mussels have evolved the ability to recognize the presence of the crabs and respond the way they do to native predators, by adding layers to their shells. Got It? Two of Darwin’s observations that led to the theory of natural selection are that organisms in a population from each other and that traits can be on their offspring. The fitness of an individual is defined as its success in and compared with other individuals in the same population. An adaptation is a trait that increases an individual’s relative to others in the population who do not have the trait. When humans manipulate the environment and cause the evolution of traits in a population of domestic animals or plants it is called . The evolution of rapid alcohol metabolism in a population of fruit flies in a lab occurred in the presence of high alcohol levels could occur because individuals in their rate of alcohol metabolism.

In your own words, explain the concepts from the four observations discussed in 12.2 using the moth as an example. In other words, how does the moth illustrate the first observation, the second observation, etc.?

Your response must be at least 200 words in length.

Question 3

Explain how homology is different from convergent evolution and give examples. Briefly define homology and convergent evolution in your explanation.