Chapter 8. Chapter 8: Evolution and Natural Selection

Review & Rehearse

Review & Rehearse
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You must read each slide, and complete any questions on the slide, in sequence.

Instructions

Review the visual summaries and answer the essay questions below.

Make sure to enter a brief response that completely answers each question and explains your reasoning. When you click "Submit," you will be provided instant feedback, allowing you to check if your response is correct.

(This activity contains 21 total essay questions. Each new question will be revealed once you complete the preceding question.)

Question 8.1

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Fruit flies reach maturity in about two weeks, so a scientist can include many generations during an experiment such as that on starvation resistance. With an organism having a late age of maturity, it would require many years to complete a study such as the starvation resistance experiment.

Question 8.2

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It was though that: (a) as in the Biblical account, the earth was only about 6,000 years old; (b) the earth had not changed very much over time, with the exception of the occasional earthquake, flood, or volcanic eruption; and (c) all species, including humans, were created at the same time, and these species would never change or die out.

Question 8.3

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Darwin found many fossils that were very similar to species still living in the same area. He also noticed that finches on the Galápagos Islands, which he assumed were all of the same species, had different physical characteristics on different islands. Biologists at the Zoological Society in London later discovered that they were 13 different finch species. All of these species closely resembled a single species of finch on the closest mainland, in Ecuador.

Question 8.4

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Darwin noticed a striking similarity between fossils found in an area and the animals living in that area. An example was the fossils of the extinct glyptodont, which was estimated to be 10 feet long and 4,000 pounds, and lived in the same area that was now occupied by armadillos. This suggested to Darwin that glyptodonts were extinct relatives of the armadillos, contradicting the belief in unchanging species.

Question 8.5

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In 1858, Darwin received a letter from Alfred Russel Wallace, a biologist studying in Malaysia, that presented a clear description of evolution. A joint presentation of Darwin’s and Wallace’s work at the Linnaean Society of London soon followed. In 1859, Darwin published The Origin of Species, which was an immediate success and ensured Darwin’s place in history as the father of evolutionary theory.

Question 8.6

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Evolution is a genetic change in a population over time. A change in allele frequency in the population occurs as a result of natural selection, mutations, genetic drift, and migration.

Question 8.7

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Three causes of mutations are: (a) exposure to high-energy radiation; (b) exposure to certain chemicals in the environment; and (c) spontaneous mutation—that is, a random genetic change occurring by chance.

Question 8.8

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Genetic drift is a random change in allele frequency in a population that is not influenced by the allele’s impact on fitness. Smaller populations have fewer individuals with each type of allele, so any random loss of individuals is more likely to cause a change in allele frequency in that population. An example is the founder effect, in which individuals leave one population and create a new, smaller population in a different geographic region. This new population is likely to have a different allele frequency than the originating population.

Question 8.9

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Fixation for an allele occurs as a result of genetic drift, when an allele frequency becomes 100% in a population. As a result, there is no genetic variation for that gene, which can be detrimental because the population may be less able to adapt to environmental changes.

Question 8.10

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Evolution is genetic change in a population over time—specifically, a change in allele frequency over time. Natural selection is a cause of evolution in which alleles that increase reproductive success become more prevalent in a population, while those that do not favor reproductive success become less prevalent in a population.

Question 8.11

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The frequency of a recessive allele does not change in a population as long as the allele does not affect reproductive success. If a recessive allele does affect reproductive success, its frequency will change. Those recessive alleles that adversely affect reproduction will decrease in frequency, and those that improve reproductive success will increase in frequency.

Question 8.12

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Fitness is a measure of the reproductive success of individuals. (a) An organism’s fitness depends on the environment in which it lives. The organism may be fit in one environment, but less fit in another environment. (b) Fitness is relative to that of other organisms in the same environment. The fitness of one individual is measured against that of other individuals with specific genotypes and phenotypes. (c) The relative fitness of an individual is measured against that of other individuals of the same species in the same population. Individuals who reproduce more frequently than others are more fit, irrespective of the age at which they die.

Question 8.13

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The use of an antibiotic creates environmental pressure, and bacteria possessing one or more alleles imparting resistance to that antibiotic are more likely to reproduce than those that do not possess alleles for antibiotic resistance. Over time, those bacteria with antibiotic-resistance alleles become more frequent in the population. Should another antibiotic be introduced, other alleles present in the population that impart resistance to that antibiotic will become more frequent over time.

Question 8.14

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(a) Natural selection requires time, sometimes more time than is allowed by the rate at which changes are occurring in the environment. The world’s environments are ever changing and require continual evolutionary change if organisms are to maintain fitness. No single perfect organism for all environments will ever evolve. (b) Since mutations occur irrespective of the needs of the organism, new mutations (i.e., new alleles) may not be those required for the organism to adapt to changes in its environment. (c) There may be more than one allele suitable for selection in an environment, and thus there is no single optimal adaptation.

Question 8.15

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Answers can include the selective breeding of a wide range of animals and plants, such as cats, dogs, chickens, pigs, horses, cows, sheep, and goats, and apples, tomatoes, potatoes, cucumbers, corn, lettuce, strawberries, grapes, peppers, and cucumbers. Each organism chosen as an example should have been modified by humans from its natural state so that it possesses traits desired by humans.

Question 8.16

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Normally, babies weigh between 7 and 8 pounds at birth as a result of stabilizing selection. Modern medicine has enabled very-low-weight, premature babies to survive, and very large babies to be delivered through Caesarean births. This has prevented selection against extremes in the size of babies born and may lead to a greater number of babies of extreme size (small or large) becoming more prevalent in the future.

Question 8.17

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The forelimbs of mammals have been modified to form, for example, wings in bats, flippers in porpoises, forelegs developed for running in horses, and arms with hands for grasping in humans. These are referred to as homologous structures, since the bones of the forelimbs of mammals have been modified to form a number of different structures to perform vastly different functions.

Question 8.18

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(a) The soft parts of animals are not well preserved in fossils. (b) There are very few environments where erosion and decay do not occur, and thus very few with the conditions required for the formation of fossils. The result is a large number of gaps in the fossil record.

Question 8.19

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Scientists in the field of biogeography have observed that species will migrate to nearby locations and adapt to their new habitats. Different species living less than 100 miles apart and occupying drastically different habitats resemble each other more than they resemble species many thousands of miles away that occupy very similar habitats. This suggests shared common ancestors among species that are close together geographically. An example is found on the continent of Australia, occupied by marsupials. Many niches in Australia that are occupied by marsupials would otherwise have been occupied by placental mammals. For example, the Tasmanian wolf, now thought to be extinct, occupied the same type of habitat as gray wolves. However, the Tasmanian wolf is much more closely related to other marsupials living in Australia than to gray wolves found thousands of miles away.

Question 8.20

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Analogous structures are structures having similar functions that developed as a result of convergent evolution but were formed from different starting materials in unrelated species. For example, the wings of insects, birds, and bats are all used for flying, but developed independently in each group of animals. Homologous structures develop in related species in which the same structures are modified to perform different functions. An example is the modification of the bones making up the forelimbs of mammals. All mammals have a common ancestor, but the bones of the forelimbs have been modified for different functions, such as flying, swimming, and walking.

Question 8.21

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A molecular clock is a way for biologists to determine how long two different species have been evolving separately. The longer two different species have been evolving separately, the greater the differences in their genetic sequences. For example, humans and chimpanzees share many genes, while the fruit fly and humans share much fewer genes.

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