AP Biology : Understanding Types of Selection

Study concepts, example questions & explanations for AP Biology

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Example Question #1891 : Ap Biology

The various species of Galapagos finches utilize their beaks to access food resources in many ways. Those with longer beaks use their beaks to penetrate cactus fruit and eat the inside, while those with short beaks tear apart the base of the cactus and eat insect larvae and pupae.

Cacti in the Galapagos fail to produce any fruit for several generations, causing finches with long beaks to die. With these environmental changes, selection will favor those with short beaks over those with long beaks. What type of selection is described?

Possible Answers:

Directional selection

Sexual selection

Disruptive selection

Stabilizing selection

Correct answer:

Directional selection

Explanation:

The answer is directional selection. This environmental change will cause a shift in the mean beak size of the population towards smaller beaks because their food resource was not affected. There is selection against long beak size only.

Example Question #1 : Understanding Types Of Selection

Which of the following is an outcome of artificial selection?

Possible Answers:

Artificial selection increases the variation of traits within a population 

Artificial selection tends to favor wild type traits

Artificial selection increases the compatibility between organisms and their environment

Artificial selection can result in dramatic changes to a species in a relatively short period of time

Correct answer:

Artificial selection can result in dramatic changes to a species in a relatively short period of time

Explanation:

Artificial selection is a process by which humans have modified species over many generations by selecting and breeding individuals that possess desired traits. The result of such a selection is a narrowing of the traits originally present from its ancestors. The goal of artificial selection is generally to make certain desired traits fixed in the population, with an allele frequency of 100%. These traits are relatively arbitrary, and may be wild type, recessive, advantageous, or disadvantageous. The key factor is a decrease in genetic variability in a small population.

Examples of artificial selection include the derivation of individual breeds of dog and the production of genetically specific lab mice.

Example Question #3 : Understanding Types Of Selection

Which of the following is most affected by natural selection?

Possible Answers:

Merlot grapes

Poodles

Influenza

Angora rabbits

Correct answer:

Influenza

Explanation:

Influenza (the flu virus) is a real-life example of natural selection that is most evident during an annual time period when the influenza virus infection peaks. The repeated or cyclical nature of flu outbreaks is caused by the natural selection of the virus for traits that allow it to evade the immune system and replicate, a process that actually results in new distinct strains of influenza virus.

In contrast, the other answers are all examples of artificial selection by which the specific variety or breed has been purposely selected for specific traits. Poodles and Angora rabbits have been purposely bred by humans to select for traits that affect their fur. Merlot wine is produced from a certain strain of grapes that have been bred for their taste. Note that artificial selection differs from genetic modification, as no new traits have been introduced to these strains.

Example Question #3 : Understanding Types Of Selection

A species of moth that has a predominantly darker coloration in industrial urban areas, predominantly lighter coloration in rural areas, and limited intermediate coloration in both habitats. This is an example of what type of selection?

Possible Answers:

Artificial selection

Stabilizing selection

Disruptive selection

Directional selection

Correct answer:

Disruptive selection

Explanation:

Disruptive selection occurs when conditions in a habitat favor individuals that are on the extremes of the phenotypic range, instead of individuals with intermediate phenotypic traits. In the example provided in the question, dark-colored moths and light-colored moths each have regions of the habitat where they gain an advantage, but intermediate grey coloring is not favored in any region of the habitat. As a result, both extremes will experience positive selection, while the intermediate will decrease, giving rise to a disruptive selection trend.

Stabilizing occurs when the intermediate phenotype is favored over either extreme. Directional selection occurs when a single extreme is favored over any other phenotype. Artificial selection is the result of human manipulation of breeding, selecting for chosen traits.

Example Question #2 : Understanding Types Of Selection

Which type of selection is depicted by the graph if the solid line represents the original population and the dashed line is the population following selection?

Stabilizing_selection_image

Possible Answers:

Artificial selection

Disruptive selection

Directional selection

Stabilizing selection

Correct answer:

Stabilizing selection

Explanation:

Stabilizing selection removes extreme variants, as they do not provide a survival advantage in a given environment, and increases the frequency of the intermediate phenotype. For example, the bone density of a species of bird is likely to experience stabilizing selection. Bones that are too dense will inhibit the bird's ability to fly, and bones that are too light will be brittle and prone to injury. Stabilizing selection moderates the influence of these two factors and selects for the intermediate phenotype that is neither too heavy, nor too weak.

Disruptive selection shows an increase the in the frequency of extreme traits and a decline in the intermediate trait. For example, if two extremes are white and black coloration, disruptive selection will act against grey coloration and favor both white and back. Directional selection favors only one extreme, for example favoring black over grey and grey over white. Artificial selection occurs when humans interfere with breeding habits to promote the inheritance of a specific trait.

Example Question #6 : Understanding Types Of Selection

The various species of Galapagos finches utilize their beaks to access food resources in many ways. Those with longer beaks use their beaks to penetrate cactus fruit and eat the inside, while those with short beaks tear apart the base of the cactus and eat insect larvae and pupae.

Finches with medium beak size feed on seeds. These birds aren't specialized to feed on insect larvae or cactus fruit like finches with short or long beaks. What type of selection would be observed if deforestation removed all the trees from the area where these birds obtained seeds?

Possible Answers:

Sexual selection

Multi-directional selection

Disruptive selection

Directional selection

Stabilizing selection

Correct answer:

Disruptive selection

Explanation:

In this example the extreme beak sizes are favored over the mean; however the mean beak size does not change. There is selection against the mean, without affecting the value of the mean. This type of selection is known as disruptive selection.

Example Question #5 : Understanding Types Of Selection

A population of giraffes lives in a forest with trees of varying heights. Following a flood in the area, all shorter plants were destroyed and only tall trees remain. After many generations, it is noted that giraffes living in this forest have longer necks, on average, than giraffes living in the same forest prior to the flood. 

What type of selection has occurred?

Possible Answers:

Destabilizing selection

Directional selection

Stabilizing selection

Extreme selection

Disruptive selection

Correct answer:

Directional selection

Explanation:

Directional selection is natural selection that favors a phenotype at an "extreme" for a particular trait. In this example, we are looking at the height trait. Individuals with taller phenotypes exhibit greater fitness due to their ability to reach the food source more easily. The new environment favors taller giraffes, and thus, after many generations, the phenotype of the population as a whole will be skewed towards the tall end of the height spectrum.

Stabilizing selection favors the average phenotype for a given trait. Disruptive selection favors both extremes of a phenotype, but favors against the average.

Example Question #6 : Understanding Types Of Selection

White mice are homozygous for a recessive pigmentation trait. Brown mice are homozygous dominant for the same pigmentation trait, and beige mice are heterozygous for the trait. In a particular ecosystem, natural selection favors the beige mice because they blend in with the brush and leaves. What concept explains to this type of selection? 

Possible Answers:

Directional selection

Frequency-dependent selection

Heterozygote advantage

Balancing selection

Founder effect

Correct answer:

Heterozygote advantage

Explanation:

Heterozygote advantage occurs when heterozygotes at a particular locus, such as pigmentation, have greater fitness than do both kinds of homozygotes. If the heterozygote favors an intermediate phenotype, such as beige mice, it is also stabilizing selection. Frequency-dependent selection occurs when the fitness of a phenotype depends on how common it is in the population. Together, heterozygote advantage and frequency-dependent selection lead to a balancing system. The founder effect occurs when a few individuals of a population become isolated and form a new population whose gene pool differs from the original population. Directional selection occurs when conditions favor individuals at one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or another.

Example Question #7 : Understanding Types Of Selection

Due to the abundance of blue-bellied lizards, they are easier for predators to find. Predators such as hawks become adapted to spotting these blue-bellied lizards and prey on them more frequently than yellow-bellied lizards. When the blue-bellied lizard population begins to decline, the yellow-bellied lizard population begins to rise (due to more abundant resources and the fact that they are preyed on less than the blue-bellies). Which of the following best describes the relationship between the two types of lizards?

Possible Answers:

Intersexual selection

Relative fitness 

Frequency-dependent selection

Disruptive selection 

Heterozygote advantage

Correct answer:

Frequency-dependent selection

Explanation:

This relationship displays a frequency-dependent selection. Frequency-dependent selection occurs when the fitness of a phenotype depends on how common it is in the population. When blue-bellied lizards are preyed on the most because they are most abundant, the yellow-bellied lizard population grows quickly. However, as the blue-bellied lizard population declines, predators will begin to prey on the yellow-bellied lizards. As the yellow-bellied lizard population declines, the blue-bellied lizard population will increase, and so on. The number of blue or yellow-bellied lizards depends on the relative amount of lizards of a different phenotype, demonstrating frequency-dependent selection.  

Example Question #3 : Understanding Types Of Selection

What is true regarding mutations?

Possible Answers:

Duplications, inversions, and translocations are all types of mutations

Mutations are changes to a cell's genome

All of these

Mutations may be harmful

Correct answer:

All of these

Explanation:

Mutations are changes to a cell’s genome and include inversions, duplications, translocations, and deletions, among other unplanned changes to the DNA. Mutations can be harmful to the organism, but can also have beneficial or neutral impacts. Mutation is an important means of evolution because it introduces new genetic combinations into a genome, allowing for the potential of new functions. 

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