GRE Subject Test: Biology : Evolutionary Factors

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

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Example Question #2 : Natural Selection

Members of a species of red fox have teeth of varying sharpness. Foxes with very sharp teeth are able to kill large prey for food, while foxes with very dull, strong teeth are able to crush eggs and small animals. Foxes with teeth of medium sharpness, however, cannot get food and many die before they are able to reproduce. Over time, the fox population shows a greater proportion of individuals with either very sharp or very dull teeth. Which type of natural selection best describes this situation?

Possible Answers:

Vestigial selection

Directional selection

Stabilizing selection

Artificial selection

Disruptive selection

Correct answer:

Disruptive selection

Explanation:

In this scenario, the two extreme phenotypes are selected for, while intermediate phenotypes are selected against. This is disruptive natural selection. Over time, disruptive selection results in a decreased frequency of "middle" phenotypes and an increased frequency of the two groups at the extreme ends. This is a process that can eventually lead to speciation.

The opposite is process stabilizing selection, in which the extreme variations are selected against in favor of more "average" phenotypes. Directional selection occurs when only one end of the spectrum is favored, such as sharp teeth but not dull teeth. Artificial selection involves human intervention in selecting desirable traits. Vestigial selection is not a type of natural selection.

Example Question #2 : Natural Selection

In a certain species of feline, all males are much larger than females. Members of either sex that are of intermediate size struggle to find mates. What principle best describes this phenomenon?

Possible Answers:

Disruptive selection

Stabilizing selection

Genetic drift

Bottleneck effect

Directional selection

Correct answer:

Disruptive selection

Explanation:

Large size is favored in males and small size is favored in females, but intermediate size is always disfavored. The result is an increase in the two extreme phenotypes (either large or small) and a decrease in the average phenotype. This type of trend is known as disruptive selection.

Stabilizing selection occurs when the extreme phenotypes are disfavored, and the average or intermediate phenotype is preferable. Directional selection occurs when only one extreme phenotype is advantageous, for example if only large felines were able to find mates. Genetic drift is the phenomenon by which the allele frequencies of a population change by chance, due to independent assortment or other random events. The bottleneck effect occurs when an outside event, such as disease or natural disaster, diminishes the original population such that the allele frequencies of the new population differ from those of the original.

Example Question #1 : Evolution

What is the definition of "fitness" in terms of evolution?

Possible Answers:

The organism's ability to attract the most mates

The ability of an organism to contribute its genes to future generations

The organism's health

The organism's ability to attain resources while in competition with other organisms of its species

The ability of an organism to survive its environment

Correct answer:

The ability of an organism to contribute its genes to future generations

Explanation:

An organism's evolutionary "fitness" depends on its ability to reproduce and create viable offspring, or contribute its genes to future generations.

Even if an organism is in perfect health, it is considered to have very low fitness if it cannot produce viable offspring. In an evolutionary sense, the perseverence of certain genes in a population defines the favorability of those genes. An increased prevalence of certain genes can be interpreted as evolution. The activities of a single individual (aside from reproductive viability) are relatively ineffective in determining its ability to pass on its genes to future generations.

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