GRE Subject Test: Biology : Understanding Natural Selection and Fitness

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

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Example Questions

Example Question #1 : Understanding Natural Selection And Fitness

Which of the following is an example of natural selection?

I.  Horses are bred for strength and endurance, and over time, the population of horses is more robust.

II.  A late spring storm kills all the young plants in a region, but they are spared outside the storm zone.

III.  Ancient ancestors of giraffes instinctively wanted to have longer necks to reach food higher in the trees, leading to the present appearance of giraffes.

IV.  A flower that happens to be more attractive to pollinators is more likely to have reproductive success. 

V.  A mutation of a bacterium caused by exposure to ultraviolet light causes the originally red colonies to be yellow instead.

Possible Answers:

III

V

IV

I

II

Correct answer:

IV

Explanation:

It is always difficult to rephrase "survival of the fittest" in some new, clever way. The flowers which BY CHANCE have developed a different color, pattern, or odor that better attracts pollinators are indeed more likely to experience reproductive success and pass on these genes to their offspring. Competing plants might do well for a while, but they are already disfavored, and further environmental changes may put them even more at risk (or have no effect, or again favor them over the presently more attractive plants).

The horse choice is an example of intentional breeding—artificial selection.

The storm option does not imply any condition in any of the plants which conferred an advantage against freezing to death, or even any difference between species of plants; it is more akin to a question about mass extinction than to one about evolution.

The giraffe choice relates to the Lamarckian fallacy of being able to pass on acquired characteristics; species that are more successful just plain "luck out" relative to environmental stresses.  

The bacterial response discusses a mutation without likely survival implications for the bacterium.

Example Question #2 : Evolution

Vertebrates are evolutionarily adapted to terrestial life. Which one of the following adaptations is LEAST likely to contribute to this land-based predominance?

Possible Answers:

Internal lungs

Development of legs

Impermeable outer skin

Internal fertilization

Short loops of Henle

Correct answer:

Short loops of Henle

Explanation:

Vertebrates have adapted to terrestial living due to their ability to maintain water inside their bodies, despite no longer being immersed in water. The loop of Henle in the nephrons is designed to concentrate urine, reabsorbing water without unnecessarily excreting it. The longer the loops descend into the medulla, the more concentrated the urine becomes. Shorter loops would not concentrate urine as much, and thus would not contribute to a vertebrate's adaptation to land-based life.

Internal lungs, impermeable skin, and internal fertilization would all protect vital processes from interference by the external environment.

Example Question #3 : Evolution

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:

Disruptive selection

Stabilizing selection

Directional selection

Vestigial selection

Artificial 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 #4 : Evolution

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

Genetic drift

Stabilizing selection

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 #2 : Understanding Natural Selection And Fitness

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

Possible Answers:

The ability of an organism to survive its environment

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

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

The organism's health

The organism's ability to attract the most mates

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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