MCAT Biology : Evolution

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1025 : Biology

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

I

IV

II

V

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 #1 : Natural Selection

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:

Impermeable outer skin

Development of legs

Internal lungs

Short loops of Henle

Internal fertilization

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 #1 : 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:

Artificial selection

Stabilizing selection

Disruptive selection

Vestigial selection

Directional 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 #1 : 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

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 #1 : Natural Selection

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

Possible Answers:

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

The organism's health

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

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.

Example Question #1 : Speciation

A scientist studies three populations of frog (populations A, B, and C) that live in the same rainforest. He notices some interesting similarities between the three groups. What would be the best evidence that A and B have a more recent common ancestor than A and C or B and C?

Possible Answers:

Populations A and C often fight over territory

Populations A and B share similar mtDNA sequences, but differ in their nDNA sequences

Populations B and C eat different things

Populations A and B are not native to the rainforest, but came from two different areas that were very similar to the rainforest

Populations A and B are both blue

Correct answer:

Populations A and B share similar mtDNA sequences, but differ in their nDNA sequences

Explanation:

Mitochondrial DNA (mtDNA) is only inherited directly from a mother to her offspring and can be used to directly track lineage of a population or species. Nuclear DNA (nDNA) is inherited from both the father and mother of the offspring; it can be used to track lineage as well, but mtDNA similarity is enough to conclude a close relationship between the two populations described in the question.

Color, diet, and location are all distinguishing features of the populations and help characterize their niche in the ecosystem. Diet and location (territory) are not heritable traits, and do not signify ancestry. Color is genetic, but could result from convergent or divergent evolution. mtDNA similarity is the strongest available evidence for a close ancestral link between populations A and B.

Example Question #1 : Understanding Speciation

A certain class of protein is found to exist in several different species. The amino acid sequence of this protein is compared between a large number of species. The greatest number of amino acid differences will be found between species of different __________.

Possible Answers:

families

genera

orders

phyla

Correct answer:

phyla

Explanation:

The higher the taxonomic group, the less similar the members are. This is true for appearance, behavior, and genetics. The order of taxonomic groupings, from most general to most specific is: kingdom, phylum, class, order, family, genus, species.

Of the given answers, phyla are the highest taxonomic rank. Species of different phyla would show the greatest genetic difference. In contrast, genera are the lowest taxonomic rank of the given answers; species of the same genus would show the least genetic difference.

Example Question #2 : Evolution

Which of the following is not necessary for designation as a chordate?

I.  A notochord

II.  Vertebrae

III.  A post-anal tail

IV.  Gill arches 

V.  A dorsal, tubular nervous system

Possible Answers:

I

III

II

V

IV

Correct answer:

II

Explanation:

Chordates must have, at some point in their embryogenesis, all features except vertebrae. Although the vast majority of chordates are vertebrates, Amphioxus has only a notochord. This primitive organism never develops vertebrae. Man has gill arches, and remnants of these are seen in certain congenital malformations. The notochordal remnants can give rise to the tumor known as a chordoma. The brain has ventricles (it is tubular) and it is certainly dorsal. Finally, human tail persists in about one per million live births.  

Example Question #1 : Animalia

Which of the following statements about chordates is true?

Possible Answers:

They are protostomes

Their anus forms from the blastopore

All chordates are vertebrates

They lack a coelom 

Correct answer:

Their anus forms from the blastopore

Explanation:

The phylum chordata has a few key characteristics. They are deuterostomes, meaning that the anus arises from the blastopore. They have a coelom that arises from the mesoderm during development, and at some point they have a tail, pharyngeal slits, and a notochord.

Vertebrata is a subphylum of chordata, so not all chordates are vertebrates (though all vertebrates are chordates).

Example Question #2 : Evolution

Which of the following is false of the phylum chordata?

Possible Answers:

They have a notochord at some point in their development

They have bilateral symmetry

They have a coelom

They are protostomes

Correct answer:

They are protostomes

Explanation:

All chordates are deuterostomes, not protostomes. During early development, the formation of the blastopore leads to the growth of the digestive tract. In protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.

All chordates have a notochord during development, which eventually gives rise to the nervous system. Chordates also have a coelom, or fluid filled cavity, somewhere within the body and they all exhibit bilateral symmetry.

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