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MCAT Speciation, Reproductive Isolation and Phylogenetic Trees Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on allopatric and sympatric speciation, prezygotic and postzygotic barriers, adaptive radiation and reading phylogenetic trees, with explanations.

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Quick concept primer

A species is a group of organisms that can interbreed and produce viable, fertile offspring. Speciation occurs when populations become reproductively isolated. In allopatric speciation, a geographic barrier separates populations; in sympatric speciation, isolation arises within the same range, for example through polyploidy in plants or a change in mating preference. Parapatric speciation involves adjacent populations along a gradient.

Prezygotic barriers act before a zygote forms: habitat, temporal, behavioral, mechanical and gametic isolation. Postzygotic barriers act afterward: hybrid inviability, hybrid sterility (mules) and hybrid breakdown. Adaptive radiation is rapid diversification from a common ancestor into many niches. Divergent evolution makes related species different, convergent evolution produces similar traits in unrelated lineages (analogous structures), and homologous structures share ancestry. On a phylogenetic tree, branch points show common ancestors and closer nodes mean closer relationships.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Two populations of squirrels are separated when a canyon forms, and over time they can no longer interbreed. This is an example of

  1. sympatric speciation
  2. allopatric speciation
  3. convergent evolution
  4. genetic drift without selection
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Answer: B. allopatric speciation

A geographic barrier that separates populations, followed by divergence and reproductive isolation, defines allopatric speciation. Sympatric speciation occurs without geographic separation. Convergent evolution produces similar traits in unrelated species, which is not described here.

Question 2

Two species of fruit flies live in the same orchard, but one mates at dawn and the other at dusk. Which barrier does this represent?

  1. Postzygotic hybrid sterility
  2. Prezygotic temporal isolation
  3. Prezygotic mechanical isolation
  4. Postzygotic hybrid inviability
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Answer: B. Prezygotic temporal isolation

Differences in the timing of mating prevent gametes from meeting, so no zygote forms. This is temporal isolation, a prezygotic barrier. Mechanical isolation involves incompatible anatomy, and postzygotic barriers act after fertilization.

Question 3

A horse and a donkey mate and produce a mule that is healthy but sterile. This is an example of

  1. hybrid sterility, a postzygotic barrier
  2. gametic isolation, a prezygotic barrier
  3. behavioral isolation, a prezygotic barrier
  4. hybrid breakdown in the second generation
Show answer and explanation

Answer: A. hybrid sterility, a postzygotic barrier

A zygote forms and develops into a viable adult, so the barrier is postzygotic. Because the mule cannot reproduce, gene flow between horses and donkeys is blocked, and the two remain separate species. Mules are sterile mainly because of mismatched chromosome numbers that disrupt meiosis.

Question 4

Which scenario is the best example of sympatric speciation?

  1. A river changes course and divides a population of frogs
  2. A plant population undergoes polyploidy and the offspring cannot breed with the parent population
  3. Birds colonize an island far from the mainland
  4. A mountain range rises between two lizard populations
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Answer: B. A plant population undergoes polyploidy and the offspring cannot breed with the parent population

Polyploidy can instantly make a new species within the same geographic range, because the tetraploid plants cannot produce fertile offspring with diploid parents. The other options involve geographic separation, which is allopatric speciation.

Question 5

The wings of bats and the wings of insects both allow flight but evolved independently. These structures are

  1. homologous
  2. vestigial
  3. analogous, showing convergent evolution
  4. the result of divergent evolution from a winged common ancestor
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Answer: C. analogous, showing convergent evolution

Analogous structures share a function but not a common evolutionary origin, and arise when similar selection pressures act on unrelated lineages. Homologous structures share ancestry, such as the forelimbs of bats and humans, even when their functions differ.

Question 6

Darwin's finches diversified from a single ancestor into species with different beak shapes suited to different foods. This is best described as

  1. convergent evolution
  2. adaptive radiation
  3. genetic bottleneck
  4. hybrid breakdown
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Answer: B. adaptive radiation

Adaptive radiation is the rapid diversification of a lineage from a common ancestor into many forms that exploit different ecological niches, often after colonizing a new environment. A bottleneck is a sharp reduction in population size.

Question 7

On a phylogenetic tree, species X and species Y share a node that is more recent than the node shared by Y and species Z. What can be concluded?

  1. Y is more closely related to X than to Z
  2. Z is the direct ancestor of Y
  3. Y is more closely related to Z than to X
  4. X evolved from Y
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Answer: A. Y is more closely related to X than to Z

The more recent the shared node, the more recent the common ancestor and the closer the relationship. Branch tips represent living (or sampled) taxa, so no species on a tip is the ancestor of another. Only shared ancestors, not direct descent, are shown.

Question 8

Which statement about the biological species concept is a limitation?

  1. It works for asexually reproducing organisms such as bacteria
  2. It cannot be applied to fossils or to organisms that reproduce asexually
  3. It requires that every species is geographically isolated
  4. It ignores reproductive isolation
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Answer: B. It cannot be applied to fossils or to organisms that reproduce asexually

The biological species concept defines species by the ability to interbreed and produce fertile offspring, so it is difficult to test in fossils and meaningless for organisms that reproduce asexually. It is built around reproductive isolation rather than ignoring it.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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