AP Biology : Understanding Reproductive Barriers

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #151 : Evolution And Genetics

Which of the following is not a geographic barrier to speciation?

Possible Answers:

Canyons caused by erosion

Newly formed streams

Mountain ranges

Plant polyploidy

Oceans

Correct answer:

Plant polyploidy

Explanation:

Plant polyploidy—the presence of more than one genotype—is not a geographic barrier. Plant polyploidy helps enable speciation.

Example Question #1 : Understanding Reproductive Barriers

Two populations of the same species over time grow distant from one another. At what point will these two populations be considered different species?

Possible Answers:

When the populations begin to eat different foods

When they are no longer able to interbreed

When the two populations have not been in contact with one another for two hundred years

When the distance between them exceeds the carrying capacity

When there is a physical barrier, such as a river, between the two populations

Correct answer:

When they are no longer able to interbreed

Explanation:

Speciation is the event that occurs when two populations of a particular species can no longer interbreed. Speciation is not defined by physical barriers or by the time that two populations are separate from one another. In fact, two populations of the same species can be apart any distance or time, and if they can still interbreed they are considered the same species even if they look completely different.

Note that carrying capacity refers to the maximum number of individuals in a population that the natural resources of the surrounding environment can support. It is, essentially, the maximum healthy population size, and is not a measure of distance as implied in the answer choice.

Example Question #1 : Understanding Reproductive Barriers

A horse and a donkey can successfully mate to produce an offspring known as a mule. Mules are sterile, display a mixture of donkey and horse traits, and have a different number of chromosomes from both parent species. Would mules be classified as their own species?

Possible Answers:

No; mules are too similar to the parent species to be considered their own species

No; due to their inability to reproduce successfully mules would not be classified as a new species

Yes; the fact that they have a different number of chromosomes makes them their own species

Yes; mules have enough of their own traits to be considered an independent species

Correct answer:

No; due to their inability to reproduce successfully mules would not be classified as a new species

Explanation:

Classifying species can be a difficult and particularly gray area. The key to this question, however, is that mules are sterile. This is an example of post-zygotic reproductive isolation. Hybrids that are sterile are not considered unique species.

Simply identifying traits or chromosome number is not enough to determine species. The defining factor of speciation is that the organism can no longer reproduce with the parent species, but can reproduce with other individuals of the species. Note that all breeds of dog still fall in the same species, despite the variety of different traits displayed across breeds, because they can produce offspring. Chromosome number also cannot solely determine species. Some genetic disorders result in aneuploidy in humans, in which individuals have too many or too few chromosomes.

Example Question #2 : Understanding Reproductive Barriers

A researcher has discovered two species of frogs that live in the same location. In the lab he is able to generate a viable hybrid by combining gametes from each species, however, no such hybrids exist in the wild. Which of the following methods of reproductive isolation might explain this phenomenon?

Possible Answers:

Hybrid breakdown

Temporal isolation

Habitat isolation

Gametic isolation

Correct answer:

Temporal isolation

Explanation:

Out of the choices, only temporal isolation can explain this phenomenon. Temporal isolation describes a type of reproductive isolation that comes about because the two species mate at different times.

Habitat isolation cannot explain this occurrence because we are told that the two species live in the same area. Hybrid breakdown also cannot be correct because the researcher was able to generate a viable hybrid (so he should reasonably be able to find first generation hybrids in the wild). Gametic isolation is also not correct because a hybrid was successfully generated. 

Example Question #21 : Speciation

Which of the following is a NOT potential outcome from a speciation hybrid zone?

Possible Answers:

The continued production of hybrid individuals

A strengthening of reproductive barriers, and eventual extinction of the hybrids

All of these are potential outcomes

A weakening of reproductive barriers, allowing the two species to fuse

Correct answer:

All of these are potential outcomes

Explanation:

A hybrid zone is a geographic region where allopatric populations happen to come back in contact with one another. There are three potential outcomes for the hybrid zone over time. It is possible for the two species to re-integrate due to a weakening of reproductive barriers. In this scenario, the two species become one again. It is also possible that the reproductive barriers will strengthen, resulting in an inability to produce the hybrid offspring and a retention of species identity. The final option is a stable intermediate in which the species remain separate, but are still naturally able to create hybrid offspring.

Example Question #22 : Speciation

Different fish living in the same pond mate at different times during the year. What type of reproductive isolation is this?

Possible Answers:

Habitat isolation

Mechanical isolation

Temporal isolation

Behavioral isolation

Gametic isolation

Correct answer:

Temporal isolation

Explanation:

This is an example of temporal isolation. Temporal isolation means that different species are kept reproductively isolated because they mate at different times.

Example Question #23 : Speciation

While donkeys and horses have ignored each other historically in the wild, hybrids can be made through artificial insemination. The resulting hybrids are unusual in that they are generally viable, but always sterile. Because of their sterility, a blended lineage of the two animals cannot exist. What evolutionary force is this an example of?

Possible Answers:

Pre-zygotic barrier

Behavioral barrier

Post-zygotic barrier

Geographic barrier

Mechanical barrier

Correct answer:

Post-zygotic barrier

Explanation:

Reproductive barriers can be divided into two major categories: pre-zygotic or post zygotic. The former is an issue that arises before fertilization such as two animals not being compatible anatomically or a behavioral incompatibility (think of bird mating rituals, or imagine a mouse trying to reproduce with a whale). A Post-zygotic barrier is a reproductive barrier that prevents the zygote from forming, or hampers the developmental process. It does not prevent the animals from mating however. Thus, The sterile mule example falls under the post-zygotic category since the animals were not prevented from mating. 

Example Question #24 : Speciation

What are the two major classifications of barriers to reproduction?

Possible Answers:

Pre-zygotic and inviability 

Pre-zygotic and post-zygotic 

Reproductive isolation and inviability 

Reproductive isolation and post-zygotic 

Correct answer:

Pre-zygotic and post-zygotic 

Explanation:

Barriers to reproduction are behaviors and physiological processes that maintain distinct species and prevent hybridization. There are two major classifications of barriers to reproduction: pre-zygotic barriers and post-zygotic barriers. Pre-zygotic barriers prevent hybridization before fertilization and post-zygotic barriers prevent it after fertilization.

Example Question #1972 : Ap Biology

What is true regarding temporal isolation?

Possible Answers:

It is a barrier that prevents fertile individuals from meeting 

Resulting zygotes are inviable

It is a post-zygotic barrier to reproduction 

It is a barrier resulting from different mating behaviors 

Correct answer:

It is a barrier that prevents fertile individuals from meeting 

Explanation:

Temporal isolation is a pre-zygotic barrier to reproduction. In instances of temporal isolation, fertile individuals do not meet one another, thereby isolating them reproductively. This includes distinct habitats, physical barriers, and different timing of sexual maturity.

Example Question #26 : Speciation

Which is not an example of behavioral isolation?

Possible Answers:

Pheremones that attract only one species of insect

Species of fish living in different areas of a pond due to differential salt concentrations

Male plumage on a bird species that is only found to be attractive by females of the same species

Two species of bird having distinct male mating dances

Correct answer:

Species of fish living in different areas of a pond due to differential salt concentrations

Explanation:

Behavioral isolation is a type of pre-zygotic barrier to reproduction that isolates species through different mating rituals. “Species of fish living in different areas of a pond due to differential salt concentrations” is not an example of behavioral isolation because there is not difference in mating behaviors that reproductively isolates the two species of fish. The rest of these examples demonstrate barriers to reproduction that originate with mating behavior.

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