AP Biology › Understanding Reproductive Barriers
Which of the following is a NOT potential outcome from a speciation hybrid zone?
All of these are potential outcomes
The continued production of hybrid individuals
A weakening of reproductive barriers, allowing the two species to fuse
A strengthening of reproductive barriers, and eventual extinction of the hybrids
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.
Which is not an example of behavioral isolation?
Species of fish living in different areas of a pond due to differential salt concentrations
Two species of bird having distinct male mating dances
Pheremones that attract only one species of insect
Male plumage on a bird species that is only found to be attractive by females of the same species
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.
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?
Temporal isolation
Habitat isolation
Gametic isolation
Hybrid breakdown
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.
Which of the following is not a geographic barrier to speciation?
Plant polyploidy
Mountain ranges
Oceans
Newly formed streams
Canyons caused by erosion
Plant polyploidy—the presence of more than one genotype—is not a geographic barrier. Plant polyploidy helps enable speciation.
Which of the following is not an example of a reproductive barrier?
For a certain species of walrus, males defend large groups of females called harems. Typically, one male's harem will consist of about 20 females, and he will mate with all of them. Some male walruses don't have harems.
Two species of cicada live in the same area. Species A is sexually mature every 7 years. Species B is sexually mature every 2 years.
Two species of birds live in the same area. Species A has dull gray plumage and has developed an elaborate courtship dance. Species B has vibrant orange plumage and has no courtship dance. Females from Species A choose mates based on their dance, and females from Species B choose mates based on their plumage.
Horses and donkeys are different species, but can breed with each other to produce mules. However, mules are sterile (they cannot have offspring of their own).
A reproductive barrier is something that prevents two populations from interbreeding, eventually leading to speciation. The cicada populations are an example of temporal isolation--the two species do not interbreed because they do not breed at the same time. The horse and donkey example shows hybrid sterility--the two species can produce offspring, but the offspring cannot have offspring of their own. The bird species are an example of behavioral isolation--the two species have different courtship rituals (dance vs. plumage) which leads to minimal interbreeding.
Which of the following is not true regarding zygote mortality?
Fertilization does not occur
It is an example of post-zygotic reproductive isolation
Fertilization occurs between gametes
After fertilization, the resulting zygote is inviable
Zygote mortality is an example of post-zygotic reproductive isolation. In this type of reproductive isolation, gametes from two species may fuse, however the zygote is inviable shortly after fertilization due to improper development.
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?
No; due to their inability to reproduce successfully mules would not be classified as a new species
No; mules are too similar to the parent species to be considered their own species
Yes; mules have enough of their own traits to be considered an independent species
Yes; the fact that they have a different number of chromosomes makes them their own species
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.
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?
Post-zygotic barrier
Pre-zygotic barrier
Mechanical barrier
Behavioral barrier
Geographic barrier
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.
In cases of mechanical isolation, what is the barrier to reproduction between two species?
Genital incompatibility
Mating behavior
Habitat
Zygote inviability
Mechanical isolation is a type of pre-zygotic barrier that maintains distinct species through genital incompatibility. In this type of reproductive isolation, the genitals of a male and female of the same species allow for copulation and reproduction, while the genitals of individuals of different species mechanically will not allow for reproduction.
What are the two major classifications of barriers to reproduction?
Pre-zygotic and post-zygotic
Pre-zygotic and inviability
Reproductive isolation and post-zygotic
Reproductive isolation and inviability
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.