This section describes mechanisms by which species may become reproductively isolated before a zygote can be formed. In some cases mating between species cannot occur, while in other cases mating occurs, but fails to produce a zygote. Show
Spatial IsolationThe most obvious form of reproductive isolation is spatial isolation. If members of two populations never encounter each other, they will never mate and no gene flow will occur. Spatial isolation occurs between populations that are separated by great distances, but it can also take place between populations that inhabit different parts of the same area. Geographical IsolationGeographical isolation occurs when a physical barrier separates populations. This can happen in two ways: 1) A previously continuous population can be divided by the appearance of a barrier, an event known as vicariance; 2) A population may also be divided when some members cross an existing physical barrier, known as dispersal. In some cases there is no barrier other than great distance. For example, some species of frogs inhabit areas from the Northeastern US to Mexico. Within this distribution, neighboring populations are not isolated from each other and may interbreed. However, populations from the two ends of the distribution never meet and cannot mate. Habitat IsolationFor habitat isolation to occur, populations do not need to be separated by great distance. Instead, they must occupy different habitats, even within the same area. For example, if two populations of flies exist in the same geographical area, but one group lives in the soil and another lives on the surface of the water, members of the two populations are very unlikely to meet and reproduce. Temporal IsolationTemporal isolation represents another way in which populations living in the same area can be prevented from mating. Different populations may be ready to mate at different times of the year. For example, two populations of plants may produce flowers in different seasons, making mating between the populations impossible. Behavioral IsolationIn many animals, courtship displays and rituals are vital to reproduction. Such behaviors can be very specific, varying between closely related species. Male behaviors such as courtship calls, songs, and dances will only be recognized by females of the same species. Some species of crickets are morphologically identical, but can be distinguished by the fact that females will only respond to the mating songs of males of their own species. Males of other species are ignored. Recommended textbook solutions Biology1st EditionKenneth R. Miller, Levine 2,591 solutions Hole's Human Anatomy and Physiology13th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis 1,402 solutions
Human Resource Management15th EditionJohn David Jackson, Patricia Meglich, Robert Mathis, Sean Valentine 249 solutions Campbell Biology9th EditionJane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman 655 solutions All AP Biology ResourcesWhich of the following is not a geographic barrier to speciation? Possible Answers: Canyons caused by erosion Oceans Mountain ranges Newly formed streams Plant polyploidy Correct answer: Plant polyploidy Explanation: Plant polyploidy—the presence of more than one genotype—is not a geographic barrier. Plant polyploidy helps enable speciation. 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 two populations have not been in contact with one another for two hundred years When the distance between them exceeds the carrying capacity When the populations begin to eat different foods When there is a physical barrier, such as a river, between the two populations When they are no longer able to interbreed 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. 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; 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 No; mules are too similar to the parent species to be considered their own 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. 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 Habitat isolation Temporal 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. Which of the following is a NOT potential outcome from a speciation hybrid zone? Possible Answers: A weakening of reproductive barriers, allowing the two species to fuse The continued production of hybrid individuals A strengthening of reproductive barriers, and eventual extinction of the hybrids All of these are potential outcomes 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. Different fish living in the same pond mate at different times during the year. What type of reproductive isolation is this? Possible Answers: Gametic isolation Habitat isolation Mechanical isolation Behavioral isolation Temporal 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. 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: Geographic barrier Behavioral barrier Post-zygotic barrier Mechanical barrier Pre-zygotic 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. What are the two major classifications of barriers to reproduction? Possible Answers: Reproductive isolation and post-zygotic Pre-zygotic and post-zygotic Reproductive isolation and inviability Pre-zygotic and inviability 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. What is true regarding temporal isolation? Possible Answers: It is a post-zygotic barrier to reproduction Resulting zygotes are inviable It is a barrier that prevents fertile individuals from meeting 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. Which is not an example of behavioral isolation? Possible Answers: Pheremones that attract only one species of insect Two species of bird having distinct male mating dances Male plumage on a bird species that is only found to be attractive by females of the same species Species of fish living in different areas of a pond due to differential salt concentrations 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. All AP Biology ResourcesWhat are the two main types of barriers in reproductive isolation?Reproductive isolation can be either prezygotic (barriers that prevent fertilization ) or postzygotic (barriers that occur after zygote formation such as organisms that die as embryos or those that are born sterile).
What are 3 types of barriers that can lead to reproductive isolation?Any of the factors that prevent potentially fertile individuals from meeting will reproductively isolate the members of distinct species. The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering.
Are there barriers to reproduction?We were able to identify five potential barriers to reproduction including ecogeographic isolation, phenological differences, differences in pollinator assemblages, differential pollen adhesion, and low levels of hybrid seed development.
What are the two types of isolation?a) Temporal isolation. Individuals of different species do not mate because they are active at different times of day or in different seasons. b) Ecological isolation. Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences.
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