The Interbreeding of Species
Organisms can be divided into groups called species, based on similarities in physical form, biochemistry, behavior, and genetic makeup. One such characteristic is the potential of organisms to interbreed with each other in nature to produce viable offspring. Obviously, a lion will mate with a lion and not with a mouse, but what prevents species that are very similar to each other, from interbreeding? There are barriers to reproduction which isolate the gene pools of species. These barriers are called prezygotic and postzygotic barriers, based on whether the barriers exist before a zygote, a fertilized egg, is formed or after one is formed.
The western spotted skunk and the eastern spotted skunk are examples of species which are closely related to each other and yet do not interbreed, hence the species are kept separate. Their barrier to reproduction is known as temporal isolation, because they breed at different times of the year. The western spotted skunk mates in late summer while the eastern spotted skunk mates in late winter.
Two related species may live in the same area but rarely meet each other because they occupy different habitats. Two species of garter snakes living in the same jungle may rarely encounter each other because one is aquatic and the other lives primarily on land. Just as habitat isolation is a barrier to reproduction between species, mechanical barriers may exist when species live together. In a field containing many flowering plants, species of plants will remain isolated because their anatomy is adapted to certain insects or animals that transfer pollen between the same species of plants.
Differences in behavior, especially in courtship behavior, can cause reproductive isolation between species. This is likely the most important barrier between closely related organisms. Eastern and western meadowlarks inhabit the same area of the United States, and are almost identical in appearance. Yet they remain separate species, partly because of the difference in their songs. On the Galapagos Islands, another species of bird called the blue-footed booby will mate only after a certain courtship behavior occurs in which the male lifts its feet one at a time, displaying the bright blue feet characteristic of the species.
Even if different species can undergo the physical act of mating, a zygote, or fertilized egg, rarely results. The sperm may not survive in the reproductive tract of the female or the molecules on the egg's surface may not adhere to the molecules on the sperm's surface. The latter situation occurs among aquatic animals which release their eggs and sperm into the water, where fertilization takes place.
In the case that the egg of one species is successfully fertilized by a different species, postzygotic barriers exist to prevent the hybrid offspring from becoming a viable, fertile adult. If fertilization does occur, the genetic differences between species may result in an undeveloped offspring. This has occurred among some species of frogs living in the same regions and habitats. Hybrids that do develop fully and are strong are usually unable to have offspring because they are sterile; they are unable to form normal sperm or eggs. The cross between a horse and a donkey, for example, produces a sterile hybrid called a mule. The species still remain separate because, except very rarely, the mule cannot breed with either species. Finally, there are also some cases where the hybrids are viable and fertile, but their offspring in turn are not. These second-generation offspring are weak or sterile.
Although reproductive barriers do exist between species, there are some cases where interbreeding transcends these barriers. In the natural world, coyotes can interbreed with dogs and wolves and their offspring are fully developed and fertile. When we leave the natural environment which animals live in and turn to the laboratory or a zoo, we find that fertile hybrids can sometimes be produced between species that do not interbreed in nature. We also find that these reproductive barriers do not apply among some organisms, specifically organisms which can reproduce themselves. The banana plant and certain lizards reproduce asexually, as do some fungi and protists. Bacteria do transfer genes but in a very minimal manner compared to the gene transfer that occurs during sexual reproduction. So as you can see, the reality of intra-species breeding and asexual reproduction in the natural world makes it evident that a species cannot be defined by its potential to interbreed.
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leaves out essential information.
A.The structure of flowering plants allows certain insects to access the plants' pollen.
B.Different species of flowering plants will not interbreed due to their structural adaptations to certain pollinators.
C.The isolation of species of flowering plants is dependent on the transference of pollen between plants.
D.Certain insects or animals are the pollinators that contribute to the isolation of flowering plant species.
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