Members share genes with other members of their species, and not with members of other species. Although this definition clearly is attractive, it has problems. Can you test it on museum specimens or fossil data? Can it explain the existence of species in a line of descent, such as the well-known lineage of fossil horses?
Obviously not. In fact, one cannot apply this definition easily, or at all, with many living organisms. What if species do not live in the same place?
What about the hybrids that we know occur in zoos? These problems are serious enough that some biologists recently argued for a return to the morphological species concept. So what is the best way to define a species? Most scientists feel that the biological species concept should be kept, but with some qualifications.
It can only be used with living species, and cannot always be applied to species that do not live in the same place. The real test applies to species that have the potential to interbreed. Most importantly, the biological species concept helps us ask how species are formed, because it focuses our attention on the question of how reproductive isolation comes about. Let us first examine types of reproductive isolation, because there are quite a few. Types of Reproductive Isolation There are many barriers to reproduction.
Each species may have its own courtship displays, or breeding season, so that members of the two species do not have the opportunity to interbreed. Or, the two species may be unable to interbreed successfully because of failure of the egg to become fertilized or to develop. This suggests a simple and useful dichotomy, between pre-mating or prezygotic i. Remember that a zygote is the cell formed by the union of two gametes and is the basis of a developing individual.
Prezygotic isolating mechanisms Ecological isolation: Species occupy different habitats. The lion and tiger overlapped in India until years ago, but the lion lived in open grassland and the tiger in forest. Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females. Evolutionary changes in mating rituals, such as bower construction, can contribute to speciation. Satin bowerbird photo courtesy of Graeme Guy. These damselfly penises illustrate just how complex insect genitalia may be.
Image adapted from Eberhard, W. Sexual Selection and Animal Genitalia. Harvard University Press. Offspring inviability or sterility: All that courting and mating is wasted if the offspring of matings between the two groups do not survive or cannot reproduce.
The process of reproductive isolation is shown in figure 2. Figure 2: Reproductive Isolation. The four types of pre-zygotic isolating mechanisms are mechanical isolation, ecological isolation, temporal isolation, and behavioral isolation.
In mechanical isolation , the mating is restricted by the differences of the morphological characteristics. For example, the snails with right-coiling shells cannot mate with the snails with left-coiling shells.
The changes occurred in that gene may cause speciation. The ecological isolation occurs due to geographical barriers. It is also called the geographical isolation and is described above in this article. The temporal isolation occurs when the populations breed in different time periods or breeding seasons. Therefore, no interbreeding may occur between the two types of frogs.
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