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Natural selection can cause one species to evolve into two or more species. How can this occur? Speciation requires reproductive isolation, the situation whereby one population of a species becomes isolated and can reproduce only with the individuals in that population (remember the premises of Hardy-Weinberg). The different environment of the isolated species will pose different selective pressures on the population, and, given enough time, natural selection may result in evolution of a new species.
Two types of isolation are generally recognized:
  • Allopatric: When species are divided by a physical barrier, they have undergone allopatric isolation. The most prominent type of allopatric isolation is geographic isolation: Volcanoes, tectonic plate shift, formation of new rivers, species blown by storms to island, etc. can cause geographic isolation. This type of isolation is probably the most common form of reproductive isolation, and, consequently, the most important.
  • Sympatric: When no physical barrier divides a population, reproductive isolation can still occur. In this case, daughter species arise within the home range of an existing species. One type of sympatric isolation is behavioral isolation. In this case, a genetic variation may arise within a species that causes behavioral changes.
    For example, changes may cause some individuals to feed at night. Eventually, these individuals will become nocturnal, essentially isolating themselves from the diurnal individuals. Adaptation may cause changes in eyesight, coloration, and other traits to allow these organism to survive. This may lead to speciation. Another example of sympatric isolation is physiological and anatomical isolation: Variations in physical attributes may prevent breeding among individuals within a population, which may eventually lead to speciation. Consider one example from above: although all dogs are the same species, they can be very different. The Chihuahua will find it difficult, if not impossible, to mate with the Great Dane due to differences in their sizes. Thus, given enough time, the two strains may eventually evolve into two different species.

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