Most known species existing on the Earth reproduce sexually. Members of a single species cross almost exclusively amongst themselves. They mostly do not cross with the members of other species, or at least their crossing does not yield progeny. Barriers preventing crossing between species and thus ensuring reproductive isolation are basically of two types, external and internal. External barriers are formed, e.g., by mountain ranges, which separate the areas of occurrence of the two species; internal barriers consist, e.g., in the number and shape of chromosomes, which differ in the two species and thus prevent meiotic division, necessary for the formation of sex cells, from progressing to its conclusion. In some cases, it is not easy to decide which type of barrier is involved. For example, most biologists would classify as an external barrier the incompatibility caused in many species of insects by infection of part of the population by parasitic bacteria of the Wolbachia genus, which is capable of preventing reproduction of an infected individual with an uninfected individual or an individual infected by a different strain of this bacteria. However, if the cause of the infection were a virus hiding directly in the DNA of the cell, most biologists would probably consider the resulting reproduction barrier to be an internal barrier.
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