Mechanisms of MacroevolutionThe following deals with mechanisms of evolution above the species level. Speciation -- Increasing Biological Diversity Speciation is the process of a single species becoming two or more species. Many biologists think speciation is key to understanding evolution. Some would argue that certain evolutionary phenomena apply only at speciation and macroevolutionary change cannot occur without speciation. Other biologists think major evolutionary change can occur without speciation. Changes between lineages are only an extension of the changes within each lineage. In general, paleontologists fall into the former category and geneticists in the latter. Modes of SpeciationBiologists recognize two types of speciation: allopatric and sympatric speciation. The two differ in geographical distribution of the populations in question. Allopatric speciation is thought to be the most common form of speciation. It occurs when a population is split into two (or more) geographically isolated subdivisions that organisms cannot bridge. Eventually, the two
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