hybrids. Therefore, if the barrier separating newly formed species broke down and the two species came into contact, they would remain phenotypically distinct due to the evolution of reproductive isolation. Hybrid inviability and sterility are examples of 'post-zygotic' isolating mechanisms because the isolating mechanism affects individuals after a zygote is formed. Alternatively, allopatric populations could evolve 'pre-zygotic' isolating mechanisms that prevent successful mating or zygote formation, such as courtship differences or gametic incompatibilities.
The role of geographic isolation in the generation and maintenance of biological diversity has been long recognized by naturalists and evolutionary biologists, even well before publication of Charles Darwin's The Origin of Species in 1859. By the early twentieth century, however, geographic separation was considered only a secondary process of speciation, largely by population geneticists who felt that most new species arose together in sympatry. It was not until the publication of several seminal volumes by Ernst Mayr, and subsequent research during the period known as the 'modern synthesis' of evolutionary biology that allopatry gained wide acceptance as a common mechanism of speciation.
In his pioneering book, Systematics and the Origins of Species, Mayr coined the term 'allopatry' to describe two populations isolated in different geographic areas. Mayr then argued that a severe reduction of gene flow between allopatric populations plays a fundamental role in speciation. This hypothesis involved the development and synthesis of three important ideas. First, Mayr championed a new concept of the species based on biological factors (the 'biological species concept'), defining species as groups of interbreeding populations that cannot interbreed with other similar groups. Second, Mayr called attention to the fact that most species exhibit substantial geographic variation, forming distinct races, varieties, and subspecies; the most distinct varieties were often found in geographic isolation with respect to the main range of the species. Mayr asserted that geographic variation represented adaptation to local environments and that the features that distinguish local geographic variants are usually the same kind that typically differentiate reproductively isolated species. In other words, Mayr felt that distinct geographic variants represent intermediate stages in the process of specia-tion. Mayr then reasoned that completely geographically isolated populations would ultimately evolve reproductive isolation as a by-product of adaptive divergence. He hypothesized that natural selection, brought about by differential environmental or ecological conditions, causes an accumulation of adaptive genetic changes between allopatric populations that eventually result in negative epistasis, causing inviability or sterility in
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