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Modes of Speciation

Author(s): Tadzia GrandPre, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Parapatric Speciation

Both allopatric and peripatric speciation occur when populations are physically separated and therefore do not exchange genes. In contrast, parapatric speciation involves the evolution of reproductive isolation within a population where gene exchange is possible because there is no physical barrier separating individuals. However, although the population is continuous, it does not interbreed randomly and the degree of gene exchange is modest. For example, if the population of a single species expands its range, some individuals may grow in new areas with distinct environmental conditions. Over time, the "subpopulation" that inhabits the new area, or a fringe region adjacent to a new area, has potential to become genetically distinct from the rest of the population as it adapts to the new environmental conditions. As genetic differences accumulate, gene flow between the incipient species may slow. Eventually, if selection pressures are strong enough, reproductive isolation will evolve and the subpopulation will become a new species.

One of the best documented examples of parapatric speciation involves the evolution of tolerance to heavy metals in the subpopulation of grass Anthoxanthum odoratum. Populations of A. odorantum growing in close proximity to abandoned mines were observed to have diverged from neighboring populations as a result of selection pressure for heavy metal tolerance. In addition to evolving tolerance to heavy metals, the new populations self-pollinate and flower at different times. These characteristics reproductively isolate the heavy-metal tolerant populations.