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Modes of Speciation

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

Polyploidy

While many models of sympatric speciation remain somewhat controversial among evolutionary biologists, most (if not all) agree that sympatric speciation can occur through polyploidy, the inheritance of more than two basic genome copies as a result of errors during cell division. Polyploids (organisms with more than two duplicate sets of chromosomes) are named according to the number of chromosome sets they have within their nuclei: a triploid has three sets of chromosomes (3n), a tetraploid has four sets (4n), etc.

Polyploid organisms are immediately reproductively isolated from their diploid parent species. For example, a failure during meiosis can cause diploid (2n) parent species to produce tetraploid (4n) offspring. However, crosses between a diploid and a tetraploid will result in inviable or sterile offspring due to abnormal chromosomal pairings during mitosis or meiosis. In fact, for the tetraploid organism to successfully reproduce, it must fertilize itself or mate with another tetraploid. Thus, polyploidy can result in instantaneous speciation in a single genetic event. Polyploidy also provides an immediate way for hybrids of otherwise incompatible parent genomes to produce fertile offspring.

Polyploidy is very common in plant species. It is estimated that 35% of flowering plant species are polyploid. While relatively rare, polyploidy is also found in animal species, such as the frog Hyla versicolor, a tetraploid derived from the diploid Hyla chrysoscelis.