When species originate through genome-level changes involving primary species, the term "secondary" speciation has been applied (Grant, 1981). Although debate continues on defining terms and especially in determining types of polyploids (Soltis and Rieseberg, 1986), a "taxonomic" approach to defining the different kinds of secondary speciation is applied here. Thus, secondary speciation can occur through genome multiplication within a lineage (autopoly-ploidy), through hybridization between lineages that brings together two distinct genomes but does not involve genome doubling (allohomoploidy), or through hybridization between lineages followed by genome doubling in the (usually) sterile hybrid (allopolyploidy). Although substantial secondary speciation has been documented in the ferns and among members of the Lycopodiaceae and Isoetaceae, and despite the discovery of hybrids (e.g., Tryon, 1955; Somers and Buck, 1975) and dysploidy (Marcon et al., 2005) in the Selaginellaceae, and hybrids and triploidy (Bennert et al., 2005) in the Equisetaceae, secondary speciation appears absent from these latter two families.
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