[2] Initially considered to be caused by elevated rates of mutation in hybrids, the most probable hypothesis infers that they are the result of negative (purifying) selection.
[3] Originally hybrizymes were defined as "unexpected allelic electromorphs associated with hybrid zones", a formal term proposed by renowned conservation geneticist and biogeographer David S. Woodruff in 1988.
[1] By suggesting a new definition for a phenomenon that had been previously widely observed Woodruff's interpretation bypasses the etiological connotation of alternative terms and avoids inappropriate context.
The hybrizyme phenomenon is widespread in hybrid zones of species of snails, crickets, lizards, salamanders, rodents, fish and birds.
The phenomenon has also been detected in a broad range of genetic markers such as intron haplotypes,[3] microsatellites,[7] ribosomal DNA spacer variants,[8] and anonymous SNPs.
Research on pocket gophers and Japanese freshwater crabs confirms that the phenomenon is possibly caused by simple nucleotide substitutions.
However, research on Acer species implies that high recombination rates are possible due to acceleration of genetic variation after hybridization.
[18] Several hypotheses have been proposed to account the high frequency of hybrizymes in hybrid zones such as genetic drift, elevated rates of nucleotide substitutions.
[21] The exact origin and mechanism that maintains these alleles at a high frequency is still a subject of debate and additional studies, such as Next Generation Sequencing analysis of the genomic regions involved in the phenomenon as a more trustworthy pathway to identify genes that impact the level of reproductive isolation.
Consequently, there will be recurrent removal of disadvantageous alleles for reproductive isolation and relative stabilization of hybrid zones, possibly slowing down the path of complete speciation by reinforcement.