  In many animals parental gametes unite to form a zygote that develops into an
adult with gonads that, in turn, produce gametes. Interruption of this germinal
cycle by prezygotic or postzygotic reproductive barriers can result in two
independent cycles, each with the potential to evolve into a new species. When
the speciation process is complete, members of each species are fully
reproductively isolated from those of the other. During speciation a primary
barrier may be supported and eventually superceded by a later appearing
secondary barrier. For those holding certain cases of prezygotic isolation to
be primary (e.g. elephant cannot copulate with mouse), the onus is to show that
they had not been preceded over evolutionary time by periods of postzygotic
hybrid inviability (genically determined) or sterility (genically or
chromosomally determined). Likewise, the onus is upon those holding cases of
hybrid inviability to be primary (e.g. Dobzhansky-Muller epistatic
incompatibilities), to show that they had not been preceded by periods, however
brief, of hybrid sterility. The latter, when acting as a sympatric barrier
causing reproductive isolation, can only be primary. In many cases, hybrid
sterility may result from incompatibilities between parental chromosomes that
attempt to pair during meiosis in the gonad of their offspring
(Winge-Crowther-Bateson incompatibilities). While WCB incompatibilities have
long been observed on a microscopic scale, there is growing evidence for a role
of dispersed finer DNA sequence differences.
