One of the biggest questions in evolutionary biology is how biological species arose.Since biological species are groups of interbreeding natural populations reproductively isolated from other such groups (Mayer,1942),reproductive isolation is the key process of the speciation.Among various isolating barriers,postzygotic barriers ranging from maladaptation to inviability or sterility in the offspring have attracted biologists attention because of the problem of the evolution of the "unfit" offspring.