Positive selection alone is sufficient for whole genome differentiation at the early stage of speciation process in the fall armyworm

Background: The process of speciation inherently involves the transition from genetic to genomic differentiation. In the absence of a geographic barrier, the whole genome differentiation may occur only when the homogenizing effect of recombination is overcome across the whole genome. The fall armyworm is observed as two sympatric strains with different host-plant preferences across the entire habitat. These two strains exhibit a very low level of genetic differentiation across the whole genome, suggesting that whole genome differentiation occurred at an early stage of speciation. In this study, we aim at identifying critical evolutionary forces responsible for the whole genome differentiation in the fall armyworm. Results: We found that these two strains exhibit a low level of genomic differentiation (Fst = 0.0176), while 91.3% of 10kb windows have genetically differentiated sequences (Fst > 0). We observed that a genomic reduction in migration rate due to combined effects of mild positive selection and genetic linkages to selectively targeted loci are responsible for the whole genome differentiation. Phylogenetic analysis shows that positive selection generates the whole genome differentiation by sub-setting of variants in one strain from the other. Conclusions: From these results, we concluded that positive selection alone is sufficient for whole genome differentiation during the process of speciation. This study demonstrates that the propensity of adaptation alone determines the speciation events, suggesting that adaptive evolution is a single critical driving force for species diversity.