Geographical isolation rather than convergent evolution explains
significant high genetic but low morphological differentiations between
two cycad species
Abstract
Exploring the driving forces promoting speciation is a key question in
evolutionary biology. Cycas bifida and C. micholitzii are
two leaflets dichotomously divided Cycas species that have
morphological similarities. Their distribution areas are separated by
the RRFZ, a biogeographical barrier for Cycas species. The
hypothesis for explaining their divergence is geographical isolation or
convergent evolution. Three chloroplast DNA fragments, five low copy
nuclear genes and 16 microsatellite loci were used to test this
hypothesis and also to reveal their population genetics. The two species
had high genetic differentiation but low gene flow with a deep
divergence occurred in the late Miocene. The long-term geographical
isolation not convergent evolution could explain for the divergence of
the two species revealed from significant IBD testing, Barrier analysis
and Niche consistency detection. Subsequently, each species made its own
response to the Pleistocene climate fluctuations: a weak bottleneck
effect in C. bifida and a population expansion in C.
micholitzii. These findings imply that the role of geographic isolation
rather than convergent evolution facilitates them divergence by
constraining gene flow. Maybe they have gene flow or introgression with
their neighboring distribution of Cycas species, making them
genetically closer to neighboring Cycas species.