Patterns of evolution in Graomys griseoflavus (Rodentia, Muridae). IV. A case of rapid speciation

The South American group of rodents known as Graomys griseoflavus comprises two sibling species differing only in diploid chromosomal complement: G, g riseoflavus (2n = 36, 37 and 38) and G, centralis (2n = 42). Reproductive b arriers comprising postzygotic as well as precopulatory mechanisms prevent gene exchange between these species. We have studied genetic polymorphism i n two populations of G, centralis and four of G. griseoflavus by means of g el electrophoresis of enzymes and other proteins giving information on a to tal of 30 loci. Values of interspecific genetic identity were similar to th ose obtained for conspecific populations, suggesting that fixation of Rober tsonian fusions would have occurred without significant bottlenecks. It wou ld also indicate that the cladogenetic process must be relatively recent. F -IS, values showed no evidence of inbreeding. Fixation indices (F-ST) for t he ancestral species showed a tendency to form demes with very low gene flo w among them, while in the derived species such tendency was not shown. How ever, because of the characteristics of the region they occupy, populations are of moderate size, and genic flow is low. Lack of correlation between g ene flow levels and geographical distance between population pairs would in dicate a recent and fast colonization of its distribution areas by the deri ved species. It is possible that fixation of Robertsonian fusions occurred in a marginal deme of the ancestral species, e.g. in a parapatric geographi cal context.