The genus Cryptomys contains a number of social, subterranean rodents that are widely distributed throughout subSaharan Africa. Specimens of Cryptomys from 23 localities in south-west Zambia were karyotyped using a standard staining protocol. A minimum of five metaphases per specimen was scored for 2n and the fundamental number (NF) was determined in females. Nine new karyotypes, which may represent several new species, were identified: (1) 2n = 42, NF = 78 (Dongo, Southern Province); (2) 2n = 44, NF = 76 (Salujinga, North-western Province); (3) 2n = 45, NF = 78 (Lochinvar, Southern Province); (4) 2n = 52, NF = 86 (Chinyingi, North-western Province); (5) 2n = 54, NF = 78 (Monze, Southern Province); (6) 2n = 56, NF = 76 (Watopa, North-western Province); (7) 2n = 58, NF = 80 (Livingstone, Southern Province); (8) 2n = 58, NF = 86 (Senanga, Western Province); (9) 2n = 60, NF = 82 (Kataba, Western Province; type locality of C. damarensis micklemi). Contrary to previous reports, the specimens from Kataba and Senanga on the left bank of the Zambezi do not correspond to C. damarensis and should be considered a separate species: C. micklemi (as confirmed by molecular analyses; Ingram, Burda & Honeycutt, 2004). According to the karyotype, C. damarensis occurs only on the right bank of the Zambezi River in the Western Province. In contrast to the high karyotypic variability on the right bank of the Kafue River, it was found that C. anselli (2n = 68) is widely distributed throughout the Central province on the left bank of the Kafue River. The resulting pattern of occurrence of the different karyotypes correlates well with the extant river system configuration that separates most karyotypes. We hypothesize that geomorphological changes and in particular river system dynamics in recent geological times have played an important role in the chromosomal diversification and may have provided opportunities for speciation to occur.

A commonly held view in evolutionary biology is that speciation (the emergence of genetically distinct and reproductively incompatible subpopulations) is driven by external environmental constraints, such as localized barriers to dispersal or habitat-based variation in selection pressures. We have developed a spatially explicit model of a biological population to study the emergence of spatial and temporal patterns of genetic diversity in the absence of predetermined subpopulation boundaries. We propose a 2-D cellular automata model showing that an initially homogeneous population might spontaneously subdivide into reproductively incompatible species through sheer isolation-by-distance when the viability of offspring decreases as the genomes of parental gametes become increasingly different. This simple implementation of the Dobzhansky-Muller model provides the basis for assessing the process and completion of speciation, which is deemed to occur when there is complete postzygotic isolation between two subpopulations. The model shows an inherent tendency toward spatial self-organization, as has been the case with other spatially explicit models of evolution. A well-mixed version of the model exhibits a relatively stable and unimodal distribution of genetic differences as has been shown with previous models. A much more interesting pattern of temporal waves, however, emerges when the dispersal of individuals is limited to short distances. Each wave represents a subset of comparisons between members of emergent subpopulations diverging from one another, and a subset of these divergences proceeds to the point of speciation. The long-term persistence of diverging subpopulations is the essence of speciation in biological populations, so the rhythmic diversity waves that we have observed suggest an inherent disposition for a population experiencing isolation-by-distance to generate new species.