species could have evolved independently from distinct sources and converged in their chromosomal ideotype until they became members of the same cytodeme (including the capacity to cross-breed). It is more likely, though, that the cytodeme arose first complete with its suite of chromosomes and breeding patterns all intact and then, remaining constant in its fundamentals, it diversified into species sometimes so different as to merit generic distinction. Thus, in what may be termed the cytodeme adjunct to Darwinian theory, evolution becomes a two-stage process - first, the establishment of distinct cytodemes reproductively isolated both from one another and from all previously existing cytodemes; second, diversification within cytodeme to yield taxonomically recognisable (but not reproductively isolated) species. Whereas the second stage is Natural Selection as expounded by Darwin, the first stage is not necessarily Darwinian. In the special case of polyploidy it is known that the first stage is not Darwinian. Doubling the chromosome number of the sterile hybrid between two diploid cytodemes yields an allotetraploid which is both fertile within its bounds and essentially incompatible with all previous life forms. As yet no mechanism is known which could account for the origin of new diploid cytodemes - nor is there any proof that there are any diploid cytodemes of recent origin. Indeed, the evidence is that cytodemes are remarkably long lived. Presumably the Platanus cytodeme existed on Pangaea before that land fragmented. The present day forms of Platanus - P. orientalis in Europe and P. occidentalis in America - are fully intercompatible when brought together artificially in Botanic Gardens yielding fertile hybrids like the London plane, establishing not only that they are in the same cytodeme as one another but also that they are both in the same cytodeme as their common ancestor of approximately 200 million years ago. References Further reading Lannér, Carita. "Genetic relationships within the Brassica oleracea cytodeme: comparison of molecular marker systems." Acta Universitatis Agriculturae Sueciae. Agraria (Sweden) (1997). LIU, Yao, and Wei QIAN. "Crossability among Variant Types of Brassica oleracea Cytodeme." Journal of Plant Genetic Resources 14.2 (2013): 342-346. Gómez-Campo, César, ed. Biology of Brassica coenospecies. Vol. 4. Elsevier, 1999. Kowalski, Stanley P., et al. "Comparative mapping of Arabidopsis thaliana and Brassica oleracea chromosomes reveals islands of conserved organization."Genetics 138.2 (1994): 499-510. Mei, J., et al. "Genetic investigation of the origination of allopolyploid with virtually synthesized lines: application to the C subgenome of Brassica napus."Heredity