If the distribution of every species within a genus is drawn on a single map, it will usually be found that there are one or more areas with a marked concentration of species. Such an area is termed a centre o f genetic diversityfor that genus. Often there is a single such area for the genus (or other greater or lesser taxon) concerned, and there are progressively fewer species found as the distance from the centre of diversity is increased. This topic was first studied in detail for crop plants, particularly cereals and legumes, by Vavilov in the 1920s and 1930s,454 and an updated discussion of the topic is provided by Zeven and Zhukovsky.486 Vavilov and later workers found, moreover, that there is a frequent coincidence in the centres of diversity for many different, unrelated taxa, and that a relatively small number of major centres of diversity can be recognized in the world. Zeven and Zhukovsky defined twelve such centres (Fig. 7.4), but there are good arguments for recognizing fewer and smaller centres. All of them lie in tropical, subtropical or warm-temperate areas, and they have provided virtually all the world’s important crop plants. When the centres of genetic diversity for non-crop
166 Information from plant geography and ecology
Fig. 7.4 Zhukovsky's map of the twleve centres of genetic diversity, adapted from Zeven and Zhukovsky.486 Superimposed on this map (in black) are the three major centres in which agriculture is believed to have begun (after Harlan184).
species are similarly plotted, they are mostly found to coincide with those of the crop species.
Vavilov believed that the centres of diversity were also the centres of origin of the taxa concerned. This may well be true in some cases, especially in the tropical parts of large continental masses which have probably varied in climate rather little for many thousands of years, but in other areas there have been major changes in climate and large-scale movements of land-masses since the time of origin of modern plant taxa, and it is unlikely that there is in general a close relationship between the centres of diversity and origin. Nevertheless, the centres of diversity today probably hold the main remnants of the ancestral genetic material of the taxa concerned. The older the taxon, the less likely the coincidence of the centres of origin and diversity. Stebbins,421 however, considered that the processes of evolution have operated in the past in a similar manner to now (‘principle of genetic uniformitarianism’). Using this principle he deduced that angiosperms origi nally diversified in semi-arid zones, since those habitats now support the greatest number of species per genus.
Each monophyletic taxon has, by definition, a single centre of origin. By study of the characteristics of the taxon in its centre of diversity in relation to its characteristics progressively further from the centre, the major patterns of evolution and pathways of migration can very often be deduced. The ways in which the taxon has adapted to the different environments which it has encountered during its migrations are thus frequently uncovered. Such an ‘outward’ pattern of evolutionary migration is known as adaptive radiation. Its demonstration may be of great assistance in deciding upon the taxonomic relationships and delimitations of the various biotypes encountered within a taxon. As precise as possible a characterization of the biotypes is important, since anything less might well fail to differentiate between separate biotypes
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which are superficially similar. Frequently cytological study is important. It is commonly found, especially in temperate taxa, that plants near the centre of diversity are diploid, whereas high proportions are polyploid towards the edge of the range, i.e. polyploidy appears to have been an important aid to adaptive radiation in many instances.
It should be emphasized that the methods described above are just as often valuable in studying the migration of the variants of a single species as in studying the species of a single genus. Favarger131 has found that the cytology of a great many alpine endemics throws light on their evolutionary pathways. For example, the fact that Paradisea liliastrum is diploid in the Pyrenees but polyploid in the Alps suggests that it migrated from the Pyrenees to the Alps, where it became polyploid.
Sometimes there appears to be more than one centre of genetic diversity for a taxon. This might indicate that the taxon is polyphyletic, having arisen in more than one area. However, often all but one (or even all) of the centres are secondary centres o f genetic diversity—areas where the taxon was able to diversify (perhaps due to the presence of a large number of habitats) to a greater extent than in other areas a similar distance from the primary centre. For example, a primarily South African taxon might exhibit a secondary centre of diversity in the Mediterranean basin, or a primarily Balkan taxon might have a secondary centre in Spain.
Usually it is possible to differentiate between primary and secondary centres of diversity if thorough biosystematic investigations have been carried out. Thus the grass genus Vulpia has its primary centre of diversity in the western Mediterranean, with secondary centres in temperate North and South America and in south-western Asia. The diversification in the latter areas is based on only one of the five sections of the genus found in the western Mediterranean and, moreover, there are higher proportions of polyploid taxa in the secondary areas than in the primary one.
This decrease in variability, i.e. in the number of biotypes, away from the centre of diversity is known as biotype depletion, which, as intimated above, can be used to investigate pathways of migration. Bocher41 found that
Ranunculus glacialis, an arctic-alpine species, is far richer in biotypes in the
Alps than in the arctic. This diminution might have originated either during a northwards migration after glaciation or from a greater loss of biotypes in the north during perglacial survival. Biotype depletion is also one possible explanation for the observation that species frequently appear to be far more demanding ecologically near the edge of their geographical range than in the centre of it. In Europe this is very often manifested by the northern botanist finding that species which are confined to particular habitats in his own country are far less so restricted in southern Europe.
The concept of centres of genetic diversity (and, even more, centres of origin) has been strongly criticized in recent years by vicariance biogeographers,76 who prefer to think in terms of the ‘generalized track’ of a taxon differentiating allopatrically in different regions. Almost certainly, this is a better model than a centre of origin in some instances, a less good one in others, and in yet further cases something between the two is most accurate. As intimated earlier, a centre of origin is a more valid concept in the case of younger taxa (and hence usually at the lower levels of the hierarchy),
particularly where differentiation has occurred more recently than any known vicariance events. The evolution of modern cereals is a good example.