Chapter 7 SPACE, TIME, AND VARIATION s G gi l~ oj . some species can Indeed be said to dl C( have had a single and sudden origin, local­ o Ized and capable of being located. With is others, however, the or/gin Is no origin at 01/ d, but a gradual transformation extending over wide areas and long periods and shifting Its focus In the course of time. Between the two Is every gradation. D.C. Darlington and EK Janakl Amma!, 1945 1 n s' b c s s e ( '\;, Space, Time, and Variation! KINDS OF PATTERNS OF VARIAnON Geographic patterns of variation have historically been used to trace the ori­ gin and evolution of cultivated plants. We have seen that Vavilov (1926. 1949/50) thought that areas of maximum genetic diversity represented centers of origin and that the origin of a crop could be identified by the simple proce­ dure of analyzing variation patterns and plotting regions where diversity was concentrated. It turned out that centers of diversity are not the same as centers of origin. yet many crops do exhibit centers of diversity. The phenomenon is real and requires explanation. What causes variation to accumulate in secon­ dary centers is not too well understood, but some observable factors are: 1. A long history of continuous cultivation. 2. Ecological diversity. many habitats accommodate many races. 3. Human diversity, different tribes are attracted to different races of the crop. 4. Introgression with wild and weedy relatives or between different races of a crop. There may be other causes. but the reasons for secondary centers are hu­ man. environmental. and the internal biological dynamics of hybridization. segregation. and selection. A crop-by-crop analysis shows the situation to be much more complex than that conceived by Vavilov. Many crops did not originate in Vavilovian centers at all, and some do not have centers of diver­ sity; several can be traced to very limited and specific origins, and others seem to have originated all over the geographical range of the species. It seems evident that if a crop originated in a limited area and did not spread out of it, the center of origin and the center of whatever diversity there may be would coincide. Both space and time are involved, and different crops have different evolutionary patterns. The main patterns can be classified as follows: Endemic. Crops that originated in a limited area and did not spread ap­ preciably. Examples: Brachiaria def/exa in Guinea (Fig. 7-1), Ensete ven­ tricosa in Ethiopia, Digitaria iburua in West Africa, Setaria geniculata in ancient Mexico (Callen, 1967), and Panicum sonorum in modern Mexico. Semiendemic. Crops that originated in a defmable center and with limited dispersal. Examples: Eragrostis tel and Guizotia abyssinica are Ethiopian domesticates; both are grown on a limited scale in India. Basic to the Ethio- I See Table 3-1, Chapter 3, for a listing of scientific and common names for cultivated plants. 137 138 CROPS AND MAN SPA( pian diet, they are not important elsewhere. African rice (Oryza glaberrima) is another example (Fig. 7-2); the center of origin is probably the flood ba­ sal \\ sin of the central Niger (Port'eres, 1956) whence it was distributed to Sene­ earl gal, southward to the Guinea coast, and eastward as far as the Lake Chad Anci area. Some of the minor tubers of the South American highlands, such as time Oxalis tuberosa, UI/ucus tuberosus, and Tropaeolum tuberosum, would also fall into this class (Leon, 1964). and East spp. in II over GL ~ DIGITARIA EXILIS _ DIGITARIA IBURUA II Inf:N\:{J BRACHIARIA DEFlEXA •I l1] Fig. 7-1. Some lesser millets of West Africa. Brachiaria dli!/lexa and Digitaria iburua are en­ Fig. demic crops. ri, SPACE, TIME, AND VARIATION 139 Monocentric. Crops with a definable center of origin and wide disper­ sal without secondary centers of diversity. Examples: Arabica coffee and hev­ ea rubber. Crops of this class are mostly new plantation or industrial crops. Ancient widespread crops usually develop secondary centers, but this takes time. OUgoeentrie. Crops with a definable center of origin, wide dispersal, and one or more secondary centers of diversity. Examples: the whole Near East complex of barley, emmer, flax, pea, lentil, oats, chickpea, Brassica spp., etc.; all have secondary centers in Ethiopia and some also have centers in India and/or China. Noneentrie. Crops whose patterns of variation suggest domestication over a wide area. The suggestion may be misleading, of course, but centers ~ YAM BELT GLABERRIMA RICE _ PRIMARY CENTER _ SECONDARY CENTERS _ MAIN RICE BELT [WW!!!WiJ INTERMITTENT RICE CULTURE '-----~--~----' Fig. 7-2. Distribution of major areas of cultivation of yams and African rice. The g/aberrima rice has a semiendimic variation pattern. 140 CROPS AND MAN sp, are either not apparent or anomalous. Examples: sorghum, common bean, and Brassica campestris. we ter Af du NONCENTRIC CROPS fOi do Aside from the subjective questions of how endemic is "endemic" and gel how widespread is "widespread", the categories are clear and self-evident except for the one called "noncentric". It seems to me that some crops sim­ ply do not have centers, and the concept of centers of either origin or diver­ ac wr sity as universal phenomena can be called into question. Sorghum is selected as an example of a noncentric crop; i.e., neither in~ a center of diversity nor a center of origin is evident from the distribution of variation alone. Vavilov had indicated that Ethiopia was a center of diversity-center of origin for sorghum, but he did not know sorghum well and did not understand African crops. To some extent, Ethiopia is a center of diversity for the durra race and is certainly the main center of diversity for the durra-biocolor race, but these are only 2 of 15 races according to the classification of Harlan and deWet (1972). All other cultivated races are rather poorly represented in Ethiopia. The region from eastern Nigeria through Chad and western Sudan is -~z, a center of diversity for the caudatum, guinea-caudatum, and durra­ caudatum races, but not for the races of Ethiopia. The region from western ,~ Nigeria to Senegal is the center of diversity for the guinea race, and while there are some durras in the drier zones, other races are poorly represented. The area from Tanzania to South Africa is the center for the kafir race and although guineas and guinea-kafirs are found, they are not especially varia­ ble. There is no area in Africa where the diversity of even several races is highly concentrated. Snowden (1936) suggested that the several major races of sorghum had been ennobled separately from distinct wild races. This may, indeed, have been the case, although we have yet to see much evidence either for or against the theory. Multiple ennoblements are probably common, however, and must be dealt with by geographers and students of plant domestication. The distribution of the races of sorghum in Africa is shown in Fig. 7-3 to 7-6. The patterns are remarkably consistent and clear-cut and presuma­ bly mean something with respect to the origin and evolution of sorghum. Harlan and Stemler (1976) attempted to reconstruct the history of sorghum domestication using this and other distributional information. The results are shown in Fig. 7-7 and 7-8. While we show a center of origin labelled Early Bicolor in Fig. 7-7, this region was not selected because of any clues given by variation patterns in modern cultivated sorghum. It was chosen be­ cause (i) archaeological evidence suggests African agriculture originated north of the equator, (ii) the West African race of wild sorghum is a forest grass with an adaptation quite different from that of the crop, and (iii) the region outlined includes the most massive stands ofwild sorghum adapted to a savan­ Fi! na habitat north of the equator. SPACE, TIME, AND VARIATION 141 Today the most common cultivated race in the region is caudatum, which we consider to be a relatively new race. One could describe the pattern in terms of a series of secondary centers, e.g., West Africa for guinea, southern Africa for kafir, Sudan-Chad-Uganda for caudatum, and Asia-Ethiopia for durra, but these are not centers of diversity for sorghum; they are centers for only certain races of sorghum. Some crops clearly have centers and others do not. Variation patterns must be analyzed in each crop separately before generalizations can be made. t The idea of a noncentric crop is not new and was well documented by a distinguished member of Vavilov's professional team. E.N. Sinskaya (1928) wrote a monograph on some of the cruciferous crops; she made the follow­ ing observation about Brassica campestris: The geographical distribution of the forms of colza, as it may be pictured on the basis of data available at the present time, points in no way to the existence of a special centre of diversity. To every region corresponds a definite ecotype. The introduction into cultivation has taken place, and is still taking place, in every region independently of any "centre". The cultivated forms are identical is .­ n Ie i. ld a­ is ld ve lSt lSt -3 a­ n. m Its ed les ,e­ ih lSS on m- Fig. 7-3. Distribution of guinea and half-guinea sorghums in Africa: guinea, • ;guinea-kafir, o ; guinea-caudatum, ~; guinea-durra, e. 142 . CROPS AND MAN SPACE. TTh with the local weeds; the local climatical ecotype being rrrst distributed as weed, becomes afterward a cultivated crop. Crop Nothing since has been found to change that impression.