Origin of the common bean, Phaseolus vulgaris

Origin of the Common Bean, Phaseolus vulgaris

H O W A R D SCOTT GENTRY 1

A wild leguminous vine growing in Mexico and Central America is identified as conspecifie with the cultivated Phaseolus vulgaris. The exact origin of the common cultivated bean has not certainly been known. Evidence from comparative morphology, geographic distribution, ecology, genetic relationship, and archeologic history all indicate the wild vines are progenitors of the common American bean. Collections of both wild and cultivated seeds graphically illustrate the evolution of the cultivated beans and display evidence for multiple origins in Mesoamerica. The recognition of the wild beans and their acquisition pro- vides agriculture with a renewed genetic re- source for the protein deficient world.

Introduction The wild ancestor of the common bean,

Phaseolus vulgaris L., has not been known until recent times, and no account has been written of its occurrence in North America. De Candolle, in his classic account of the origin of cultivated plants in 1882 (5), was not certain whether the cultivated forms came from the Old or New World. In 1935 Vavilov (13), because of the rich assort- ment of varieties found in North America, nominated Mesoamerica as the center of origin of the common bean. His collectors in the Americas (1), however, failed to lo- cate the wild progenitor. It appeared that the forerunner of the domesticates was perhaps already extinct, like ancestral maize, which with beans formed the nutrient basis of American civilizations. Burkart in 1952 (2, p. 545, 546) described Phaseolus aborigineus from Argentina as the wild ancestor of the cultivate. In 1953 he reduced this to a subspecies of P. vulgar~ (3). In 1948 Oliver Norvell, an astute student of beans, annotated several collections in the U.S. National Herbarium as wild forms of P. vul-

t Botanist, Crops Research Division, Agricul- tural Research Service, United States Depart- ment of Agriculture, Beltsville, Maryland.

Received for publication May 2, 1968.

garis. The earliest of these was collected by Edward Palmer in 1896 in the "vicinity of (Ciudad) Durango," Durango. Is this strip- ling vine species, with its small twisting pods and small seeds, really the progenitor of our succulent snap beans and our bush dry beans?

In conjunction with the bean germ plasm program of the Crops Research Division of the U.S. Department of Agriculture, it was decided to give first priority to the collec- tion and investigation of the wild relatives of Phaseolus vulgaris. From the time of Vavilov onward, plant breeders have found that regions of origin provide the richest as- sortment of genes for improvement of crops, and especially for developing resistance to diseases and to insects, a primary aim of our bean program. Two field trips during the falls and winters of 1966 and 1967 were spent in collecting and note-taking on wild and cultivated beans in Mexico. The following account attempts to enlighten the old problem of origin and, in so doing, compares some morphologic, geographic, anthropo- logic and genetic relationships between the Mexican wild forms and their nearest relatives, the cultivates.

General and Specific Characters of the Wildling

The wild plants of Phaseolus vulgaris are annual or, more rarely, short-lived perennial climbing vines, flowering and fruiting the first year. Germination is with the onset of the summer rains and by late fall the slender vines are 8 to 10 feet long, scattered and sprawling in or over bushes and shrubs. The flowers are generally lavender, some- times pink, and more rarely white, as at Tepoztlan. The flowers also vary in size and in the proportions of the organs, e.g., the long wing petals characteristic of the population above Cofradia near the western base of Volcan de Colima. Stems and leaves vary in size and indument, according to site and to genetic factors. The pods, from 2 or 3 to 8 or 10 to a raceme, hang characteristically from the divergent axillary rachises,

55

56 ECONOMIC BOTANY

Fig. 1. A wild bean vine hanging upon a Solanum shrub, 10 miles S of Uruapan, Michoacan. The broad leaflets are characteristic of both wild and cultivated forms.

obscure among the twigs and leaves of the protective thickets (Figs. 1-3). The pods also vary in size and coloring, a red or pur- plish brindling being common, but nearly all are twisting dehiscent, ejecting the seeds violently when they reach a certain point of dryness. Older plants with several branches or shoots, as the perennials, bear as many as 240 pods by actual count (Fig. 1).

The seeds are most striking in their variability of shape, color, patterning, and size

(Fig. 7). Various gray and brown mottled patterns were found most commonly, but solid yellows, duns, and blacks occur, some of them regionally characteristic as will be discussed later. Most of the wild vines have small seeds, measuring about 5 mm long, 4 mm wide, and 2-2.5 mm thick as a general average. However, some individual vines, one in about several hundred, have much larger seeds, as the following samples show:

GENTRY: ORIGIN OF PHASEOLUS VULGARIS 5 7

Fig. 2. Vine from the Durango population for the plant press. Some of the peduncles are longer than the petioles, contrary to a key character of authors for P. vulgaris.

Uruapan (pop. mean) 5 X 3.6 X 2 mm or 36 cubic mm (22153):

Tepoztlan (single pl.) 10 X 5.5 X 3 mm or 275 cubic mm (22497)

La Garrita (single pl.) 9 X 6.5 X 5 mm or 292.5 cubic mm (22266)

e Herbarium specimens of such numbers cited here are deposited in the U.S. National Her- barium or the U.S. National Arboretum herbarium.

Oaxaca (single pl.) 14 X 8 X 5 mm or 560 cubic mm (22505)

The latter two have larger seeds than some of the common cultivates.

Perennialism is frequent in the wild bean populations of Nayarit, Jalisco, and Michoa- can. Such plants develop corky stems up to one-half inch in diameter. The roots thus far examined are always fibrous, without tuberous thickening, and they are shallowly

58 ECONOMIC BOTANY

Fig. 3. A bag of beans as picked for seed from the Tetitlan lava beds, Nayarit. The pods are longer than average while the variability in brindling is common to other populations as well.

deployed. Only one perennial vine was observed in Oaxaca, Gentry 22505, which had very large black seeds. No perennials were observed in the Morelos and Guerrero populations. Field observations indicate that perennialism may tend to be eliminated by local environmental stresses, as in Durango where frosts are of regular occurrence and

may kill the plants. Pods picked there on November 2 from frost-bitten vines yielded seeds with poor viability percentages. Ex- tended drought is of annual occurrence in the Oaxaca Valley and the Guerrero bean localities. Ground fires may eliminate perennial individuals, especially if late in seeding.

The bush form (Fig. 11) has not been

GENTRY1 O R I G I N O F P H A S E O L U S V U L G A R I S 59

Fig. 4. Phaseolus vulgaris: Floral organs and seeds of the Mexican wild form, from Gentry 22180, and cultivated form, "rabbit eye," and the Argentine subspecies aborigineus, P.I. 266910. a--banner, b---wing, e---free stamen, d--keel, e--pistil, f--bractlets, g--calyx split along dorsal; all • 6. Flower and seeds X 4.

found among the wild bean populations. Its short stature makes it unfit to compete in thicket communities, where it would be de- prived of the sunlight so essential to growth and maturity. If it were to grow wild in the open, it would be a ready prey to grazing animals. The bush habit is reported by Yarnell (15) to depend upon three major genes and other modifiers; see also Frazier et al. (6). These genes may have appeared

originally as mutations or just chance recom- binations after man had begun to cultivate beans. As a cultivate the bush habit appears in the Oaxaca Valley archaeology about 1000 BPZ (Flannery, personal communica- tion). Its short stature with more determi- nate seeding and quicker maturation than the pole forms made it ideal for open plot

a Before Present.

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cultivation. Ancient and modem man have made great capital with it. As the genes sort, so will man profit or lose.

The wild bean vine has the following diagnostic characters that taxonomists iden- tify with Phaseolus vulgaris: large, broad, but thin leaflets; persistent broad bractlets covering the calyx; the lower lobes of the broad calyx shorter than the tube; the two upper calyx lobes greatly reduced ("united"); the keel of the corolla spiraled and enfolding the stamens and pistil together; the pistil distally pubescent with the stigma terminal and lateral on the inner curvature; pod dehiscent, linear, mildly curved, little com- pressed or terete, many-seeded, pendulant on long racemes.

The cultivated varieties differ from wild progenitors. They are relatively short-lived annuals with thicker stems, have either bush or vine habits, shorter peduncles, larger fleshier pods with delayed dehiscence not violently ejecting the seeds, and larger seeds more permeable to water (9). There are many other distinguishing characters in the cultivated Phaseolus vulgaris, such as the great variety in seed coloring, and physio- logical adaptations to local environments. But these are due to mans' selection and have minor weight in taxonomy.

Altogether, no morphological characters separate the wild Phaseolus vulgaris as a distinct species from the cultivated beans. The diagnostic characters from stem to pod are common to both groups. Although the wild forms may appear quite distinct in nature, when grown side by side in the greenhouse, only the twisting pod valves and qualitative characters, mainly in size, distinguish them. Occasional wild plants ex- hibiting unusual robustness (i.e., larger pods and seeds) bridge the differences between the two groups, Gentry 22266, 22497, 22505. Whether such plants are spontaneous mu- tants or crosses made by insects between wild and cultivated forms is unknown. The similar floral morphology of cultivated and wild forms is apparent in Fig. 4.

The wild and cultivated forms of Phaseo- lus vulgaris are closer morphologically than either is to P. vulgaris subsp, aborigineus. The latter is readily separable by its bractlets, only half as wide as the species, and by more acuminate leaflets. In the three sam-

ples observed from Argentina and Venezuela, the seeds are intermediate in size between those of the wild and cultivated North American forms. Burkart and Brucher found that aborigineus crossed readily with cultivated vulgaris, giving fertile F 1 and F9 generations (3).

Distribution and the Ecologic Niche

The wild bean was studied and collected in 25 localities in Mexico during 1966 and 1967. It was found growing spontaneously and widely scattered in the states of Sinaloa, Durango, Nayarit, Jalisco, Colima, Michoa- can, Guerrero, Morelos, and Oaxaca. The map in Fig. 5 shows the general distribution of the wild Phaseolus vulgaris populations, based on herbarium and seed collections of the writer and on collections of others. With the South American subspecies aborigineus, there is a general, but discon- tinuous, distribution of wild beans through- out the Americas from the Tropic of Cancer to the Tropic of Capricorn. It may provoke wonder that a plant so widespread has so long remained generally unknown to bota- nists, who for more than a hundred years have assiduously been gathering the tropical American floras. However, the wild bean is secretive or inconspicuous in the particular habitat that nurtures it. The following observations describe the environment of the wild bean niche in ecologic terms.

The 25 localities show similar ecologic features characteristically in secondary vegetation at elevations between 2500 ft and 6000 ft above sea level (Fig. 6). In this subtropical region, temperatures are generally equable, but frosts occur at higher elevations during November to late Febru- ary. The annual rainfall averages range from about 20 inches (500 mm) in Durango to 60-70 inches (1500-1850 mm) in Nayarit and Colima, with 80% or more falling from May through October. The growing period is from May to November, after which soils dry out and the vines mature. In the wetter localities, where occasional showers persist into December, flowering and pod maturation are later than in drier localities at the same latitude and elevations. This indicates that the timing of maturation is controlled more by the rainfall regime than it is by the length of day in these latitudes. In its early

G E N T R Y : O R I G I N O F P H A S E O L U S V U L G A R I S 61

Fig. 5. The known distribution of wild Ph~eolus vulgaris in North America.

growth stages this delicate wild bean is shade tolerant. However, for good develop- ment, the slender vines appear to need sunlight, because the best fruiting vines are found about the open crowns of shrubs, whereas under tall closed shrubbery the vines are s~mted and yield poorly. They are not found under forest or in other climax communities. The shrub associates, which the vines use as climbing supports, are all common perennial pioneers that repossess clearings, open rocky slopes, or other disturbed subtropical sites. These thickets also protect the palatable vines from grazing animals. Some of the more common shrubs and small trees are listed below, the first ten or twelve being most common and characteristic:

Higher Elevations Verbesina greenmanii Urb. Wigandia caracasana H. B. K. Solanum ferrugineum Jacq. Hyptis spp. Ipomoea tree species Heliocarpus spp. Lippia pringlei Briq. Montanoa patens Gray Montanoa grandiflora (D.C.) Sch. Bip. Annona Erythrina spp. Lagascea helianthi[olia H. B. K. Acacia pennatula (S. & C. ) Benth. Acacia [arnesiana (L.) Willd. Bocconea arborea S. Wats. Buddleia Dodonaea viscosa L.

62 ECONOMIC BOTANY

Fig. 6. A habitat view 10 miles S of Uruapan, Michoacan. The dense shrubbery completely hides the scattered wild bean population.

Compositae Lippia bushes Solanum spp.

Lower Elevations

]atropha curcas L. Psidium guayava L. Pisonia capitata (Wats.) Acacia farnesiana (L.) Standl. Willd. Buettneria aculeata Jacq.

Ipomoea arborescens (H. & B.) Don Urera caracasana (Jacq.) Griseb. Guazuma ulmifolia Lam. H eliocarpus Erythrina spp. Ricinus communis L. Lysiloma divaricata (Jacq.) Macbr. Malvaceae Montanoa sp. Opuntia spp.

GENTRY: ORIGIN OF PHASEOLUS VULGARIS 63

Bursera spp. Ficus spp. Mimosa spp.

These secondary communities are re- garded as indicators, but not guarantors, of wild bean occurrence. Although some of these arborescent species are always present when the wild beans are found, there are many shrubby slopes and glades without the wild bean vines. The association appears to be more indicative when other legumes are present. Prominent among these are the leguminous vines of Canavalia, Phaseolus adenanthus Meyer and Teramnus uncinatus (L.) Sw.

The composition of the association changes in minor constituents, according to the in- cidence of more local species. Among these in western Mexico are Phaseolus acutifolius Gray and members of the genera Desmo- dium, Rhynchosia, Dalea, Cologania, Mi- mosa, and many others. In Guerrero the wild vulgaris vines occur frequently along with teosinte, Euchlaena mexicana Schrad., which may serve as support for the bean vines; a highly suggestive condition in refer- ence to the food-gathering, food-growing transition stage towards maize-bean agriculture.

In Oaxaca I found the wild bean growing about Anonas, Baccharis glutinosa Pers., Alnus, Arundo donax L., Buddleia, a riparian community along arroyos in a region which appeared too arid elsewhere for the meso- phytic wild bean. About Tepoztlan in Morelos, the wild bean was found along the lava stone fences bordering maize and other plantings and also in clearings on the steep oak slopes with Tripsacum, other grasses, and numerous herbaceous perennials in or out of the oak forest. Here, human disturbance of the vegetation was very evident. Two large-seeded wild bean vines, found twining on maize, appeared to be the inter- mediates of a cross with beans planted with the maize. A farmer considered the wild bean, which he called "frijol de raton," a weed. A more indigent family living above on the oak slopes of the mountain had an Indian name for the wild bean and stated that they ate it.

The soils of these plant communities are assumed to be generally acidic because of the frequency of basaltic rocks and the pre-

vailing organic litter. However, neutral and even alkaline soils are not to be summarily excluded. The Durango wild bean site in the margin between grassland and an evapo- rative basin is indicative of alkaline soil. Generally the soils are coarse, rocky, and well-drained. In drier climates, the bean populations are restricted to local sites with deeper soils that tend to retain moisture. Many of the shrub associates endure drought better than the feeble bean.

In addition to the nurse-plant thickets, seed coverage by soil appeared to be a vital survival factor from one generation to the next. Plants are not found upon bare sur- faces, but grow out of shrub litter or among rocks and sand as along washes, where flood waters mix and pile surface materials that apparently cover the seeds. The seeds are self-camouflaging among the litter and grav- els. This is especially obvious with the mottled patterns and may largely explain why they are so prevalent among wild populations.

Many of the bean localities owe their shrub community makeup to man-made clearings; this is the responsive kind of vegetation that takes over abandoned living sites and milpas. Because of this condition, I first tended to regard the wild beans as sur- vivals from early cultivates. Some of them may well be, for instance, the Durango population adjacent to a former lake, where man may have been held by a permanent source of water. The grassland community is distinct from other bean habitats observed. The bean seeds from there, Gentry 22042, which are larger and more uniform than generally obtained elsewhere, sustain this inference. Incidentally, this locality should be explored for archaeological sites. However, as more wild bean localities were found, this interpretation became doubtful for several reasons. While some of the wild beans are found near ruins and are in vegetation successional to human disturbance, not all of them are. Successional plant communities existed long before man arrived in America, and they can be observed today on local sites disturbed by land slides, fires, floods, volcanism, hurricanes, and wild animals. Man has only accentuated and ex- tended such communities, not created them.

Furthermore, the dominant and charac-

64 ECONOMIC BOTANY

teristic plants of these associations were never cultivated by man; most of them, Verbesina, Hyptis, Viguiera, Heliocarpus, Lippia, Ipomoea, Acacia, have no utilitarian value beyond firewood. They, like the Old World Arundo and Ricinus adventives, have become man-followers. Like maize, cultivated beans have lost the ability to compete and survive in wild plant communities. In a few untended generations they disappear. Descendants may survive by introgression with their wild relatives, as they seem to be doing in a few observed instances. If there are survivors of cultivates, these are most likely from the very early stages of domesti- cation before man had selected out the hereditary system having to do with survival in nature, such as dehiscent pods.

One can summarize from the above that the wild bean requires moist, rich, well- drained soil, protection and vine support by shrub associates, and 20-40 inches of rain through the growing season in equable temperatures prevailing between 2500 and 6000 ft elevations in tropical latitudes. It is shade tolerant through seedling stages, but needs sunlight for maturation, which is generally concomitant with drying soils in November and December. As a sheltered component of an aggressive plant community, it became a man-follower, and, because of good yield of palatable seeds, it was collected for the pot and eventually planted.

Anthropological History Archaeological investigations during the

last two decades have thrown a great deal of light on the history of the cultivated bean in North America. Kaplan has made special studies of the archaeological bean collections (7, 8, 9). The carbon 14 method of dating places these collections from 1000 to 2300 BP in southwestern United States and 4300 to 7000 BP in eastern Mexico. The later reports of McNeish and Smith (10, 12) on the Tehuac~in Valley confirmed the more northern finds. All of the materials were identified as cultivates. The above dates are far earlier than any from other parts of the world. The next early date is about 2200 BP from Peru, "coinciding with the introduction of maize" (9). In the Old World Phaseolus vulgaris is not certainly known until post-Cohimbian time.

The evolution of cultivated Phaseolus vulgaris appears to be represented by the seeds in Fig. 7. From a central core of wild seeds, the cultivates have been arranged to radiate outward, the yellows, buffs, and pintos being from northwestern Mexico, the blacks, reds, browns opposite being from southeastern Mexico and Central America. They have also been graded according to size; the smallest cultivates from Indian plantings are near the center, and the largest seeds about the periphery are represented mainly by modern cultivates. Finally, by sorting according to color and seed shape, there appear lines of geneology, the smallest mem- ber of which points approximately to its apparent ancestor among the wild populations. A similar alignment could have been arranged with the white beans, but as they occur everywhere and no wild white has been found, they were placed about the margin. The three largest wild beans appear to be hybrids and have also been placed marginally. The geographical-historical correlation is further indicated in the following three cases.

The Durango collections, Gentry 22042, are light yellow with a veiny pattern. Yel- low color, some with a veiny pattern, also shows in seeds of the west Mexican populations of Copala, Sinaloa, Gentry 22277, Lagunilla, Nayarit, Gentry 22241, and La Garrita, Jalisco, Gentry 22180. The yellows of eastern Guerrero lack the veiny pattern. The predominant colors in the "creole" cultivates of northwestern Mexico are pale yellow and buffs. Common and widespread among them are the veiny yellows, known by such names as "frijol azufrado" in south- ern Sonora and adjacent Chihuahua and Sinaloa, and the "frijol garbancillo" and "frijol bolito," more southward into the central plateau region. These are small-seeded bush beans. Other common varieties represented by the "pintos" and the "frijol bayos" appear to be more modern selections, with larger and quicker cooking seeds. The geographic coincidence of color characters in wild and cultivated forms becomes more significant with two other observable cases.

Another color pattern commonly found in cultivates on the Jaliscan Plateau and central Mexico is generally called "ojo de liebre" (rabbit eye). It has dark arcuate bars partly

Fig. 7. Tile evolution of cultivated Phaseohls vidgaris as iadieated by the seeds. For explana- tion see text.


Fig. 8. Three ca~es of correlation between wild and cultivated beans. 1--The veiny yellows and buffs of northwestern Mexico. 2--The "rabbit eye" pattern in central Mexico. 3---The black beans in the southeast. Solid shading indicates occurrence of putative wild ancestors.

circling on the light-colored faces of the beans. It appears less frequently among the beans cultivated by Chiapas Indians in a smaller, more primitive looking variety. This distinctive color pattern was also found among the wild bean populations in Colima, Gentry 22184, Jalisco, Gentry 22199, and Guerrero, Gentry 22544. This is a second instance where local cultivates have their origins indicated by wild relatives still growing as respective neighbors.

A third similar connection between the wild beans and their cultivated descendants appears in the small black beans. Among wild populations black beans occur in eastern Guerrero, Morelos, and Oaxaca, Gentry 22541, 22492, 22395. They were not found in the western and northern wild populations. Again it is significant that black beans are the prominent varieties planted in the adjacent region of southeastern Mexico and Guatemala. The coincident distributions of these three cases are outlined on the map in Fig. 8.

The dominant color variety cultivated through Central America south of Guatemala is a solid red. This is the color preferred by most people in Costa Rica, Nicaragua, and Honduras. More modern improved varieties of other colors, as the blacks, are resisted by the local consumers. The small red bean is found at least as far north as Oaxaca and Puebla. The wild counterpart of this red has not been observed by the writer. It may occur among the wild population of Central America, or it may be a genetic derivative of the small black beans, some of which it closely resembles in size and form. It is the one predominant geographic color form that remains unobserved among the wild beans.

In addition to phylogenetic connections, these diffusion patterns indicate multiple origins for the common bean cultivates. Be- cause of the numerous local varieties found in archaeological sites, Kaplan arrived at a similar conclusion (9). The bean was not just some chance superior mutant a family once found, picked, planted, and passed on

66 ECONOMIC BOTANY

to fellow tribesmen. If archaeologists and other historians have interpreted early cultures at all well, it seems reasonable that, in the long incipient stages of American agriculture, the semiagriculturists picked and planted wild beans many times in many localities. The innovation was the practice of planting, not the bean itself. Compare Saner (11).

The apparent remarkably long persistence of these early forms of beans is attributable in part to two factors. The first is biological in that the respective genetic systems are well adapted to their regional environments, or they would not have survived. The other is the strong conservatism in eating habits and preferences, which are long-conditioned by human social structures. Compared to the Mexican cultures, the Africans have ex- pressed their social conditioning in quite a different way. In the relatively brief time they have had the American bean, they have selected out forms of the gayest colors possible.

In Mexico wherever bean cultivation has been regular for centuries a complex bean culture has developed. In the larger markets it is not unusual to find 25 or more varieties offered (Fig. 9). These varieties by trial and error have been found suitable to local farms and for various special dishes desired by the consumer. Thus the white bean is generally preferred with pork, a black bean in some regions for the tortilla sandwich, and still other varieties preferred for "frijol de la olla," boiled beans, or as "refritos," fried beans. There is a small variety, either black or red, discussed above, which is preferred for soups. I recall one in Oaxaca which makes a thick soup quite unmatched for flavor by any cultivated in the United States, according to my taste. There are varieties that are eaten as green beans in the pod like our snap beans and others of which the fresh green seeds are eaten, the pod valves having been removed. Still another variety is eaten, pod and all, when fully mature; the consumers report that the dry pods are without fiber and sweet tast- ing. Bean fodder is fed to livestock. The bean varieties are always given names, and because the same variety may bear different names in various localities and in different languages, the names far outnumber the

Fig. 9. Threshing common beans with sad- dle horses in the highland grassland of Durango.

varieties. Perhaps the most significant name heard on all my journeys was the one in a corner of the Oaxaca Valley. The wild bean there in the Zapotec tongue was called "chaneca" meaning the ancient one! Like maize, the bean in all its nuances of use re- flects the complexities engendered by a truly ancient and specializing culture.

The historical relationship between North American and South American Phaseolus vulgaris is not very clear. On present archaeological evidence, cultivation was much earlier in Mexico, 7000 BP in the Tehuadm Valley vs. 2500 BP in the Nazca Valley, Peru. Were the cultivated beans carried by immigrant people to South America, or was cultivation practice applied to the native wild bean already there? Study of the wild and cultivated beans of South America might resolve whether they are pre- human wildlings, as Burkart regards his subspecies aborigineus, or are rather escapes from early cultivates, as their seeds suggest.

The Genetic Evidence and Potential

Burkart and Brucher (3) reported crosses between cultivated Phaseolus vulgaris and the wild P. vulgaris aborigineus. The off- spring were fertile and produced an Fz generation. Freytag working in Honduras reported (letter 12, July, 1967) crosses between wild P. vulgaris from Mexico, Gentry 22042, 22153, 22180, 22234, 22241, 22277, and bush and vine forms of the Mexican culti-

G E N T R Y : ORIGIN OF PHASEOLUS VULGARIS 67

Fig. 10. Market in Uruapan, Michoacan, with dry beans at retail in one of the many stalls.

vates. The Fl's were having "terrific de- velopment" at the time of his letter. Norvell at Palo Alto, California, has reported (let- ters) making numerous successful crosses between wild P. vulgaris he collected in Mexico and our cultivated varieties. These reports indicate close compatibility in the respective germ plasms. I accept them as genetical evidence, confirming the morphological, for treating the wild and cultivated forms as conspecific. This with the historical evidence leaves little doubt for consider- ing the wild Mexican beans as the progenitors of the common bean. They are all Phaseolus vulgaris L.

The Amerindian traditionally still selects and saves seed for his next planting out of his current harvest, or in case of crop failure, that of his neighbor. Phaseolus has a cleis- togamus flower, which means it is self- fertile and selections are easily made with seeds from individual plants. Because of the limited segregates carried in the succeeding selfing generations, pure lines are soon estab- lished by avoiding seed of the undesirable variants. The occasional cross-pollinations, made by strong bees capable of opening the dosed flowers, result in new genetic com- binations, or new choices for the planter, according to circumstances. Some of the obvious characters the Indian selected, judg- ing from the widespread indigenous varieties, were large seeds, large pods, flavor, quick-cooking properties, fancy seed colors, and bush forms. The local environments im-

posed restrictions on his choices. For instance, more arid localities have short rain seasons requiring early maturation, as is found in the bush beans now popular for both summer and spring seasons. The later maturing pole types for dry bean use are still largely restricted to late-maturing winter harvest varieties in the more humid regions with longer rain seasons. Thus, the evolution of the common bean proceeded under climatic controls as well as by human selection (Figs. 9 and 10).

The bush habit involved a major genetic change in plant organization, and it resulted in the loss of many genes and further nar- rowed the planter's selection of variants. As the bush bean diffused to other localities, the inadvertent selections resulted in many endemic varieties, narrowly tolerant, and genetically limited within themselves as breeding stocks. The genetically rich re- source in the early ancestors was innocently subverted to dead ends. One obvious result is the weak, diseased, and pest-ridden varieties common in many parts of tropical America, and which can no longer supply a food need for the expanding human populations. The Amerindian did not breed beans in our sense of the word. He did use intel- ligently what nature offered him. He had no knowledge of genetics and made no crosses. He had no knowledge of plant diseases; so he could not breed for disease resistance. Until very recently the only breeding stocks available to bean breeders have been those supplied by the American Indians. With his beans we inherited the strength of his toils and the weakness of his knowledge as well.

The arresting part of this story is that wild beans have not been used for crop improvement for several thousand years. Our recent collections of bean germ plasm mean that geneticists can make a new start at improving one of the world's important food resources. A new era in bean breeding is just beginning. This is a prediction based on our experience with other crops. When genes from the primitives have been trans- fused into worn-out cultivates, there is a resurgence of crop and profit. This has been demonstrated in wheat, barley, melons, squashes, potatoes, tomatoes, sesame, and doubtless others unknown to me. Plant

68 ECONOMIC BOTANY

Fig. 11. Indian farm in the Peten highlands of Guatemala with bush beans growing beyond the fenced garden.

Fig. 12. A bush bean variety grown by Indians near Peten, Guatemala. Note the short internodes and the terminal racemes of large nontwisting pods.

habits and structures have been modified to increase yields, to tolerate aridity and other climatic stresses, to conform to machine harvest, and to resist diseases and insects that plague our fields.

Resistance to Fusarium root rot has recently been reported by Burke, Silbernagel, & Zaumeyer in wild stocks (4). Their work confirmed that of Oliver Norvell, who collected the wild beans of Mexico in the late 1940's and early 1950's. He has since con- ducted disease screening work and has found resistance among the wildlings to some of the bacterial diseases (personal communica- tion). This is just a beginning as the important genetic reconstructions of our cultivates frequently requires a generation of man to refine. Interspecific hybrids among the cultivated species have been difficult to obtain, except between P. vulgaris • coc- cineus. Will our wild stocks have better cross-ability than our cultivates and provide easier bridges to other gene pools, as those found in the drought-hardy and more disease-resistant teparies? Perhaps recombina- tion with some of the endemic varieties would regain the flavor largely lost in our modern varieties, which were selected more for yield, quick-cooking properties, and sales appeal. A new genetic feed-back might eliminate the flatulence problem which appears to block the adoption of the American bean by the protein-deficient peoples of India. It will be interesting to watch the

genetic discoveries and even more interesting to engage in seeking them. The hungry tribes left us a great legacy. How about re- turning the favor to the hungry hordes of today?

L i t e ra tu re Ci ted

1. Bukasov, S. M. 1930. Cultivated plants of Mexico, Guatemala, and Colombia. Bull. Appl. Gen. and Plant Breed. Supp. 47: 151-176. Leningrad. Trans. H. J. Kid&

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Documents

Origin of the common bean, Phaseolus vulgaris