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II. Selection in Domesticated Crops

II. Selection in Domesticated Crops

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ing, and threshing. But this is not selection by man. It is clearly the operation of

natural selection within the environment of man’s cultural practices, involving

no active selection by man himself, conscious or otherwise. It is proposed here,

at least for annual seed crops, that Darwin’s “unconscious selection by man” be

termed “nonspecific selection by man,” and that Darlington’s “unconscious

selection by the cultivator” be regarded as “natural selection for adaptation to


Darwin recognized the close adaptation by natural selection of numerous

varieties of wheat to various soils and climates even within the same country;

“that the whole body of any one sub-variety ever becomes changed into another

and distinct sub-variety, there is no reason to believe. What apparently does take

place, is that some one sub-variety . . . which may always be detected in the

same field, is more prolific than the others and gradually supplants the variety

that was first sown” (p. 389).




We now recognize several categories within Darwin’s general processes of

selection by man and natural selection. These are discussed here in relation to

annual seed crops.

I . Selection by Man in Annual Seed Crops

Conscious selection by man relates especially to fruiting organs and seed; to

larger ears of wheat, larger cobs of maize, or heads of sunflowers, all undoubtedly contributing to an improved harvest index and grain yield in the early years of

domestication. The choice of seed size, color, and flavor was also a basis for

selection, notably in rice and beans. An early and important case is the selection

of the dwarf habit in the naturally climbing common bean (Phaseolus vulgaris)

by the American Indians (A. M. Evans, 1980). The dwarf mutant, in nature or

mixed cultivation, would have been effectively lethal because of suppression by

taller plants (Smartt, 1969; Hamblin, 1975); it could not have emerged by natural

selection, but man has preserved and propagated it as a key mutant. A similar

situation has occurred in rice with the development of short, high-yielding types

which are rapidly eliminated in mixtures with tall, low-yielding types (Jennings

and Aquino, 1968; Jennings and Herrera, 1968; Jennings and de Jesus, 1968).

However, in many instances it is difficult to distinguish selection by man from

natural selection within the changed environment that man has provided. How

effectively did early cultivators select for better yield and to what extent did

better yields arise through the natural selection of genotypes producing more

seed? In present-day annual seed crops, it is certainly very difficult to select by




subjective judgement those plants capable of higher seed yield in pure culture

(Bell, 1963; McGinnis and Shebeski, 1968; Walker, 1969; Hamblin, 1971). The

genetic worth of any plant is confounded by the influences of its immediate

environment, including its neighbors (Hamblin et al., 1978). Percival (1921)

quotes Virgil and other classical writers as advocating the selection of the best

ears for use as seed, but we do not know to what extent this influenced yield per

unit area. Nevertheless, many effective selections have occurred in the past; we

have records for a century or more of successful cereal varieties being developed

from single ear or plant selections by farmers, such as wheat cultivars; Chidman,

Hunter’s White, and Squarehead (United Kingdom); Canadian Fife (Canada);

Fultz (United States), and Purple Straw and Prior barley (Australia). Each was an

arbitrary choice of a plant which, in the eye of the observer, looked more

productive and indeed proved to have features which were sustained in pure

cultures over a range of environments. But the unsuccessful and unrecorded

conscious selections for yield in cereal crops doubtless occurred in great


Wherever conscious or methodical selection has been practiced, correlated

secondary selection for other characters has been almost inevitable. Thus the

selection of the dwarf habit in beans was probably accompanied by unplanned

and unrecognized selection for determinate growth, a feature of plant architecture no less important than reduced height itself. Increased cob size in corn

initially increased yields over its progenitors (Galinat, 1965; Wilkes, 1977).

However, once the modem types had become established, secondary selection

probably was adverse to productivity in pure culture, reducing any gain resulting

from the conscious component of selection. Large maize cobs, selected in the

field, were almost inevitably from tall, broad-leaved, highly competitive plants

that had exploited the habitat of their neighbors (see the work of Gardner and coworkers at Nebraska, considered in more detail on p. 115-1 16).

A striking example of such secondary selection was reported by Wilcox and

Schapaugh (1980), who selected “phenotypically superior single (soybean)

plants, based on a visual estimate of seed yield and lodging resistance.” No

change in seed yield or lodging resistance occurred, although the selected plants

were significantly taller and later than the unselected control plants. It seems the

selected plants were chosen only because of competitive advantage resulting

from their tallness and longer growth period. When competing against like

neighbors, these taller and later plants showed no advantage. However, it must

be remembered that only recently has man become obsessed with yieldhnit area.

In many situations, these competitive features would provide benefits in terns of

animal feed, building materials, and so on (Hamblin and Rosielle, 1983).

It is conceivable that nonspecific selection by man may have occurred among

horticultural plants, for example the propagation of a fruit tree because of its

general form, cropping capacity, fruit quality, and so on, without any precise



definition or evaluation in the mind of the observer. It is difficult, however, to

identify nonspecific selection of plants within annual seed crops. If the selection

was made in the field, some particular feature, whether number or size of fruits

or seeds, growth form, vigor, or some other specific factor, probably would have

been the basis of selection.

Today’s growers of annual seed crops are most interested in yield per unit

area. This is a form of conscious selection for yield, not within the crop but

between cultivars within the region. This was also the case when hexaploid

wheats replaced tetraploids over northern Europe and when tetraploid cottons

from the Americas replaced most of the diploid cottons of the Old World. In each

instance further selection for adaptation to new environments occurred.

2 . Natural Selection in Annual Seed Crops

Within man’s crops, natural selection is always potentially operative. Darwin

emphasized that “natural selection . . . a power incessantly ready for action, is

as immeasurably superior to man’s feeble efforts as the works of Nature are to

those of Art” (p. 77). Natural selection results from the relationship of each plant

to its physical environment and to its neighbors. If the plants within a crop are

genetically variable in even the slightest degree some biotypes will increase and

others will decrease in ensuing generations. The relative advantage of a particular biotype may change with the environment (e.g., for barley, Harlan and

Martini, 1938; for beans, Hamblin, 1975; for rice, Adair and Jones, 1946).

Two mechanisms of natural selection may be recognized (Nicholson, 1962):

environmental selection and selection through competition. Nicholson considered that the views of Darwin and Wallace toward natural selection were based

primarily on one or other of these mecharismeDarwin’s on selection through

competition and Wallace’s on environmental selection.

a. Environment Selection. Nicholson wrote that environmental selection “removes all individuals which are not sufficiently potent to withstand the severe

conditions to which the species is exposed from time to time, and so leads

towards the production of a population in which all individuals can survive, even

under these severe conditions” (p. 65). Examples of environmental selection

abound in man’s crops. It occurs when early flowering individuals in a crop are

prevented from setting seed by a late frost, when late flowering plants produce

no seed because of hot, dry conditions, when some plants grow poorly on an acid

soil or under saline conditions, or in any one of numerous circumstances within

the physical environment.

Cotton differs from most annual seed crops in that it entered cultivation as a

perennial shrub. It thus continued, confined to the tropics, for several millennia.

When taken to temperate areas, in less than 600 years natural hybridization and

selection for earliness and avoidance of frosts gradually led to a branched shrub



of strictly annual habit (Hutchinson, 1965). Similarly, maize, originally confined

to the subtropics of the Americas, progressively extended northward into areas

with shorter seasons and severe winters because of interspecific crossing and

environmental selection (for earlier flowering forms having different photoperiodic responses (Galinat, 1965).

At the experimental level, a barley composite involving crosses between 31

cultivars (Allardand Jain, 1962) rapidly adapted to the climate at Davis, California. The population shifted strongly to earliness in heading date for 5 generations

and then more slowly for the next 15, a directional selection. At the same time

there was a steady elimination of the “tails” (either the earliest or latest) and

variance decreased, stabilizing selection around the optimum heading time for

the locality. Similar environmental selection has been recorded for other climatic

features, especially length of day, and to soil features, such as texture, pH,

fertility, and salinity. “All those who have closely attended the subject insist on

the close adaptation of numerous varieties (of wheat) to various soils and climates even within the same country” (Darwin, 1868, p. 388).

Environmental selection within cultivated crops also occurs from the influence

of man’s cultural practices (Harlan er al., 1973). Although it may be claimed that

these practices are “artificial,” the plant responses and the selection of the “fit”

are truly natural selection in the Darwinian sense. Successful plants must be

more suited to, or less harmed than other plants by man’s treatment of the crop.

In primitive agriculture, emergence from varying depths of planting was critical;

in many earlier crops, and still today in some species, resistance to damage

during temporary grazing gave selective advantage. There was advantage also

for those plants that responded better to man’s cultivation practices, to his

soil-water storage, or to his irrigation practices. More recently, those plants that

respond to artificial fertilizers or are less harmed by pesticides are at an advantage. Faced by these practices, some plants will show a relative gain and others a

reduction in prolificity (number of seeds produced).

However, seed number per plant by itself will not ensure that any individual is

represented in successive crops. A high proportion of each plant’s seed must go

into the bulk seed used for sowing the next crop. This attribute is distinct from

prolificity but just as significant in determining the pattern of natural selection.

Initially seed must be retained on the plant until the crop is cut, but then must be

threshable yet not so light as to be lost during winnowing. The seed must retain

viability until the following sowing time, but it then must germinate without

problems of hard-seededness, physiological dormancy, or undue delay. It must

emerge and establish from varying depths of sowing. This list of selection filters,

doubtless far from complete for many seed crops, indicates the powerful selection pressures that inevitably occurred from the earliest years of domestication,

and future natural selection will follow with the adoption of new cultural practices. Natural selection in response to many of these features of the crop environ-



ment must have occurred within a single, or at most a few, generations; the most

ill-fitted plants, such as those that shed all their seed before the harvest, would

have no descendants in the next generation of the crop.

b. Selection through Competition. If any plant within a crop takes up more

water, nutrients, or light than another at the expense of that other plant, it will

have the potential to be advantageously represented by its progeny in the ensuing

generations; it will be selected through competition. Successful competitors

within seed crops have been selected for the following:

i . For the Annual Habit. In perennial plants, part of the assimilates are distributed to storage organs, that is, to underground organs or enlarged stem bases

(which may themselves be harvested, but are not relevant to this article). Storage

competes directly with seed production. Where annual seed crops are descendants of perennial species, selection toward the annual habit has occurred simply

because of the capacity of annuals to produce more seeds. However, remnants of

perenniality remain in some annual crops, so that “ratooning,” the growth of a

second crop from the bases of the first, is possible. Many sorghum cultivars and

also some rice varieties, especially those that tiller freely, can be ratooned. But

within seed crops evolution toward a strictly annual habit has been continuous,

so that seed ripeness and plant death are coincident or nearly so. The avoidance

of diversion of resources to vegetative organs combined with the adaptive value

of an annual growth pattern in regions of limited season has led to the development of annual seed crops in many families and genera (Chang, 1976).

ii. For Tallness. The most universal factor for which natural selection has

occurred in crops has been plant height. Even slight superiority, through advantageous competition for light, can give a plant sufficient yield increment to

ensure its dominance in a few generations. Fischer (1978) found that each centimeter of superiority in height among spaced wheat genotypes (40 X 40 cm)

gave a yield advantage of 0.58%. At normal densities this advantage is considerably enhanced (Jensen and Federer, 1964; Khalifa and Qualset, 1974). Similar

results have been obtained in a segregating population of wheat. During bulk

breeding, Khalifa and Qualset (1975) found that the mean height of the population increased from the F, to the F, and that the shortest types were eliminated.

There was also a negative relationship between F, height and pure culture yield.

Data from other cereals have been similar. In a segregating barley population,

there was a positive relationship between single plant height and yield in the F,

population; this was reversed in the F, plots (Hamblin and Donald, 1974).

Perhaps the most striking data are those available for rice. When two tall, leafy

varieties of rice were mixed in equal proportions with three semidwarf, erect

cultivars (each providing 20% of the seed sown), the low-yielding tall varieties

suppressed the high-yielding semidwarf varieties within four generations to less

than 0.5% of the population (Jennings and de Jesus, 1968). Similar results

occurred with segregating rice populations (Jennings and Herrera, 1968). In




maize, dwarf types yielded less when grown in competition with tall types than

they did in pure culture (Pendelton and Seif, 1962).

Thus over the millennia, and probably quite early in the history of cropping,

annual seed crops became tall. Percival (1921) instances Triticum uestivurn to

150cm, T. turgidurn to 180 cm, and even T . compacturn (Club or Dwarf Wheats)

to 140 cm. Yet under marginal soil and climatic conditions, even these tall

wheats were superseded and replaced by the still taller rye (Secale cereale).

Under fertile conditions in the United States, maize attained heights of more than

3 m by the mid-twentieth century; Goldsworthy (1970) records a local sorghum

of more than 4 m in height in Nigeria.

The advantages of height in competition are not confined, however, to cereals.

We have estimated (from the data in Table I and Fig. 1 of Schutz and Brim,

1967) that each centimeter of height advantage in soybeans gave Jackson (a tall

variety) a yield increase of 2.5%, and 4% of mean yield, when competing in hills

46 cm apart with the short varieties Hill and Lee. The advantage was 0.7-1%

when competing in rows 107 cm apart. In pure culture, Hill and Jackson had

similar yields whereas Lee was 13% higher yielding than Jackson, the tall variety. These results are partly confounded by maturity effects, but in all cases the

taller lines were more competitive than the shorter lines, and the late lines were

more competitive than the early lines.

Tallness in seed crops had certain advantages to early cultivators, which

continues in the village agriculture of many regions. It gives stem material of

value for fuel, bedding, building, and thatching purposes, so that tallness is

esteemed. Also tallness is at an advantage when competition with weeds is

severe (Pal et al., 1960). It tends also to be linked, in the minds of many

growers, with greater yield; as will be discussed later, this is a fundamentally

mistaken belief in weed-free situations. But there is little reason to believe that

tallness developed through selection by man, because it dominates through natural selection within a few generations of its appearance within a crop.

Finally, an equilibrium is reached when the tallest plants suffer grain loss or

collapse because of wind damage. Further, these tallest plants tend to have

reduced harvest indices (Rosielle and Frey, 1975; Donald, 1981; Hamblin and

Rosielle, 1983), so that they yield less grain than their slightly shorter neighbors.

iii. For a Leafy Canopy. The third powerful influence of natural selection

within seed crops is for a canopy of wide, horizontal or floppy leaves. Such

plants are able to intercept light preferentially. The large, subcircular, horizontally disposed leaves of cotton and the large leaves of many sunflower cultivars

permit considerable light interception even by crops of low leaf area index (LAI).

The influence of canopy structure on competitive ability was demonstrated by

the comparative behavior of wheat varieties of different leaf habit (Tanner et al.,

1966). In a weedy situation, the floppy-leaved varieties were able to suppress

weeds and yield well whereas those of erect leaf habit were depressed in yield. In



a weed-free situation, the yields were reversed. Similarly, in an F3 population of

barley at strongly competitive spacing (Hamblin and Donald, 1974), there was a

positive correlation between leaf length and yield. Reference was made earlier to

the almost total suppression of short, erect-leaved rices by tall, floppy-leaved


The leafhess of successful competitors must also be subject to stabilizing

influences. Maize or rice plants cannot continue to grow taller and leafier indefinitely. The disadvantage of tallness and leafhess, especially of tallness beyond

that needed for successful competition, is the tendency of the crop to lodge,

leading to a disorganized light profile, reduced seed production, and harvest

problems. The trend to leafhess and strong competitive ability also tends to be

associated with heavy water use, prolonged growth, lateness in maturity, and a

decreased harvest index (Donald, 1981).

An unexpected feature of the evolution of wheat under domestication has been

the much lower photosynthetic rate per unit area of leaf of modem wheats

compared to primitive species of Triticum and Aegilops (Evans and Dunstone,

1970; Khan and Tsunoda, 1970; Evans and Wardlaw, 1976). Evans and coworkers suggested that this falling rate is caused by the reduced surface/volume

ratio of the mesophyll cells, which Kranz (1966) had shown to have become

progressively larger during domestication. However, Evans and Dunstone

(1970) found that the leaf size had increased more than the photosynthetic rate

had fallen, so that the photosynthesis per leaf was much greater in modem

wheats. They also noted a positive relationship between leaf size and grain size

and reasoned that selection for yield would lead progressively to increased grain

size, increased leaf size, larger cell size, and lower photosynthetic rate. Khan

and Tsunoda (1970) take the view that the change in leaf size and photosynthetic

rate is caused by an improvement in the environment of plants under agriculture

which has selected for a mesophytic habit from a wild xerophytic habit. Similar

changes with domestication have occurred in cotton and tomatoes (Stebbins,


An alternative explanation has been offered for this enigma of falling photosynthetic rates during the evolution of wheat under domestication (Donald,

1981), based on competitive relationships. Within the crop canopy, plants with

large, usually wide, floppy, drooping leaves would have had strong competitive

ability for light and a clear selective advantage throughout domestication. As

long as the photosynthetic rate per leaf was sufficiently maintained, a progressive

increase in leaf size ensured natural selection, with a consequent relaxation of

selection pressure on photosynthetic rate per unit of leaf area. Less leafy plants

were at great selective disadvantage because of shading by their leafy neighbors

which, although they might have been “physiologically weaker,” were “ecologically powerful.” Thus there would again be stabilizing selection between

directional selection for larger leaves in the competition for light and opposing




directional selection for an adequate or advantageous rate of photosynthesis. It is

proposed that modem wheats have leaf sizes and photosynthetic rates representing the outcome of this selection. Selection for yield, it is suggested, has not

been involved as the initiating factor, and man has played a role in the falling

rates of photosynthesis only through his crop production.

iv. For Tillering or Brunching. The selective advantage of free tillering is

illustrated by the response of wheat to poor establishment, which was doubtless

common in early agriculture. Abundant tillering compensates for low plant uumbers. When the stand density of wheat at Adelaide was reduced from 184 to 35

plants/m, the number of tillers per plant increased from 5.5 to 13.7, and the

number of grains per plant increased from 46 to 215, with no significant change

in yield per square meter (Puckridge and Donald, 1967). Similar results were

obtained by Bremner (1969). In both studies, a genetically uniform cultivar was

used; it is clear that if a genetically diverse crop were depleted in plant numbers

for any reason, free-tillering genotypes would have great selective advantage

over less tillered kinds. Under domestication there seems to have been little

reduction in the tillering capacity of wheat or barley. Modem cultivars are

capable of producing 30 or more fertile tillers (ears) per plant at wide spacing,

although only 2 or 3 are produced when competing at crop densities (Puckeridge

and Donald, 1967).

In contrast to wheat and barley, there has been a strong trend in maize and

sorghum toward single-stemmed plants. One may ask why this should be so,

because they are all graminaceous plants of basically similar vegetative structure.

The probable explanation lies in the culture of these four species by man. Wheat

and barley have always been harvested as a plant community, with sickle,

scythe, mower, or header, and there is no recognition of the individual plant. In

contrast, for many millennia maize and sorghum have been hand-harvested by

pulling the cob or by cutting off the inflorescence. Here lay the opportunity to set

aside the largest cobs or heads for seed the following season. This would lead

indirectly to the preferential selection of sparsely tillered and ultimately of single

stemmed plants, an instance of secondary selection by man.

The phenomenon of branching in dicotyledonous crops is ecologically parallel

to tillering in cereals. The competitive ability of soybean cultivars was assessed

in several experiments by their growth in pure cultures and mixtures (Mumaw

and Weber, 1957). The most consistent feature linked with the competitive

success of a cultivar in mixtures was the branching growth habit, even exceeding

the influence of a 12-cm height advantage of some cultivars. In all comparisons

over 2 years, branching varieties contributed about 60% of the total yields of the

mixtures, and nonbranching varieties contributed about 40%.

v. For Seed Size. In most natural communities, small seeds have marked

selective advantage over larger seeds. Small-seeded annual plants probably will

yield a greater number of seeds than large-seeded plants: small seeds are more



easily dispersed and buried; grazing animals can pick up small seeds less easily,

are less likely to crush them while chewing, and are more likely to pass some of

the seed through the digestive tract undamaged. Most of these selective advantages disappear in the crop situation; there are no grazing animals, and dispersal

and resowing do not depend on natural forces. Large seeds have one powerful

selective advantage, namely, they produce large seedlings (Evans and Bhatt,

1977) which have strong competitive ability over smaller seedlings. In the wild

state this advantage probably is reduced or lost because of defoliation by animals, but within a crop larger seedlings give rise to larger plants with more seed.

When wheat seeds of 45 mg were sown alternately with small (27 mg) seeds of

the same variety, the large seeds produced plants with seed yields 57% greater

than those from the small seeds (Christian and Grey, 1941).

However, in mixtures the advantages of large seed size do not necessarily lead

to survival. Hamblin (1975) found that the relative competitive advantage of

bean varieties (Phaseolus vulgaris) changed with environment; large-seeded

types yielded relatively more when competition was most severe (i.e., when

yield/plant was small), but in nearly all situations the smallest seeded variety

produced the most seeds per plant (was more prolific), although it was never the

highest yielding variety and often yielded only moderately. He also showed that

as the small-seeded type dominated the mixture so the average yield of the

population fell. In another study (Hamblin and Morton, 1977) involving segregating populations, natural selection was always for small seeds, and in three out

of four cases it was for increased seed numbers. The results were obtained at crop

densities and contrasted markedly with the results obtained at low density, illustrating the importance of not extrapolating from one situation to another with

changed competitive relationships (Donald and Hamblin, 1976; Donald, 1981).

A similar result was found for a bulk cotton population that was grown without

conscious selection for 10 generations; there was a linear increase in seed numbers and a linear decrease in seed size over the generations (Quisenberry et al.,


These stabilizing factors for seed size (i.e., the competitive advantage of large

seeds and the greeter prolificacy of plants with small seeds) would, despite

fluctuations in their significance from season to season, lead to a loose equilibrium for seed size. But early in the history of cropping, another factor was

superimposed: the conscious selection by man of larger and plumper grain,

features associated in the minds of growers with high yield. Several Greek and

Roman writers emphasized the importance of retaining large grain from the

harvest to be sown the following year (Percival, 1921). Large grains could

readily be separated during winnowing or by shaking grain on a shallow tray.

Thus an added and powerful selective advantage, unrelated to field performance,

lay with plants producing larger seeds. Although continuing recombination and

segregation also would have ensured stabilizing selection for small-seeded



prolific genotypes, the equilibrium size presumably would have tended to increase.

The doubling of seed size from the wild wheats (Triticum rhaoudar, to 20 mg)

to the cultivated wheats (T. aestivum, 40-45 mg) was doubtless partly the result

of active selection for seed size by man, but the increase in competitive advantage of large seeds and seedlings under wheat cultivation as compared to that in

the wild state was probably notable. In some crops there was intense selection of

individual seeds, within the harvested crop, for color, shape, and greater size;

this was particularly so in the common bean (P. vulgaris) and also in many grain

legumes. Not only were distinct and strikingly different races of beans developed

in nearby villages, but there was a fivefold range in seed size under domestication (Evans, 1973), probably far beyond the limits of any equilibrium relationship resulting from natural selection.

Perhaps man’s concern for selecting large seeds in cereals was reduced,

eventually, because it added little to yield. For example, in Christian and Grey’s

study (1944), there was no difference in yield between crops established wholly

from large seed (45 mg) and those established wholly from small seed (27 mg) of

the same genotype. If genetic selection has been made for large seed size, there is

usually a compensating decrease in the number of grains per plant (Grafius et al.,

1976; Hamblin and Morton, 1977). Indeed the data of Grafhs er al. indicated

that the best means of increasing yield was to select for grains per head and to

allow seed size to vary more or less randomly. Although man has taken a strong

interest in larger wheat seed, the influence of natural selection has been so allpervading and continuous that his direct influence, at least until plant breeding

began, may have been quite limited.

vi. For Speed of Germination. In the wild, the irregular or protracted germination of wheat or of any other annual grass is a partial protection against

uncertain climatic conditions, so-called false starts to the rainfall season. But

cultivated wheats germinate more rapidly and evenly than do wild wheats (Evans

and Dunstone, 1970). This evolution to speedy germination was wholly because

of natural selection. Within a crop sown in a prepared seed bed, plants that

emerged first had strong competitive advantage over their neighbors. In a drilled

barley crop (cv. Clipper) at Adelaide, plants that emerged 1 day earlier gave rise

to seedlings 15% heavier at day 17 (from sowing) and to plants 14% heavier at

day 70. There was a reduction of 14% in the mean number of grains per plant for

each delay of 1 day in emergence (Soetono and Donald, 1980). In that instance,

the differences in day of emergence were caused principally by individual seed

environments (depth, soil physical condition, and so on), but the same effect of

delay would occur when prolonged germination was a genetic character. Thus

there would be a progressive selection under crop conditions toward rapid and

simultaneous germination, a feature, albeit imperfect, of most modem annual

seed crops.



vii. For Root Characters. Knowledge of the role of root systems in natural

selection under domestication is seriously lacking. If it is reasonable to extend to

the root system our understanding of the features of plant tops giving selective

advantage through competition, one would suggest that just as a tall, leafy,

tillered plant secures an undue share of the light, so plants with a widely ramifying root system would be at a selective advantage by absorbing water and

nutrients more rapidly and more extensively than could neighboring plants with a

restricted system.

Passioura (1972) has shown that restricting root growth early in a plant’s

development reduces preflowing water use, and he has suggested that this characteristic may be important in situations where a crop is maturing grain in dry

situations. However, O’Brien (1979) has pointed out that reduced root development in early stages of growth may lead to problems of nutrient uptake. It is not

possible, with our current knowledge, to make any generalizations about root

growth and crop development (Fischer, 1981).


It is evident that all crops have been subject to many similar selective and

evolutionary processes during domestication. They have become adapted to both

the natural environment of the region and the manmade environment of local

cultural methods. Despite the diversity of environments in the cropped areas of

the world and the specific responses by individual crop species, there have been

many common trends in all crops. Various writers have pointed to features which

they regard as typical of wild plants and which commonly disappear under

cultivation. These especially include morphologically wild characters associated

with seed dispersal and seed burial (awns, brittle rachis, shattering pods, pointed

seeds, wings, spines, etc.). Positive responses have included the development of

synchronous ripening, rapid and simultaneous germination, and larger seeds.

Selection through competition has been recognized in its more generalized expression; those genotypes that are prolific and produce more seed, and in the next

generation more seedlings, contribute an ever-increasing proportion of the


However, there has not been adequate recognition of the influence of competition in ensuring the success of plants of common architecture in all seed crops,

irrespective of species, soil, or climate. Their structure can be clearly designated:

tall, free branching (or tillering), a dense canopy of large, horizontally disposed

leaves, and indeterminate habit. Although these may be combined and expressed

in many variant ways, they integrate to give a plant of strong competitive ability.

Turesson’s (1922) concept of an ecotype, “a product arising as a genotypical

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II. Selection in Domesticated Crops

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