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IV. Selection, Evolution, and Crop Yield

IV. Selection, Evolution, and Crop Yield

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some instances, as evolution continues seed yield advances; in other instances, it


“Natural selection, it should not be forgotten, can act solely through and for

the advantage of each being,” wrote Darwin (1868, p. 184). Within seed crops it

ensures the greater abundance of certain genotypes in ensuing crops, without

implications for yield by the entire seed-producing community. And therein lies

the dichotomy: on the one hand the performance of the individual competing in a

mixed community, and on the other the performance of that same individual

genotype growing as a pure crop stand, each plant competing against like


During selection by man, the bases for many of his choices were, by selfdefinition, advantageous. If he preferred and selected large mottled seeds, then

any increases of these in the next generation were advantageous, by his standards. A secondary effect, such as a reduction in the number of seeds produced

per plant, might reduce his yield per unit area, however. When this occurred, he

may have accepted it; more probably he was unaware of it.

When man has sought to select for yield deliberately, he has usually based his

attempts on the performance of individual plants or individual shoots. Selection

of larger wheat ears, corn cobs, sunflower heads, or plants with more inflorescences was considered a route toward higher yields, and no doubt was highly

successful during the early years of domestication. But, as considered later, such

selection had progressive limitations.

Natural selection for adaptation to the farm and to the farmer’s practices also

offered fum prospects that the progeny would contribute to increased seed yield.

Again we emphasize the distinction between biological prolificacy (many

seeddplant) and crop-to-crop survival that ensures representation in the bulk

seed sown for the next crop (see Section III,B,2a). Plants that have both high

biological prolificacy and strong crop-to-crop survival will dominate because of

exacting natural selection for performance within the agricultural environment.

It is especially in relation to natural selection by interplant competition that

evolution and increased yield do not go hand-in-hand. There is growing experimental and circumstantial evidence that successful competitors within mixtures

of biotypes may be poor producers in pure stands. Instances in which these

features are either unrelated or negatively related are recorded in many seed

crops, particularly in the cereals, and they are discussed in the following section.




The successful plant within a genetically uniform crop growing in a uniform

environment will be the plant suffering the least competition from its neighbors.



It follows then that the crop should be comprised wholly of plants of low

competitive ability, interfering with each other’s growth only minimally (Donald, 1968a,b). Such plants are, in Darwinian terms, unfit plants because of their

weak capacity to compete or survive against other plant types in natural communities or in crops of mixed genotypes.

A further postulation may be made: strong competitors, which in general are

tall, leafy, and freely branched, not only suppress short, erect, unbranched plants

but when grown in a pure community may so interfere with each other’s growth

(as, for example, by strong mutual shading) as to perform relatively poorly. If

this is so, then one not only may expect that performance in mixtures will be no

guide to performance in a pure stand (a nil relationship), but further that there

will be instances of a negative relationship between performance in the two

situations. Both these situations have been reported frequently.

1 . Wheat

The first recorded case of yield reversal for varieties grown in pure and mixed

culture was described by Montgomery (19 12). He found that one variety rapidly

dominated the mixture but that it was not necessarily the one that was highest

yielding in pure culture. Similar results have often been reported (Engledow,

1925; Klages, 1936; Christian and Grey, 1941; Laude and Swanson, 1942;

Khalifa and Qualset, 1974). The results of Khalifa and Qualset (1975) on a

segregating wheat population, suggesting that competition was eliminating the

short, high-yielding lines, have already been discussed (see Section III,B ,2,b,ii).

2 . Rice

Perhaps the most striking case of a negative relationship between competitive

ability and yield among crop varieties is that reported between tall and semidrawf

rice cultivars in the Philippines (Jennings and de Jesus, 1968). The strong competitive ability of these tall leafy varieties was discussed earlier (Section

II,B,2,B,ii and iii), but these successful cultivars in mixtures were poor producers in pure culture, When the dwarf erect types were almost eliminated, the

lower yielding of the two tall cultivars reduced the other tall cultivars to a low

frequency in the mixture. Similar results were obtained by Sakai (1955) and

Akihama (1968). Jennings and Herrera (1968) also demonstrated a negative

relationship between competitive ability in mixtures and yield in pure culture for

segregating populations.

3. Barley

Harlan and Martini (1938) grew 11 barley varieties at 10 centers across the

United States for 4-10 years; the seed harvested at each site was resown at that



site the following year. One variety rapidly dominated the mixture, but the

particular variety varied with the site. In several instances, the variety most

successful in terms of farm use in the region was reduced to a very low frequency. The poor competitive ability of genotypes that are high yielding in pure

culture has also been reported by Suneson and Wiebe (1942), Suneson (1949),

Wiebe et al. (1963), and Allard and Adams (1969).

On the basis of morphology (long leaves, tall plants), Hamblin and Donald

(1974) suggested that the high yield of individual F, plants was the result of high

competitive ability. This situation was reversed in pure culture F, plots, where

high yield was associated with short, small-leaved plants. The negative relationship between competitive ability and pure culture yield for these lines was

confirmed by direct measurement (Hamblin and Rowell, 1975).

4 . Oats

Smith et al. (1970) examined all pair combinations of five oat varieties. The

tallest variety (Rodney) was the lowest yielding in a pure culture but the most

competitive in mixed culture. In pure culture, however, a variety of intermediate

height (Brave) had the highest yield; nonetheless, competitive ability was closely

related to plant height.

5 . Maize

The yield of brachytic maize genotypes grown in alternate rows with normal

maize genotypes was less than when this maize was grown as a pure stand

(Pendleton and Seif, 1962). The mass selection experiments for yield of Gardner

and co-workers (Gardner, 1961, 1968, 1969; Lonnquist et al., 1966, personal

communication) can be interpreted in terms of selection for increased competitive ability (our interpretation) rather than for high-yield potential (Gardner’s

interpretation). These workers mass-selected single plants on the basis of plant

yield, using a grid system to give local control of environmental variation

(Gardner, 1961). The selected individuals provided the progeny for the next

round of hybridization and selection. Eventually the different generations were

tested for yield, and it was found that yield increased linearly with time for

several cycles and then reached a plateau. The plots used were either single or

double unbordered rows in which the within-row spacing was the same as the

between-row spacing. The height of the populations increased in step with the

yield. This would mean that the most advanced generations were surrounded by

shorter, earlier generations whose yield potential would not have been fully

expressed because of interrow and intergeneration competition. Ultimately, the

plants became so tall that increased competitive advantage was offset by the

disadvantages of increased lodging, and no further yield increase was observed.

Increased yield from mass selection is a rare event; therefore it is important




that further work be done to determine whether mass selection for yield was

effective, or whether selection was in fact for competitive ability. This is particularly important in view of the contrasting CIMMYT results to be considered in

Section V,D.

6. Sorghum

Averaged over 16 hybrids and two environments, Kern and Atkins (1970)

found significant yield depression for short hybrids bordered by tall hybrids and

significantyield increases for tall hybrids bordered by short hybrids. On average,

a k m difference in height between rows increased or decreased yield by 0.2%

for the taller and shorter hybrids, respectively. Kern and Atkins (1970) considered that small yield differences between genotypes of different heights were of

doubtful validity unless the data were obtained from bordered plots.

7. Soybeans

The performance of soybean cultivars in pure culture was not necessarily

related to survival in mixture (Mumaw and Weber, 1957). Survival was related

to branching pattern and height. Branching in dicotyledons may be ecologically

parallel to tillering in graminaceous crops. Similar results were obtained by

Schutz and Brim (1967) and by Hinson and Hanson (1962) in which height and

maturity were the dominant factors. Mumaw and Weber (1957) concluded that

“a relatively high yield of a variety in pure stands was not necessarily an

indication of its ability to survive in mixed populations.”

8. Beans (Phaseolus vulgaris)

The experiments of Hamblin (1975) have already been considered (Section

II,B,2,b,v). In summary, he found that yield in pure culture and survival in

mixtures were not related.

9. Sunflowers

Working with eight varieties of sunflower, Fick and Swallers (1975) found

that in one-row plots the tallest variety had the highest yield, and that there was a

close correlation between height and yield (Fig. 1A). When three-row plots were

used the height/yield correlation disappeared (Fig. 1B) and the yield rankings

changed markedly. In three-row plots, there is still a suggestion that the shortest

genotype suffereda competitive disadvantage. Also, it was probably sown at a

density too low for maximum yield.





Height (cm)

FIG. 1A. The effect of height on yield of single rows of eight varieties of sunflower. These. rows

are not bordered. Note there is a strong correlation between height and yield.




Height (cm)

FIG. 1B. The effect of height on yield of the same varieties in 3-row plots, where data was

obtained from the center TOW only. These rows are bordered. Note there is no correlation between

height and yield, although it appears that the center row of the shortest genotype is still suffering

interplot competition. (Data from Fick and Swallers, 1975; Table I.)

10. Cotton

Significant genotype-neighbor interactions were found for yield of four cotton

varieties grown as single rows (Moran-Val and Miller, 1975). Competitive ability was in part related to height and the authors cautioned that “If competitive

effects of the magnitude observed in this study are generally present in yield

trials, however, fully bordered plots would be indicated.”

1 1 . Comments on the Results Presented in This Section

The papers just discussed, relating yield in pure culture to competitive ability

in mixtures, suggest that there is frequently zero or negative association between

these two factors. Although this negative association has not always been found

between yield and competitive ability (e.g., Johnston, 1972; Blijenburg and

Sneep, 1975), it is so common that it cannot be ignored. In the cases where a

positive association has been found between yield and competitive ability, the




comparisons are often between adapted and unadapted types so that the observed

result is to be expected. It may be argued also that the results obtained using

mixtures of varieties are artifacts caused by the small sample of varieties used in

many of the experiments discussed.

We believe that the latter argument is not correct for several reasons. First,

there is the sheer number of experiments involved; the result has been observed

so often that it is unlikely to be an artifact. Also the argument that results from

mixture experiments using varieties may be artifacts applies equally whether the

relationship between yield and competitive ability is positive or negative. Second, and more convincing, there are studies involving segregating populations

(Jennings and Herrera, 1968; Hamblin and Donald, 1974, Khalifa and Qualset,

1975; Hamblin and Rowell, 1975) in which yield in pure culture was not associated with competitive ability in mixtures for a whole range of species. In these

experiments the lines used were related and chosen at random. Third, the results

make evolutionary and biological sense (see the next section and Section


12. Principles Governing Competitive Success and Yield

It follows from the foregoing studies that successful plants for pure-stand grain

yield are often poor competitors (however, we do not equate poor competitive

ability with physiological deficiency). In the study of wheat by Christian and

Grey (1941), seed size was the feature giving competitive success; in the study

by Suneson (1949), it was a prolific root system; in those for rice (Jensen and de

Jesus, 1968) and barley, (Hamblin and Donald, 1974) it was height, leafhess,

and leaf length; and for soybeans (Mumaw and Weber, 1957), it was a branching

habit. In each instance of yield reversal, it was the shorter, less leafy, less

branched cultivars or segregates that gave higher yields in pure stands.

Three propositions may now be stated:

1. That, under domestication, competitive plants gained dominance through

the natural selection of “the fit”; in some instances, man’s purposeful

selection of successful competitors has maintained the place of these


2. That, in pure crop stands, highly competitive plants give lower seed yields

than do less competitive individuals, especially at high plant densities.

3. That, because natural selection has favored competitive plants with reduced capacity for yield in pure culture, various natural selection processes

must be reversed by plant breeding and selection.

In the Darwinian sense of fitness, it is the unfit plants that will succeed in the

pure culture crop situation. Acceptance of this proposition imposes on the plant

breeder the need to avoid using selection on individual plant performance in



situations where there is significant competition among plants of differing genotypes. In this circumstance, poor competitors, whatever their potential merit in

pure stands, will give depressed yields. The unfit plant must be allowed to

express its potential in pure culture; such plants will perform poorly in mixtures

with other genotypes, as in a segregating population in rows or hill plots of

mixed genotypes. They will also perform poorly in spaced plant situations (Donald and Hamblin, 1976). Individual plant yields in those circumstances may be

positively misleading for predicting crop yields (Wiebe et af. (1963). However, a

crop comprised of a single genotype of weak competitive ability still depends on

effective exploitation of the environment for high yield. If the annual plants to be

used in crops are weak competitors, then the number of plants per square meter

must be increased to ensure that they compete with one another sufficiently to

exploit the environment fully.

It may seem a paradox to propose that productive annual crops will be comprised of weakly competitive plants and to say these plants must be sown at a

density sufficient to ensure that they will compete intensely with one another.

But these are not incompatible thoughts or objectives. Each is aimed at increased

crop efficiency, the first by reducing the pressure of each plant on its neighbors

through plant form, and the second by increasing the pressure of the whole

community on the available resources through an increased population density.

Thus, testing should not merely avoid competition between different genotypes.

The lines to be evaluated must be tested in pure stands at densities sufficiently

high to ensure that there is interplant competition of considerable intensity.







The relationships between biological yield and seed yield in seed-producing

annual crops display important differences, as is illustrated in Fig. 2. The upper

graph (Fig. 2A) shows the general relationships found in cereals (Donald, 1963).

Biological yield increases with density until it reaches a plateau. This is maintained up to very high densities unless crop failure occurs from lodging or the

advent of disease among the attenuated plants. Grain yield increases to a maximum at a density approximating the minimum density giving full biological

yield. To the extent that when maximum seed yield is attained there is maximum

exploitation of the environment in terms of biological yield, cereals are efficient

in ensuring their prolificacy.

On the other hand, the maximum yield of seed cotton or lint by cotton crops is

achieved at a density at which the biological yield is still rising steeply (Fig. 2B).

The biological yield of the highest yielding crop falls far short of the capacity of

the cultivar to produce dry matter. In this regard it is an inefficient crop, which

will eventually be replaced to advantage by cultivars giving increasing lint yields

up to full biological yield. It may be that this contrast between an annual such as










FIG. 2. The general relationship between biological yield (BY) and grain yield (GY)for cereals

(afterbnald, 1%3)aodforcotton(dataderivedfromKirkeral.. 1%9;Fig. 14,16,19,21 (BY);21

(GY).(GY is the fruit weightlha; seed yield data was not available).

wheat and an annual such as cotton relates to the period for which they have been

cultivated (i.e., wheat for some millennia, annual forms of cotton for little more

than 500 years) (Hutchinson, 1965).

The future trend in the relationship between biological yield and grain yield of

annual crops may follow the pattern shown in Fig. 3. If nonbranched plants are

used, the density @lants/ha) required to give the full biological yield will be

greater. Because of the improved canopy structure, the biological yield attainable

will be greater, although perhaps not markedly so (10% in the example shown).

The main contribution to potential yield will be an improved harvest index,

perhaps 0.35-0.40 to 0.50 in cereals, representing an increase of about 25% in

grain yield. The influence of an increased biological yield of 10% and an increased harvest index of 25% would be a 37% increase in grain yield. The extent

to which the trend toward increased harvest index is displayed in annual seed

crops of diverse growth form is discussed later in this article. The same trends are

evident in each crop, and there is no instance in which newer cultivars demonstrate a reversal of the relationship. However, whether harvest index itself is a

useful selection criterion, or a useful description of past selection trends, has yet

to be critically determined (Hamblin and Rosielle, 1983).







II fi'



FIG. 3. Present relationship (solid lines) and possible future relationship (broken lines) between

biological yield (BY) and grain yield (GY).



In this section we briefly review crop evolution, including recent plant breeding efforts for a series of annual seed crops. This will allow a basis for proposing

a generalized ideotype for all annual seed crops.


Wild wheats, both diploid and tetraploid, are annuals and have primitive

features associated with effective seed dispersal and burial (Bell, 1965). Less

easily defined but no less important characteristics of these wild wheats relate to

growth form, permitting survival and seed production under close grazing by

sheep or goats. Many are fine stemmed and prostrate, so that some ears are borne

on a nearly horizontal stem only a few centimeters above the ground and can lie

within a grazed sward. Under cultivation, these primitive characters of wheat

were at a selective disadvantage and have been lost, except for the brittle rachis

and enclosed grain of T. munucuccum (Bell, 1965). Taller, more erect, and more

leafy plants had major competitive advantage and probably became dominant

quickly. Bruegel's paintings (sixteenth century) depict shoulder-high wheat

crops. Although having a tendency to lodge, these tall crops were feasible in

premechanized agriculture because most of the grain would be recovered during

hand harvesting.

In early wheat breeding programs, improved disease resistance was a major




contributor to increased yields (Athwal, 1971). Yield potential also increased

(Austin et al., 1980; L. T. Evans, 1980; Perry and Reeves, 1980; Kulshrestha

and Jain, 1982). However, this yield potential increased markedly with the

incorporation of the Norin 10 gene in wheat. These short, fertilizer-responsive

wheats had been grown in Japan long before scientific plant breeding and were

successfully used in Italy 60 years ago (Athwal, 1971). However, it was only

when Vogel at Pullman and then Borlang at CIMMYT used this material that the

so-called semidwarf wheats made such an impact on world wheat yields in the

1960s and 1970s. These wheats are resistant to lodging and have many tillers and

grains per spikelet.

Reduced stature and resistance to lodging are characteristics sought in the

“all-crops ideotype;” increased number of grains per spikelet in the semidwarf

wheats defines the expression of increased yield. On the other hand, the freetillering and relatively broad, lax leaves of these highly successful semidwarf

varieties are a challenge to the all-crops ideotype.

Many workers believe that only a small number of tillers is needed to give both

maximum yields and sufficient plasticity to permit adaptation to the environment

(MacKey, 1966; Hurd, 1969; Bingham, 1972; Jones and Kirby, 1977). This

view was extended by Donald (1968a,b), who described a wheat ideotype for

high grain yields with a short, strong stem, few small, erect leaves, a large ear in

relation to the total dry matter (i.e., a high harvest index), an erect ear, awns, and

a single culm. (In view of the authorship of that article, this wheat ideotype

conforms to the common ideotype for all annual seed crops described in this

article). Atsmon and Jacobs (1977) have produced uniculm wheat lines of medium height, high harvest index, and resistance to lodging; they appreciably outyielded the standard cultivar of the region. Further evidence for the potential of

controlled tillering was presented by Islam and Sedgley (1981), who examined

the effects of manually detillering wheat plants in the field to give biculms. The

performance of these was compared with normal-tillered control plots of the

same variety. The detillered plants outyielded the controls by 14 and 22%,

respectively, in 1978 and 1979.


Barley (Hordeurn distichurn) is a cereal with a growth form and physiology

similar to wheat, so that most considerations of canopy structure, tillering, and

leafiness are applicable to both species. Cultivated barleys vary in height from

brachytic forms (40 cm), widely grown in the Middle East, to tall forms (1.5 m)

(Reid and Weibe, 1968). They tiller freely at low density, especially the tworowed types, but there is a mutant form with a single stem per plant (uniculm).

There are considerable differences in leaf size, with extremes of leaf shortness in

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IV. Selection, Evolution, and Crop Yield

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