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CHAPTER 3. THE CONVERGENT EVOLUTION OF ANNUAL SEED CROPS IN AGRICULTURE

CHAPTER 3. THE CONVERGENT EVOLUTION OF ANNUAL SEED CROPS IN AGRICULTURE

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98



C . M.DONALD



AND J. HAMBLIN



“once only” event, when certain major mutations ensured that a species was

adapted into the agricultural environment and altered unfavorably for survival in

the wild (Mangelsdorf, 1965), although this may be an oversimplification in

some cases (Harlan, 1971). These mutations include such well-known features of

crop plants as indehiscent fruiting bodies, which allow easier and more complete

harvesting, and soft seededness, which ensures simultaneous germination of all

seeds. The third phase of evolution within the domesticated species is a continuing phase. The crop changes under selection pressures, both natural and manimposed. It is the role of agronomists and plant breeders to optimize the environment and plant type to ensure maximum crop production within this

environment.

The world’s field crops are commonly grouped into cereals, pulses or grain

legumes, fiber crops, oil crops, root crops, and rubber. This classification, based

partly on botanical considerations and partly on crop product, has little ecological basis. However, many of these crops, such as the cereals, the grain legumes,

and some of the oil crops, are annual seed crops with parallel ecological features.

Increased understanding of the factors governing crop photosynthesis and respiration, distribution of assimilates, and seed growth permits us to compare and

contrast the performance of annual seed crops. This may be in terms of their

branching, leafiness, light profile, photosynthesis, biomass, flowering, seed setting, grain filling, harvest index, and yield, and/or in terms of agronomic factors

such as soil fertility, plant density, and plant arrangement. At first sight, it may

seem difficult to compare cotton with maize or sunflower with wheat, but such

comparisons provide a major challenge to our thoughts. For instance, why are

some annual seed crops so much more productive than others? And what can be

done to remodel less efficient crops?

Annual seed crops provide most of man’s food and, in some countries, a

significant part of the animal feed as well as industrial products of great importance (fibers, oil, etc.). They are cultivated from the equator to near the Arctic

circle and are adapted to diverse edaphic situations. Some are extremely tall

(more than 4 m) whereas others are dwarf, some are climbers with tendrils.

When these patterns of adaptation and the morphological and physiological

characters are surveyed, Can we perceive common features, either plant or

cultural, that may be exploited to increase seed yield per hectare in all the various

annual seed crops and environments?

We consider that the yield potential of annual crop species will increase at a

faster rate than occurs with empirical selection for yield if suitable ideotypes are

identified. A considerable list of common features and practices that influence

yield in all annual seed crops can indeed be identified, and it may be possible to

design a basic ideotype for all these crops, involving principles of crop physiology and associated agronomic practices equally applicable to any annual seed

crop. Such a common model or ideotype is formulated here.



CONVERGENT EVOLUTION OF ANNUAL SEED CROPS



99



A sharp distinction is drawn between the ecology of annual field crops grown

for their seed and that of most horticultural crops. Horticultural plants are cultivated for their fruits, unripe seeds, roots, stems, or leaves. “We believe,”

remarks Schwanitz (1966, p. 29), “that the transition from normal to giant

growth is the most important step in the evolution of wild species into cultivated

plants.” The modem apple, tomato, celery stalk, or lettuce heart are examples.

However, it is important to recognize the large environmental as well as the

genetic component in the improvement of horticultural crops. Letluce and many

other species may be so widely spaced that the plants are almost noncompetitive.

Alternatively, there may be deliberate reduction in the number of harvested

parts, as in flower or fruit growing where huge blooms and fruits of some species

are produced by deliberate thinning. Schwanitz’s view of gigantism through

evolution under domestication is clearly valid with respect to horticultural plants;

there has been remarkable progress toward gigantism of the harvested part within

markedly more favorable environments. Under horticultural conditions, natural

selection is heavily suppressed, but selection by man has been highly effective.

The range of varieties within a single species (e.g., within Brussicu oleruceu, the

cabbage, cauliflower, brussel sprouts, kohlrabi, kale, etc.) illustrates this point

most vividly.

For annual seed crops, however, Schwanitz’s views regarding gigantism of

plant parts do not hold. The prime need for cultivators of these crops has always

been the quantity of seed in the bag or basket, of the crop yield per unit of land,

rather than the size of the individual seed or the seed yield per plant. In modem

seed crops, a reduction in individual plant yield to as little as 5% of the yield of

like plants growing in isolation is usual (Donald, 1963), yet for a long time

agricultural scientists failed to recognize the significance of competition within

these monocultures. With competition of such intensity, the extent of natural

selection is limited only by the genetic variability between the plants and by any

inequalities of the immediate environment of the individuals. If a drought occurs,

there will be intense competition for water. If water is abundant, there may be

equally intense competition for nitrogen. If water and all nutrients are freely

available, there will be extreme competition for light.

Nothing is farther from reality than the following analysis by Schwanitz,

which is a common viewpoint (italics ours): “Cultivated plants are also exposed

to the influence of their environment; they too are threatened by frost and

drought, pest and disease. But man has been careful to protect the plants that are

useful to him from excessive hazard. By tilling and fertilising the soil, by

regulating the water supply and eliminating the struggle for life, and by protecting the plants from pests and disease, he has created an artificial environment

that favours the plant more and above all exposes it to less rigorous requirements

than those met in nature. Hence natural selection in cultivated forms is less

harsh than among wildplants” (p. 116). Only in a few horticultural crops is that



100



C. M. DONALD AND J. HAMBLIN



statement generally acceptable. For cultivated seed crops, the seedling environments have been substantially improved; nonetheless, intense competition

among plants rapidly develops. This competition more than any other factor has

governed their evolution. Many of the generalizations on crop evolution are

largely based on horticultural crops and simply do not apply to annual seed

crops, the principal component of man’s agriculture.

Our views are essentially similar to those of Clements et al. (1929, p. 77),

who state that “Competition is keenest when individuals are most similar

and . . . make nearly the same demands on the habitat and adjust themselves

less readily to their mutual interactions,” and also that “The closeness of competition between plants of different species varies directly with their likeness in

vegetation or habitat form.”



II. SELECTION IN DOMESTICATED CROPS

A. CHARLESDARWIN’S

VIEWS



Darwin (1868) grouped the selection forces operating among domesticated

animals and plants into three loosely defined categories: methodical or con-



scious selection by man “according to some pre-determined standard”; unconscious selection by man through retaining ‘better’ animals or plants (‘better’ in

the eyes of the herdsman or the cultivator) and natural selection, occurring

without any purposeful intervention by man. He drew no particular distinction

between the operation of these three forms of selection among animals and

plants.

Darwin readily illustrated conscious selection with horticultural crops: the

development of large gooseberries, double flowers, and early maturing peas, and

the selection of high sugar content in beets;hence his tribute to man’s capacity to

select methodically from within a varying population those features or attributes

he values. However, he used the term unconscious selection when man selected

for general superiority of animals or plants without attempting to define the

specific factors for which selection occurred. Consequently, some confusion has

arisen in the subsequent use of this term. Though the herdsman might choose a

‘better bull’ or the cultivator might choose a ‘better plant’ without any predetermined standard or without evaluating any array of desirable features, he neverthelessdoes make a perfectly conscious and deliberate choice. The term unconscious selection seems scarcely appropriate.

Darlington (1%9) uses Darwin’s term unconscious selection but gives it a

different meaning. He speaks of “unconscious selection by the cu1tivator”as the

transformation of the crop by selection during cultivation, tilling, sowing, reap-



CONVERGENT EVOLUTION OF ANNUAL SEED CROPS



101



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

agriculture.’’

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).

B. SELECTION

WITKIN ANNUAL

SEED CROPS



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



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