FIGURE 21. The outcome of selection for high and low oil content in the Illinois corn experiment
(From Woodworth et al. 1952).
Ecological genetics and evolution: essays in
honour of E. B. Ford (1971)
Plant Evolution in Extreme Environments
A. D. Bradshaw
Artificial Selection Experiments
To understand how such evolution can come about we need to appreciate the power of selection. A starting point is the famous long term selection experiment, the Illinois corn experiment, which was started in 1900 and has continued ever since. Artificial selection for low and high oil and protein content has been carried out on an ordinary unselected open pollinated population, the Buff White variety of maize. The population size has been maintained at several thousands of plants and in each generation the twelve highest or lowest plants out of sixty analysed are selected to give rise to the next. The course of selection for oil content is shown in fig. 2.1. The significant feature is the long and continued response to selection which has taken the selected populations to levels of oil completely outside that of the range of the original population (Woodworth, Leng & Jugenheimer 1952). The rate of progress under selection has continued almost undiminished over the whole period.
FIGURE 21. The outcome of selection for high and low oil content in the Illinois corn experiment (From Woodworth et al. 1952). |
A similar experiment has been carried out in ryegrass Lolium perenne where flowering time has been selected (Cooper 1960) (fig. 2.2). There is the same picture of change although in this case the basic population is only 125 plants and only four are selected for the next generation. One aspect of the experiment is that in the first generation two different sets of parental plants were chosen as starting points. The effect of these has continued throughout.
 FIGURE 2.2. The outcome of selection for heading date in ryegrass (from Cooper 1960). |
The characters that have been selected in these two experiments are controlled by a large number of genes. The degree of response achieved concurs with what might be expected from genetic theory. It also agrees with work on Drosophila. Such patterns of change are likely with a series of additive genes, scattered through an initial population (Lee & Parsons 1968). In the restricted ryegrass populations the effect of choice of starting material is very evident and is similar to that found by Hosgood & Parsons (1967). There is little sign of restriction of release of variation due to linkage: this can be explained by chromosome numbers which are effectively at least twice that of Drosophila.
The Illinois corn experiment with its relatively large population is the model most relevant to what may happen in a natural population. Plant populations are usually large in size. And it must also be remembered that the size of a natural population cannot be measured solely in terms of the number of observed adjacent individuals: it must take into account individuals farther away supplying new genetic material by gene flow. In plants, gene flow due to pollen movement by wind or insects is leptokurtic, so that there is a small amount of gene flow over relatively large distances (Bateman 1947) sufficient to be important in supplying new variability (see p. 41).
Proceedings of the Eighth International Grassland Congress
held at the University of Reading, England, 11-21 July 1960. p. 41-44
Selection for Production Characters in Ryegrass
J P Cooper
(Welsh Plant Breeding Station, Aberystwyth, Wales)
|
Fig. 1. Response to selection for ear emergence in Kent ryegrass. |
Continued response to selection in later generations is illustrated in Figure 1 for date of ear emergence in Kent ryegrass. Selection for this character was started in Irish and Kent perennial ryegrass in 1954 and is now in the fifth generation. A similar picture is presented by both varieties; 5 generations of selection have resulted in extensive response well outside the range of the original populations, and as early as the fourth generation, the full range of Kent had been obtained from Irish and vice versa. Even 4 initial plants of the same phenotype have released more variation than is expressed phenotypically in the original variety, confirming the great heterozygosity of the initial parents. Appreciable genetic variation still exists within the selection lines and response is still continuing.
There are, however, dangers in selecting for a single character at a time. Unfavourable correlated responses in other characters may occur. In selection for date of ear emergence, for instance, male and female sterility have appeared as correlated responses in 2 of the lines, decreasing the possible selection differential (7). Similarly, selecting for rapid leaf appearance has invariably led to smaller leaves and vice versa (9); this correlation may, well prove an obstacle to selection for high total leaf area.
Genotype/environment interaction may also act as a brake on progress, as in selection for dry matter production in Italian and Irish perennial ryegrass, where a strong interaction occurs between family production and level of cutting.
Agronomic assessment
The efficiency of selection for any character, however, depends also on the accuracy with which it can be measured. Such characters as date of ear emergence, habit of growth, disease resistance, and winter greenness can be recorded fairly accurately on individual seedlings or spaced plants (1). The case of production in the sward, however, is more difficult. Assessment of production on the basis of spaced plants, either by eye or by cutting, is not always correlated with yield under sward conditions, particularly where a legume is included (10, 16).
Selection techniques must therefore be developed to assess individual plants and their progenies under controlled competition and defoliation, preferably with a legume. A simulated sward technique involving close spacing of seedlings (2.5 in.) combined with different levels of defoliation has been developed at Aberystwyth for this purpose. It has been used for comparing different varieties and families, and since records are taken on individual seedlings, it can be used as a screening technique for selecting parents.
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