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).