Hard (and soft) selection

The physical conditions tolerated and the resources utilized by organisms play a vital role in determining their distribution and abundance. Collectively, these traits can be called ecological phenotypes, implying a relationship with the r–K continuum. Extremes of physical conditions cause death after various periods of time irrespective of the density or the frequency of organisms, that is, populations are subject to hard selection in the sense of Wallace (1981).

The difference between hard and soft selection is well illustrated using an example presented by Wallace (1981) on two methods whereby a plant breeder can obtain seed while carrying out a program of artificial selection for increased yield. The breeder plants his experimental seed in a randomized series of test plots (Figure 3.1). At maturity, there are two procedures whereby selected heads may be chosen, which arise because of variation of soil conditions, other environmental variations among plots, or genotype differences.