ion to the critical roles of yield stability and nonstop selection in mitigating and reducing crop losses. Cotton is used as a case study to illustrate both the potential to gradually incorporate resistance to pathogens much faster than in conventional breeding and the necessity of nonstop selection to improve the yield and homeostasis of cultivated varieties, especially during the annual propagation of breeder seed.
Research that led to the identification of yield stability as the critical factor that connects yield potential at the single-plant level with crop yield conditions has its origins in the development and applications of the Honeycomb field selection designs . The time course of this research, through some key publications that defined the principles which underlie the present cotton case study, can be briefly described as follows: An early important article  demonstrated the existence of a negative correlation between a plant’s yielding and competitive ability and showed that because of this, progress through selection in a breeding program is achieved when the evaluated plants are grown at distances excluding any interplant competition. Consequently, the unit of selection ceases to be the conventional, densely sown plot, becoming the individual plant grown in the absence of interplant competition. Subsequent work  measured for the first time the correlation between yielding and competitive ability, which was found to be high and negative. This negative correlation was interpreted as the main cause of variety degeneration and of selection inefficiency during the early, segregating generations of a breeding program. In the traditional densely-sown plots, where cultivar reproduction commonly occurs, there is preferential and gradual proliferation of genotypic forms that are high competitors-reduced yielders at the expense of high yielders-weak competitors. Another key finding was that the conditions, which maximize crop yield are those that minimize interplant competition.
The unique properties and establishment process of the Honeycomb field selection designs (Figure 1 and Figure 2), conceived to accurately evaluate individual plants through elimination of the masking effects of soil heterogeneity and interplant competition, have been extensively treated in . A subsequent article  defined interplant competition as the interference with the equal sharing of growth resources among plants because of genetic and acquired differences. It was demonstrated that in the crop stand, the gains from competition do not overcompensate for the losses from it. In the prevalence of big genetic or acquired differences in the crop stand, crop yields are reduced, a concept of great relevance as to the reasons for the observed gap between actual and potential yields. The article established the use of the Coefficient of Variation (CV) of single-plant yields to measure the acquired and the genetic competition. Under dense stands, CV strongly correlates with crop yield (the lower the CV, the lower the unequal sharing of resources within the crop stand, the more uniform the stand, and the higher the yield). An early verification of this notion appeared in  and . Under conditions that exclude interplant competition, CV was found to reflect the departure from normality of the yield distribution and to estimate the load of deleterious genes in breeding populations .
A further essential point was the analysis of the crop yield potential into three independent genetic components [19,20] estimated reliably in the absence of the masking effects of interplant competition and soil heterogeneity. The three components were defined as (i) the yield potential per plant, estimated by the mean progeny yield , (ii) the stability, estimated by the standardized progeny mean yield , where and represent the mean and the standard deviation of the entry/line where each plant belongs, and (iii) the adaptability or responsiveness to inputs, estimated by the standardized selection differential . The independence among the three genetic components was key fo