Genetic Variations and Trade-Offs
We observe apparent overall declines in some nutrients, combined with apparent increases in a significant minority of individual foods and nutrients. A possible explanation for this observation is changes in cultivars during the period represented by data published in 1950 and 1999. Cultivars commonly are selected for yield, growth rate, pest resistance and other attributes, but seldom have they been selected for nutrient content. It is well accepted in agricultural research that selection for one resource-using function may take resources away from other resource-using functions. For example, there are often trade-offs between growth rate and pest resistance [22], between yield and resistance to herbivory [23] and between the number of seeds and their size [24]. As we will illustrate, cultivars selected for yield, rapid growth or other non-nutrient characteristic may suffer resource limitations in their abilities to extract soil minerals or transport them within the plant, or in their abilities to synthesize proteins, vitamins and other nutrients. Such trade-offs are usually unpredictable in size, however, and because of ever-present genetic variability, some fraction of cultivars will show enhanced contents of individual nutrients.
