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We demonstrate a novel case of selection for heterozygosity in nature, involving inadvertent human selection on a population of domesticated plants. Amerindian farmers propagate cassava (Manihot esculenta) clonally by cuttings. Seedlings also appear spontaneously in fields, and farmers allow them to grow, later using some for cuttings. These‘volunteers’ contribute new genotypes. However, many are inbred, whereas multiplied clones are highly heterozygous. We demonstrate the selective retention of heterozygous volunteers. When farmers weeded fields, they killed small volunteers, but retained large ones. Plant size and heterozygosity were correlated, and both increased after weeding. The process we document allows maintenance of genotypically diverse and heterozygous clonal stocks. Demonstrating heterosis in nature usually requires large sample sizes, but novel features of our system allowed escape of this constraint. Traditional agroecosystems offer unusual opportunities for the microevolutionary studies required to give on-farm conservation of genetic resources a solid scientific basis. [ABSTRACT FROM AUTHOR]

The article presents a rejoinder to the paper "Commentary on Calcagno et al. (2006): Coexistence in a Metacommunity: The Competition-Colonization Trade-off Is Not Dead," by Frederick. R. Adler. The authors stated that Adler was able to illustrate the effect of preemption on the limits to similarity and dissimilarity by plotting the area in which coexistence is possible. They mentioned that a trade-off between survival and competition does not induce a positive effect of preemption on coexistence, and that only the competition-colonization trade-off is responsible for that.