Reticulate evolution and the formation of organisms utilized by humans

The previous two chapters have reflected the wealth of examples of organisms that benefit humankind by providing clothing, tools, containers, building materials, food, and much more. In the present chapter, I will conclude the discussion of beneficial organisms by surveying a set of plant species and species complexes from which H. sapiens derives nourishment. Thus, I will highlight the reticulate nature of our food sources by discussing data for plants from which we harvest roots, seeds, fruits, stems, or leaves. Of the 20 lineages to be discussed, some reflect major, worldwide crop species such as rice and wheat, while others reflect much more geographically limited species, such as cassava and grapefruit. Similar to the animal examples in Chapter 6, a limited geographic distribution of a plant cultivar does not reflect an unimportant role. Instead, some of the more geographically restricted plant lineages are of primary importance for local populations of humans, in terms of income and/ or nourishment.

Humans have not only affected the evolution of plant-derived food sources through the intentional use of native taxa, causing subsequent genetic exchange between wild and cultivated lineages, but they have also inadvertently provided avenues for the evolution of hybrid forms useful for domestication. For example, Hughes et al. (2007) tested the hypothesis that "Backyard gardens, dump heaps, and kitchen middens are thought to have provided important venues for early crop domestication via generation of hybrids between otherwise isolated plant species." Edgar Anderson and Ledyard Stebbins constructed the conceptual framework for this hypothesis in a series of publications (Anderson 1948, 1949; Anderson and Stebbins 1954). Specifically, Anderson and Stebbins hypothesized that disturbance, with or without human involvement, could lead to the sympatry of previously separated taxa and the production of [disturbed] habitats that the hybrid progeny could occupy. This "hybridization of the habitat," as Anderson (1948) termed it, could thus lead to both the origin and survival of introgres-sive genotypes.

Hughes et al. (2007) tested for the effects from human-mediated habitat disturbance on the evolution of a south-central Mexico food source, the leguminous tree genus, Leucaena. They accomplished this through a combination of genetical, geographical, archaeological, and ethnobotanical analyses. In regard to the role of introgression between wild species in the production of domesticated lineages, their analyses determined that: (1) predomestication cultivation and multiple transitions from wild to cultivated forms led to the sympatry of 13 Leucaena species; (2) introgressive hybridization among the various species—brought into sympatric associations through humanmediated disturbances—resulted in numerous recombinant lineages; (3) the degree of introgres-sive hybridization estimated for this cultivar/ wild complex exceeded any other domesticated complex in Mexico; however, (4) other plant species complexes utilized in Mexico, such as Agave and Opuntia, reflected exactly the same avenues (i.e., "backyard hybridization") through which domestication had proceeded. Therefore, anthropogenic hybridization of the habitat (Anderson 1948) may have played a significant role, not only in Mexican agriculture, but also throughout the history of Mesoamerican crop development (Hughes et al. 2007). Indeed, the fundamentally important role that human-mediated genetic exchange has played in the origin of plant lineages utilized as food sources will be illustrated repeatedly in this chapter.

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