Ancient genomes reveal early Andean farmers selected common beans while preserving diversity.

All crops are the product of a domestication process that started less than 12,000 years ago from one or more wild populations. Farmers selected desirable phenotypic traits (such as improved energy accumulation, palatability of seeds and reduced natural shattering) while leading domesticated populations through several more or less gradual demographic contractions. As a consequence, the erosion of wild genetic variation is typical of modern cultivars, making them highly susceptible to pathogens, pests and environmental change. The loss of genetic diversity hampers further crop improvement programmes to increase food production in a changing world, posing serious threats to food security. Using both ancient and modern seeds, we analysed the temporal dynamics of genetic variation and selection during the domestication process of the common bean (Phaseolus vulgaris) in the southern Andes. Here, we show that most domestic traits were selected for before 2,500 years ago, with no or only minor loss of whole-genome heterozygosity. In fact, most of the changes at coding genes and linked regions that differentiate wild and domestic genomes are already present in the ancient genomes analysed here, and all ancient domestic genomes dated between 600 and 2,500 years ago are highly variable (at least as variable as modern genomes from the wild). Single seeds from modern cultivars show reduced variation when compared with ancient seeds, indicating that intensive selection within cultivars in the past few centuries probably partitioned ancestral variation within different genetically homogenous cultivars. When cultivars from different Andean regions are pooled, the genomic variation of the pool is higher than that observed in the pool of ancient seeds from north and central western Argentina. Considering that most desirable phenotypic traits are probably controlled by multiple polymorphic genes, a plausible explanation of this decoupling of selection and genetic erosion is that early farmers applied a relatively weak selection pressure by using many phenotypically similar but genetically diverse individuals as parents. Our results imply that selection strategies during the past few centuries, as compared with earlier times, more intensively reduced genetic variation within cultivars and produced further improvements by focusing on a few plants carrying the traits of interest, at the cost of marked genetic erosion within Andean landraces.