Detecting multiple origins of domesticated crops.

C rop domestication has long been studied both as a model for understanding the process of evolution (1) and for gaining insights into the history of human civilization (2). In recent decades, a wealth of neutral molecular markers (e.g., SNPs, microsatellites, amplified fragment length polymorphisms) has become available for many crop species, permitting genomewide examinations of genetic diversity in crops and their wild relatives. Quite commonly, these surveys reveal that the present-day representatives of a crop all show close genetic similarity to a specific geographical subset of a wild species (e.g., ref. 3; Fig. 1A); this pattern is typically interpreted as evidence that the crop was domesticated a single time and in a single geographical region. Interestingly, this picture of a single, geographically localized crop origin does not always match the picture inferred from archaeological data. In particular, for Old World cereal crops, archaeobotanical data have indicated that domestication was likely a geographically diffuse and protracted process, involving long-term predomestication plant use across wide areas of the Near East (4). As such, the present-day diversity of a crop might well be expected to reflect multiple, geographically disparate origins of domestication (Fig. 1B). Why then, do multilocus genetic studies reveal monophyletic origins of Old World cereal crops? A study by Allaby et al. (5) in this issue of PNAS suggests that, given certain population parameters, the genetic signature of a single origin can arise even if multiple domestication events have occurred. Allaby et al. (5) have conducted a series of simulations in which they ask the following question: given a true domestication history that involves two independent events (Fig. 1B), how often do multilocus, neutral markers lead to the inference of a single domestication origin (Fig. 1 A)? Their results suggest that with a sufficiently protracted domestication period the genetic evidence of multiple domestications events may be lost. The key to this ‘‘protracted model’’ lies in the power of genetic drift: if domestication occurs over a long period, with effective population sizes of just a few hundred individuals for many generations, then drift may be strong enough to eliminate the genetic traces of multiple domestications, leaving only a single event detected in a crop’s present-day genetic diversity.