Evidence for human‐caused founder effect in populations of Solanum jamesii found at archaeological sites: I. Breeding experiments and the geography of sexual reproduction

PREMISE: Plant domestication can be detected when transport, use, and manipulation of propagules impact reproductive functionality, especially in species with self‐incompatible breeding systems. METHODS: Evidence for human‐caused founder effect in the Four Corners potato (Solanum jamesii Torr.) was examined by conducting 526 controlled matings between archaeological and non‐archaeological populations from field‐collected tubers grown in a greenhouse. Specimens from 24 major herbaria and collection records from >160 populations were examined to determine which produced fruits. RESULTS: Archaeological populations did not produce any fruits when self‐crossed or outcrossed between individuals from the same source. A weak ability to self‐ or outcross within populations was observed in non‐archaeological populations. Outcrossing between archaeological and non‐archaeological populations, however, produced fully formed, seed‐containing fruits, especially with a non‐archaeological pollen source. Fruit formation was observed in 51 of 162 occurrences, with minimal evidence of constraint by monsoonal drought, lack of pollinators, or spatial separation of suitable partners. Some archaeological populations (especially those along ancient trade routes) had records of fruit production (Chaco Canyon), while others (those in northern Arizona, western Colorado, and southern Utah) did not. CONCLUSIONS: The present study suggests that archaeological populations could have different origins at different times—some descending directly from large gene pools to the south and others derived from gardens already established around occupations. The latter experienced a chain of founder events, which presumably would further reduce genetic diversity and mating capability. Consequently, some archaeological populations lack the genetic ability to sexually reproduce, likely as the result of human‐caused founder effect.