Maize evolution under domestication is a process that continues today. Case studies suggest that Mexican smallholder family farmers, known ascampesinos, contribute importantly to this, but their significance has not been explicitly quantified and analysed as a whole. Here, we examine the evolutionary and food security implications of the scale and scope under whichcampesinosproduce maize. We gathered official municipal-level data on maize production under rainfed conditions and identifiedcampesinoagriculture as occurring in municipalities with average yields of less than or equal to 3 t ha⁻¹. Environmental conditions vary widely in those municipalities and are associated with a great diversity of maize races, representing 85.3% of native maize samples collected in the country. We estimate that in those municipalities, around 1.38 × 10¹¹genetically different individual plants are subjected to evolution under domestication each season. This implies that 5.24 × 10⁸mother plants contribute to the next generation with their standing genetic diversity and rare alleles. Such a large breeding population size also increases the total number of adaptive mutations that may appear and be selected for. We also estimate thatcampesinoagriculture could potentially feed around 54.7 million people in Mexico. These analyses provide insights about the contributions of smallholder agriculture around the world.