A broad genetic base for a crop species such as wheat (Triticum spp.) may be important to reduce genetic vulnerability and assure long‐term potential for genetic gain. The number and diversity of original ancestors can provide insight into relative genetic diversity within and among gene pools. Based on the coefficient of parentage values between cultivars and ancestral lines, the relative genetic contributions of ancestral lines to spring wheat (Triticum aestivum L.) cultivars were computed for successive 25‐yr time periods. The cultivars were from Canada (47), the USA (133), and Mexico (90). Genetic backgrounds were studied for important changes in the ancestral base over time and to determine trends that continue today. About 124 ancestors, tracing to 32 countries on five continents, were found to have contributed to the North American spring wheat gene pool. Canadian western red spring cultivars released before 1950 had an intermediate level of ancestral diversity. Their post‐1975 cultivars have narrow ancestral diversities probably due to strict quality regulations. They consist primarily of ‘Thatcher’ germplasm modified by a set of pest resistance genes carefully managed in backcross schemes. Before 1950, the U.S. hard red spring wheat cultivars traced to the same Thatcher ancestors but also to ‘Kota’ and ‘Yaroslav’. Many more ancestors were added to this base after 1950 and especially after 1975. This was a side effect of the incorporation of pest resistance, most notably from sources ‘Gabo‐Timstein’, ‘Frontana’, and ‘Kenya 58’, and reduced height sources, most notably CIMMYT cultivars with new ancestry from ‘Kenya 324’ and Turkey. CIMMYT cultivars have also expanded their genetic base over time, with the systematic incorporation of new ancestry from various sources including winter wheats. Introduction of novel alleles from wild ancestors or other, sources should contribute to future broadening of the genetic base, maximizing genetic gains, and reducing genetic vulnerability of the gene pools.