In several cases, estimates of gene flow between species appears to be higher than predicted given the strength of interspecific barriers. However, as far as we are aware, detailed measurements of reproductive isolation have not previously been compared with a coalescent-based assessment of gene flow. Here, we contrast these two measures in two species of sunflower, Helianthus annuus and Helianthus petiolaris. We quantified the total reproductive barrier strength between these species by compounding the contributions of the following prezygotic and postzygotic barriers: ecogeographic isolation, reproductive asynchrony, immigrant inviability, pollen competition, hybrid seed formation, hybrid seed germination, hybrid pollen fertility, and hybrid seed set. The estimated probabilities that a reproductively successful hybrid is produced: range from 10^-4 to 10^-6 depending on the direction of the cross and the degree of independence among reproductive barriers. Population genetic estimates of gene flow estimated between these sunflower species (N_e m = 0.34-0.76) are due mainly to their large effective population sizes (N_e >10^6). The interspecific migration rate (m) is very small (<10^-7) and an order of magnitude lower than that expected based on our reproductive barrier strength estimates. Thus, even high levels of reproductive isolation (> 0.999) may produce genomic mosaics.