Melampodium ranges from Arizona south throughout Mexico and down into Central America and northern South America. One species, M. divaricatum, has become invasive in many other countries, having been reported in Cuba, Brazil, the Philippines, and elsewhere. Most of the interest centers on Mexico, where 39 species occur in many different ecological zones. A new database of Asteraceae has now made it possible to present comprehensive distribution maps that include new collections accumulated (and accurately identified) during the past several decades. The number of new data points for species in some cases has been doubled. These new data make it possible to evaluate biogeography and speciation in the genus with greater precision. Species have adapted to many ecological zones and vegetation types within Mexico, which illustrates the broad adaptations that have occurred in consort with chromosomal change.