Several species and infraspecific taxa of Viburnum have been recognized at the southern edge of its geographic range in the neotropics, from central Peru through Bolivia to northern Argentina. Using multiple lines of evidence and analytical approaches, we support the recognition of just a single species, V. seemenii, which we infer to have recently moved southward through the central Andes. We highlight the ongoing evolution of leaf ecomorphs in the region and a case of incipient speciation. We show that what has previously been recognized as V. seemenii f. minor has recently occupied the drier Tucuman-Bolivian forest region from Samaipata in Bolivia to Salta in northern Argentina. Plants in these populations form a well supported clade with a distinctive genetic signature and they have evolved smaller, narrower leaves. We interpret this as the start of a within-species divergence process that has given rise to the different leaf ecomorphs described by Donoghue et al. (2022) for the entire neotropical Oreinotinus clade. Specifically, the southern populations are in the process of evolving the small, glabrous, and entire leaf ecomorph that has evolved in four other montane areas of endemism. As expected based on our studies of leaf ecomorphs in Chiapas, Mexico, these southern populations experience generally drier conditions, with large diurnal temperature fluctuations. A population in Central Peru is noteworthy in also showing a tendency towards smaller leaves. In a central portion of the range of V. seemenii, characterized by wetter climatic conditions, we may also be seeing the beginnings of the differentiation of the leaf ecomorph with larger, pubescent, and toothy leaves. Within this region, the initial stages of the evolution of the leaf forms appear to be associated with geographic separation, whereas in other regions, it may initially have been driven by differentiation along elevational gradients, with corresponding differences in flowering times.