Data from: Caught in the act: Incipient speciation at the southern limit of Viburnum in the Central Andes

A fundamental objective of evolutionary biology is to understand the
origin of independently evolving species. Phylogenetic studies of species
radiations rarely are able to document ongoing speciation; instead, modes
of speciation, entailing geographic separation and/or ecological
differentiation, are posited retrospectively. The Oreinotinus clade of
Viburnum has radiated recently from north to south through the cloud
forests of Mexico and Central America to the Central Andes. Our analyses
support a hypothesis of incipient speciation in Oreinotinus at the
southern edge of its geographic range, from central Peru to northern
Argentina. Although several species and infraspecific taxa of have been
recognized in this area, multiple lines of evidence and analytical
approaches (including analyses of phylogenetic relationships, genetic
structure, leaf morphology, and climatic envelopes) favor the recognition
of just a single species, V. seemenii. 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 beginning of a within-species
divergence process that has elsewhere in the neotropics resulted
repeatedly in Viburnum species with a particular set of leaf ecomorphs.
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 predicted based on our studies of leaf
ecomorphs in Chiapas, Mexico, these southern populations experience
generally drier conditions, with large diurnal temperature fluctuations.
In a central portion of the range of V. seemenii, characterized by wetter
climatic conditions, we also document what may be the initial
differentiation of the leaf ecomorph with larger, pubescent, and toothy
leaves. The emergence of these ecomorphs thus appears to be driven by
adaptation to subtly different climatic conditions in separate geographic
regions, as opposed to parapatric differentiation along elevational
gradients as suggested by Viburnum species distributions in other parts of
the neotropics.