Altitudinal gradients, plant hybrid zones and evolutionary novelty.

Altitudinal gradients are characterized by steep changes of the physical and
biotic environment that present challenges to plant adaptation throughout
large parts of the world. Hybrid zones may formwhere related species inhabit
different neighbouring altitudes and can facilitate interspecific gene flow and
potentially the breakdownof species barriers. Studies of such hybrid zones can
revealmuch about the genetic basis of adaptation to environmental differences
stemming from changes in altitude and the maintenance of species divergence
in the face of gene flow. Furthermore, owing to recombination and transgressive
effects, such hybrid zones can be sources of evolutionary novelty. We
document plant hybrid zones associated with altitudinal gradients and
emphasize similarities and differences in their structure. We then focus on
recent studies of a hybrid zone between two Senecio species that occur at
high and low altitude on Mount Etna, Sicily, showing how adaptation to
local environments and intrinsic selection against hybrids act to maintain it.
Finally, we consider the potential of altitudinal hybrid zones for generating
evolutionary novelty through adaptive introgression and hybrid speciation.
Examples of homoploid hybrid species of Senecio and Pinus that originated
from altitudinal hybrid zones are discussed.