The genus Geum contains a number of morphologically and ecologically distinct species which, nevertheless, have not evolved genetic breeding barriers. Ecological and geographical separation, reinforced by aspects of pollination biology, generally maintain the distinctness of these species in the wild.
Two Geum species are native in Britain:
Geum urbanum (Wood Avens) is a woodland species and is widespread and common throughout most of Britain.
G. urbanum is a sciophyte, i.e. a true 'shade plant', reaching its maximum photosynthetic rate at around 50% full daylight. Although sometimes seen in open ground, often as a result of felling operations and where there is still little competition, it is primarily a plant of shadier, and often drier parts of woods, where there is little competition even from other woodland herbs.
As can be seen, the two species differ markedly in the colour, shape and size of the flowers and of the individual petals. Vegetative differences (not shown) are less marked, though G. urbanum is usually much the taller, with ascending flower-branches. Neither species shows much variation other than through hybridisation. Morphologically they seem quite different species.
At the junction of the two habitats is an intermediate zone, occupied by morphologically intermediate but fertile plants (shown below).
Here the hybrid plants themselves are relatively uniform (or were when the site was photographed). Presumably they are exchanging genes with both G. urbanum in the wood and G. rivale in the meadow, but selection may also be actively favouring physiologically and morphologically intermediate plants.
It should also be noted that genes may well be passing from one species to the other via this intermediate zone of hybrids. No biometric (or molecular) work has been carried out at this site, but such work might well reveal evidence of introgression.
But what if the habitat structure is not stable? What if the two species are brought together by disturbance?
Management practices may involve opening up areas of woodland. If the ground is low-lying or poorly drained, or if heavy machinery increases soil compaction, areas suitable for G. rivale may be opened up within populations of G. urbanum. Rather than a band of intermediate ground that develops a hybrid zone, there may be a mosaic of microhabitats. Spatial and ecological separation of the two species may completely break down to produce a hybrid swarm.
In these conditions, hybrid plants may exhibit any combination of parental characters, may backcross continually to either parent, until there is a single, highly variable population in which pure examples of either species are infrequent to absent.
Interestingly, habitat recovery after such disturbance may result in the re-establishment of populations of the two species. Maybe there will be colonisation from unaffected populations in the vicinity, but it also seems that selection can act upon hybrid swarms to favour parental morphotypes. The result is that a location with a large hybrid swarm may, a few years later, contain populations of the parents and only scattered hybrids. Closer examination of the two species may, however, reveal greater variability or atypical characters compared with "pure" populations.
The two species are also partially isolated from each other by their modes of pollination (see Taylor's papers, given in the references below).
It follows that crossing between the species may be less than expected. Taylor also notes that crosses may often fail when G. rivale is the potential female parent.
Perhaps evolution of breeding barriers between the species has progressed further than is generally acknowledged.
All photographs: Howwood, Renfrewshire, 1983