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BIODIVERSITY REFERENCE
 
Geum urbanum L.
Geum rivale L.
 
Wood Avens, Herb Bennet
Water Avens
 
   A brief account of hybridisation between these species   


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.



 
 
 
Geum rivale (Water Avens) is a plant of riversides and wet meadows, common in northern Britain but becoming much more local in the south.

It demands moist conditions and tolerates only partial shade.



 

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.


 
 
However, sometimes the habitats of the two species may adjoin, as shown here, where woodland borders a wet meadow. Geum rivale can be seen in the foreground.

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.



 


A typical hybrid plant (Geum × intermedium). The flower is closer to G. urbanum in general appearance but the petals are broader, rounded rather than pointed, and tinted orange; the flower is saucer-shaped rather than opening flat or remaining shortly tubular; the styles are intermediate in length; the flower-stalks are somewhat pendulous.


 

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.
[These observations based on field experience of the page author, regrettably without detailed, quantified data.]


 

POLLINATION BIOLOGY

All the above assumes that proximity = cross-pollination. This is not wholly true.



The two species are also partially isolated from each other by their modes of pollination (see Taylor's papers, given in the references below).

  • G. rivale is pollinated primarily by bees, less often by flies and beetles. As the flower matures, elongation of the stamens ensures it self-fertilises if not already cross-pollinated. (The flowers are protogynous, i.e. the stigmas mature before the stamens.) It begins flowering a little earlier than G. urbanum, so early pollinations will be within the gene-pool of the single species.
  • G. urbanum is noted by Taylor as having few insect visitors, mainly flies (see photograph above). It is only weakly protogynous and soon self-fertilises.

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


References / Information Sources
•   Briggs, D., & Walters, S.M., (1997). Plant variation and evolution, 3rd ed., Cambridge University Press, Cambridge.
•   Taylor, K. (1997) Biological flora of the British Isles: Geum urbanum L. Journal of Ecology 85: 705-720.
•   Taylor, K. (1997) Biological flora of the British Isles: Geum rivale L. Journal of Ecology 85: 721-731.



© A.J. Silverside
Page first hosted at www-biol.paisley.ac.uk/bioref/, November/December 1998; transferred to lastdragon.org December 2012, last modified May 2014
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