A hybrid zone is created when hybrids between two taxa flourish in the marginal area (hybrid zone) that lies between two parents and is more common with plants where fertility barriers are weaker than in animals
(see below). Marked environmental gradients such as those that occur in relation to exposure (e.g. near the sea, or in zones of varying flooding frequency, or across altitudinal zones on mountains) create marginal sites where hybrids can be more frequent than either parent. Examples of such zones can often be found in populations of willow, birch, poplar and pine, as well as in many herbaceous species where genetically differentiated populations meet and produce offspring of mixed ancestry. These zones of hybridization can be considered as marginal regions as they may lie along regions of environmental transition, with one taxon being dominant at one end and the other parent better suited to conditions at the other end.
Marginal coastal areas are commonly the location of records of hybrid species. Crosses between creeping willow (Salix repens) with S. cinerea and S. caprea are found in coastal areas, just as are many other willow hybrids that occur in alpine regions. Similarly, sand couch grass (Elytrigia repens) forms numerous hybrids with other closely related species, e.g. E.juncea and E. aetherica (Stace, 1997), which occupy specific zonations in the salt marsh.
The hyperoceanic Orkney Islands to the north of Scotland provide a striking example of a hybrid becoming the dominant form in a marginal area. The partially drained pasture lands of these islands have a saturated soil profile for a large part of the winter and are too wet for the overwintering survival of the common ragwort (Senecio jacobea) which is not tolerant of prolonged flooding. The pastures are too dry in summer for the closely related S. aquaticus (Forbes, 1976). They are, however, highly suited to the hybrid between these two species. In the hyperoceanic conditions of the Orkney Islands stands of pure S. jacobea, as recognized morphologically, are relatively rare and confined to the drier areas of a limited number of coastal sand dunes while the other parent S. aquaticus is widespread along lakeside and stream margins. The area occupied in the Orkney Islands by the hybrid between these two species, namely the partly drained pastures, is therefore much greater than that occupied by either parent and the hybrids might be described as a hybrid swarm (Fig. 4.8).
4.5.1 Transient and stable hybrids
A constant question that arises in relation to the hybrids that inhabit the marginal zones between their parents is
whether or not the hybrids are a transient phenomenon or whether they represent an evolutionary advance producing populations with greater fitness than their parents. Two points of view exist in relation to hybrids. The first is that hybrids are essentially transitory but manage to exist in a stable tension zone where selection against them is balanced due to a constant gene flow between the parent species, possibly aided by the vigour that is associated with heterosis in F1 and other early hybrid generations (Barton & Hewitt, 1985). The second viewpoint asserts that selection depends on the environment favouring opposite traits in the two parental habitats or favouring well-adapted hybrids within a bounded region (Campbell & Waser, 2001). Experimental studies to determine whether or not selection of hybrids is environment dependent have been carried out on a number of species. A study in the Netherlands where S. jacobea occurs abundantly in sand dunes and S. aquaticus only infrequently at lake edges showed that maternal effects played a role in the fitness of experimentally produced F1 hybrids, with offspring from S. jacobea mothers exhibiting higher fitness than those from S. aquaticus mothers (Kirk et al., 2005). This is the reverse of the situation described above for Orkney where S. aquaticus is the more frequent parent. This study concluded that the natural hybrids were not distributed in zones where they were most fit with respect to nutrient and water regimes. It was also found that different hybrid generations differed in fitness. It was therefore concluded it was the greater heterosis in early generation hybrids which created their temporary superior fitness and which may be contributing to hybrid swarm stability.
Support for the second model can be found in studies of progressive introgression where the hybrids have gradually spread and replaced the parent plants over wide areas, as has happened in central Florida with hybrids between two taxa in the Piriqueta caroliniana complex (Cruzan, 2005). It is not yet clear which model is most appropriate for the vast expansion and maintenance of the S. aquaticus X S. jacobea hybrid in Orkney where the frequency of the parent species differs from the study in the Netherlands.
The term hybrid swarm is often used to describe transient populations that arise from time to time and then disappear, as can be found in the hybrids between wood avens (Geum urbanum) and water avens (G. rivale). The hybrid (G. intermedium) forms swarms that link the parents with a range of intermediate populations. Even in arctic habitats, fertile hybrids are common in several well-studied genera including Draba and Saxifraga (Brochmann et al., 1992).
Hybrid zones are often brought about by habitat disturbance or environmental change. In particular, human disturbance can lead to the breakdown of ecological isolation. In plants, the static nature of populations results in many species being closely associated with one particular habitat and therefore adaptation to a particular set of local conditions is ecologically advantageous. The resulting divergent evolution between adjacent populations, which still possess some degree of interfertility, is referred to as parapatric speciation. The difference between allopatric and parapatric speciation is that while the former requires isolation to take place, parapatric speciation takes place despite a certain amount of gene flow between populations. The selective forces favouring a particular biotype in its own region must be sufficiently great to prevent it being swamped by neighbouring biotypes. One of the best-known examples of such a hybrid zone is the hybridization between Iris fulva and I. giganticaerulea in the Mississippi delta where human disturbance of the natural vegetation by drainage schemes has created a marginal habitat for the parents but where the hybrid between these two species predominates. As long as the intermediate zone lasts, the hybrid will survive and can be looked upon as a stable hybrid (Emms & Arnold, 1997).
On the mountain slopes of Tiryal Dag, north-east Turkey (Fig 4.9-4.12), there is a clear example of habitat-mediated selection for an F1 hybrid in the ecotone zone between two species of Rhododendron. The parents are, respectively, the low-altitude (mostly < 2000 m) R. ponticum and the high-altitude (mostly > 2000 m) species R. caucasicum, which form extensive hybrid populations wherever their distributions overlap, on mountain slopes between 1800 and 2200 m. Molecular techniques confirmed that the hybrid individuals occurring on the mountain were F1 hybrids. The hybrids produce viable seed as does the F2 generation and their backcrosses. However, all non-F1 hybrid derivatives appear to be eliminated in the hybrid zone at Tiryal Dag as a result of post-germination selection. Thus on this mountain all three taxa are shrubs, and are dominant components of the vegetation within their respective altitudinal habitat ranges. The Fj hybrid Rhododendron X sochadzeae is often the commonest shrub within the centre of its habitat range, but at the upper and lower edges of this range it occurs mixed with R. caucasicum and R. ponticum (Milne et al., 2003).
Black Sea ^
Fig. 4.9 Location ofTiryal Dag in eastern Turkey.
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