Geographical modes of pollinatordriven speciation

The three main geographical modes of speciation are allopatric, whereby divergence involves populations that are not linked by gene flow; parapatric, in which divergence occurs between adjacent habitats in the face of some gene flow; and sym-patric, in which populations diverge in situ through an instantaneous genetic barrier to gene flow or extremely assortative mating. All three modes may apply to pollinator-driven speciation, although only conventional allopatric speciation is strongly supported empirically.

Allopatric speciation is amply supported by the frequent allopatry of sister species and the tendency for range overlap to increase after divergence (Barraclough and Vogler 2002). Selection resulting in local adaptation is increasingly recognized as a significant influence on allopatric divergence, in contrast to previous models which emphasized neutral change resulting from genetic drift. In the context of pollinator-driven speciation without reinforcement, such divergence occurs in response to landscape-level changes in the pollinator fauna. Indeed, studies of sister taxa demonstrate that the mosaic of long-proboscid fly pollinators in southern Africa (Fig 16.1) has promoted allopatric plant speciation. Typically, one member of a species pair is pollinated by a long-proboscid fly species, whereas the other member, in a different geographical area, is pollinated by either a different fly species (Plate 6a and b) or other pollen vectors, such as hawk moths (Plate 6 c-f; Goldblatt and Manning 1996; Johnson and Steiner 1997; Johnson et al. 1998, 2002b; Goldblatt et al. 2001, 2004). In some cases, ancestral pollination systems can be inferred from phylogenies. For example, in Zaluzianskya (Scrophulariaceae), pollination by long-proboscid flies in Zaluzianskya microsiphon (Plate 6d) is a derived condition, representing a shift from hawk-moth pollination (Johnson et al. 2002a), whereas in Lapeirousia (Iridaceae), the reverse shift (fly to moth) occurred during the evolution of

Lapeirousia pyramidalis (Goldblatt and Manning 1996). Although reciprocal translocations and phe-notypic selection experiments have yet to be performed to verify the adaptive basis of trait divergence in most of these instances, the observed patterns are consistent with the Grant-Stebbins model of allopatric pollinator-driven speciation.

Parapatric speciation, whereby divergence of contiguous populations occurs despite some gene flow, has been linked closely to ecological selection and reinforcement. Inferences of parapatric spe-ciation based on the proximity of extant sister taxa are always dubious, as distributions can change substantially after speciation (Barraclough and Vogler 2002). Goldblatt and Manning (1996) argued for parapatric speciation in the radiation of the southern African iris genus Lapeirousia. Sister taxa of Lapeirousia typically occupy closely abutting habitats that differ chiefly in substrate. These taxa also tend to differ in their modes of pollination, which Goldblatt and Manning interpreted as the outcome of selection for isolating barriers at the contact zone. However, if the pollinator fauna itself differs between substrates, the linkage of pollinator and soil specialization in Lapeirousia may be simply be a case of parallel adaptations to the local environment. Whether these adaptations occurred in the face of some gene flow or in fully allopatric populations is thus almost impossible to gauge.

Sympatric speciation has always been controversial and can be excluded as a possibility in almost all cases of pollinator-driven speciation. Although a single individual can theoretically found a new lineage in a population of self-compatible plants, it is extremely unlikely that a mutant plant, no matter how novel its floral traits, would attract a completely different set of pollinators compared with plants in the rest of the population, and thus result in a new species.

One special case that may involve sympatric speciation deserves consideration, namely, sexual deception in orchids (Schiestl and Ayasse 2002; Schiestl et al. 2003; Mant et al. 2005). In these orchids, which attract male Hymenoptera chiefly by imitating the sex pheromones of female insects, a mutant with a novel fragrance could attract a different and non-overlapping set of pollinators

(Schiestl and Ayasse 2002; Mant et al. 2005). However, despite common belief that sexually deceptive orchids attract male Hymenoptera of a specific species, molecular data show that gene flow between sexually deceptive orchid species is reasonably common (Soliva and Widmer 2003). Thus even in sexually deceptive orchids, specia-tion probably usually occurs allopatrically within a geographical mosaic of Hymenoptera species in response to selection favouring a shift to locally effective pollinators. Given that these orchids occasionally attract more than one pollinator species, this shift could occur according to Stebbin's principle of ''transfer of function via an intermediate stage of double function.''

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