Speciation by hybridization

There is undeniable evidence that hybrid species are sometimes established, and indeed it has been estimated that 11% of plant species have evolved in this way. Some of the most elegant recent studies have been conducted by Rieseberg and co-workers on North American sunflowers. Two species of sunflower, Helianthus annuus and Helianthus petiolaris, have overlapping geographic ranges in western USA and have given rise to three hybrid species, Helianthus anomalus, Helianthus paradoxus, and Helianthus deserticola (Figure 12.2). Evidence supports a ^combinatorial' model for the speciation of these hybrids (Figure 12.3). Under this model crossing events between the two parent species lead to novel chromosomal arrangements, as a result of chromosomal breakages (Rieseberg et al. 1995). The novel rearrangements cause reproductive isolation between the hybrids (Rieseberg et al. 1999). Some novel rearrangements carry a selective advantage, which allows invasion of new habitats (i.e. heterozygote advantage, Rieseberg et al. 2003). Thus, while the parent species prefer heavy clay and dry sandy soils, respectively, H. anomalus and H. deserticola are found in more xeric habitats than their parents, while H. paradoxus is found only in saline habitats. In the recombinatorial model then, hybridization both creates novel adaptive variation and simultaneously a mechanism for reproductive isolation.

Another route for hybridization to lead to speciation is through allopoly-ploidy; the creation of hybrids with double (or more) of the normal chromosome complement (Figure 12.3). Doubling of chromosome number allows a hybrid to escape infertility problems at meiosis by giving each parental chromosome its own complementary partner. The origin of several allopolyploid species has been documented in the last several decades, often as a result of species being introduced outside of their native range, leading to novel hybridizations. For example, the diploid species Tragopogon pratensis, Tragopogon dubius, and Tragopogon porrifolius were introduced into North America at the beginning of the twentieth century, and in about 1940 gave rise to two tetraploid hybrids in eastern Washington: Tragopogon miscellus (from Tragopogon pratensis X Tragopogon dubius) and Tragopogon mirus (from T. dubius X T.porrifolius). Both these polyploids have since expanded their range considerably (Levin 2000).

In recent years a major concern about the likelihood of hybrid speciation— the low fitness of hybrids and their consequent inability to persist—has been somewhat quelled by new data (Arnold and Emms 1998). One particularly graphic study by Peter and Rosemary Grant on Darwin's finches on the Galapagos island of Daphne Major highlights how hybrids might not always be less fit than their parent species. They studied the fitness of hybrids

Fig. 12.2 The sunflowers H. annuus and H. petiolaris(top), which have given rise to three hybrid species (bottom). Of these, H. anomalus and H. deserticola are found in xeric habitats, while H. paradoxus is found in saline habitats. Photos courtesy of Loren Rieseberg.

between three finch species (Geospiza fuliginosa, Geospiza fortis, and Geospiza scandens) over a number of years. In most years the hybrids were less fit than their parent species because they had intermediate beak sizes that were not best adapted to the types of seeds available (Figure 12.4). However,

Fig. 12.3 Two ways in which hybridization can give rise to new species. (a) The recombinatorial model, in which chromosomal breakages occur giving rise to hybrids that are reproductively isolated from their parents. (b) Allopolyploidy, in which diploid gametes give rise to fertile hybrid offspring because each chromosome retains its complementary pair.

Fig. 12.4 Hybridization between Darwin's finches on the Galapagos islands. The Cactus finch, G. scandens(a), the medium ground finch, G. fortis(c), and a hybrid with an intermediate beak shape (b). In dry years, the hybrids do not survive well because their beaks are not well adapted to utilize existing seeds. In rarer wet years, when seeds are more abundant, competition is reduced and hybrids survive better. Photos courtesy of Peter Grant.

Fig. 12.4 Hybridization between Darwin's finches on the Galapagos islands. The Cactus finch, G. scandens(a), the medium ground finch, G. fortis(c), and a hybrid with an intermediate beak shape (b). In dry years, the hybrids do not survive well because their beaks are not well adapted to utilize existing seeds. In rarer wet years, when seeds are more abundant, competition is reduced and hybrids survive better. Photos courtesy of Peter Grant.

Species 1

Species 2

Species 2

Beak size

Fig. 12.5 Hybrids are often selected against through disruptive selection (top), and this prevents introgression of the two parent species. Occasionally, however, as in Darwin's finches, hybrids may survive due to environmental change relaxing selection against them (bottom).

Beak size

Fig. 12.5 Hybrids are often selected against through disruptive selection (top), and this prevents introgression of the two parent species. Occasionally, however, as in Darwin's finches, hybrids may survive due to environmental change relaxing selection against them (bottom).

in the El Niño year of 1982-83, the rainfall on the islands dramatically increased, and a dramatic change occurred in the types of seed available to the finches. In that year, the hybrids had equal or higher survivorship, recruitment, and breeding success than their parents (Grant and Grant 1993).Thus, changes in the external environment can sometimes affect drastically the fitness of parents and hybrids. It is not inconceivable that such variations in fitness might sometimes allow a hybrid species to persist and spread into new habitats (Figure 12.5).

It is also becoming clear that hybrids frequently do persist for long periods and spread despite their normally low fitness. Some populations of the closely related Drosophila simulans and Drosophila mauritiana share large segments of DNA indicating past gene exchange between them, despite experimental evidence showing strong barriers to gene exchange (Solignac and Monnerot 1986). Such observations led Arnold (1997) to suggest that the traditional assumption of low hybrid fitness, while often true, does not apply to all hybrids and that some hybrids can be fitter than their parents in a range of niches. Even if these are established rarely they can give rise to persistent lineages. Thus, the concept of hybrid speciation, not just in plants but also in animals, is experiencing something of a revival.

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