Gazania: diversification vs. speciationPosted: September 30, 2012
The studies on Gazania by Seranne Howis are a reminder that biodiversity can’t always be divided into discrete species. Speciation may form the kind of clearly articulated branching pattern that cladists like when evolution is driven by new niches becoming available one by one. But the sudden landscape-wide diversification of a clade and its sorting by natural selection into stable entities are separate, and almost contradictory, processes – rather like an explosion and the subsequent settling of the debris. Such explosions of diversity have occurred in the Mediterranean climate zones of south-western Australia and in South Africa, as part of wholesale vegetation changes caused by the cyclical climate changes of the last few million years. Many genera were reduced to small populations in refugia during the dry periods, and rapidly diversified again in the wet periods.
The South African biodiversity hotspot has given the world several genera of ornamental plants that have been evolving in this way. Some of them are real gifts to the plant breeder because in cultivation they function as coenospecies with all their genetic diversity available for use in hybridisation. For example, Watsonia (Iridaceae) has been divided into 52 morphological species that rarely hybridise in the wild as the flowers of sympatric populations have diversified to utilise different pollinators. But they all have the same chromosome number, and can all be interbred in cultivation with the F1 generation often showing hybrid vigour and high fertility. My ‘gut feeling’ from working with Watsonia is that it’s a genus with little evolutionary depth, all the species having similar genetic architecture and closely homologous genes. Gazania is another example, in which the process of speciation may not have proceeded even as far as it has in Watsonia.
Howis showed that Gazania includes seven valid, monophyletic species – each reproductively isolated, with a distinct morphology, habitat and genetic identity – but it has the majority of its diversity in a broad complex where morphological, ecological and genetic variation are only partially correlated. The complex may be called an ochlospecies, defined as a polymorphic species with chaotic infraspecific variation intractable to formal taxonomic treatment. In her 2007 thesis, Howis tentatively called this ochlospecies by the earliest published name, G. rigens (L.)Gaertn. But Howis et al. (2009) take a more conservative course by calling it the K-R complex – possibly because G. rigens is usually applied to the stoloniferous sand-binding forms that are distinct from the others morphologically and ecologically, but not genetically.
Another model for understanding genera like Gazania might be found in Vavilov’s theory of homologous variation, which is closely related to Nabokov’s concept of homopsis. A complex that has diversified only since the Pleistocene is likely to consist of populations with similar functioning genes that determine the morphology and physiology of individual plants. This is quite apart from the fine variations in the four non-coding chloroplast sequences and two nuclear spacers used in Howis’ study. The model predicts that similar traits of morphology and physiology would appear repeatedly in response to the appropriate environmental conditions. Homopsis is a type of homoplasy in which the phenotypic similarities are due to underlying genetic similarity, instead of being due to convergence from more diverse ancestors; this would be the case with the stoloniferous ‘rigens’ forms of Gazania that do not form a genetically coherent entity since natural selection has shaped them from the same gene pool as the rest of the complex.
The many forms of Gazania introduced to Australia, and now feral here, are all within the K-R complex. For practical purposes such as gardening books and legal declaration as weeds, a Latin binomial may be demanded. There are three possibilities:
- refer simply to the genus Gazania. As none of the seven distinct species are naturalised here, in practice this would mean the K-R complex.
- use the name Gazania rigens (L.)Gaertn. to signify the whole K-R complex. This species epithet has priority under the Code, having been published as Gorteria rigens L. in 1763.
- separate G. rigens out as the name of the stoloniferous forms that have been planted for sand stabilisation on our coasts. The earliest valid name for the residue of the K-R complex would then be Gazania rigida (Burm.f.)Roessler. This would be similar to the treatment in the 1986 Flora of South Australia, but with G. rigida replacing the later synonym G. linearis.
Howis, S. (2007) A taxonomic revision of the southern African endemic genus Gazania (Asteraceae) based on morphometric, genetic and phylogeographic data. Ph.D thesis, Rhodes University Botany Department. 293 pp.
Howis, S., Barker N.P. & Mucina, L. (2009) Globally grown, but poorly known: species limits and biogeography of Gazania Gaertn. (Asteraceae) inferred from chloroplast and nuclear DNA sequence data. Taxon 58(3): 871–882.