Speciation

Abstract

Speciation is arguably the most fundamental process in plant evolution because it leads to evolutionary lines that represent the basic units of plant classification, the species, and is the means of diversification within lineages (Rieseberg & Brouillet, 1994; Coyne & Orr, 2004). Also, the plant diversity for given areas is usually presented in terms of species richness, and thus species enter prominently into discussions of conservation (Steele & Pires, 2011). Despite the central role of the origin of species in plant evolutionary biology, there is no one universally accepted species concept for plants nor is there a comprehensive, refined understanding of the process(es) of speciation. Indeed, the lack of a single species concept in plants, and the extensive biological/philosophical debates and discussions of the species concepts have historically retarded the study of speciation (Rieseberg & Willis, 2007). Species concepts have been reviewed extensively elsewhere, and thus will not be considered in detail here (de Queiroz, 1998, 2007; Coyne & Orr, 2004, pp. 447-472, and references therein; Hausdorf, 2011; Shapiro & al., 2016). Most importantly, as argued by de Queiroz (1998, 2007), despite the different concepts of species, there is general agreement that species are independently evolving metapopulation lineages. Thus, speciation can be discussed from the perspective of the factors promoting divergence into independent lineages. Having said this, however, it must be emphasized that determining the lineages may be difficult, especially when different criteria suggest incongruent species boundaries, or when certain criteria indicate divergence between populations but others do not. As indicated by Bacon & al. (2012) and others, these issues may be especially problematical in island plants where divergence and speciation are often recent. Recent divergence may not have provided sufficient time for completion of the speciation process in the sense that there has not been sorting out and divergence in the attributes commonly seen in ?older? continental species. However, as noted below, paradoxically, this is also an advantage when studying the process. In addition to speciation via divergence, so-called primary speciation, species may originate by hybridization between two differentiated populations (usually recognized as species) at the same ploidy level to give stabilized, novel phenotypes that are isolated from each of the parental species. This is most commonly called homoploid hybrid speciation (Rieseberg, 1997; Mallet, 2007; Yakimowski & Rieseberg, 2014). An increase in ploidy level is also a mechanism of speciation and may involve chromosome doubling within species (autopolyploidy) or be associated with hybridization between species (allopolyploidy), see Soltis & al. (2014) for an excellent mini review. In this chapter, the emphasis is on primary speciation because it is the most frequent, if not exclusive, mode of speciation in Juan Fernández (JF) plants.