11 - Chromosomes and evolution in New Zealand endemic angiosperms and gymnosperms

Summary

The New Zealand flora, like that of many other oceanic islands, is unique in several ways. It has a high level of endemicity (de Lange et al., 2006) but this is largely at the species level, with approximately 82% (1944/2357) of taxa being endemic. However, only 65 of the 446 indigenous genera are regarded as endemic and there was only one endemic family, the Ixerbaceae, represented by the monogeneric Ixerba brexioides (Fig. 11.1a.b), a small woody tree of northern New Zealand forests, but the family has now been merged with the Strasburgiaceae (APG, 2009). The vascular flora is also remarkably small for an archipelago the size of New Zealand (263 830 km²), in comparison, the much smaller Fijian archipelago (18 235 km²) has an indigenous spermatophyte flora of 1318 taxa in 137 families and 490 genera (Smith, 1996) and, while complete statistics are not available, the even smaller Hawaiian archipelago (16 626 km²) has an indigenous flowering plant flora of c.956 species in 87 families and 216 genera (Wagner et al., 1990). So what makes New Zealand’s flora so unusual? Perhaps the answer lies not so much in the size of the flora and its perceived diversity but rather in the problem of species and generic delimitation. Many New Zealand genera, most notably those that are species rich, either have poorly defined generic boundaries, e.g. Celmisia, Raoulia (Allan, 1961; Ward, 1997; Cross et al., 2002; Smissen et al., 2004), or hybridise readily, especially in situations of prolonged disturbance, e.g. Chionochloa, Coprosma, Epilobium and Kunzea (Oliver, 1935; Connor, 1967; Raven & Raven, 1976; Wichman et al., 2002; de Lange et al., 2005). Hybridism is a feature of the New Zealand flora and it occurs at both the species and genus level to varying degrees but at sufficient frequency to ensure that the existence of hybrids has caused difficulties with traditional taxonomic attempts to classify the flora (Morgan-Richards et al., 2009). This recognition of hybridism in the New Zealand flora is not a recent discovery; indeed, early New Zealand botanical writings indicate an awareness of hybrids as early as the mid 1860s and it was certainly postulated as both a driver for speciation and a feature of the indigenous flora by pioneering botanists such as Leonard Cockayne and Harry Allan (Cockayne, 1923, 1929; Allan, 1924; Cockayne & Allan, 1934). Although the prevalence of widespread hybridism has been questioned (Allan, 1961; Hair, 1966; Connor, 1985), its significance has come in for re-evaluation (Morgan-Richards et al., 2009), especially with the advent of DNA-based methods when many examples have been confirmed (Molloy & Dawson, 1998; Lockhart et al., 2001; Wichman et al., 2002; Gardner et al., 2004; Murray et al., 2004; de Lange et al., 2008).