The biogeographic distribution of species can be interesting from a biological or an evolutionary perspective. In the past, priority areas for conservation were often selected on the basis of the number of species in a given area. In recent years this has changed towards prioritising efforts according to entities, such as number of endemics (e.g. CAVIERES et al.
2002) or supra-specific species richness. It follows that the conservation value of a genus will be reflected by its endemic or diverse properties. Chaetanthera is principally endemic to Chile (20 of 30 species), with all but one of the remaining ten species also recorded from, but not restricted to, Chile. More interestingly, Chaetanthera has two diversity hotspots (see Chapter V, Section 1.2); in Coquimbo and Santiago. They are both located somewhat to the north of the main Chilean biodiversity hotspot (situated between 34°S and 40°S, according to BARTHLOTT et al. 1998). By collating hotspots of species – that is the number of species
coexisting in the same area (ecological or geographical) – one can consider the environmental influences driving the sympatric aggregation of related taxa.
VII Discussion & conclusions of systematic analysis 117
Species radiations are evidence of past dynamic change in a genus. There are examples in Chaetanthera of historical radiations, spanning outwards from both hotspots. The variation in leaf indumentum of the limbate, indistinctly petiolate-leaved species complex within Chaetanthera subgenus Tylloma, (Chapter V, Section 1.3) or the shared lamina morphology of several suites of annual species in Chaetanthera subgenus Chaetanthera (Chapter V, Section 1.4) are two examples of past radiation events. However, Chaetanthera also has some excellent examples of current dynamic change, as evidenced by polymorphism within the collected material. From an evolutionary biology perspective, areas where species boundaries break down or are incomplete are as interesting as those areas where there is distributional congruence amongst different endemic taxa.
Polymorphism is a well known phenomenon, in particular heterophylly (Ranunculus subgenus Batrachium; Polygonum amphibium after BRIGGS & WALTERS 1984). Phenotypic variation and plasticity can often be an indicator of a plant’s response to stress. In Chaetanthera glabrata, the observed leaf polymorphism is demonstrably linked to the climatic phenomenon of El Niño (Chapter VI, Section 2). There is some evidence in the literature regarding the potential effect of the El Niño on phenotypic plasticity of plants. The effect of ENSO-linked growth has been recorded for 2 woody Prosopis species (P. pallida and P. chilensis) from the coastal regions of Peru and Chile respectively (LÓPEZ et al. 2006). Changes in primary productivity in vegetation in Coquimbo (Chile) as a result of ENSO events were observed by SQUEO et al. (2006). They also found that El Niño has the greatest
impact at low elevations, while the effect is buffered by cooler air temperatures at mid- elevations. In the instance of El Niño-driven polymorphism, it is important to realise that the hydration stress typically experienced by the plants occupying arid/semi-arid regions is alleviated, not that the El Niño event itself is a cause of stress (after HOLMGREN et al. 2006).
Thus, in the case of C. glabrata the leaf polymorphism (= increased vegetative productivity) is a result of removal of hydration stress. C. glabrata avoids the huge competitive surge in plant species recruitment (HOLMGREN et al. 2001) during El Niño events by having a very
rapid germination response (1-3 days after hydration).
Phenotypic mosaics in taxa with porous genomes – i.e., those taxa that are incompletely separated and experience gene flow – are especially challenging for evolutionary taxonomy, raising issues such as character conflict in phylogenetic studies, biased sampling of traits in morphological studies and cryptic cases of ecological speciation. These are all issues raised by the case studies of C. albiflora – C. linearis and C. chilensis – C. elegans. A good explanation of analytical issues encountered when studying taxa with porous genomes is given by LEXER et al. (2009).
As outlined in Chapter VI, Section 3, a large scale shallow cline of hybrids stretching over 300 Km between 30° – 33°S exists between two parent species: Chaetanthera albiflora and C. linearis. The apparent introgression of the upland southern species into the lowland (coastal) northern species, to the exclusion of the southern parent in northern upland areas, indicates that natural selection favours the hybrid genotype in that environment.
Not all polymorphism is so easily explained. Complex patterns of variation between only a few species can be generated by incompletely isolated taxa and subsequent reticulate hybridisation and introgression. The perennial scapose Chaetanthera present one such scenario, as analysed in Chapter VI section 4. C. chilensis, a highly polymorphic, wide- spread, southern Chilean montane-mid elevation species seems incompletely isolated from the montane micro-species C. elegans. C. elegans has an active genetic effect radiating out
from its localities in southern montane areas. Additionally, these two species have generated a stable distinctive hybrid, C. x serrata. C. x serrata prefers more lowland, southerly niches than the parents, and is characterised by having stolons and poorly developed carpopodia. It is unclear whether the hybrid itself produces viable seed. It seems likely that, even with a reduced viability in the hybrid offspring, the swarm could be maintained via backcrossing (introgression) with the parents. Although the analyses implied that several montane micro- species could be involved in a reticulate configuration, a reassessment of the material would be necessary first. The use of AFLP’s to calculate genetic distances and polymorphism in metapopulations is one technique that can aid understanding of spatial variation within and between populations (e.g. Sisybrium austriacum, JACQUEMYN et al., 2006). Analysis of cpDNA variation is increasingly used as a tool to aid interpretation of plant evolutionary biology. Directional introgression, initially indicated by multivariate analysis, between two Populus species has been successfully demonstrated using single nucleotide polymorphisms from both nuclear and chloroplast genomes (HAMZEH et al. 2007). Variation in the intergenic
cpDNA regions was also studied in Crataegus hybrids but with less clear results (ALBAROUKI
& PETERSON, 2007). It is possible that these techniques might also be suited to this Chaetanthera complex.
Hybrids often occur at ecotones or boundaries between different habitats (HARRISON 1993). Phenotypic variation as a result of active hybridisation between two (or more) species is recorded in the literature concerning native Chilean species, although it has not been considered in terms of ecological boundaries found in Chile. For example, various hybrids within the Chilean Calceolaria (Scrophulariaceae) are said to be ephemeral or exist as stable swarms (EHRHART 2005) where the parents coexist sympatrically, but little is recorded about
the ecological significance of the sympatric zone. Similarly, the species-rich genus Haplopappus forms only a few hybrids (7 recognised hybrid forms) where the species distributions are sympatric (KLINGENBERG 2007). Chaetanthera has two instances of species
instability over different boundary zones. The significance of hybridisation events over the arid – semi-arid boundary in Coquimbo seems to be largely undocumented, although the high number of species, especially endemics, characterising this region is well known. The hybrid swarm between the annuals C. linearis and C. albiflora seems stable (not ephemeral) and selection appears to favour the hybrid genotype in ecozones where the parent species are less fit. The north-south climatic/hydrological boundary zone between Maule and Chillán seems to be a locus of change in the caespitose C. chilensis/ C. elegans species. The genus Baccharis L. (Astereae) forms 27 hybrids, scattered throughout Chile (HELLWIG 1990),
including a hybrid specific to the Chillán area. Nothofagus obliqua and N. glauca also form natural hybrids in this area (DONOSO & LANDRUM 1979). The dwarf hybrid shrub
Haplopappus glutinosus x paucidentatus also occurs as hybrid swarm reported from the Chillán area.
VII Discussion & conclusions of systematic analysis 119