Geographic Variation in Eucalyptus diversifolia (Myrtaceae) and the Recognition of New Subspecies E. diversifolia subsp. hesperia and E. diversifolia subsp. megacarpa

Abstract

Patterns of geographic variation in morphological and chemical characters are documented in Eucalyptus diversifolia Bonpl. (soap mallee, white coastal mallee). This species is found in coastal and subcoastal Australia from southern Western Australia to Cape Nelson (western Victoria), with a number of disjunctions in the intervening region. Morphological data from adult plants collected at field localities and seedlings grown under uniform conditions were analysed using univariate and multivariate methods, including oneway ANOVA, multiple comparison tests, non-metric multidimensional scaling (NMDS), nearest neighbour networks, and minimum spanning trees. Seedling material was tested for isozyme polymorphism, and adult leaf flavonoids were analysed using liquid chromatography. Morphological and chemical characters are also documented in E. aff. diversifolia, a closely related but unnamed taxon restricted to ironstone outcrops near Norseman (WA), and putative E. diversifolia- E. baxteri hybrids from Cape Nelson. Congruent patterns in data sets distinguish three groups of E. diversifolia adults and progeny: (1) those to the west of the Nullarbor disjunction; (2) South Australian populations to the east of this disjunction; and (3) those from Cape Nelson. Formal taxonomic recognition of the three forms at subspecific level is established, namely E. diversifolia subsp. diversifolia, E. diversifolia subsp. hesperia, and E. diversifolia subsp. megacarpa. Patterns of geographic affinity between populations are consistent with a hypothesis of genetic exchange between normally disjunct regional populations of E. diversifolia via coastal land-bridges exposed during periodic times of low sea level since the mid Tertiary.