Evidence for convergent evolution among phylogenetically distant rare species of Tetratheca (Elaeocarpaceae, formerly Tremandraceae) from Western Australia

Abstract

Morphological and molecular investigations of taxon relationships among rare species of Tetratheca Sm. occurring near Koolyanobbing, Western Australia, have confirmed the distinctness of T. aphylla F.Muell., T. harperi F.Muell. and T. paynterae Alford and identified three new rare taxa from collections affiliated with T. aphylla and T. paynterae. The recognition of these taxa at specific and sub-specific ranks is based on their different degrees of morphological and molecular divergence, combined with geographic disjunction. Cladistic analysis of nrDNA internal transcribed spacer and cpDNA trnL-trnF sequences from a range of Tetratheca species from Western Australia and the eastern states indicates that T. aphylla, T. harperi and T. paynterae belong to three separate evolutionary lineages and that the endemism displayed among these taxa to small, disjunct ranges within the same geographic area, is a result of in situ speciation due to historical fragmentation. These results exemplify the extremely high conservation value of the Yilgarn banded ironstone ranges. The superficial similarity among the study taxa in having a ‘leafless’ habit can be seen to be adaptive convergence in response to the marginal and semi-arid environments in which they occur, and this character is highly homoplastic within the genus.