Evolution of the coastal neospecies Zieria prostrata (Rutaceae) and its relationship to the Zieria smithii species complex

The endangered plant Zieria prostrata J.A.Armstrong (Rutaceae) is known from only four coastal headlands in northern New South Wales, Australia. The discovery of another headland Zieria form, Z. sp. aff. smithii, raised questions about the taxonomic status of Z. prostrata and its relationship to the Z. smithii Jacks. species complex. Morphometrics was used as the primary tool for investigating the relationship between Z. prostrata and the Z. smithii species complex, while a genetic study utilising RAPD markers was used to assess the validity of the distinct evolutionary lineages implied by the morphometric analysis. Z. prostrata formed a distinct group in phenetic space based upon the morphometric data, but with an incomplete discontinuity between it and nearby populations of Z. smithii based upon the genetic data, implying that Z. prostrata may be considered a distinct but incipient species. While the morphometric data set suggested that Z. sp. aff. smithii may be worthy of subspecific status, the genetic data revealed that each headland population is likely to have originated independently from inland populations of Z. smithii. Therefore, the morphological similarity among populations of Z. sp. aff. smithii does not reflect evolutionary relatedness, but rather is likely to be a consequence of parallelism and active or rapid speciation. Z. sp. aff. smithii is therefore considered to be a headland ecotype of Z. smithii.