Coastal dwarf variants of the red swamp Banksia (Proteaceae) represent parallel ecotypic evolution rather than a divergent evolutionary lineage of single origin

Abstract

Morphological variation is common within widely distributed plant species and often raises interesting taxonomic and evolutionary questions. We sampled eight typical inland populations and two coastal dwarf populations of Banksia occidentalis to assess whether the coastal variant warrants taxonomic recognition. Genomic (single nucleotide polymorphisms) and morphometric analyses were undertaken to assess the variation within and between variants. Genetically, the coastal populations were more closely related to geographically adjacent populations of the typical variant than they were to each other, indicating that each coastal population has originated independently. Morphologically, the coastal populations also did not represent a cohesive phenotypic group, with both falling outside the range of variation for the typical variant, but also not overlapping each other. Wide morphological, but not genomic, variation was identified along a coastal–inland transect. Combined, these data indicate that the morphological variation in these coastal populations is the result of parallel ecotype evolution from experiencing similar environmental conditions and does not represent the diversification of a distinct evolutionary lineage that would have warranted taxonomic recognition. Our study highlights the consequences of strong selection pressures in marginal environments and provides fascinating insight to the long evolutionary history of the flora in this ancient Australian landscape.