ABSTRACT
The algae are a non-monophyletic group of highly numerous organisms which exhibit extremely diverse morphologies. It is estimated that there are >350,000 species of algae globally, although only a fraction of this number have been described. Here we use the red algae (Rhodophyta) to demonstrate the approaches that have been taken in their identification and classification. The red algae are an ancient and morphologically highly diverse group with about 5,800 described species. Different approaches have been employed in their classification, including anatomical, biochemical and physiological studies, but molecular studies have also had a profound impact on our understanding of their evolution. Ordinal classification has increased from four orders recognised in the nineteenth century to 30 currently recognised. Based primarily on morphological observations, some orders have considerably more species than others; for example, the largest is Ceramiales with c. 2,300 species, with some of its genera being very large (>200 species). We explore in the red algae what factors, many of them unique to this group, may have led to high levels of speciation
(reproductive isolation). In red algae, levels of genetic uniqueness are shown to be correlated with reproductive isolation and not always with morphological distinctness. There is also evidence that red algal populations are highly differentiated over small distances. In addition, red algae have unique reproductive systems that may lead to the easy acquisition of reproductive isolation. In order to obtain a greater understanding of the causes and mechanisms of reproductive isolation in red algae, we propose that a new line of research targeted at reproductive incompatibility be explored. We conclude that the continuation of a multifaceted approach, including molecular techniques, population studies and cell biology remains necessary to illuminate evolution in the red algae.
