Our understanding of species concepts goes to the very heart of the way Nature is organised. It is endlessly complex and is probably a little different for each and every species. Both practical and theoretical issues arise from it that have broad relevance to the taxonomy, biogeography, systematics and evolutionary concepts of most marine organisms, not just corals.
Species have long been considered to be the ‘building blocks’ of Nature, blocks that have a time and place of origin, which can evolve, and which maintain themselves as discrete entities by being reproductively isolated. This is a logical concept, but one which breaks down, for most corals, when confronted with the realities of geographic variation. The alternative, where most species have none of these attributes, is initially less intuitive but ultimately is highly explanatory of what can actually be observed in the real world.
Concepts of what species are have long been debated, a debate driven as much by misinterpretation as information. The debate, however, has mostly been ignored by taxonomists (who often believe they ‘know’ what their species are for one reason or another) as well as most other biologists (who often consider it ‘armchair philosophy’ and thus largely irrelevant to the needs of reality). Be that as it may, species have variously been considered to be (a) self-defining natural units, (b) human-defined units composed of assemblages united by common descent, or (c) genetically-defined units resulting from Darwinian natural selection and/or genetic (reproductive) isolation. The last involves a group of concepts, sometimes called the ‘neo-Darwinian synthesis’ or the ‘biological species concept’ (depending on a wealth of detail) which embodies the notions of the building blocks referred to above.
The concept of the syngameon, first recognised by botanists and introduced to the marine world through corals, is important for the understanding of the evolutionary mechanisms of corals and the geographic patterns they form. A syngameon, by definition, is a reproductively isolated unit. In concept, so are neo-Darwinian species. In reality, syngameons are nothing like any of these concepts of species as they incorporate geographic variation and the spectrum of genetic links geographic variation creates among different species.
A syngameon may be a single species, or it may be a cluster of different species which have variable genetic links (genetic flow through cross-fertilisation) with other members of the syngameon. Where a syngameon contains several species, a single component species may be distinct at a single geographic location but, because it intergrades with other species at other locations, it will become submerged in a mosaic of variation at other locations. The geographic range and morphological variation of the single ‘species’ will depend on taxonomic decisions. These decisions will be arbitrary if they impose divisions in natural continua rather than reflect natural units. The syngameon as a whole is not morphologically distinguishable unless its component species are determined genetically or experimentally (through cross-fertilisation trials) in every part of the species’ possible distribution range.