'Reticulate Evolution' of CORALS

Beautiful hard corals of the Ningaloo Reef - Ningaloo Kayak Adventures, Coral Bay 2013

A small animal that, collectively, is the foremost integral member of the ‘Coral Coast Crew’, is of course, CORAL!

The purpose of this blog is to briefly introduce you first to the evolutionary history of the ‘modern corals’. Corals are a major constituent of the Coral Coast and are essentially what structurally and fundamentally makes up the Ningaloo Reef ecosystem.

The term ‘coral’ is generally used for both ‘soft’ and ‘hard’ corals, sometimes encompassing other colonial Cnidaria (also commonly called Coelenterata). All, except the Stylasterina, of the four extant orders are partly or wholly zooxanthellate (corals which have symbiotic blue-green algae called zooxanthellate in their tissues) and are consequently constrained to sun-lit shallows and warm water.

In brief, the evolutionary history of ‘modern corals’ may be divided into three geological intervals:

(1) Paleogene – the survivors of end-Cretaceous and Late Palaeocene extinctions proliferated into a diverse cosmopolitan fauna;

(2) Miocene – subdivision of this fauna into the broad biogeographic provinces we have today and when the immediate ancestors of most extant species (primarily Indo-Pacific) evolved.

(3) Plio-Pleistocene to present – global glaciation mode and when modern distribution patterns emerged.

Compared with most other major taxa (groups) of animals, coral genera are long-lived in geological time and have low extinction rates: nearly half of all extant genera extend as far back as the Oligocene and nearly a quarter extend back to the Eocene.

Species are the fundamental units (or ‘building blocks’) of Nature defined in a limited geographic space, however, this concept breaks down when applied to corals over large geographic ranges.

The fundamental reason for this is that coral species exist as interlinked patterns in geographic space that change continuously so that variation within a single species becomes indistinguishable from the variation between comparable species. The majority of coral species do not exist as geographically or taxonomically definable units. Therefore, it may be necessary to consider evolutionary change in a different manner from that which has become generally accepted in both popular and scientific literature. ‘Reticulate evolution’, the development of a network of closely related taxa within and at the species level, is a fundamentally distinct concept (paradigm) which involves a different way of looking at what species are, the geographic patterns they make, and their change over evolutionary time.

Stunning hard coral garden inside the lagoon of the Ningaloo Reef - Migration Media, Ningaloo Reef Dive, Coral Bay 2013

As with most other forms of life, coral evolution can only be inferred from other studies - palaeontology, taxonomy, biogeography and genetics. Palaeontology shows something about how life was in the distant past and how it has changed, whilst genetics has the potential to reveal a great deal about mechanisms of evolutionary change. Biologists depend to a large extent on taxonomy and biogeography to provide an insight into how species evolve. Geographic space and evolutionary time interact.

Evolutionary changes will include distribution and genetic changes occurring in response to fluctuations in ocean currents. As a result, a coral species are genetically as well as geographically changed irregularly over the species’ geographic range and evolutionary history. The species may break apart and then re-form into a slightly different unit, creating a ‘reticulate’ pattern in both geographic space and evolutionary time.

A hypothetical view of reticulate evolutionary change within a group of species. 

Australian Institute of Marine Science 2013

The reticulate ‘re-packaging’ occurs constantly at all scales of space and time and is not confined to a single phylogeny or evolutionary clade but involves many simultaneously. Reticulate evolution is hence a mechanism of slow arbitrary change acting on genetic composition is under physical environmental control changes patterns of genetic connections. This is again in sharp contrast with a major aspect of the traditional view where evolution is largely controlled by competition between species, resulting in morphological changes.

The concepts of reticulate evolution and the traditional view of evolution are not compatible – they are two paradigms which become increasingly mutually exclusive with increasing amounts of space and time.

Image: Australian Institute of Marine Science, 2013

These are hypothetical representations of the same gene pool under different regimes of ocean currents. (A) The gene pool forms a single cohesive species with strong currents, (B) currents are decreasing and the gene pool forms a single species but some parts of it are partly reproductively isolated (represented by overpasses), (C) currents are weak and the species is broken up into isolated pockets.

Reticulate evolution is primarily driven by changes in surface circulation patterns causing changes to the dispersal patterns of larvae. If dispersion by all the ocean currents stopped, every reef, island and headland would be genetically isolated. The corals of each location over time would gradually become distinct from those of every other through the processes of Darwinian natural selection, amplified by genetic drift and mutations. In time, every location would develop a unique reef fauna and every coral species would have a distribution range of just that situation. Thus, if currents remained constant throughout evolutionary time there would be general uniformity in species and their distribution. Nevertheless, the variations in ocean currents due to geo-climatic cycles result in constant changes in dispersion and genetic connectivity: they generate reticulate patterns. It is argued that the concept of reticulate evolution is vastly explanatory about coral taxonomy observations and biogeography and is starting to be supported by genetic studies.The reticulate evolution concept is also strongly supported by what is known of coral reproduction.

More on these members and their coral reefs of the ‘Coral Coast Crew’ are to feature in the following blogs!

Bibliography:

Arnold, M.L. & Fogarty, N.D. (2009) Reticulate evolution and marine organisms: the final frontier?, International journal of molecular sciences, 10 (9), 3836-3860.

Frank, U. & Mokady, O. (2002) Coral biodiversity and evolution: recent molecular contributions, Canadian Journal of Zoology, 80 (10), 1723-1723.

Stanley, J.G.D. & Fautin, D.G. (2001) Paleontology and evolution: The origins of modern corals, Science (New York, N.Y.), 291 (5510), 1913-1914.

Veron, J.E.N. (2013) coral info sheets, Australian Institute of Marine Science, viewed April 2013, <http://coral.aims.gov.au/info/coral-reefs.jsp>.