Evolution of species
For over two centuries species have widely been regarded as the fundamental units, or ‘building blocks’, of Nature – units that can be named, described and studied. When this concept is applied to corals over large geographic ranges, it breaks down. Thus, a species is only a species in limited geographic space. When that space is progressively increased (as, for example, when studying a particular species in one country, then in progressively more distant countries), it becomes progressively not that species.
The fundamental reason for this is that coral species exist in geographic space as interlinked patterns that change continuously so that variation within a single species becomes indistinguishable from with variation between similar species. The majority of species do not exist as geographically or taxonomically definable units. This creates a dilemma, for humans cannot easily communicate in terms of continua: they need discrete units of some form or other. In short, the units we have created in order to communicate are, at best, only an approximation of what actually occurs in Nature.
The issues that arise make it necessary to consider evolutionary change in a way that is different from that which has become generally accepted in both popular and scientific literature. This is ‘reticulate evolution’, a fundamentally distinct concept which involves a different way of looking at what species are, the geographic patterns they make, and their change over evolutionary time. The concept itself is simple, once grasped.
Reconstructing evolutionary change
With corals, as with most other forms of life, evolution cannot be studied directly, it can only be inferred from other studies - palaeontology, taxonomy, biogeography and genetics. The first of these subjects, palaeontology, builds the ‘big picture’ summarised above. It shows something about how life was in the distant past and how it has changed, but it reveals nothing about the mechanisms of change. The last of these subjects, genetics, has the potential to reveal a great deal about evolutionary mechanisms, but this requires vast amounts of study – studies that are only just beginning. To a large extent, biologists depend on taxonomy and biogeography to provide an insight into how species evolve.
Evolutionary change can seldom be observed directly: it must be re-constructed, as is commonly done using family ‘trees’. Family trees are also one way of envisaging reticulate evolution. Such a tree might have a main trunk for the family, several large branches for genera, and many fine branches for the species. Species that are currently alive will be the tips of the uppermost branches and these will be trimmed to uniform height, to represent present time. The branch tips, each representing a single species, can be converted into distribution maps, each map representing the present-day distribution of one species (as in the species maps of Coral ID). If a single branch (or species) is then sliced into a sequence of horizontal layers, and each layer is turned into a distribution map, each map will indicate the distribution of the species at a progressively distant points in time. If the maps are viewed like the pages of a book, the pattern will change sequentially back in time. These changes will not be just distribution changes, they will also be genetic changes occurring in response to changes in ocean currents. As a result, a species (or map) at one point in time is not the same as it is in another point in time: it has been genetically as well as geographically changed. These changes do not occur uniformly, they occur irregularly over the species’ geographic range and evolutionary history. Geographic space and evolutionary time interact. The species may break apart and then re-form into a slightly different unit. This creates a ‘reticulate’ pattern in both geographic space and evolutionary time.
Ocean currents and reticulate patterns
When a species breaks apart (as a result of weak ocean currents, see below) it becomes many species. When it re-forms (as a result of strong ocean currents) it may again be a single species or it may be more than one species. It may also contain genes from other species. This is reticulate ‘re-packaging’, and it occurs constantly at all scales of space and time. The re-packaging is not confined to a single phylogeny or evolutionary clade, it involves many clades simultaneously. A hypothetical view of evolutionary change in a small phylogeny (left) correlated with distribution patterns (maps, right). At the bottom (Time 0), currents are strong (and the sea level is low). The phylogeny exists as a single widespread species. At Time 1, currents are weak and the phylogeny has broken up into six geographically isolated indistinct or ‘sibling’ species. (Two of these species, labled 2 and 3, are doomed to subsequent extinction.) At the top (Time 2), currents are strong again. The phylogeny has been re-packaged into two widespread species which are reproductively isolated at this time. The species on the right is part of the phylogeny of another species (coloured green). Note: (a) there is no distinction between geographic (sympatric) and non-geographic (allopatric) origination, (b) evolution is driven by environmental change, not biological competition. Hypothetical representations of the same gene pool as  under different current regimes. (a) currents are strong and the gene pool forms a single cohesive species, (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.
The illustrations (above) of reticulate evolution show why this concept is sharply contrasted with the traditional view of evolutionary change. The diagram shows that species (a) have no time of origin, (b) have no place of origin, (c) are semi-arbitrary bits of continua rather than units, and (d) are continually re-packaged in space and time.
Importantly, reticulate evolution is under physical environmental control, not biological control. The physical control changes patterns of genetic connections: it acts on genetic composition. 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. Reticulate evolution is therefore a mechanism of slow arbitrary change rather than a mechanism for progressive improvement.
Reticulate evolution is primarily driven by changes in surface circulation patterns causing changes to the dispersal patterns of larvae. If currents remained constant throughout evolutionary time, the oceans would be divisible into source areas (where larvae come from) and destination areas (where larvae go to) and there would be general uniformity in species and their distribution. However, with the exception of the most major currents, which are driven by the direction of rotation of the Earth, currents are not constant. Sea levels are known to fluctuate over 100 metres, oceanic passages are opened and closed by tectonic movements, and the Earth goes through cyclical climate changes due to variations in the tilt of its axis and variation in the shape of its orbital motion around the sun (Milankovitch cycles). These, and several other types of geo-climatic cycles, cause variations in ocean currents. These changes open and close genetic contacts: they generate reticulate patterns.
To understand how changing ocean currents effect genetic connectivity, it is helpful to imagine what would happen to dispersion if all the ocean currents stopped. Every reef, every island and every headland would be genetically isolated. In time, through the processes of Darwinian natural selection, augmented by genetic drift and mutations, the corals of each location would gradually become distinct from those of every other location. In time, every location would develop a unique fauna and every species would have a distribution range of just one location. There would be millions of species worldwide. If the very opposite is now imagined – where ocean currents are so strong and so variable that the corals of every location came into frequent genetic contact with those of every other location – every species would eventually become dispersed to every location. All species would be found everywhere they could grow and there would only be a small number of species worldwide. It is certain that these imaginary extremes never happened, but what has happened is that the Earth’s climate has oscillated within these extremes, causing constant changes in dispersion, constant changes in genetic connectivity – causing reticulate patterns to arise.
The concept of reticulate evolution is highly explanatory of the observations about coral taxonomy and biogeography and it is also strongly supported by what we know of coral reproduction and is starting to be supported by genetic studies.
Reticulate and neo-Darwinian evolution
The subject of evolution, now covered by hundreds of books since Darwin’s (1859) epic The Origin of Species, is dominated by varying emphases on three broad notions: (a) that species exist in Nature as more-or-less reproductively isolated units of some kind, (b) that these units are comprised of individuals that compete in different ways or are modified by environment and (c) that the ‘fittest’ individuals survive. From these notions there has been an assumption that species have a time and place of origin (where phylogenies divide) and that they are units where geographic space and evolutionary time only interact in a non-reversible manner (where phylogenies divide irreversibly). 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.
There is a point, however, where the paradigm of reticulate evolution interlinks with the traditional Darwinian view of competition and survival. Clearly, coral species compete, perhaps for the same resource (such as food, or living space), or in predator-prey relationships. These sorts of competition affect relative abundance, but only in specific environments, times and places. In other environments, times and places, the dynamics are likely to be different. Thus, one species of coral may continually out-compete another on the east coast of Africa, but this may have little bearing on how they compete on the Great Barrier Reef or how they compete on the east coast of Africa after an environmental upheaval or disease epidemic. For a start, the ‘species’ will not be the same in the two regions. Nevertheless, competition does affect survival. The outcomes will initially be of local significance, but if repeated in many places, they can presumably cause total extinctions. They will not, however, create evolutionary change unless the competing species are, in fact, cohesive units of some kind.
The same issues apply to adaptations to environments: species can adapt to environmental pressures if they are cohesive units of some kind. If any given species is mobile enough to get its genes around the space it occupies in few-enough generations to be able to maintain genetic cohesion, it will be able to adapt as a single unit. Clearly, this can happen with highly mobile organisms like most birds and large mammals. For other organisms, the larger the space they occupy and/or the less mobile they are, the less cohesive they will be. Marine organisms that rely on larval dispersal clearly have minimal chance of cohesion.
It can generally be said that species that are alive today are only those small fragments that have not gone extinct. Ever-changing reticulate systems are not prone to extinction and are resistant to major evolutionary movements. For this reason, evolutionary change in corals large enough to be revealed in the fossil record is extremely slow. At the present point in the Earth’s history, sea levels have been approximately constant for 18,000 years and there has also been an extended interval of climatic stability. At present, the Earth is likely to be in a period of weak ocean currents that are providing the mechanism for re-packaging species into small units. The stage in a re-packaging sequence reached by each individual ‘species’ will be different as it will depend on genetic dissimilarity with other ‘species’, dispersal ability and abundance. These factors all vary geographically. The decision as to what an individual species is will, in concept, change with evolutionary time, but it will always be arbitrary.