Ever wondered why with an endless possibility of patterns and permutations, there is always somehow a few fish with consistently repeated pattern templates? A few species that share the same overall appearance but not quite the same. That confusing group of fish that you can never really figure out because they just look so similar? Speciation is at work, or has been at work for thousands of years. How two similar looking species came into existence isn’t simply a matter of coincidence.
Like Darwin and his finches, evolution and speciation is rampant in the underwater world, and can be seen in certain groups of fishes. Speciation is an evolutionary process in which new biological species arise. Speciation doesn’t happen for no reason. There usually has to be a driving force, or a barrier that brings about this change. There are four main geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another. They are allopatric, peripatric, parapatric and sympatric.
Admittedly we’re not scientists, and our understanding of evolutionary speciation is rudimentary at best. However it is sufficient enough to explore some of these processes and hence try to make sense of what’s going on. The pictorial illustration above shows basically the four types of speciation that takes place. All forms of speciation occur in reef fishes.
In allopatric speciation, the original species population is dispersed via a “barrier”. This barrier is usually an intrinsic force, such as an emergence of a mountain range, or the lowering of sea level. Whatever the case, these forces eventually split up the original population into two or more isolated, disjointed groups. Lower sea levels during glacial periods serve to be a very effective barrier that divide formally widespread species, which sets the stage for evolution.
The barrier effect is pivotal and instrumental in the evolution of geminate, or sister species. A twin species occurs when the separated population evolve and develop distinct characteristics. Over time the difference may be so vast such that when the barriers are removed, for example by rising sea levels, the two populations may not be able to mate with each other. At this point, the genetically isolated group has emerged as a new, distinct species.
This is seen in a variety of reef fish spanning multiple genera. An example of a species complex featuring related sisters is the “xanthurus complex” of butterflyfish in the genus Chaetodon. Referring to the picture above, four similarly marked but genetically and phenotypically distinct species suggest that they all diverged from one ancestral member. If we look at their geographical distribution, you will see that none of them overlap in range significantly, which supports the theory of allopatric speciation.
In the Indian Ocean Chaetodon madagaskariensis (A), the body has pronounced chevron shaped markings and a crown on the nape. Chaetodon mertensii (B) ranges across Oceania and is similar, but lacks the crown on the nape. Instead, it is replaced with a smudge and the fins are coloured yellow instead of orange. The East Indian and Indo-Pacific C. xanthurus (D) bears similar coloration to C. madagaskariensis, but the chevron markings are less discernible, and instead features an extensive crosshatch pattern. C. paucifasciatus (C) is found in the Red Sea, which is also in the Indian Ocean as with C. madagaskariensis. However the Red Sea is a rather special place with a high rate of endemism and we will go back to this at a later time*. C. paucifasciatus is red instead of orange or yellow.
This same sister species complex of allopatric distribution can also be seen in Centropyge argi and its peers. Although all four members are found in the Atlantic Ocean, they do not overlap in range. Over time defining characteristics such as DNA as well as tail and body coloration set these apart both phenotypically and genotypically.
Allopatric speciation is also observed in fishes with disparate ranges, such as two entirely different oceans. Indian Ocean fishes often have twin species in the Pacific, and vice versa.
Using Chaetodon as an example again, and with reference to the table above, you can see that many species share a twin in another region. Chaetodon interruptus and C. unimaculatus for example are similar, but genetically distinct and do not overlap in their range. Let’s back track quickly to when we were talking about Chaetodon paucifasciatus and C. madagaskariensis previously.
Both C. paucifasciatus and C. madagaskariensis occur in the same general ocean. The Indian Ocean. However C. paucifasciatus is decidedly different, with red instead of orange coloration. The Red Sea has a remarkably high rate of endemism, and can be considered as an “isolated range within a range”. To understand how this happens, let’s take a look at the history of the Red Sea and it’s geographical position proximal to the Indian Ocean.
About five million years ago, the northern portion of the Red Sea was connected to the Mediterranean. As land levels rose over the years, the gateway to the Mediterranean was cut off, effectively turning the Red Sea into a huge evaporation pan, increasing the salt concentration as evaporation took place. Sometime later, the southern end at the Straits of Perim opened up, bringing with it water as well as fish from the Indo-Pacific. However because of the very narrow and shallow opening, the salinity and temperature of the Red Sea differed from the adjacent Indian Ocean.
Fiften thousand years ago, during the last ice age, a huge portion of water was locked up in glacial ice caps, effectively bringing down the sea level by about a hundred meters. This would have cut off the Red Sea from the Indian Ocean once more and set the stage for evolution. As such, the Red Sea today is home to a wide range of endemic species that bear close resemblance to their once Indo-Pacific sisters. Chaetodon paucifasciatus, Chaetodon striatus, Zebrasoma xanthurum, Acanthurus sohal, Pseudanthias heemstrai, Paracheilinus octotaenia and Cirrhilabrus blatteus are some of the many endemics present in the Red Sea. Even species who have yet to evolve are slowly starting to show minor differences in coloration, such as Chaetodon auriga.
Because we saw an influx of fish from an adjacent area rush into the Red Sea and eventually getting trapped, and forced to speciate, this is no longer regarded as allopatric speciation, but rather, peripatric speciation. Peripatric speciation occurs when an organism separates from the main populace by entering an adjacent niche habitat, but yet do not occur in overlapping ranges. The Red Sea is home to many peripatric species as mentioned above.
The third model for divergent evolution is parapatric speciation. Sister species who undergo parapatric speciation often have adjacent ranges that overlap only in a very narrow strip where the two ranges meet. In allopatric and peripatric speciation, the members of the same complex never meet, versus the narrow contact zones of overlap in parapatric species.
Chaetodontoplus poliorus and C. mesoleucus is a great example of parapatric speciation. The former differs from the other only by having a grey tail, and was thought to be a colour variant of C. mesoleucus. In 2009, Rocha and Randall separated the two species and the grey-tailed species was officially recognised as C. poliorus. The range of C. poliorus encompasses the extreme east of the East-Indian region, ranging from Papua New Guinea, to Palau, up to the Solomon Islands. C. mesoleucus exhibits an opposite geographical range, ranging westwards up to Japan, and the Indo-Pacific. The two species co-occur at a narrow contact zone in the Moluccas, Halmahera and the Bird’s Head Peninsular, triangulated on the map above in red.
The last and final speciation process is sympatric speciation. Sympatric speciation occurs when a new species evolve from a single ancestral species while inhabiting the same geographical region. The overlap in range for both species may be large, or even identical. Sister species who meet this criteria are often the result of sympatric speciation, and share the same range.
Because observing sympatric speciation is difficult, unlike allopatric speciation, the theory is rather difficult to prove. It is challenging to prove, with the theory of natural selection, how two species could arise from the same ancestor without the formation of any barriers, especially if they were allowed to inter-breed. Recent mechanisms such as gene flow as well as studies on actual cases have proven that sympatric speciation does occur.
In the Atacama Desert, two distinct but very closely related palms occupy the same island, but grow in very different soil types. Being palms, they are are able to cross-fertilise by air-borne pollen, except this has not occurred. Although highly similar, no hybrids of the palms are produced, suggesting that they have already speciated within their niche, barrier free and without any intrinsic forces.
On the reef, sympatric species are not easy to pick out. One possible example may be members of Paracheilinus. Paracheilinus females are very difficult to identify, and look highly similar to members of the same genus. Males often flash elaborate neon colours and display beautiful finnage during courtship to attract the females, and it is this distinctive coloration that is unique to each species that allows the females to correctly identify their corresponding mate. Pressure to develop more unique coloration has brought upon multiple geographical variations within a single species, which may end up distinct in the future.
Even closely related species such as P. filamentosus and P. flavianalis may have speciated in the past based on this premise, although concrete evidence for the lay-man like us may prove to be rather elusive.