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Revealing Relationships

A cura di , 2000. print format
Pubblicato
Ron Coleman, 2000

Versione Español English
Zazzle Dazzle Them!

Zazzle Dazzle Them!

(This article was originally published in Cichlid News Magazine, Jan-00 pp. 32-34, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).

While few cichlids could ever be regarded as dull, some groups of cichlids are particularly intriguing. The lamprologines are just such a group. Cichlid aquarists and scientists alike are drawn to these natives of Lake Tanganyika for all sorts of reasons. There are plenty of forms, including roughly 85 described species (all from Lake Tanganyika except for six species found in the Zaire River system) and there are more species being described all the time (Bills and Ribbink 1997). They come in a range of sizes from tiny dwarf shell-dwellers to species over a foot in length, and they exhibit a diversity of interesting behavior.

Lepidiolamprologus attenuatus

Recent cladistic analyses have taken place on Lepidiolamprologus attenuatus. Photo by Don Danko.

If you have followed these fishes for the last few years, you are aware that generic names applied to them keep changing: sometimes they are all lumped in the genus Lamprologus, while at times they are split into separate genera, such as Altolamprologus and Neolamprologus. Why is this?

These name changes reflect developments in our understanding of the evolutionary relationships among these fishes. The good news is that several teams of researchers have recently devoted substantial effort to sorting out these fishes; the bad news is that the results don't all agree with each other, so the process is ongoing. In the mid-1990s two teams of researchers (Sturmbauer et al., 1994, from the State University of New York at Stony Brook and Kocher et al., 1995 from the University of New Hampshire) applied modern molecular techniques to the lamprologines, but got surprisingly different results. More recently, Melanie Stiassny (American Museum of Natural History, New York) re-examined a large number of lamprologines using more traditional techniques of inspecting morphological structures. Her findings support some of the previous molecular work, but also suggest that even more bizarre things may be going on. Understanding this research and other controversies in cichlid systematics require a quick introduction to modern systematics.

Systematics is the study of the evolutionary relationships among organisms. Like all branches of science, this field is not without disagreements, but in the last two decades at least some agreement has been reached on the basic approach and philosophy of systematics. That methodology is phylogenetic systematics, often called "cladistics." The goal of a cladist is to produce a phylogeny for a group of organisms, i.e., a depiction of the evolutionary relationships of that group. The purpose of the phylogeny is to illustrate which taxa (e.g., species, genera, etc.) are most closely related to which other ones and to illustrate the evidence we have to support that relationship. An important point to remember is that there is a single, true phylogeny, i.e., these organisms evolved in a certain way. The problem comes in trying to reconstruct that pattern of evolution.

As scientists gather more data, our understanding of these relationships grows. We might realize that creatures that we once thought were closely related are not so closely related at all. Thus, a currently accepted phylogeny may change, sometimes dramatically.

Cladistics has a few central principles, one of the more important being that a taxon can consist only of the evolutionary descendants of a common ancestor. So what? This means that if you are naming a group, such as a family, or a genus, you can only include those species which descended from a common ancestor. Also, you must include all of the descendants. This makes the group "monophyletic," the basis for cladistics. This may seem a bit esoteric or even blatantly obvious (of course you want to include only things that are closely related to each other in a group) but this wasn't always the way things were done. It also means that no matter how much you like the name of something, if it doesn't belong in a group (because it isn't closely related to the other taxa in the group) you have to remove it from the group and call it something else.

The second key point of cladistics is that we decide how closely taxa are related solely on the basis of what are called shared derived characters or "synapomorphies." A synapomorphy is an evolutionary novelty, that is to say, something peculiar that didn't exist before. It might be a strange bump on the head or a particular way two bones join together or even a unique behavior. If two organisms share a very peculiar trait, we assume that the reason they have this trait is because their common ancestor had it. For example, imagine that two fish both have a bright iridescent stripe along the base of the dorsal fin. The odds that such a peculiar trait evolved twice is very low. It is far more likely that the common ancestor to these two fish had the bright iridescent stripe and "passed it on" to its descendants.

Now imagine that we are examining four species of fish. Two species share the peculiar trait of having extremely long pelvic fins, three have the bright iridescent stripe, and the fourth has neither of these peculiar traits. Additionally, all four species have a pointed tail.

Altolamprologus compressiceps

Altolamprologus compressiceps is an excellent example of an extremely "derived" or specialized lamprologine. Fish and Photo by Don Danko.

The cladist would hypothesize that the two with extremely long pelvic fins are closest relatives, or "sister taxa." The third species, which does not have long pelvic fins, but shares the bright iridescent stripe with the other two, is the sister group to the group consisting of the two species with the long pelvic fins. Finally, we also can conclude that all four species form a monophyletic group because they all share the peculiar, derived character of having a pointed tail.

Cladistics would be easy if real organisms were so neat and tidy. Unfortunately, the real world is vastly more complex. Sometimes, characters evolve more than once independently. For example, it is quite possible that pointed tails have evolved a number of times in cichlids (and indeed they have). To deal with conflicting data, cladists rely on the principle of "parsimony," meaning that the path that takes the fewest total steps is most likely the one that evolution took. So, imagine three taxa. The first and second share fifteen uniquely derived characters. But, the second taxon shares two unique characters with the third taxon. Which two species are most closely related? Because of parsimony (the 15 outweighs the two), we assume that the first two species evolved from a common ancestor and the "unique" traits shared by the second and third species each evolved twice, once in the second taxon and once in the third. So, the first and second species are sister taxa, and the third species is the sister taxon to the group consisting of the first and second species.

In the case of sorting out the lamprologines, the first task was to find peculiar traits shared by all members of the group. When Stiassny scrutinized the bones of the head and the tail, she found such traits. The particular arrangement of bones below the eyes, in the pelvic region, and in the tail are unique to this group. Further, lamprologines have a unique type of scale - with tiny tooth-like structures over the entire surface of the scale and a unique arrangement of the teeth. Together these characters and others give us evidence that all the lamprologines are a single evolutionary lineage, i.e., a monophyletic group.

Next, Stiassny found evidence that Variabilichromis moorii (formerly called Neolamprologus moorii) is the sister group to all other lamprologines. This finding agrees with the molecular results of Sturmbauer et al. (1993) and will likely stand the test of time. But things get messy after this point.

Neolamprologus sexfasciatus

Neolamprologus sexfasciatus represents still another example of the broad diversity seen among the lamprologines of lake Tanganyika. Photos by Mikael Karlsson.

Previous studies have divided the lamprologines into several genera: Lamprologus, Lepidolamprologus, Neolamprologus, Altolamprologus, Telmatochromis, Julidochromis and Chalinochromis. When Stiassny examined many species from each of these genera she found a peculiar thing: certain, but not all, members of the genera Lamprologus, Lepidolamprologus, Neolamprologus and Altolamprologus contained a peculiar bony element in the jaw, which she called a "labial bone." This presents a problem, because it suggests that the 26 species which share this character form a natural evolutionary group, excluding the other members of those four genera. Stiassny does not offer a new name for this group as yet, other than to call it the "ossified" group, and she stresses that further work is needed; however if she is correct, then the current scheme of using the names Lamprologus, Lepidolamprologus, Neolamprologus and Altolamprologus will have to be abandoned or at least radically adjusted.

This work on lamprologines is just one example of a recent surge in interest in cichlid systematics. Indeed, papers are coming out almost too fast to keep up with! In the short term, we are in a period of instability for the names of cichlids, which is a nuisance to hobbyist and scientist alike. In the long run, however, we are gaining a much deeper understanding of these fascinating fishes.

References

Citazione:

Coleman, Ron. (Agosto 29, 2001). "Revealing Relationships". The Cichlid Room Companion. URL consultato in data Luglio 30, 2014, da: http://www.cichlidae.com/article.php?id=158&lang=it.