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Do "fishes" grow in trees?
Our phylogenetic trees help to understand the order in Nature. This order is based on what character states living organisms have (and not on what they do, nor where they live, nor on how we use them). More precisely, this order is based on what living organisms share: trees show this share of characters between species. It exemplifies “who shares what and with who”. A cat and a dog share a bulbous nose, hair, and external ears, characters that a lizard does not have. A mackerel and a seabass share spines on their fins, a character that a trout does not have.
Since 1859, year of publication of Origin of Species by Charles Darwin, we know that what is shared by species which do not breed now has been inherited from a common ancestor. Then, while showing “who shares what and with who”, our trees explicit “who is related to who”. Trees explain the relative degrees of relationships. A mackerel and a seabass are more closely related than they are to a trout, as the first ones share features showing that they are closely related.
Mackerel and seabass belong to the Acanthomorpha by the presence of peculiar spines in front of anal an dorsal fins (salmon does not have such spines and does not belong to the Acanthomorpha). Mackerel, seabass and trout are teleostean fishes as they all possess a peculiar type of caudal skeleton.
(Photograph by B. Chanet).
Trees are then tools to construct classifications, but they can be used for many other purposes:
With trees, it is possible to give an age for the origin of features.
First, a tree provides the relative chronological order for characters. Thanks to trees, we know that the skull appears before the jaws which occurred before four limbs and before hair. Furthermore, with fossils, segments of the tree can be reasonably dated. Even uncertain, an absolute datation is then possible.
The oldest known fossil John Dory (Zeus faber) tells us that their family, the Zeidae, already existed 36 million years ago. If the tree is reconstructed by the comparisons of DNA sequences, time of divergence can be specified more accurately. The tree then is a tool to describe the history of living creatures
These trees tell about the history of the geographical distribution of diversity.
Species we classified are often limited to some areas. Trees can then tell us the history of the multiple fragmentations of these areas on a long time. Relationships of many groups of vertebrates provide information about: i) the dislocation of the Gondwana for instance (Gondwana was a supercontinent formed by South America, Antarctica, Australia, India, Africa and Madagascar during the palaeozoic times), ii) ancient migrations. The geographical distribution of several arctic pleuronectid (flatfishes) species in Atlantic and Pacific Oceans is explained by multiple transoceanic migrations. In the same manner, labrids interrelarionships enligthen the understanding of the distribution of the seas during Holocene glaciations.
A labrid species : Ballan's wrasse, Labrus bergylta.
Trees show us that some features have been secondarily lost.
Acanthomorphs, for instance, lost the spines several times : it occurred in flatfishes, in seahorses and related forms (as Corythoichtys present in the homepage of this site) and puffers (Tetraodontiformes).
A puffer (by V. Maran).
At the opposite, trees can show as well that some things can be easily acquired several times.
Then, during the evolution of Acanthomorpha, a dorsal extending onto the head occured several times : in flatfishes, in oarfishes (Lampridiformes), in remoras (Echeneidae) and some Scorpaeniformes. In the remora, this part of the dorsal fin modified in a sucker used by the animal to be attached to bigger ones (sharks, rays, boats …).
A topknott (Zeugopterus punctatus).
An oarfish (Regalecus glesne).
A suckerfish (Remorina albescens).
Still within the Acanthomorpha, some antifreeze proteins in blood occurred several times in not closely related groups, all living in very cold waters. Within Acanthomorphs, notothenioids (genus Notothenia) (see Lecointre and Ozouf-Costaz (2004 ) for more details), in Polar cods (genus Boreogadus), in sculpins (genus Myoxocephalus) and an arctic flatfish (genus Pseudopleuronectes) possess antifreeze proteins. Moreover, some molecules with the same function are present as well in non acanthomorph species, such arctic smelts (genus Osmerus), the atlantic herring (Clupea harrengus) and even in some insects... As these animals are not closely related, it is possible to say that such proteins appear independently. The tree tells us that some organic dispositives are quite, evolutionary, easy to set up.
Finally, trees can help us to take decisions in for conservation.
The quality of a species for biodiversity does not only depend on its rarity, its esthetical of symbolic value or on the use we are doing with it. A tree can show us that a species or a group of species is the last representative of an ancient lineage and possesses rare features. Regarding the diversity of characters that we have to protect in Nature, the tree is precisely the appropriate intellectual tool because it shows us the historical distribution of features.
Chondrosteans form a group of ray fined fishes related to Teleosteans. Sturgeons and paddlefishes belong to this group.
Sturgeons are well known and highly esteemed (caviar is made of their eggs) and all the Chondrosteans are now endangered by over-fishing and environment modifications (dams, pollution ...). Protecting these animals and their environment, it is not only preserving a ressource, but it is also keeping alive proofs and twigs of the tree of Evolution.
- Phylogenetic Relationships, Evolution of Broodcare Behavior, and Geographic Speciation in the Wrasse Tribe Labrini. Reinhold Hanel, Mark W. Westneat and Christian Sturmbauer, J. Mol. Evol., 2002(55), pages 776-789.