The phylogeny is the study of the formation and the evolution of the living organisms in order to establish their relationship. The phylogenesis is the term more used to describe the Généalogie of a Espèce, of a group of species but also, on an intraspecific level, the genealogy between Population S or individuals.

One usually represents a phylogeny by a Phylogenetic tree. The proximity of the branches of this tree represents the degree of relationship between the Taxon S, the nodes the common ancestors of the taxons. In a tree worked out by Phénétique, the length of the branches represents the genetic distance enters tax; in a tree worked out by Cladistique (cladogramme), one places on the branches the evolutionary events (derived characters) having taken place in each line.

Presentation

The Systematic , the study of biological diversity for its phylogenetic Classification, concentrates, in the light of the recent discoveries, on a phylogenetic Classification replacing the traditional Classification now. Traditional classification establishes groups or tax according to a simple criterion with total resemblance. A phylogenetic classification supposes that one gathers the living beings according to their family ties. Any systematic group (or “taxon”) thus contains close living beings between them genetically (what is not always correlated with a phenotypical resemblance total). The family ties between two members of a taxon are increasingly closer than the family ties between an unspecified member of the group and an living being external with the group (it happens that this external member is however very resembling because of the phenomenon of evolutionary Convergence). To reconstitute the family ties between living beings, phylogeny proceeds according to two techniques: the phenetic and the cladistic .

The phénétique one

The phénétique one rests on the basic postulate that the degree of resemblance is correlated with the degree of relationship. It thus supposes to quantify the resemblance between the living beings to be classified.

This method appears not very relevant when one applies it to the morphological characters because of the Analogie S: certain resemblances between living beings or let us tax cannot indeed be allotted to a common ascent. One speaks then about analogy. The principle used to explain this phenomenon is evolutionary convergence: two let us tax different living in ecological Niches similar or on which the natural selection had a similar impact could be similar. The wings of the birds and the bats are similar characters as wings, because these two wings are not inherited a winged common ancestor. Moreover it is very difficult to quantify morphological resemblances numerically.

On the other hand, the phénétique one becomes relevant since one compares a very great number (with the statistical direction) of characters because the number of similar natures becomes negligible among all the characters whose resemblance is indeed due to the relationship. Thus this technique is very powerful when one applies it at the molecular level. The systematicians have thus more and more recourse to molecular methods to compare tax them and to rebuild phylogenies. With this intention, they compare various molecular components from alive like DNA, ARN or the Protéine S. Indeed, DNA, ARN and proteins are molecules Polymère S. Each residue of the molecule (Nucléotide for the DNA and the ARN or Amino-acid for protein) can be regarded as a character. It is thus possible to compare the sequences at several living beings and to quantify their resemblance by a simple percentage that one compares to the genetic distance between the two let us tax to which the two living beings belong. The results are represented in a Phylogenetic tree, which one could name phénogramme, where the length of the branches depends on the genetic distance and thus represents the degree of relationship between tax studied.

The cladistique one

The cladistique one initiated by Hennig treats on a hierarchical basis the compared characters. In fact are gathered in same a taxon only the living beings which share characters homologous S: when a resemblance between two tax can be allotted to a common ascent, one speaks about homology. The forelimbs of all the Tétrapode S, which they are arm or wings, are homologous.

Thus the wing of the bat and the bird are homologous as forelimbs, and not as wings. The common ancestor of the bird and the bald person mouse had indeed already four legs but its forelimbs were not wings. This common ancestor is indeed also that of the Lézard S, of the Crocodilien S. the forelimb “wing” appeared later independently in the two lines Chiroptère S and Oiseau X…

The homologies in fact are seen like shared evolutionary innovations (Synapomorphie S): if the same homologous character is shared by two let us tax it is that both tax inherited it their common ancestor. This homologous character thus appeared in the line leading to this common ancestor. Any living being having this homologous character thus goes down from this common ancestor. Any living being not having this homologous character does not go down from this common ancestor and is thus distant genetically.

The cladistique one thus rests on the identification (often difficult) of the homology of the characters. It is relevant at the morphological level (and is thus the only means of classifying the fossil species whose DNA is seldom preserved) as at the molecular level. The results are represented in a phylogenetic tree or cladogramme in which each node represents a common ancestor and where the synapomorphies are represented on the branches of which the length is arbitrary. Two let us tax are all the more related as they divide a common ancestor near in the tree. Here also, therefore, let us tax them find themselves gathered according to their family ties.

Joint use of phenetic and the cladistique one

For a long time sometimes violent discussions opposed holding of one or other technique. Today the phénétique one and the cladistique one is often used jointly as being two independent methods. When their results are convergent, very solid phylogenies are obtained.

The joint use of these two methods revealed the existence in the traditional Classification of many groups nonfounded on the family ties and which are thus regarded as nonlegitimate and do not owe any more beings used in Taxonomie. The use of phenetic molecular and cladistic as well as confrontation of the trees obtained was largely allowed by the modern methods which are amplification by PCR and the Séquençage, allied with powerful computational tools which make it possible to automate these methods.

Example of changes in the phylogenetic tree due to the use of these techniques:

• the group of the reptiles. Were gathered within this one the Crocodilien S (makes some genetically close to the Oiseau X) and the Lézard S, Serpent S and tortoise S (distant genetically from the birds).

Example of the use of the Gene 16s for the studies of phylogeny of the Procaryotic .

See too

• Reigns of alive the
• Phylogéographie
• the Phylogenetic tree of alive the
• molecular Phylogenesis
• Phylogeny

External links

• To manufacture a phylogenetic tree

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