Usuário(a):Ayanamartins/Filogenética molecular

Este artigo foi inicialmente traduzido, total ou parcialmente, do artigo da Wikipédia em inglês cujo título é «Molecular phylogenetics».

Filogenética molecular, também conhecida como sistemática molecular, is the use of the structure of molecules to gain information on an organism's evolutionary relationships. O resultado de uma análise filogenética é expresso na forma de árvore filogenética.

Técnicas e aplicações[editar | editar código-fonte]

Todo organismo vivo contém DNA, RNA e proteínas. Organismos aparentados normalmente apresentam mais semelhança na estrutura molecular desssas substâncias. Quanto menos aparentados os organismos maior a dessemelhança entre as suas moléculas. A filogenia molecular utiliza esses dados para construir um "árvore de parentesco" que mostra uma possível história evolutiva de vários organismos. Entretanto, apenas nas últimas décadas, tem sido possível isolar e identificar a estruturas dessas moléculas.

A abordagem mais comum é a comparação de sequências gênicas usando técnicas de alinhamento de sequência. Outra aplicação de filogenética molecular é em DNA barcoding, no qual a espécie de um indivíduo é identificada a partir de pequenos trechos do seu DNA mitocondrial. Esta técnica também pode ser utilizada em testes de paternidade ou para obtenção de evidências (perfil genético) em ciências forenses.

The effect on traditional biological classification schemes in the biological sciences has been dramatic as well. Work that was once immensely labor- and materials-intensive can now be done quickly and easily, leading to yet another source of information becoming available for systematic and taxonomic appraisal. This particular kind of data has become so popular that taxonomical schemes based solely on molecular data may be encountered.

Panorama teórico[editar | editar código-fonte]

Early attempts at molecular systematics were also termed as chemotaxonomy and made use of proteins, enzymes, carbohydrates and other molecules which were separated and characterized using techniques such as chromatography. These have been largely replaced in recent times by DNA sequencing which produces the exact sequences of nucleotides or bases in either DNA or RNA segments extracted using different techniques. These are generally considered superior for evolutionary studies since the actions of evolution are ultimately reflected in the genetic sequences. At present it is still a long and expensive process to sequence the entire DNA of an organism (its genome), and this has been done for only a few species. However it is quite feasible to determine the sequence of a defined area of a particular chromosome. Typical molecular systematic analyses require the sequencing of around 1000 base pairs. At any location within such a sequence, the bases found in a given position may vary between organisms. The particular sequence found in a given organism is referred to as its haplotype. In principle, since there are four base types, with 1000 base pairs, we could have 4¹⁰⁰⁰ distinct haplotypes. However, for organisms within a particular species or in a group of related species, it has been found empirically that only a minority of sites show any variation at all and most of the variations that are found are correlated, so that the number of distinct haplotypes that are found is relatively small.

In a molecular systematic analysis, the haplotypes are determined for a defined area of genetic material; ideally a substantial sample of individuals of the target species or other taxon are used however many current studies are based on single individuals. Haplotypes of individuals of closely related, but supposedly different, taxa are also determined. Finally, haplotypes from a smaller number of individuals from a definitely different taxon are determined: these are referred to as an out group. The base sequences for the haplotypes are then compared. In the simplest case, the difference between two haplotypes is assessed by counting the number of locations where they have different bases: this is referred to as the number of substitutions (other kinds of differences between haplotypes can also occur, for example the insertion of a section of nucleic acid in one haplotype that is not present in another). Usually the difference between organisms is re-expressed as a percentage divergence, by dividing the number of substitutions by the number of base pairs analysed: the hope is that this measure will be independent of the location and length of the section of DNA that is sequenced.

An older and superseded approach was to determine the divergences between the genotypes of individuals by DNA-DNA hybridisation. The advantage claimed for using hybridisation rather than gene sequencing was that it was based on the entire genotype, rather than on particular sections of DNA. Modern sequence comparison techniques overcome this objection by the use of multiple sequences.

Once the divergences between all pairs of samples have been determined, the resulting triangular matrix of differences is submitted to some form of statistical cluster analysis, and the resulting dendrogram is examined in order to see whether the samples cluster in the way that would be expected from current ideas about the taxonomy of the group, or not. Any group of haplotypes that are all more similar to one another than any of them is to any other haplotype may be said to constitute a clade. Statistical techniques such as bootstrapping and jackknifing help in providing reliability estimates for the positions of haplotypes within the evolutionary trees.

Características e pressupostos da filogenética molecular[editar | editar código-fonte]

A sistemática molecular consiste essencialmente em uma abordagem cladística: presume que a classificação corresponde à história filogenética, e que todos os taxa válidos devem ser monofiléticos.
A sistemática molecular frequentemente usa o pressuposto do relógio molecular que considera que a similariade quantitiva no genótipo é suficiente para estimar quão recente é a divergêncai genética. Particularmente, em relação a especiação, este pressuposto pode estar errado se:
1. alguma pequena modificação genotípica tiver atuado impedindo o cruzamento entre dois grupos de organismos, ou
2. nos diferentes subgrupos dos organimos considerados, a modificação genética ocorrer em taxas diferentes.

These characteristics and assumptions are not wholly uncontroversial among biological systematists. As a cladistic method, molecular systematics is open to the same criticisms as cladistics in general. It can also be argued that it is a mistake to replace a classification based on visible and ecologically relevant characteristics by one based on genetic details that may not even be expressed in the phenotype. However the molecular approach to systematics, and its underlying assumptions, are gaining increasing acceptance. As gene sequencing becomes easier and cheaper, molecular systematics is being applied to more and more groups, and in some cases is leading to radical revisions of accepted taxonomies.

História da filogenética molecular[editar | editar código-fonte]

Os cientistas pioneiros em sistemática molecular foram: Charles G. Sibley (aves), Herbert C. Dessauer (herpetologia) e Morris Goodman (primatas), seguidos por Allan C. Wilson, Robert K. Selander e John C. Avise (que estudaram vários grupos). Trabalhos utilizando eletroforese de proteínas começaram por volta de 1956. Although the results were not quantitative and did not initially improve on morphological classification, they provided tantalizing hints that long-held notions of the classifications of birds, for example, needed substantial revision. In the period of 1974–1986, DNA-DNA hybridization was the dominant technique.^[1]

References[editar | editar código-fonte]

↑ Ahlquist, Jon E., 1999: Charles G. Sibley: A commentary on 30 years of collaboration. The Auk, vol. 116, no. 3 (July 1999). A PDF or DjVu version of this article can be downloaded from the issue's table of contents page.