Evolutionary "tree" or evolutionary "net"? / "arbore" filogenetic sau "retea" filogenetica?

Since the time of Darwin, the evolutionary relationships between organisms have been represented as a tree, with the common ancestors at the base of the trunk and the most recently evolved species at the tips of the branches. Microbiologists have argued that this representation doesn’t really hold true for microbes, which often exchange genes among different species. Their claim has been that the evolution of these organisms is better represented by a net, rather like that of a tree.
Initially these speculations were based on discoveries made in medical microbiology; namely that genes for resistance to antibiotics were found to move from one bacterial pathogen to another, but recent analysis of DNA sequences suggests that horizontal gene transfer has also occurred within eukaryotes.
Horizontal gene transfer (HGT), also Lateral gene transfer (LGT), is any process in which an organism transfers genetic material to another cell that is not its offspring. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g. its parent or a species from which it evolved. Most thinking in genetics has focused on the more prevalent vertical transfer, but there is a recent awareness that horizontal gene transfer is a significant phenomenon.
Horizontal gene transfer is a potential confounding factor in inferring phylogenetic trees based on the sequence of one gene. The original metaphor of a tree no longer fits the data. For example, given two distantly related bacteria that have exchanged a gene, a phylogenetic tree including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to include as wide a range of genes for phylogenetic analysis as possible.

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Inca de pe timpul lui Darwin, relatiile filogenetice dintre organisme au fost reprezentate sub forma unui arbore, avand la baza trunchiului stramosul comun, iar speciile mai recent aparute spre varful ramurilor. Microbiologii sustin ca acest tip de reprezentare nu este valabila si pentru speciile de microbi, care schimba frecvent gene intre ele. Ei pretind ca procesul evolutiv al acestor organisme este reprezentat mai fidel sub forma unei retele, decat sub forma unui arbore.
Initial, aceste speculatii s-au bazat pe descoperirile facute in domeniul microbiologiei medicale, si anume pe observatia ca genele raspunzatoare de rezistenta la antibiotice se transfera de la un microorganism patogen la altul, de la o specia la alta. Insa cercetari mai recente au scos la iveala faptul ca acest transfer are loc si in cazul organismelor Eucariote, nu numai Procariote.
Transferul orizontal al genelor (TOG), sau transferul lateral al genelor (TLG), este orice proces prin care un organism transfera material genetic unei alte celule care nu este descendent direct al organismului respectiv. Prin opozitie, transferul vertical are loc cand un organism primeste material genetic de la parintele sau direct sau de la o specie din care a evoluat. Majoritatea cercetatilor de pana acum s-au bazat in mod preferential pe transferul vertical, insa se pune din ce in ce mai mult accent pe transferul orizontal.
Transferul orizontal al genelor este un factor problematic in alcatuirea de arbori filogenetici pe baza analizelor genetice. Vechea metafora a arborelui nu mai corespunde datelor. De exemplu, fiind date doua specii de bacterii inrudite de departe, dar care au schimbat intre ele o gena pe cale orizontala, un arbore filogenetic incluzand cele doua specii le va arata ca fiind inrudite de aproape, deoarece au in comun gena respectiva, asta in ciuda faptului ca restul genelor lor sunt diferite. Din acest motiv, este indicat sa se includa in analizele filogenetice cat mai multe gene posibile, nu numai una singura, si a miscora astfel probabilitatea de a gresi.