Simulation of evolution / Simularea evolutiei

Despite the plethora of modern genetic tools, something that is little changed since Darwin's time is our reliance on evolutionary outcomes to unravel the process of evolution. The lack of evolutionary intermediates leaves the door ajar for the proponents of intelligent design. But a new technique could help fill the gaps. It involves the construction of evolutionary intermediates in the lab, and the search for viable paths between them. The resulting 'fitness landscapes' map viable routes between accessible evolutionary paths.

There are some bacteria that have a version of a particular enzyme that makes them 100,000 times more resistant to certain antibiotics (like penicillin). We know that there are five differences that separate this version of the enzyme from the basic version, and we know what those mutations are. In theory, if the mutations happened one at a time, there are 120 possible ways that the enzyme could go from the original form to the resistant form. (For example, mutation 1 could have happened first, mutation 2 second, mutation 3 third, mutation 4 fourth, and mutation 5 fifth, or mutation 2 could have happened first, mutation 1 second, mutation 3 third . . . or mutation three could have happened first . . . and so on until all the possibilities are exhausted. )

Scientists then were able to construct possible intermediate forms of the enzyme - varieties that contained some, but not all 5, of the mutations, and test their resistance to the antibiotic. What they found was that 12 of the 120 possible paths from the original form to the new form increased resistance with every additional mutation. That's pretty cool - it shows that not only could natural selection drive the changes in this enzyme, but also that there are 12 different ways it could have happened.

The current studies show mutation-by-mutation paths, with selective values for each step, in the development of some new traits. (Like it or not, a 100,000-fold improvement in antibiotic resistance is a new trait.) Some of the other studies cited in the paper look at even more things, like the possible effects of taking paths that are longer than the shortest possible paths investigated in the bacterial resisitance study, by looking at things like the effect of a muation that is gained then lost along the way.

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In ciuda multitudinii de instrumente genetice avute la indemana, exista ceva ce nu s-a schimbat de pe vremea lui Darwin, si anume dependenta totala de rezultatele evolutiei, in demersul de a elucida mecanismele evolutiei. Lipsa stadiilor intermediare lasa usa deschisa adeptilor Intelligent Design. Insa o noua tehnica ar putea suplini lipsa. Aceasta implica construirea de stadii intermediare in laborator, precum si cautarea de legaturi viabile intre aceastea, permitand trasarea de traiectorii evolutive viabile.

Exista anumite bacterii care poseda o versiune a unei enzime care le confera o rezistanta de 100.000 de ori mai mare fata de anumite antibiotice (ca penicilina). Se stie ca exista 5 diferente care separa aceste varietati ale enzimei de enzima clasica, si se stie care sunt cele 5 mutatii care produc aceste modificari. Teoretic, daca mutatiile nu s-au produs simultan, atunci exista 120 de cai posibile prin care enzima originala se poate transforma in varianta rezistenta. (De exemplu, mutatia I s-a produs prima, apoi mutatia II a 2-a, apoi a III-a, apoi a IV-a si apoi a V-a; sau mutatia II putea fi prima, mutatia I a 2-a, apoi a III-a, si asa mai departe pana cand toate posibilitatile se vor fi epuizat.)

Astfel, oamenii de stiinta au reusit sa construiasca forme intermediare ale enzimei, forme care contineau numai cateva din cele 5 mutatii, iar apoi sa le verifice rezistenta fata de antibiotic. Au descoperit ca 12 din cele 120 de traiectorii evolutive posibile, de la varianta originala la cea rezistenta a enzimei, confereau rezistenta din ce in ce mai crescuta fata de antibiotic, odata cu fiecare mutatie succesiva. Acest lucru nu numai ca demonstreaza implicarea selectiei naturale in transformarea enzimei, ci si faptul ca sunt 12 traiectorii diferite prin care acest lucru se putea produce.

Studiile arata pas cu pas, mutatie cu mutatie traiectoriile urmate, mentionand valoarea selectiva a fiecarui pas. Alte studii privesc posibilele efecte ale luarii in calcul a unor traiectorii evolutive mai lungi decat cele mai scurte cai posibile, asa cum au fost ele cercetate in studiul initial. Acest lucru s-a facut luaind in considerare, printre altele, efectul datorat dobandirii unei mutatii, si pierderii ei ulterioare.