Evolution in the lab: the Lenski experiments
An elegant experiment in evolution has been carried out by Richard Lenski and colleagues at Michigan State University and elsewhere. For more details on this I strongly recommend that you go to his home page:
http://www.msu.edu/∼lenski/, and also to an article describing his experiments and others:
http://www.msu.edu/∼lenski/sciencearticle.html The basic idea is simple. Bacteria reproduce rapidly, so they are ideal for evolutionary experiments. They can also be cloned easily, so in 1989 Lenski set up 12 flasks that contained cloned members of an original E. coli bacterium (this is a bacterium we have in our intestines). Into each flask he put a small amount of glucose (a type of sugar). Since then, daily, he
has let the bacteria run through the sugar, then the next morning he would transfer some of the survivors in each flask to a new flask. By 2007 he had run through more than 40,000 generations of bacteria (i.e., several per day). This is the equivalent of nearly a million years
of human evolution!
The point, of course, is to determine how mutation and other variational mechanisms would change the properties of the bacteria. The answer: a lot! He has frozen samples every now and then, so that direct comparison is possible. The current bacteria average twice the size of their ancestors, and are 70% more efficient at metabolizing the glucose. Most of the improvement occurred early on, but the changes are not identical in each of the flasks. In fact, now that gene sequencing is fast and efficient, the scientists can follow the precise changes that occur.
Since the original experiments, others have tried different environments. Julian Adams at the University of Michigan also started with genetically identical E. coli, and grew them in a device called a chemostat that made sure that all the lines had identical conditions. However, commonly, he found that in a given flask more than one strain of E. coli would develop. He found that initially the E. coli evolved to metabolize glucose more efficiently. After a while, however, the amount of glucose was too much to metabolize, so the E. coli added another pathway that generated acetate as a waste product. Enough acetate was produced that a mutant emerged that used the acetate as fuel. Pretty impressive adaptation!
Even more recently, Lenski and collaborators reported that a variant had evolved around the 31,000th generation that was able to use citrate. This is not possible in wild-type E. coli, but investigations by the team showed that there had been at least three separate mutations
required; the first two were non-adaptive (i.e., neutral drift), but the last one combined with the first two to allow a major biochemical change. Such “potentiating” mutations can be seen in the digital life simulation Avida (available at
http://avida.devosoft.org/), in which
one sees many cases where neutral or even somewhat harmful mutations allow subsequent development of new capabilities.
Πηγή:
https://www.astro.umd.edu/~miller/teach ... ture08.pdf
Τα έχουμε περάσει και άλλοι αυτά, κάποιοι μάλιστα με το ίδιο ακριβώς κείμενο. Μην ανησυχείς, μόλις περάσουν μερικές σελίδες ή εμφανιστεί κανένας καινούριος να του ανοίξει κουβέντα θα κάνει ρησέτ και θα σπαμάρει πάλι τα ίδια.
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