Almost there...

Tuesday, June 10, 2008

Replaying Evolutionary History in the Lab

I'm just going to start posting all of the super-cool science news I find rather than just thinking, "Dang, that's so cool!" because it seems like every time I want to go back and find some article I read, El Goog fails me. Already this week I couldn't find the article on orange cat genetics that I've found three other times in the past. I'm not sure what the magic set of keywords is to make that page come up, but I've looked at a lot of other pages and none of them focus exclusively on orange kitties and their X chromosomes.

Anyway, today's "OMGScienceIsCoolSQUEEE!" article is all about a researcher tracking e.coli in the lab across thousands of generations and having them make a really unexpectedly complicated evolutionary leap.
A major evolutionary innovation has unfurled right in front of researchers' eyes. It's the first time evolution has been caught in the act of making such a rare and complex new trait.

And because the species in question is a bacterium, scientists have been able to replay history to show how this evolutionary novelty grew from the accumulation of unpredictable, chance events.

Twenty years ago, evolutionary biologist Richard Lenski of Michigan State University in East Lansing, US, took a single Escherichia coli bacterium and used its descendants to found 12 laboratory populations.

The 12 have been growing ever since, gradually accumulating mutations and evolving for more than 44,000 generations, while Lenski watches what happens.
Profound change

Mostly, the patterns Lenski saw were similar in each separate population. All 12 evolved larger cells, for example, as well as faster growth rates on the glucose they were fed, and lower peak population densities.

But sometime around the 31,500th generation, something dramatic happened in just one of the populations – the bacteria suddenly acquired the ability to metabolise citrate, a second nutrient in their culture medium that E. coli normally cannot use.

Indeed, the inability to use citrate is one of the traits by which bacteriologists distinguish E. coli from other species. The citrate-using mutants increased in population size and diversity.

"It's the most profound change we have seen during the experiment. This was clearly something quite different for them, and it's outside what was normally considered the bounds of E. coli as a species, which makes it especially interesting," says Lenski.
Rare mutation?

By this time, Lenski calculated, enough bacterial cells had lived and died that all simple mutations must already have occurred several times over.

That meant the "citrate-plus" trait must have been something special – either it was a single mutation of an unusually improbable sort, a rare chromosome inversion, say, or else gaining the ability to use citrate required the accumulation of several mutations in sequence.

To find out which, Lenski turned to his freezer, where he had saved samples of each population every 500 generations. These allowed him to replay history from any starting point he chose, by reviving the bacteria and letting evolution "replay" again.

Would the same population evolve Cit+ again, he wondered, or would any of the 12 be equally likely to hit the jackpot?
Evidence of evolution

The replays showed that even when he looked at trillions of cells, only the original population re-evolved Cit+ – and only when he started the replay from generation 20,000 or greater. Something, he concluded, must have happened around generation 20,000 that laid the groundwork for Cit+ to later evolve.

Lenski and his colleagues are now working to identify just what that earlier change was, and how it made the Cit+ mutation possible more than 10,000 generations later.

In the meantime, the experiment stands as proof that evolution does not always lead to the best possible outcome. Instead, a chance event can sometimes open evolutionary doors for one population that remain forever closed to other populations with different histories.

Lenski's experiment is also yet another poke in the eye for anti-evolutionists, notes Jerry Coyne, an evolutionary biologist at the University of Chicago. "The thing I like most is it says you can get these complex traits evolving by a combination of unlikely events," he says. "That's just what creationists say can't happen."


So, not merely evolution, but replicable evolution. That's frikkin' cool!

4 Comments:

  • That's extremely cool science.
    First, How cool is the experiment itself. Let's take 12 colonies of bacteria and see how they evolve of many years. That by itself is incredibly neat.
    Second, they just slammed a big creationist point. There was always a question of how complex systems, like eyes, or in this case metabolic systems occured, because the intermediate systems aren't much use to the creature, but apparently they do happen and we can see how they happen.

    Were there any black obelisks near the population at any given time ?

    By Blogger Chris, at 3:28 PM  

  • This is the point I was looking for.
    http://www.newscientist.com/channel/life/dn13683-evolution-myths-half-a-wing-is-no-use.html

    This experiment back up the point that like bacterium flagella, complex features can and do evolve.

    By Blogger Chris, at 3:38 PM  

  • a really unexpectedly complicated evolutionary leap.

    Interesting? Yes. Cool? Undoubtedly. Unexpectedly complicated evolutionary leap? Not so much.

    The point of this experiment was to follow natural selection. A couple of mutations (rare or not) in sequence is neither particularly complicated nor has it shown to be any more of an "evolutionary leap" than brown-eyed parents producing blue-eyed offspring.

    A major evolutionary innovation

    As usual, the popular press botches the science. In fact, "the inability to use citrate is one of the traits by which bacteriologists distinguish E. coli from other species". In other words, this species diverged from one that already had the trait. Reactivating a trait that your genes already encode is not exactly the bacterial equivalent of Einsteinian relativity.

    It's the first time evolution has been caught in the act of making such a rare and complex new trait.

    No such "making" has been demonstrated; it is so "rare" that the other species have it? Non sequitur.

    "The thing I like most is it says you can get these complex traits evolving by a combination of unlikely events," he says. "That's just what creationists say can't happen."

    Ideological nonsense.

    just slammed a big creationist point.

    Again, not at all. Creationists do not deny recessive traits (or even micro-evolution) any more than evolutionists deny the possibility of a universal creator.

    By Blogger Michael, at 12:23 AM  

  • This comment has been removed by the author.

    By Blogger Michael, at 12:56 AM  

Post a Comment

<< Home