Friday, 18 March 2011

Maybe you're not runner up - but the first to win the race.

I've spent the last few days wondering what I would write about in here, deciding to just “write” is harder than I thought it would be. However I was chatting away with a friend of mine on MSN this evening while I was studying for a medical microbiology exam on Monday when she said to me; “but Darwin was years ago! Don't you know everything about evolution already?” Sure we've learned a lot but how much is left to learn?

Just because its been over a hundred and fifty years since Darwin published his iconic theory of Natural Selection doesn't mean the controversy has stopped. His theories shock the world and have become the underlying principal of all studies of life, uniting the fields of biology from genetics all the way to ecology and everything in between. Essentially the theory boils down to the fact that we are all modified descendants of our ancestors and that therefore all species on the planet are related through a complex tree of life. A beautifully simple and yet elegant idea that provides the ultimate explanation for the mind blowing diverse beauty that is life on our little blue marble.

This diversification of life is driven by a process called natural selection which works on the basis that there are only so many resources on our planet and all living organisms are fighting each other for them. Those which can claim these resources survive and those which can't do not. Every so often a random change in an organism happens by mutation of their DNA, therefore changing a gene slightly. If this change makes them better at claiming one of these resources then they are more likely to survive, therefore more likely live to pass it on to their offspring. Who would in turn pass it on again and so forth. Eventually this change becomes the norm.

Of course these types of changes happen very slowly, we're talking hundreds of thousands of years or at least for most types of organisms. There is one group where we can watch evolution occurring in a test tube and of course these would be our good friends bacteria! Compared to many organisms on the planet bacteria reproduce stunningly fast anywhere between minutes and hours. Escherichia coli for example is a common bacteria and is present in everyone and without it you would have one very sick tummy! Under optimal conditions the “doubling time” which believe it or not is the time taken for the population to double is 20 minutes. Pretty fast yeah?

There are hundreds of different strains of bacteria just like there are different strains - or breeds as they are more commonly called - of cats and dogs. A recent study using this bacteria has shown that just because you are one of the first to have a mutation that makes you better at something and doesn't mean you will win the evolutionary race. The experiment took two strains of E.Coli and put them together in a petri dish and let them get on with it. As in the real world in this dish the bacteria would have all been competing for resources which take the form namely of glucose, a sugar from which they derive their energy.

The group of highly respected evolutionary researchers headed by Mr Richard Lenski and Jeffrey Barrick grew E.Coli in a lab for 50,000 generations! Around the 31,500 generation mark they discovered that some of the bacteria had developed the ability to derive energy from citrate as well as glucose. The team came up with an elegant and sophisticated set of experiments to see which were better at surviving. They took four different pairs of E.Coli strains and decided to see who would win the battle for resources.

Certain strains grew better than others and became the dominant strains present, this was due to a mutation in a gene called topA which is a transcription factor meaning it controls how other genes are expressed. They also developed mutations in a related gene called rbs which increased the amount of DNA and RNA being synthesised. Of course one would think that these guys would win right? Quickly they developed two mutations that dramatically increased their ability to live...and yet that wasn't what happened.

While these guys had the “big gun” mutations the other strains happily growing along side them gained mutations as well. While these weren't as dramatic but were also important. Eventually it was the quiet underdogs that won out. Believe it or not the strains which had developed the topA and rbs mutations literally had a little rest! While we're not sure why it appears that these genes decided it was time for a nap and while they were resting the slow and steady crowd took over and out numbered their rivals. The long list of experiments done by the team showed that this ability to survive better isn't just down to one mutation, but all of the small added up together.

But truth be told, we don't really know that much about how evolution works on this kind of scale, its a dynamic fluid process and predicting it is a difficult things to do. What this kind of study shows is that the whole genome has to be taken in to account when looking at evolution and not just the one or two genes that are responsible for a certain feature changing. The change over time replies on the fact that one mutation is followed by others and its these later mutations which could play a critical role in making these changes successful!

Of course this makes sense when you have a little thing about it! Take a look at a photo of yourself next to your relatives and you will notice certain features run in your family. You might be tall like your dad, or have your mothers hair, or your grandfathers nose. These characteristics are passed down...but the ability to track them down to one gene has eluded scientists. Height is thought to be determined by at least seventeen genes and chances are many more. Like the example here it could have been one mutation then the many that followed that gave us the variability in human hight.

In the hundred and fifty odd years since Darwin first proposed evolution as an explanation for the diversity of life, we have learned a hell of a lot but we are still attempting to understand the basic molecular mechanisms by which it happens. We have an unthinkable amount still to learn about the nitty gritty way that evolution works, but you know what I think- that it is the wonderful thing about it all!

Robert J. Woods, Jeffrey E. Barrick, Tim F. Cooper, Utpala Shrestha, Mark R. Kauth and Richard E. Lenski. 2011. Second-Order Selection for Evolvability in a Large Escherichia coli Population. Science. 18th March 2011. 331: (6203) 1433 - 1436.

1 comment:

  1. Thank you for the good writeup. It in fact was a amusement account it. Look advanced to far added agreeable from you! By the way, how could we communicate?