Evolution – aiming to an objective?

This is a translation of Bastian Greshakes article “Evolution – auf ein Ziel hin?” in the German blog posts


 Werner Arber is at the Lindau meeting for the tenth time this year and again he has been giving a lecture. His topic for this year’s meeting was “Genetic and Cultural Impacts on the Course of Biological Evolution”and he mingled notorious information with little new one.

I’m full of bacteria. And most of them are friendly. Only some become pathogens.” – Werner Arber, 2010

Because who watched – like me – some of his older lectures in the Lindau media library, can see the main features of this lecture yet in the 2007 lecture «Darwinianevolution as understood by scientists of the 21st century».

What of course doesn’t turn the lecture itself bad at all. It’s worth yet for the hand-drawn foils. And yes, by that I mean those sheets that you put on an overhead projector to throw them on the wall. Completely independent from beamers and laptops (and eventually missing adapters that you need to attach the laptop to the beamer.)

And that not just on principle but because Arber uses the advantages of such a projector: They give him the oppurtunity to hold his lecture relatively free without having to plan every detail precisely. On overhead transparencies you can draw something even during the lecture, in powerpoint slides this possibility is restricted.

Arber first explained in his lecture that genetic variation is the driving force behind evolution. Without variation there can be no selection which he called trend-setter of evolution. I find his choice of words is an infelicity of expression. His term can be understood in a manner that evolution has an target that it aims for. And from there it is yet a tiny step to postulate that there is intelligent design.

But of course Arber didn’t mean it that way, because the direction is always just the “best adaption possible” through which then selection is achieved. He also addressed the issue of mechanisms that ensure genetic variation. Genes are made of the region in which the later gene-product is encoded and the control-regions (which don’t neccessarily need to be in spacial proximity).

A changed sequence in the gene is thereby – at least with a modern view – the mutation. In classical genetics you just call it a mutation if there is an altered phenotype that is passed on to further generations. But luckily we’re not stuck in classical genetics. Such mutations can then either be harmful (which is mostly the case), have positive impacts (the rarest case) or just be neutral, which means they have no effect on the success of reproduction.

Next to mutations also the recombination of genes (entire stages are copied, deleted etc.) and the horizontal gene transfer enrich genetic variation. For the microbiologist Arber the latter case is of course especially interesting: In horizontal gene transfer, genes are submitted from one organism to another. And that sometimes even crossing the borders of different species. So far it was common belief that this is only possible in bacteria because they can even perform this step controlled to some extent. But through retroviruses this is potentially possible in any other organism as well. In this context Arber shaped the dictum that we as organisms might not just have one common ancestory but also a common future through the transfer of genes. Afterwards Arber passed on to the genes he calls “evolution genes”. Those cause like restriction enzymes directly a genetic variation or just modulate the frequency with which such variation is caused. 

“Natural reality takes actively care of biological evolution”

 

And this sentence made me listen attentively because again there is such a comparison of evolution aiming in a certain direction or even having intentions. Or wanting to award nature such a purpose. Yet the thought of a second-order-selection is not as incongruous as it seems at first glance. Restriction enzymes exist and they take care of variation and its preservation. Still I would not say that natural reality actively enforces that. 

I find it more likely that in the course of evolution exactly such enzymes were formed. And even if they did not have an aim in the proper meaning of the word they brought their carriers of the gene an advantage in fitness. Variation causes – also if that’s rarely the case – positive changes. And if you have to adapt to changed life conditions it can be an advantage to have variation. If you want to know more about this topic, Werner Arber gave the Lindaublog a great interview in 2009 “Looking back”  and here is a further lecture and an interview with him.

7 comments on “Evolution – aiming to an objective?

  • Corneel says:

    Hi Jessica, thanks for your post. There were two things I’d like to comment on

    Restriction enzymes exist and they take care of variation and its preservation.

    No they don’t, because they cut proteins, not DNA. Perhaps you meant something different? (transcript splicing, I would guess)

    Re. so called evolution genes. To be honest, I do not believe such things exist. Sure, there are several factors that generate genetic variation at a high frequency(transposons for example). But because the majority of mutations are deleterious, they are in fact a nuissance. I think that the most parsimonious explanation is that we cannot get rid of them, and have to make do with what we’ve got.

    Reply
  • Corneel says:

    Man, I wish there was an editing function. Of course restriction enzymes cut DNA; I am mixing stuff up. Please ignore my first comment.

    Still, these enzymes have a proper function in bacteria: they cut viruses into little pieces. Their primary function is not to generate mutations.

    My second comment still stands.

    Reply
  • Hey Corneel,
    as Jessica was so kind to translate my post from the german lindau-blog i’ll give it a try to answer on your remarks :)

    regarding your first comment: i think restriction enzymes can help to improve the genetical variation. This is what we use them for in the lab every time, cutting things open and insert our own stuff. i’m not sure about how often this happens in nature but i’d guess this happens there too. so they add to variation.

    regarding your second comment: as i’m studying bio-informatics i’m not that sure that the most parsimonious explanation is always the right explanation. Often one should try to find the most likely explanation ;)

    At a first glance i was thinking the same about the «evolution genes» but i think there is a trade-off we should not forget: Although most mutations are harmful there can be situations where it may be positive to have lots of variation around.

    If you are living in a stable environment you’ll have no need for much variation because there may be something like an optimal solution for each problem.

    But if you live in an environment that changes a lot and frequently it may be a bad thing to have no or not much variation in your genes as a change in enviroment may render your gene-set totally useless and then, basically you are fucked ;)

    So at some point it may be a positive thing to have those «evolution genes» that add variation. So i think it is a trade-off between “risk of harmful variation added” and “risk of being wiped out because of lack of variation”.

    maybe one should try to simulate this trade-offs. :)

    Reply
  • Corneel says:

    Hi Bastian,

    Thanks for your reaction.

    You said:
    Often one should try to find the most likely explanation ;)

    I wholeheartedly agree, but I think “evolution genes” aren’t very likely either :-)
    I am familiar with the argument that variation helps population survive, which is exactly why I don’t believe in it: An abundance of variation is good for populations, but as an individual it is no good to have mutational agents wreaking havoc in your genome. That means your argument essentially depends on group selection, and that only works in special circumstances.
    We know that many adaptations exist to minimise the number of mutations (consider e.g. proofreading of DNA polymerase). Wouldn’t you agree that it doesn’t make sense that at the same time genes exist that actually promote mutations?

    Reply
  • Hey Corneel,
    you should be right that those genes should not be too likely.

    And at first glance i also thought that you are right about the group selection. I know that it works only under certain conditions and is itself quite unlikely to happen.

    But maybe one has to step down and not up? Down to the gene-level in a sense of the idea of the selfish gene dawkins proposed?

    A second level selection/evolution-gene itself would have a selective advantage from being spread in times of an instable environment i would think. But this are unsorted thoughts from the edge of the meeting. I’ll think about this :)

    About the idea that it would not make sense to have genes that promote mutations while also having ones that stop or minimize mutations:

    I’m not so sure about this. Having different types of genes with contradicting functions is not that unsual i think.

    This should give an organism more control about processes, especially if one can active and deactive the genes. In a lecture at my university i heard about the theory that there are those switches that could be turned off/on to produce more variation when needed but i can’t remember ad hoc the evidence they found for this.

    So i’d say you are right: it’s not very likely to happen, but i’m still not convinced that there is a reason why this should not happen at all.

    Reply
  • Corneel says:

    Well, I am still open to alternative explanations, may the evidence arise, so I’d say we’re in agreement

    Thank you for your thoughts.

    Reply
  • I have a intriguing question, restrictions enzymes(discovered by Werner Arbor ) are the part of Host controlled restriction modification system which restricts the entry of foreign DNA in to the host cell and also modifies it’s own DNA from the digestion.I’m wondering when there is a Host controlled restriction and modification system in all prokaryotes then how the Horizontally transferred genes escape these enzymes and are rampant

    Reply

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