Monday, April 14, 2008

Direct evidence to Dawkin's selfish gene

The biophysics department here at Hopkins had invited Dr. Alexander Van Oudenaarden from MIT physics to give a student invited seminar. I was a bit familiar with his work through my junior thesis. He is trying to understand noise and random behavior of cellular signaling pathways through experiments and computer simulations.

The talk had a lot of good things, but what caught my attention the most was a topic I could directly relate to the heading of the post. In our cells (except mine!) , or in any other eucaryotic cell there is a signaling pathway called the GAL switch. When the cell is put in an environment which lacks any other nutrients except galactose (which is a kind of sugar present in milk) the GAL switch operates and produces carriers which help the transport of galactose from outside of the cell in the cytoplasm (inside of the cell). I genetically lack that system and that's why I cannot consume milk which is a major source of galactose.

Suppose we start with a bunch of cells which at time t = 0 do not have the machinary to transport galactose in. We then supply galactose to this culture. What we expect to find and do find is that the internal GAL switch starts functioning and builds up the transport machines which take the galactose in. This makes a lot of evolutionary sense, that to a given change in environment, the cells are able to cope up accordingly. (case A)

Now suppose we start with a bunch of cells which at t = 0 already have the machinery present to transport galactose in. And suppose now we put this culture in a solution where there is a lot of galactose (case B) or there is no galactose (case C), what we would expect is that the cells would continue living with their existing machines in case B and that they would shut off the factory of transporting galactose in case C (since there is none in the environment).

Counter intuitively what we find experimentally is that in case A, there will be some cells which do not produce the machinery, in case B there will be some cells which shut off their machines and in case C, there will be some cells which will keep on maintaining the machines though they are rendered useless. This phenomena is no accident and is a persistent feature of the culture. It is counter intuitive because the cells which are going against the popular vote are eventually going to die because of their apparently stupid decision. If the organism were to evolve such that the selfish unit of evolution were the individual itself, this kind of suicide does not make much sense.

Cleverly when the experiments were performed in a medium/solution where the concentration of external galactose was changed in time, the lab found that this suicidal behavior makes a lot of sense for the gene! During evolution of the yeast, the species was exposed to a time varying environment the nature of which we surely don't know. The gene's way of predicting the future environment or differently put, the gene's way of coping up with random changes in the future environment was to produce individuals of all kinds some of which are unsuitable for the current environment (but might be suitable for the future). By maintaining a population which has some members which are suitable for the present and some members which are not, the gene ensures that for any reasonable change outside of the cells the species as a whole is not wiped out. In other words (I suppose) the gene makes sure that it does not follow a path to extinction by maintaining a pool of suicidal members (who potentially may cope better with random change in the environment in the future).

I thought this is a direct evidence to the selfish gene rather than a selfish individual or a selfish species. And that made me happy :-) This is a direct evidence because neither selfish individual nor selfish species hypothesis can explain this kind of a behavior.

And to conclude, it is important to give some definitions as I understand them. By selfish X (X = gene, individual, species) we understand that X is the unit which will try to survive in case of competition by perhaps eliminating other members of X. For example in case of food shortage, a selfish individual theory would imply that individuals will kill each other to survive so that they can acquire sufficient amount of the limited resource for themselves, a selfish species on the other hand would perhaps imply that humans would kill off other animal and plant species so that they don't have any competition for food resources. The most counter intuitive selfish gene would imply that individuals would take actions in such a way that individual genes are retained in further generations.

5 comments:

Anonymous said...

it's not counter intuitive (atleast to me). it's just not intuitive :P

Roger Waters said...

Apparent suicidal behavior is something I wouldn't expect form selfish machines such as Yeast cells :-)

Hence counter intuitive. But if you disagree it's okay. At least it's not readily explainable from the selfish individual model and that's good enough :-)

Karthik Shekhar said...
This comment has been removed by the author.
Karthik Shekhar said...

Interesting post, I was in half a mind to type those questions (rather doubts) that occurred to me but then you called up and we spoke on the phone.

I guess one needn't (atleast you and me) worry about arguing against the species centric view of evolution. However, there are definitely some issues/themes towards which more evidence needs to be established to foster the gene centric view of natural selection. Everything seems to point towards the selfish gene theory, I agree. But let us extend our skepticism a little more to enhance the pleasure :-). It will definitely make the ale much sweeter. I shall try to enumerate those sometime later in a coherent manner.

Roger Waters said...

I will definitely agree. I guess this is where I am reminded of Sudeep's opinion that 'we are never taught scientific method as a methodology'. It actually is immensely pleasing to find holes in one's argument and then trying to fix them, and then finding further holes :-)

I mailed the professor and asked him 'if his experiments are evidence of the selfish gene hypothesis'. I hope he responds.

But thanks for the discussion, it is always helpful to see what doubts other people come up with in something I imagined as self explanatory :P