Muller... his ratchet... and sex... :D

And what may i fill this blog post with?

Lets talk for a while about evolution.... I know, i know... that is not relevant to neurobiology... or at least, not directly relevant... But then most of what i say is not directly relevant to anything... :P

So! Evolution...

Evolution as a term refers to anything changing with time. Evolution in the context of biological systems also refers to change with time... However, in biological systems, it is not evolution if it is not hereditary... Thus any change that can be passed on from one generation to the next, contributes to evolution.... This is all pretty basic stuff but i like recapitulating basics... :)

Anyway, the question is not whether or not evolution occurs nor is its necessity questioned... The question is the mechanism of evolution. HOW the hell does it take place? Now i know you're going, "what's he talking about?? Everyone knows about natural selection!!" And you're right... in essentials. Evolution does seem to depend heavily on natural selection... What is not generally known, however, is that selection cannot possibly be the only mechanism driving evolution. The reasons for this are complicated and full of mathematical jargon and so i shall not delve on them. The point is to talk about the theoretical necessity for studying evolution in greater detail and with more rigour. It is no longer acceptable to say that in a population small changes occur in the genome and these, when passed on, result in speciation and thus, evolution... We must also resolve to prove, without reasonable doubt, that such changes occur or indeed are possible and that they, and they alone, are responsible for the immense diversity of species that we observe today. This is not in the least an obvious problem.

To demonstrate the complexity of this problem, let us take the example of Recombination and the role that it plays in shaping evolution. It is known that, of the millions of species that exist on earth today, only about 400 are asexual and have remained so throughout their evolutionary history. Saying that this difference in numbers of sexual and asexual species is vast is to drastically understate the situation. Obviously, there is some evolutionary benefit to be gained from recombination (which is the direct result of sexual reproduction)... But what can this advantage be?

On the face of it, recombination is an expensive process. The time and energy that go into making it work seem rather frivolous. The question, then, is to see if the benefits of recombination balance the costs or not? Such a cost-to-benefit analysis is necessary if we are ever to understand evolutionary dynamics.

As one of the reasons that recombination justifies the cost that goes into it we look at a mechanism known as "Muller's Ratchet".
What Muller did was to ask us to consider a hypothetical population of haploid, asexually reproducing individuals. Now the genomes of each of these individuals, though superficially similar, is different in its specifics. This difference is the result of random mutations only since no recombination is possible. Now if we consider that the current HIGHEST fitness individual in the population is subjected to some deleterious mutations which render him a mere shadow of his former self. That is, he is no longer as fit as he was and the SECOND fittest individual of the population now becomes the fittest. Notice that the highest fitness level in the population has gone down. Now, since there is no way that genomes from two individuals can recombine to form a higher fitness, the population has in effect forever lost the high fitness that it had and has no means of recovering its lost glory. Thus, by this mechanism, with time, deleterious mutations will continue to accumulate in the population bringing the mean fitness down with every such step. This is known as the 'Muller's Ratchet'.

Thus, as the complexity of the organism increases, recombination becomes a near necessity for the population to survive... It is somewhat akin to the winding mechanism used in old watches. It winds the mechanism again to make it work at it's optimum level...

More on this later.... I hope... :D