10.26.2009

My Anniversary Post: Evolution of Functions Requiring Multiple Mutations

I realized that I had missed the one year anniversary of this little indulgence. Unable to think of anything particularly profound to say on my own accord, I decided to share, with an attempt at illustration, something which I find truly amazing. The evolutionary means by which a new function can come about as a result of a sequence of mutations. With some of you (that small set of actual visitors to Plinydom) far better versed in these subjects than I, it is with some reluctance that I posit this little exploration but please be kind and if errors are found, I will be only too happy to correct them. I'm a visual guy. I need imagery to solidify concepts in my mind and I often engage in thought experiments intended to visually represent complex (sometimes abstract) processes. It's my way of forestalling Alzheimer's - that and the NYT crossword. So here goes...
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An argument against evolution is made that complex function changes cannot result from a process that requires multiple sequential mutations, by means of natural selection. This is part of Micheal Behe's arguments from irreducible complexity.

It's actually a question that is worth answering and the great work of a nearby academician, Dr Joe Thornton, from Oregon State seems to provide an elegant answer to this question. I find it elegant at least. The answer would appear to come from a better understanding of the various mechanisms available to evolution beyond natural selection. This is not to say that other mechanisms are not important but just to illustrate one pathway.

I know some will cringe at this falsehood but if we simplify things a great bit we can think of evolution as having 2 random and 2 nonrandom mechanisms that can effect changes for the purpose of this very high level conversation. On the random side we have genetic mutation, and genetic drift. On the nonrandom side we have natural and sexual selection. It's the combination of these forces that leads to the complexity we see from evolution. For this discussion the potential impact and contribution of sexual selection to evolution is ignored.


Imagine for a moment that this first simple drawing represents, at some time, 0, an organism's chromosome and one particular gene of interest (the blue one).

We know that a variety of factors can result in random mutations anywhere along the length of this chromosome and on occasion one may affect the sequence of base pairs within the Blue gene.

If we sampled this organism's population at a later time, 1, we might find evidence of two alleles to this gene present in the population. How can that be? Shouldn't natural selection end up favoring one over the other? Maybe but not necessarily. Depending upon the mutation and its location within the gene, the cascade of processes that the gene translation products are engaged in, etc., the effect on the proteins resulting from expression of the gene may be trivial or profound. If individuals with either allele function reasonably well, then the presence of either may have no impact on reproductive success of the organisms carrying either gene and all we would see, is an increase in genetic diversity within the population (both blue and green alleles where before only blue was present).

Life is not static so additional random mutations could pop up at any point in the same gene. Once again, any new allele may or may not confer an advantage or burden on the individual organism. If not, the mutation is essentially a neutral genetic change. Genetic diversity is increased within the population by the random process of mutation. In this case , as drawn in the next diagram, the population at some time, 2, may include three alleles - blue, green and now red.

Random mutations may increase the genetic diversity of a population but evolution's second random force may reduce it. (I say may, because deleterious mutations will generally be selected against.)

From time to time, completely random events, such as the one depicted in the next diagram may change the reproductive success of members of the population with total disregard for any genetic advantage or burden caused by a set of alleles. In other words, changes in the genetic make up of a population can result from something other than highly nonrandom natural selection. Stuff happens also would seem to apply to genetics as well as all other aspects of life.

In this example the random addition of a rock crushing the individuals with the blue and green alleles does in fact change the genetic makeup of the population. This despite any disadvantage of the green or blue allele. One doubts that the red allele would have improved the organism's crush resistance so its persistence is merely a case of luck. Now Red alleles predominate to the next generation. Random events which reduce the genetic diversity of a population are responsible for the second random effector of evolution: genetic drift. There is nothing adaptive about this change it's just at the level of 'it sucks to be you' for the blue and green alleles.

It also important to remember that up till now, there is no reason why another random mutation can't result in a return of the green or blue alleles given enough time. Evolutionary critics often cite this fact to suggest that genetic drift is not an important component of evolution since there is no net gain. That might be true if genetic drift occurred in isolation from the other forces of evolution. That of course isn't reality and in nature genetic drift is mated with the extremely nonrandom hand of natural selection.

Although genetic drift may not be able to effect the big adaptive changes we see in species, it appears to provide critical opportunities for those multi-stage changes needed to explain some complex adaptions. Genetic drift may in essence randomly tee up opportunities for complex changes by providing enough novel neutral alleles (i.e., mutations conferring no particular reproductive advantage or disadvantage) having some intermediate structure that itself, is neutral to the survival of the current generation, but very useful when combined with a later random mutation. In this example the peptide resulting from expression of the red allele is similar enough to that from the blue allele so as not to hinder the reproductive success of individuals with the red allele (neutral).

If it did, the red allele would not stay around for very long since natural selection suffers foolish alleles poorly.

But it is possible that this neutral red allele may provide a genetic beachhead for a later random mutation as is described in the drawing of a population at time, 3. Here the Red* allele represents a situation where an additional mutation has occurred somewhere in the Red allele. The eventual result is represented by the little spheroid addition to the peptide - caused by the new mutation or as a result of the combination of the new mutation and previous neutral ones.

This combination may provide a novel function previously absent, or improve upon an existing one. The same mutation might occur in an individual with the blue or green allele (resulting in a Blue* or Green* gene) but the resultant configuration might be detrimental and therefore be selected out. The Red* peptide need not be perfect in its new role just provide enough selection advantage to affected individuals for natural selection to work its magic. At this point natural selection can drive improvements in this new multi-stage mutation complex leading to further genetic wonders. Of course this refinement process is not in any way actively goal directed. Later refinements come about from the force of natural selection on subsequent random mutations within this complex that lead to improved survival.

And thus, a series of random events coupled with natural selection can give rise to the evolution of complex functions requiring multiple steps. I know I'm a geek but this is great stuff. I hope you enjoy it as well.

4 comments:

Stacy S. said...

I'm sure Jared will love it! And ... CONGRATS on your anniversary! :-)

Jared said...

Actually, it was quite accurate while maintaining simplicity. I really do like the figures; I fear I should spend more time in photoshop and trueSpace... Perhaps I'll do some more animations...

GearHedEd said...

Once, about 20 years ago when presenting an oral report to a bunch of community college students on the "Theory of Evolution" in Florida, I used myself as an example of genetic mutation.

One of the more religious students in the crowd raised her hand as soon as the Q&A session started, and challenged the concept of genetic drift and mutation. I replied that the primary method taxonomists used to classify ancient and modern species into related families was through dentition. Then I asked her how many teeth humans have. She came right back with "32!".

Except that I only have 28 teeth. Never had wisdom teeth, never will.

Then, not too long ago, I asked my dentist if she knew what proportion of the population has this expression of genetics, and she didn't know.

The mutation I have falls into the category "neutral to survival and sexual selection", and seems to be otherwise of no account, but I know that my mother and my daughter both have the same mutation, butmy son doesn't.

oneblood said...

"Actually, it was quite accurate while maintaining simplicity."

So that's why most of it made sense. Thanks Pliny. Learning doesn't take place in a vacuum.