Sunday, March 18, 2007

Commentary on Richard Dawkins Part 4 - The bioenergetics perspective

A Professor in animal physiology once told me, if a particular gene is not needed in an environment, it's better off not being expressed, or should I say "dead" in the metaphorical sense. The gene will become a fossilized remnant in our human genome or better still, become deleted.

This Professor of mine is more of a physiologist rather than an evolutionary biologist. However, I find his academic opinion interesting and refreshing nonetheless. He has a penchant for viewing the topic from the standpoint of bioenergetics. The copying of genes via the process of DNA synthesis and even the transcription of genes is energetically expensive.

Thus, if an organism were to migrate to a location where the gene(s) is/are not required for survival, the gene(s) is/are better off deleted, so that the energy resources saved can be directed for other more important purposes. Thus, the fate of the selfish gene to propagate lies in the question of its necessity, and whether it will be energetically favorable to persist with production of copies and transcription. If it's no longer needed, deleting it will save precious resources for the cell.

A rhetoric call for rebellion against the gene was made, yet the silent rebels have been waging this rebellion since the beginning of evolutionary time.


Ponder Stibbons said...

Not really. A lot of our DNA is junk DNA. DNA that gets passed on despite having no obvious use.

Genes that are 'not needed for the cell' are selected against only insofar as the welfare of the cell coincides with the propagation of the gene. This means that in cases where the welfare of the cell does not coincide with that of the gene, then even 'bad' genes will not be selected against. For example, if there is a gene that causes the death of cells only after an organism has become reproductively inactive through old age (I believe there are a few hereditary diseases in humans that are like this), then the gene will not be selected against. This is only one of many ways in which 'useless' genes can continue to be propragated. Consider also the genes that are responsible for salmons making a strenuous migration upriver, reproducing frenetically there, and then dying. This is in no way 'beneficial' to the salmons or to their cells --- they die an early death from exhaustion. However, evidently the gene complex responsible for such behaviour finds this an immensely successful method through which it propagates itself --- otherwise that kind of behaviour could not have been inherited through so many generations. Consider the gene that causes bees to sacrifice their own lives in defence of their hives (they die after stinging invaders). Certainly not beneficial to the bees themselves, or to their cells! There are many, many other examples of genes that cause the early death of the organisms they are in in order to aid their own propagation. You will not, however, find an example where a genetic trait that benefits the cell, but does not aid the propagation of the gene, will get inherited over any appreciable length of time. This is a simple consequence of the fact that the gene is what gets inherited, so no matter how successful the cells that contain the gene are, if the gene itself does not get passed on despite the success of the cells it is in, the trait will not get passed on either.

And yes, individuals migrate to different environments, upon which the selective pressures on genes will change, so the same genes that were successful in the previous environment will not be successful in the new environment. This is just another example of genes being selected through interaction with their environments. As I've said many times, there is nothing about classical population genetics (as Dawkins was presenting it) which denies the influence of the environment. Whether a gene succeeds is necessarily a function of the environment --- there is no other way to select amongst genes otherwise. This does not contradict Dawkins' central point that the ultimate arbiter of whether a heritable trait is propagated is whether the gene that induces it is sufficiently 'selfish', where 'selfishness' is of necessity determined with respect to prevailing environmental conditions (since a gene selfish in one environment may not be selfish in others --- a gene inducing kamikaze attacks in bees is being 'selfish' in the context of the social organization of the hive and the peculiar genetics of bees, but it would not be 'selfish' in the context of most other forms of animal societies). In the vast majority of cases, genes that get passed on tend to be those that benefit the organism they are in. However, when the interests of genes and organisms diverge, then gene-level selection will win out, because organisms are not the ultimate units of selection.

Socrates_Reincarnate said...

You also have to remember that junk DNA can get deleted during the natural process of crossing over (looping out). In fact, some scientists see the junk DNA as a protective layer against mutations because there is higher odds of the latter being hit. From the bioenergetics perspective, it will be energetically less expensive especially for the replication process (hence perpetuation) if the gene eventually gets deleted.

Ponder Stibbons said...

And your point is? I have said that genes that are 'good' for the cell (or the organism, or the hive, or whatever) are selected for insofar as their interests fall in line with those of the cell (or organism, etc.). Granting that the hypothesis concerning the 'usefulness' of junk DNA is correct, then that is a case where the interests of junk DNA coincide with those of the cell (if the cell survives, the junk DNA gets to propagate itself, and if the junk DNA propagates itself, it helps the cell to survive). This can be interpreted in both the gene's eye view and the cell's eye view. However, the point of moving to Dawkins' gene's eye view is that it is more generally true than the cell's eye view: it allows us to account for the many other cases where the interests of genes diverge from the interests of organisms (or cells, etc.), which is counter-intuitive in a crude view of selfish organisms/cells (why should a bee kill itself for its hive?). The gene's eye view is the only way we can understand the existence of altruistic behaviour in animals, amongst many other things.

Socrates_Reincarnate said...

Dear Ponder Stibbons:

Interesting comment. However, molecular biologists have speculated on the usefulness of junk DNA. One school of thought is that the junk DNA is a "sacrificial body" in the face of mutation. Consider it from the perspective of statistics. Since junk DNA consists of a major part of the genome, the odds of it getting hit by a mutation is high. There many types of deletional mutations, including the looping out process during crossing over. The cell will not lose out if a chunk of junk DNA is looped out.