What happened to the gemmules?

[nathan_w_cheng]

I have been reading Watson's latest book on DNA and was surprised to find out that Darwin was an advocate of pangenesis by way of "gemmules"--particles that an organism supposedly generates and accumulates as it develops and matures. These particles were supposedly what was passed on to the organism's offspring and caused the offspring to be related and similar to the parent. Thus, if a giraffe had to stretch its neck a little further to reach leaves in trees that were a little taller, the giraffe's offspring were born with longer necks.

I suppose this is really not all that surprising given the fact that Darwin had to have had some kind of understanding of heredity, and the pangenesis idea certainly was convenient for him and his theory. But I am curious to find out: In modern-day evolutionary theory, what method of "genetic feedback" has replaced Darwin's now-known-to-be-wrong pangenesis hypothesis? As far as I can tell, natural selection of random genetic mutations is the only process driving evolution in modern theory; i.e. there is no genetic feedback mechanism. How could this be? With pangenesis, evolution seems like an obvious truth; but without it, I feel like my imagination needs to do some stretching exercises.

 

[wrmgrl]

mutations

There is evidence that mutations are not as random as we are classically taught to believe. Certain parts of genes and genomes are far more prone to mutation. A lot of these so called hot spots are thought to have given rise to advantageous mutations and allowed for rapid evolution.

 

[nathan_w_cheng]

Re: mutations

Originally posted by wrmgrl
There is evidence that mutations are not as random as we are classically taught to believe. Certain parts of genes and genomes are far more prone to mutation. A lot of these so called hot spots are thought to have given rise to advantageous mutations and allowed for rapid evolution.

Can you direct me to some further reading on the topic?

Is there any indication that perhaps in earlier organisms there did exist a gemmule-type mechanism for feedback from the organism back to its DNA?

 

[wrmgrl]

DNA changes

As far as these DNA determinants that change throughout development, I have to doubt their existence. The DNA that is passed on to the offspring comes from specific cells called germ cells or gametes. These are set aside early in development and their DNA does not "change" appreciably as they develop. Mutations occur when DNA replicates, as it does requently in the germline to give rise to gametes, this allows for mutations that would give rise to nature selection, and some regions are more susceptible to mutation.
For example, all the eggs a woman will ever have are produced while she is still developing in her mothers womb. There is no evidence that the DNA in those eggs changes as selective pressures act on her during her life. If her DNA does not permit her to deal with the pressures she encounters, it will not change, and she may die. It is possible that her offspring may be able to survive those changes if mutations occured while her eggs were replicating, but those changes will not help her.
I read about hyper-mutable sites long ago, I'll try to look up some papers for you.

 

[nathan_w_cheng]

Re: DNA changes

Originally posted by wrmgrl
As far as these DNA determinants that change throughout development, I have to doubt their existence. The DNA that is passed on to the offspring comes from specific cells called germ cells or gametes. These are set aside early in development and their DNA does not "change" appreciably as they develop. Mutations occur when DNA replicates, as it does requently in the germline to give rise to gametes, this allows for mutations that would give rise to nature selection, and some regions are more susceptible to mutation.
For example, all the eggs a woman will ever have are produced while she is still developing in her mothers womb. There is no evidence that the DNA in those eggs changes as selective pressures act on her during her life. If her DNA does not permit her to deal with the pressures she encounters, it will not change, and she may die. It is possible that her offspring may be able to survive those changes if mutations occured while her eggs were replicating, but those changes will not help her.
I read about hyper-mutable sites long ago, I'll try to look up some papers for you.

Thanks; it makes sense that some sites would be more susceptible to mutation than others, given the fact that DNA replication happens in cells that all contain roughly similar arrangements of 3-D objects that can be bumped into at differnt speeds and such (e.g. as a spiral unravels, different sections of the spiral will move significantly faster than other sections, making those sections more apt to have mishaps).

I look forward to the reading.

 

[wrmgrl]

Re: Re: DNA changes

Originally posted by nathan_w_cheng
Thanks; it makes sense that some sites would be more susceptible to mutation than others, given the fact that DNA replication happens in cells that all contain roughly similar arrangements of 3-D objects that can be bumped into at differnt speeds and such (e.g. as a spiral unravels, different sections of the spiral will move significantly faster than other sections, making those sections more apt to have mishaps).

I look forward to the reading.

That is a particularly insightful observation in terms of possible methods of mutations. And these sorts of things do give rise to mutations, most of these mutations are repaired by the cells "proof-reading machinery." A particularly common way for a site to be error prone is for it to be "slippery" ie containing stretches of the same nucleotide or repeats of multiple nucleotides. If you are the proteins responsible for replicating DNA, imagine standing in a forest where all the trees look the same, as you move forward since the trees behind you look the same as the trees in front it is hard for you to know how far you've gone as you have no land marks to guide you, so you loose track of where you are in the forest and end up back-tracking and going forward again. This results in you covering the same stretch of trees more than once, and if you are a polymerase, this causes you to add extra copies of nucleotides that were not present in your template. These sorts of errors are also more difficult for the proofreading machinery to detect and repair. Similar problems can result in deletions of nucleotides as the polymerase skips them. There is a common genetic form of mental retardation that occurs this way. It is known as fragile X syndrome (the gene is on the X chromosome). Just upstream of the fragile X gene is a stretch of CGG repeats, when polymerase slips and makes more of these repeats than it should it causes the repeats to be biochemically modified (methy groups are added) and this results in transcriptional silencing of the gene. Therefore the fragile X protein is not produced and this causes Fragile X syndrome, without actually changing the coding sequence or changing the protein, but merely eliminating its production.
Of course this is not an example of natural selection occuring by selective mutation, but it is an example of a hyper-mutable site.