Talk:Evolution/Inheritance and genetics

Moved from Talk:Evolution --Brion 23:17 Jan 19, 2003 (UTC)

Inheritance and genetics as they apply to evolution

In the text of the evolution article it states:

In Darwin's time, there was no widely accepted mechanism for heritability. In modern times, the mechanism for heritability is known to be DNA. There is also the interesting possibility that proteins are responsible for some heritability.

I have a couple problems with this. First, DNA is a structural molecule and does nothing on its own. Therefore, it is not a mechanism. The mechanism of heritability is the process known as reproduction. To use an analogy, people are confusing the act of reading for the book being read.

Second, I'm trying to imagine how a protein could be a means by which traits are inherited and its awfully hard to see. In animals at least those proteins would be restricted to being in cell lines that evolve into sperm and egg. If anyone has scientific evidence for such I'd love to see a link rather than see this comment ad hoc. -- dwm.

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Yes, you're confused, but I think the text is fine as it is. First, where do you get the idea that there's some black-and-white distinction between data-carrying elements and functional elements at the molecular level? Biochemistry isn't that simple. Certain strands of RNA, for example, can catalyze reactions by themselves, without being transcribed into proteins. I don't think anyone has shown DNA doing this, but I don't think it would terribly surprize anyone either. Secondly, I think the statement is being used in a more general sense than you're interpreting: DNA is a means by which heredity takes place. Maybe "mechanism" isn't the best word, and there are certainly lots of other pieces in the whole mechanism as well, but DNA does play a central role as the primary means of long-term storage of information. I think that's all they're trying to say here. The comment about proteins is potentially confusing, but it probably refers to recent discoveries about proteins that make their way into germ cells, mostly sperm. I'll try to find a citation. --LDC

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Let me be plain. The reference chain for the protein heritability carries directly to prions, which are infectuous agents. I'm more likely to get a prion by eating my Mom as opposed to inheriting it. More so, they affect neural tissue, not germ cells. As well, even if they were capable of self-assembly in a soup of amino acids (which I *doubt*), they still are diseases and not something heritable. Finally, as all prion prteins are minor structural variants of normal proteins that are assembled from regular, central dogma controlled paths, there is no reason to believe that prions actually violate the central dogma in the first place.

As such, if I don't see a reference in a reasonable period of time, I think we'd be better off deleting that sentence as opposed to keeping it. Further, the whole concept of an inheritable protein suggests that the protein _encodes_ information that affects my cell lines in a non-germ way. Having 1 protein from my pop in a sea of billions isn't enough to claim heritability; I'll get more proteins (unwanted) from viral infections in my lifetime.

Finally, histone and histone-like binding proteins may be said to have an effec t on 'expression' of proteins, but a couple histones I get from my Dad aren't going to have much effect on a mature adult with billions of cells. Histones, can, in theory, affect 'expression', but they carry no information per se, nor can they reproduce independently. They're as dependent on the central dogma as any other protein. Which of course, begs the question: if I didn't inherit that information from my Dad, how can I make those proteins in sufficient quantity to even make a difference?

The more I think of it, the more difficult I see the task of justifying a protein based scheme of heritability. The protein has to hitch a ride in egg or sperm, else its not inherited. The protein must then appear in quantity in both germ and cell lines, and must do so without *any* central dogma support (i.e. the protein must be capable of self assembly), else the protein is a consequence of the central dogma itself, not in opposition to it. The protein in question must do something useful that helps the organism to survive, else its simply a parasite, not a property of the human condition. Those are very difficult and stringent conditions, and really shouldn't be idly tossed into the soup, if the goal of this pedia is accuracy.dave

I agree, it's a pretty speculative possibility only likely to influence to evolution of things like bacteria--certainly nothing as large as a human. So it probably doesn't need to be mentioned here. --LDC

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Ok, I altered the sentence that bothered me most to:

In Darwin's time, there was no widely accepted in-depth mechanism for heritability. In modern times, the molecule that allows for heritability is known to be DNA, and the commonly accepted mechanism of expression of heritable information is called the Central Dogma

The reason being that the basics of reproduction had to be known in Darwin's time, that having sex led to offspring who had traits inherited from parents. Microscopes existed then, I'm sure von Leeuwenhook had seen sperm by then. What wasn't known was the mechanism of heritability at a molecular detail. I don't care for my writing and would invite others to think of a better way of expressing these notions. Dwmyers

When Darwin first published evolution, Jenkins came up with a very solid objection on the grounds that any individual variations would be watered down as they spread throughout the population, to the point where they effectively didn't exist. Mendelian genetics fixed that. I think the point here was to explain that.

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Dear Graft,

You removed my reference to prions, saying that it wasn't relevent. While a discussion of the nature of inheritance is not absolutely necessary in a section about natural selection, if we are to discuss genes we should also discuss other means of heretible variation such as prions and epigenetics. Perhaps all of this should go into a different section addressing the nature of variation. (as should the comment about natural selection permitting the long term survival of life, since it is variation that permits the long term survival of life. Natural Selection = Death) adam

Hi Adam... I removed the reference to prions because I don't think it has much to do with heritable variation as it relates to evolution or natural selection, especially insofar as the forms of prions found so far seem to be diseases that kill within a generation. So, while they might be taken as a form of 'heritable variation' from a certain point of view, they don't represent a) a significantly sized class of heritable variation when compared to heritable variation due to genes, b) heritable variation if you take that to mean something transmitted consistently across many generations, and c) a mechanism for selective pressure (beyond the selective pressure induced by having some prionic disease, i.e., instantaneous death).

Genes should definitely get first priority, but heretible prions have been thoroughly examined (see True and Lindquist "A yeast prion provides a mechanism for genetic variation and phenotypic diversity" Nature, 407, pg 477 (28 september 2000). Here's a quote about Lindquist's work:

Both yeast and mammalian prions transmit phenotypes via protein-protein interactions, in which an abnormally shaped prion protein influences its normal counterpart to assume an abnormal shape. In mammalian prion infection, such abnormal, insoluble shapes cause protein clumping that kills brain cells. In yeast cells, however, the insoluble prion protein is not deadly, but it alters protein synthesis. http://www.hhmi.org/news/lindquist4

In this example, prions do have all of the traits that you listed above.

Epigenetic factors are I suppose interesting and worth discussing, but I think we'd be better off just saying that what we're talking about is DNA, not genes, since this would include chromosome rearrangements and such and would preclude a larger and unnecessary discussion of the types of DNA variations we're talking about within the body of this Evolution article. Graft

Genetic and epigenetic factors have different means of inheritance; epigenetic factors (and prions) have the ability to "convert" similar genetic factors, and this conversion ability can be influenced by environmental factors. Granted, we don't know of many examples where these were influential, so genes should get the main emphasis, but I believe that once we understand more about the mechanisms behind prions and epigenetics, we will find more examples of them.

Well, frankly I think epigenetics is a bit faddish, but I haven't read an extraordinary amount on it. At any rate I think at this point most of the interesting aspects of epigenetics rest on speculation and the people who write/study it are interested in building it up to be more than it probably is. Sure, it might have dramatic implications for evolution, but then, the far side of Alpha Centauri might be made of Swiss cheese, and think of the dramatic implications that has... Soy muy esceptico. Graft

Maybe this should be addressed in biological inheritance