h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How does mutation to our DNA help us to evolve. What I mean by this is, what type of mutations are actually meaningful.

For example, cosmic rays are spraying me all the time. My DNA in certain cells may get damaged thereoff, or maybe there is an error in the copying and such. But that may be just one of trillions of cells like cells. Is meaningful mutation only occuring say at the sperm/egg stage?... Or do mutations to our cells throughout our lives somehow get accounted for in our reproductive cells.

any mutation that gets passed on to your children that helps them, survive grow and procreate.

any sort of mutation that is passed on is meaningful

A woman has a lifetime supply of eggs in her ovaries. If any of these is damaged by a cosmic ray so that the DNA in it is mutated, then if that egg becomes fertilised the child that grows out of it will have that mutation in every cell in their body.

If any other cell in the mother's body has the DNA damaged by cosmic rays, it will not effect her children and will therefore not affect evolution.

I'm not sure how exactly sperm is produced - I know each one only lives a short time, but there must be a "master copy" of the DNA from which the sperms take their DNA. If this is damaged/mutated, then the children will inherit the mutation from the father.

Also, beyond, or rather, as well as mutation by random chance, due to environmental factors, in the spermatocytes and ovas, there is the rearrangement of some DNA/chromosomes which happens during formation of the gametes, and additionally, there can occur errors in transcription/copying of the DNA, during this process, and also as the early embryo (whilst only a couple of cells), is growing and deviding, I can't quite remember what the error rate is for miosis, and there are still in place the mechanisms within the DNA transcription process which 'spell check' the duplicated/copied DNA strands as the process occurs... But the cells in the ova and sperm can be dammaged in an individuals body, prior to conception/fertalisations, E.G., radiation, certain chemicals we take in during our lives etc...
Plus, remember, in the Female, its not just the DNA within the ova, its also the DNA within the females Mitochondria, which is passed onto, ultimately, the embryo...
By its nature, most mutations which occur in the DNA end up having no affect, either positive or negative, in terms of applying selective pressure; many substitution mutations (A T being insurted instead of a G, or a C instead of an A, etc.,), don't actually result in a change to the amino acid encoded by the triplet, as most amino acids are encoded for by several triplet variations...
Simularly, a change in a simgle nucleotide, might not be sufficient to cause any noticible structural, hence functional changes in the encoded protein...
err, basically the mutation has to give rise to a change in function of the protein encoded, or a change to the opperation of a regulatory stretch of DNA, for it to change the individuals phenotype as opposed to just their genotype, for it to then be able to potentially give a selective advantage, or disadvantage...
I've forgoten most of my detailed knowledge on genetics these day and may be a bit out of date though

Ok, so let me see if I understand correctly ........

Mutations will likely only have the possibility to affect offspring if

1) The DNA in the mother eggs gets scrambled (no metaphor intended). This is somewhat most likely since the eggs have a long exposure time.
2) The sperm somehow gets zapped on it's way there. Somewhat unlikely because of a very small exposure time.
3) Transcription errors during the copy process that developed the eggs/sperms.
4) Transcription errors during the extreme early stages of embryonic developement. (Again unlikely due to internal mechanisms designed to guard against that very thing)
5) Something about female Mitochondria being passed along.

And even if any of these happens, there is still no guarantee that a mutation will acutually alter the function of the proteins developed.

Did I get it right?

Pretty much so. You left out my rather vague statement that there must be a "master DNA" in the male from which the sperms get their copies of the DNA. So this could get mutated due to prolonged exposure to the dreaded rays.

The mitochondrians are cell components with their own DNA which is totally independent of the cell's DNA. They're almost like separate creatures with a symbiotic relationship with our cells. But they come only from the mother, so there is no father/child inheritance at all. Mitochondrian DNA reproduces asexually so there is less chance of errors.

Sperm are still effectively the descendants of cells which have been in the body since birth.

However much or little cell division happens along the way, tracing back the history of an individual sperm's DNA, at any point in time, there has always been a cells-worth of DNA around potentially getting damaged by radiation or chemicals.

>>How does mutation to our DNA help us to evolve?<<

Only to add to the above, but the larger point that I think has been missed is this: evolution occurs on the level of populations, at the level of the individual there is only variation and death. Thus take any individual, they'll have genes from both parents plus it'll be a little different. If they a) survive and b) reproduce - that individual genome will get split and get shuffled again. If they reproduce a lot (Haldane's - 5 brothers, 7 cousins) then those genes are a lot more likely to stick around and mutate in interesting or not-so-interesting variations but most of the time in inconsequential ways, as 2legs pointed out.

For humans, the "us" in the question - the 'are we still evolving?' question is a good one.

In general, children survive childhood. That's new. Go back a few hundred years and the rate of birth/death meant on average 2 parents were replaced by 2 children, now they tend to be survived by most if not all of their offspring through medicine, health, diet etc and this is why the world population is suddenly grazing 7 billion.

Secondly, in a population this widespread (global) there isn't really the isolation required to really kick-start and maintain a speciation event.

My favourite example of that is the Cichlid fish in the lower Congo river, where the speed and complex hydrology of the rapids has isolated fish of the same species on opposite bank and they are starting to speciate. Hence the origin of species.

In Humans this rarely happens any more, we build bridges or tunnel under mountains and true isolation (of the kind where no planes fly and or ships sail) is awaiting the next cometary impact or volcanic super eruption.

At the moment, for humans, the genes we all carry and the mutated variations of those genes keep getting re-mixed and passed on so genetic diversity in this world-wide population is actually flattened (think Viking genes in the north of England - there is probably more genetic similarity between one man in north England and a man from Norway, than there is from the same man in north England and his next door neighbour, who happens to be female and so has a whole different set of chromosomes and organs just to start with!)

My point is that Viking genes can be passed around and inherited but all around but it's not really converting into an evolution amongst humans. It's just upping the variation

The exception to that, is I suppose viruses - which can not only diddle with your DNA but are still an uncontrolled selection pressure affecting long-term survival.

And lo and behold there's evidence of sex-workers in sub-Saharan Africa who are genetically immune to HIV and so are their children. Consider the implications of that one for a moment in the same context that the majority of people on this board are descendants of people who survive the plagues in the middle ages in Europe.

There also sexual selection - There was a documentary on the Beeb a while back "The Incredible Human Journey" with Alice Roberts that looked at human evolution past and present. The second episode focussed on Asia and the prominent belief that Asians with their distinctive facial features may have been descended from a different lineage of humans (and there have been those: erectus, neanderthalensis, floresiensis - all extinct, only sapiens - us - have thrived.)

Much energy has been expounded on testing this hypothesis - and the result was it's nonsense. Asians are every bit as sapiens and someone from Colchester or New Orleans. The suggestion was that Asian facial features, rather than being a mutation conferring advantage etc was actually more to do with 'what women want' - and so more akin to the excess of plumage on a bird of paradise doing the macarena: Asian women like to have children with Asian men who (for a whole slew of other complex reasons) appeal to them but not insignificantly because they look, the majority of the time, like Asians.

You can watch that episode of youtube.
http://www.youtube.com/playlist?src_vid=JFQ5c_z9wU0&annotation_id=annotation_387895&feature=iv&list=PL063C7039872C3DE0

re: the 'master copy of DNA' for sperm.

Isn't that the distinction between haploid (i.e half-chromosomes) and diploid cells (double chromosomes)?

Trying to recall last years lesson on the topic, Human sex cells are hapolid cells and only ever have one complete set of chromosomes from the male or female parent - the person in question - i.e 23 chromosomes to go and join the 23 chromosomes of the female egg to form a new diploid cell which will form a new adult embryo with it's own haploid sex cells which have the genetic shuffle applied to them and so not be an exact copy.

>Mitochondrian DNA reproduces asexually so there is less chance of errors.<

Mmmm, that's not really true - asexual reproduction is usually much *more* prone to error. Having said that - that's really because we have extremely tight regulation of DNA mutations.

Higher organisms have extensive DNA repair machinery that is designed to spot errors in DNA through any form of damage.
So errors are *extremely* rare.
Lower organisms that partake in asexual reproduction make far more errors (albeit with still a remarkably error free rate generally). This is because their DNA proofreading and repair assemblies are either non-existent or much more primitive.

I'm not actually sure how mitochondrial DNA fits into this. The DNA repair complexes sit within the nucleus but I've no idea if they also exist in the mitochondris. The 'symbiotic cell within a cell' ancestor is only a theory - albeit one I also adhere to.
Much is made of the mitochondrial DNA too - but I think they only contain about 24 genes - as opposed to about 24,000 in our cells' nuclei.

Also one has to be careful with the word 'mutation'. Much of our variation comes from gene 'shuffling' and differences in expression of dominant (e.g. brown eyed) genes as opposed to subordinate (e.g. blue eyed) genes for example. So what you see as a mutation may actually a different phenotype than a mutated genotype.

(Different phenotypes have the same genes but express them differently. Different genotypes have different genes).


Interesting, but complicated question.

Interesting thread so far too

Wow! I feel like I just opened the door to my closet and found out it opened to another planet. Thank all of you for the time and care you've taken in your posts. I must admit I had a very naive and vastly oversimplified view of this topic.

One thing I've always wondered about is it seems as though the whole process happens too fast to be explained only by selection alone. It seems as though organisms adapt to their environments quicker than what it would seem statistcs has enough time to sift out. It makes me wonder if it is not infeasible somehow our own bodies can take information in from our environmental pressures and in some fashion have some sort of mechanism that actively modifies our reproductive cells in a way that we pass on an accelerated rate of beneficial change. Sort of like a biological feedback loop whereby out bodies have actually adapted to the adaption process itself. I say this again with naivete, as it may be that we already understand enough to prove that this definitely is simply bonkers.

I am also unclear regarding how the concept of Adaptation and Evolution differ from each other. Is it that it is only considered adaptation if things stay within the same species. Does evolution itself imply a change in species? What kind of time scales are required for two isolated groups to become biologically incompatible?

I appologize in advance if these questions are silly.

>I appologize in advance if these questions are silly.<

Well apology refused - excellent questions!

Animals (and plants for that matter) need to be adaptable to survive. Aside from massive environmental changes like volcanic eruptions and asteroid strikes, they must be able to adapt to less, but still fairly extreme environmental changes such as that between the summer and winter climates in temperate regions. We have evolved to be adaptable is the best way of putting it I guess. It is environment changes that make organisms express varying phenotypes. Whether that extends to changing the genes we pass on to our offspring I don't know but it seems unlikely to me. More that we just pass on our adaptability.

Extreme events that kill large swathes of the population - it is them that see those with the correct genes needed to survive or adapt pass those genes on.

As to the second question - personally I'm not really expert enough in genetics to comment really.


Incidentally regarding my previous post - I should also have pointed out that lower organisms have a more error prone DNA copying process altogether. DNA polymerase enzymes (those that make new DNA during cell division for example) in higher organisms have a 'proof reading' mechanism built into them whereas this is not the case in lower organisms. This is additional to the DNA repair complexes that exist to fix existing DNA.

>>I am also unclear regarding how the concept of Adaptation and Evolution differ from each other.<<

Lots of useful information on this website that is easy to break down and understand. This link will take you to the discussions on the "mechanics" which relates directly to your question, but you'\d do yourself no harm to begin at the begining.

Pay particular attention to the discussion of 'feathers' as that is especially relevant - namely that feathers evolved but have become adapted to flight.

http://evolution.berkeley.edu/evosite/evo101/IIIADescent.shtml

Clive, Ironically I have already been poring over that site. Not ironically, because I was searching for things related to what you had in your post. I agree! It is a great site and so far I am enjoying it very much. Thanks again

"Incidentally regarding my previous post - I should also have pointed out that lower organisms have a more error prone DNA copying process altogether. DNA polymerase enzymes (those that make new DNA during cell division for example) in higher organisms have a 'proof reading' mechanism built into them whereas this is not the case in lower organisms."

So what happens when the "proof reader" finds an error. Does it fix it or toss it?

What happens if there is an insufficient supply of the necessary amino acids, does the process just come to a halt?

First of all DNA is made of nucleotides not amino acids.

Amino acids are what proteins are made of.


If it finds an error the incorrect nucleotide in the growing DNA chain is excised and then a new correct one is inserted and the chain extension carries on.
http://en.wikipedia.org/wiki/DNA_polymerase


Secondly - if your cells have run out of nucleotides and/or amino acids then they have run out of everything and will die.
Biosynthesis of amino acids and nucleotides along with fatty acids is fundamental to the survival of any cell and so is termed primary metabolism. It is pretty much the first priority of the cell before *anything* else. So a living cell will not run out of these things as it is continually making them - if it runs out of the raw materials to make them (which is pretty much any food source) then it's trouble time - with big capital T.

re: feathers.


http://content.usatoday.com/communities/sciencefair/post/2011/09/80-million-year-old-dinosaur-feathers-found/1

It is always amusing when scientists re-discover what they have already had in their possession for some time. I wonder how long that stuff had been in the museum before someone said ....haaaaaayyy!

I never knew how complicated feathers got!

Orcus, thanks for the corrections and the link. Wish I had been smarter back then and taken a better career path. I envy the people who get to do such interesting work for a living.

>I never knew how complicated feathers got!<

The extraordinary is, rightly named, amazing but always be mindful and marvel at the ordinary too.