Science Questions

How does evolution produce new genes?

Sun, 17th May 2009

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Question

Destiny asked:

How does evolution produce new genes?

Answer

Chris Smith -   So in other words youíve got huge panoply of life on earth, all of it descended from some ancestor that must have got started something like 3.9 billion years ago, thatís when we think life started on earth, how do we have this massive and dramatic genetic diversity we have on earth today?

DNA fragmentWell the answer is that we use as our genetic material DNA and some organisms use RNA, they are two related molecules and the basis of evolution and inheritance is that that genetic material has to be copied and passed on from one generation to the next in gametes, in other words, sperms and eggs.

Now in copying the DNA, a job which is done by enzymes ó miniature machines in cells that read a DNA chain and then make an identical (hopefully) copy of it; then you end up with faithful transmission of the genetic message from one cell to another.

But occasionally mistakes occur and there are a variety of reasons why mistakes can occur. One of them is that those enzymes that do the copying make a mistake, itís like me copying a book Ė Iíve got a book opened in front of me and Iím reading off the book and making a new copy in a second book, I might miss-copy and make some mistakes.

Secondly, there are things coming in from the environment that can damage DNA. There are drugs, there are chemicals in the environment; thereís also radiation in the environment that could be ultraviolet radiation, it could be chemicals you take into your body, you could be living in Aberdeen and breathing the radon gas that comes out of the granite there. Anything radioactive that gets into your body can also damage DNA.

If that happens to the cells that are making your gametes, your sperms or eggs, that can lead to mutations ó changes in DNA. This can cause DNA to rearrange itself, bits of DNA can get copied or duplicated, and this means that thereís the opportunity for new genetic combinations to emerge; and once they emerge they can then get co-opted or changed or manipulated in order to do other jobs; and we know this happens because if you look in human DNA you can find the ghosts of genes long dead hidden in our genetic closet.

You can find for example, lots of old genes that used to make our haemoglobin which are now no longer effective, and no longer functioning. Theyíre called pseudogenes. But this is where we have copied the genes somehow and then its becomes deactivated but itís still in the genome.  Now some other process could come along and reactivate that gene and use it for something else so itís a way of making genetic diversity.

Another way this happens, is something called transposons, you have bits of genetic material that can literally jump and they take themselves out of one bit of your genetic material and put themselves somewhere else carrying bits of DNA with them, and so this is another way of rearranging your genome and producing new forms of genetic sequences which can then become other important genes.

Sacoglossan Sea SlugA final way is viruses, because viruses ó certain kinds of viruses actually physically insert their genetic material into the DNA of their hosts.  HIV does that, other viruses could Ė retro viruses do that and a really elegant example of this is that there is a certain sea slug, itís called a Sacoglossan sea slug which eats algae and when it eats that algae on the sea floor it actually gets the chlorophyll containing chloroplast ó these are tiny bodies inside the algae that contain the green pigment that enables the algae to trap sunlight, and the slug gets hold of those chloroplasts and puts them into its own skin and keeps them alive so the slug can also use the energy of sunlight to get energy.

The interesting thing is that the slug has had to steal some genes from the algae in order to power those chloroplasts in its own cells, and the only way researchers think that could happen is if a virus added the genes to the slug in the first place.

So the answer is, itís very complicated but it seems that with nature and evolution, almost anything is possible.

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Destini asked the Naked Scientists: This is a question posed to me by one of my students who is seeking to find his own understanding of the world: How does macro evolution occur? In other words, how does natural selection ADD new alleles to the genetic sequence? What do you think? Destini, Tue, 12th May 2009

All evolution will be a result of some kind of genetic error. Most genetic errors are small, like substitution or deletion of a single nucleotide.

However, some of these genetic errors can involve duplications, sometimes of small sections of DNA, sometimes of entire chromosomes (think of Down's syndrome, where 3 copies of chromosome 21 are present instead of two as a result of failure of the chromosome to split during meiosis).

I would guess that evolution may 'occur' when a duplication occured that didn't impair the hosts ability to reproduce. Even more so if the duplication somehow enhanced it.

The wikipedia entry for gene duplication talks about it in more detail that I ever could, so here's the link!

http://en.wikipedia.org/wiki/Gene_duplication fishytails, Tue, 12th May 2009

Fishytails may have the answer to some genetic change, but I think Destini's student may be looking more to the genetic codes which determine such attributes as the leg ability.

For example, say there was once a Forum Goat, which evolved on the plains. With the expansion of the species, some of these goats found themselves living at the foot of a mountainous region. They begin to venture off of the plains, into the mountains, where a nutritious plant will be available only to those adapted to this terrain. The first 'changelings' find these ventures difficult because their hoof is not adapted to this terrain, but as the generations pass, the hoof becomes more and more adapted this type of terrain. The end result, we now have two species of Forum Goat, the Plains Forum Goat and the Mountain Forum Goat, they are identical except for the differently adapted hoof.

How are gradual adaptations, such as this, genetically passed down the generations? Don_1, Wed, 13th May 2009

I am assuming that my interpretation of Destini's question is correct.

The answer actually lies in the question itself.

In the case of our Forum Goat, only those individuals which manage to move around on this new mountainous terrain with relative ease will actually venture into such a habitat, those which are unable to cope, will remain on the plains.

Look at it this way, some of us can don a pair of ice skates and manage to stay upright and even move around on the ice rink, while others, me for instance, will venture onto the ice and go arse over tit every time. We are not predisposed to ice skating, so we keep off the ice. The other show off's are, so they stay on the ice. Don't you just hate those clever buggers???

So it is with our Forum Goat. Those which have taken to the mountains will mate among themselves, thus bearing offspring with the same capabilities. These capabilities to deal with this new situation, are in fact already in the genetic code of these particular individuals, but not in the genetic code, or not so well defined in the genetic code, of those goats which have to remain on the plains because of their inability to cope with the new terrain.

Because these individuals become separated from the others, they are mating with other individuals with a similar genetic code, therefore, this will become more pronounced in the offspring. Those young which do not receive this predisposition will either perish, or will not be chosen as good mating stock by the others, or will simply lose out in the battle to pass on their genes, to the more capable individuals.

In this way, the predisposition will become more and more pronounced over the generations and result in a species separate from those which were unable to cope. This is natural selection at work, survival of the fittest. Not simply the fittest in terms of health and strength, but also the best able or predisposed to adapt to new situations. And not simply a case of the survival of the fittest individual animals, but the survival of the Ďfittestí genetic code.

So this is not a case of the individual animals changing or updating their genetic code, but a predisposition being enhanced by the interbreeding of those with that predisposition.
Don_1, Thu, 14th May 2009

Actually, I suppose I need to clarify some more...

You are all referring to natural selection, modification of genes that already exist, genetic drift, etc. The question asks how do brand new alleles get added on to what is already there?

ie: Your genetic strand is 12 proteins long.... how does it get to 14...

destini, Thu, 14th May 2009

So sorry, I will write this 100 times.

I must read questions thoroughly before answering them.

There, done it. I deleted the other 99 to save space!

This could be akin to break repair genes either 'repairing' two separate chromosomes, thus fusing them into a single chromosome, or a break repair gene inserting itself into a damaged chromosome, creating two distinct and separate halves.

Take a look at this article http://www.pnas.org/content/96/26/14899.full.pdf Don_1, Fri, 15th May 2009

Natural selection is not really responsible for causing mutations, it just acts on them after they have occurred. The mechanisms that maintain and copy DNA and RNA are not perfect. So the nucleotide sequence can be modified by environmental factors (e.g. types of radiation and certain chemicals), and also accidentally during replication. This is how genes change.

There are several different types of mutation that you can easily find out more about.

Interestingly, natural selection probably does affect mutation by selecting for better replication and repair mechanisms. _Stefan_, Fri, 15th May 2009

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