Expert: Dana Krempels, Ph.D. - 5/18/2007

Question
Dear Dana,

I wrote you an e-mail a couple of days ago regarding several issues on evolution, adaptation and Dawkin's theories. I hope you still remember me. By the way, thanks a lot for your answer.

I have been reading and re-reading your answer and still, I have a couple of questions I would like to ask.

First, if environment can affect the genotype, and thus change the genome, so that it passes to next generation, do we know by which means is this possible? Or is adaptation the product of merely chance? What I am trying to ask is: which are the mechanisms that can modify the phenotype? How do genes decide which one of alleles should be expressed?

Second, you mentioned that the way in which certain genes are "packaged" can make genes more or less likely to be expressed. What do you mean by "package"?

Finally, once the genome has been altered, how this genetic material gets passed from generation to generation? In other words, how does the gene "knows" that the phenotypic change is good for itself?

Very last, which book would you recommend me to buy from Stephen Jay Gould?

Again, I would like thank you for taking the time to answer my questions.

Jeffrey  

Answer
Hi, Jeffrey

(Of course I remember you!)

You ask some really interesting and difficult questions...some that may be too involved for a simple email.  For example, your first one:

"If environment can affect the genotype, and thus change the genome, so that it passes to next generation, do we know by which means is this possible?"

Environmental factors can cause mutations, but unless these occur in germ cells that give rise to gametes and are later are successfully incorporporated into a zygote, these mutations may be of no evolutionary consequence.  Only germline mutations (i.e., those occurring in cells that give rise to eggs and sperm) are passed on to the next generation.

But this is where it gets complicated.  The expression of genes in a zygote can actually be affected by transcription factors (usually mRNA transcripts in the cytoplasm of the ovum) deposited by the mother!  There is far more to gene expression than simple transcription and translation of a gene into a polypeptide.  But for me to explain this to you in an email would be nearly impossible.

Let's try this:  Have a look at my lecture notes that I use for one day on the genetics of development.  This *might* help clear things up...or it might just make them more confusing.  Genetics is one of the  more difficult areas of biology to learn without someone to explain things in person with plenty of pictures.  :(

"Or is adaptation the product of merely chance?"

The current thinking on this is:  yes.  Organisms cannot direct their own evolution.  Natural selection works on the phenotype of individual organisms that cannot respond genetically to environmental challenges.  MOSTLY.  But there are exceptions to this, too.

"Which are the mechanisms that can modify the phenotype?"  

No one knows for sure what all the mechanisms are that affect phenotype.  This is a very hot area of study in genetics.  One very interesting phenomenon is that of epigenesis, in which the environment causes an organism to express genes in a different way that ends up being heritable--without a change in the actual gene sequence!  But again, this is far more complicated than something I could put in a single email.  

"How do genes decide which one of alleles should be expressed?"

Genes don't decide this.  Alternative alleles may all code for functional protein products, or some alleles may code for non-functional products.  In the former case, a heterozygote will express both products, sometimes with unpredictable phenotypic results.  In the latter case, the functional protein's expression *usually* will override that of the non-functional one, which would make it dominant.  But again...not always.  I wish it were simple to explain, but if it were, there would not be massively thick textbooks on the subject.  

"What do you mean by "package"?"

There are different ways that a cell can process and "package" its DNA such that any given gene is more or less likely to be expressed.  For example, some areas of the chromosome are highly supercoiled into what's called *heterochromatin*, and the genes in the heterochromatin are usually expressed not at all, or at a much lower rate than genes found in the non-supercoiled regions called euchromatin.  

It also appears that a cell can chemically "tag" genes with substances such as methyl groups to make them less likely to be expressed.  The cell also contains numerous substances called transcription factors that bind to the beginning part of a gene and make it either more or less attractive to the enzymes that promote gene transcription (and hence, expression).

"Finally, once the genome has been altered, how this genetic material gets passed from generation to generation? In other words, how does the gene "knows" that the phenotypic change is good for itself?"

Remember that only the genome in a germline cell can be passed on to the next generation.  That means that any chance mutation expressed in an individual has to also be in that organism's germline in order to be passed on.  And that's just it:  a gene cannot really "know" in advance that it will encode a phenotype that will be adaptive in any particular circumstance.  But the chance are that if it has conferred an advantage in one generation, it may do so in the next.  The very passing on of a particular gene has a good element of chance, since only half the allele's of any given individual will make it into any single sperm or egg.

"Very last, which book would you recommend me to buy from Stephen Jay Gould?"

That's a tough one.  It might not be a bad idea to start reading Gould via his web site, and see where that leads you.  Check it out here:

http://www.stephenjaygould.org/

Many of his most famous essays are archived there, and free!  :)

I'd also recommend a textbook on Genetics, which will help you through this field.  The one I use to teach my course is here:

http://www.amazon.com/Modern-Genetic-Analysis-Integrating-Genomes/dp/0716743825

and might be interesting to you.

I wish we could talk in person, because much of what you ask is profound and nearly impossible to explain in an email.  If you're ever in Miami, you're more than welcome to sit in on my course.

I hope some of this makes sense, but if not, please ask away!

Have a pleasant and peaceful weekend!

Dana