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Evolution/Question about Natural Selection -- when can variation occur?

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Question
Dear Doctor Krempels,

The theory of natural selection (as I understand it) states that through random chance, mutations occur in the DNA producing variation. This is occasionally beneficial, giving the organism an advantage over others; if this happens, the allele will be passed onto the offspring.

What I would like to know is, at what point can this mutation resulting in variation take place? Is it something that can only happen at the most early stages of development, during meiosis, or when an embryo is still forming? Or, because the DNA is copied all the time during mitosis, does this mean that a mutation resulting in variation could happen at any time? Could something or someone be born like every other member of its species, and then a mutation could occur in their genes AT ANY TIME that would then give that organism an advantage in its environment?

Thank you very much!

Answer
Dear Allie,

Sorry for the delay in answering.  I hope this is still timely enough.

Mutations can happen in any cell, but only if the DNA in that cell is passed on to the next generation will it have direct evolutionary consequences.  In animals, that means that the mutation must occur in the germline cells, the diploid cells in the ovaries or testes that undergo meiosis to produce ova (eggs) or sperm.

Mutations that occur in an animal's somatic (body cells; i.e., not germline) cells will not be passed on to offspring, and that includes any somatic cells generated during embryo development.  
Though one could imagine a somatic mutation (probably during early embryogenesis) that might give an individual organism an adaptive advantage, that mutation will not be passed on to the offspring.  It might, however, help that organism's other genes get into the next generation, and so could indirectly influence evolution.  

The animal body has repair mechanisms that correct damage to DNA (mutations), but those mechanisms don't always work.  When they fail to repair a DNA replication error (or other type of mutation), then the cell lineage will carry that mutation along from that point on.  If it happens in a critical combination of genes (e.g., oncogenes and tumor suppressors), then the cell can become cancerous.

You can read more about the consequences of mutations in various cell lines here:

http://www.bio.miami.edu/dana/107/107F11_8.html

Those are lecture notes from my non-majors evolution class, and they might help explain this concept.

Hope this helps!

Dana

Evolution

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Dana Krempels

Expertise

I can answer questions about evolutionary mechanisms and theory, including genetic drift, mutation, natural selection, etc. I also can clear up misconceptions about evolution as it's sometimes talked about by those not well-versed in the subject (e.g., some politicians and many religious fundamentalists).

Experience

I have a Ph.D. in Biology, and presently teach Evolution and Biodiversity, Genetics, Botany, and Zoology at the University of Miami.

Organizations
House Rabbit Society Beta Beta Beta Biological Honor Society

Publications
Exotic DVM Magazine (veterinary journal)

Education/Credentials
B.S. in Biology B.A. in English Ph.D. in Biology

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