Traditionally, the individual was considered to be the smallest unit on which Natural Selection (NS) acts. Today, we usually consider the gene as being the unit of NS. Of course, we should also consider all sequences that affect the fitness even though they are not genes (even though the do not code for polypeptide). And theoretically, any sequence of DNA does have an effect on fitness because it influences the time and energy for DNA replication (although it might be negligible). The decision of considering the gene as the smallest unit of NS seems rather arbitrary to me. We might as well consider a group of genes or a given exon of even a smaller sequence.
Here are my questions:
What factors influence the minimal size of a sequence to be considered as a unit on which NS acts? Mutation rate, generation time, selection differential for this sequence, recombination rate, ...?
Could we consider a nucleotide as a unit of NS? Why?
How does the quasispecies model fit into the question of what is the smallest unit of NS? (for those interested, you will also find a very good explanation of this model in Martin Nowak's book called Evolutionnary Dynamic: exploring the equations of life)
Is it worth talking about that? Is this question biologically relevant? Or is it rather a question based on a choice of definition such as "Is a virus alive?"
As I asked several questions, let me know if I should split my post into several. Otherwise, please do not hesitate to answer only very partially to this post!
Terdon's answer makes sense to me. I should be a bit more accurate in the reas of my question. I read The extended Phenotype from Richard Dawkins quite a long time ago and if I'm not mistaken, Dawkins says the following things
A unit on which selection acts has to be:
A replicator has the 3 following properties:
- fidelity while being copied
Therefore for fidelity to be respected a unit of selection has to be a sequence which is not too long, so that it is not too often modified by recombination or mutation. He argues in this sense.
He also argues that nucleotides are not possible unit of selection. Indeed, it is hard to imagine a nucleotide being an active replicator. The word active means that it influences its not probability to be replicated. I don't think a nucleotide can do such a thing.
Unfortunately I don't have the book with me right now and I can't check what I have said, give you a citation nor a more accurate a reference. If anyone has some citations from this book, it will be welcome for the discussion!