OK. What puzzles me is that mutations that are invisible to selection such as (in humans) the major part of the genome that can confidently be referred to as junk (50% at least and maybe as much as 90%) accumulate at a regular enough rate to serve as a molecular clock. There is the possibility I believe that some of these sequences can be incorporated into functional areas of the genome (or does it just require there to be the right promoter) and thus a new source of variation.
As I understand it, drift is the process whereby an allele that is not subject to selection can fix randomly, whilst others are eliminated. In small populations drift results in loss of diversity and perhaps extinction.
What I am not seeing is how drift contributes positively to the two pillars of evolution: variation and selection. How does loss of diversity help bring new variation for selection to act on? Is it just a matter of creating space?
As you note, drift may reduce variation in small populations, so it isn’t assumed to be entirely positive. What we are saying is that drift is an important mechanism to consider when looking at how species evolve and adapt.
Drift takes neutral mutations to high enough frequency that they provide a large amount of neutral genetic variation in a population. That variation vastly increases the space of potentially beneficial states that can be reached by a small number of mutations from the existing population.
I’m not sure what exactly you’re asking. Mutation introduces lots of new variants at very low frequencies. Drift eliminates most of them but causes some to be at high frequency. The result is a large set of neutral genetic combinations that can be the basis for adaptive mutations. I guess what I don’t know is what you mean by more variation. More than what – what are you comparing the results of drift to? An organism with no neutral mutations, or an organism with an infinite population size?
Genetic drift can increase the prevalence of neutral mutations in a population. Some of these neutral mutations can interact with new mutations to result in a beneficial adaptation, an adaptation that could not have happened with the interaction between the neutral fixed mutation and the new mutation. This is called epistasis. If genetic drift were not occurring then these interactions would not occur. A static genetic background limits the number of potential evolutionary pathways.
If no genetic drift occurred then the only differences we would see between species would be at bases that are under selection. From what I can see, this scenario would result in much lower variation. As a start, you wouldn’t seen any neutral amino acid changes that could be important for adaptations in later populations.
But I’m not suggesting drift is not real, far from it. Drift is always with us. All I ask is how does the effect contribute positively to evolutionary change. If drift is defined as random, how can it? Bear with me or ignore me. I’m genuinely not seeing what it is that has elevated drift to a mantra that must always be inserted into RMNS these days. The fault could be entirely mine. Ask my wife.
I can think of a way where more drift could mean allowing a widening of standing variation (think of the normal distribution of height for example). Under strong selection, the range of variants will narrow proportionally. If the environment changes and a substantially different range of heights becomes advantageous, the current population might find itself entirely outside of this new advantageous range of heights, which means now it has to drift until some individual within this population has a height that it is within this new range where selection can start to act on it.
In this way drift can contribute to adaptability, by a sort of “pre-adaptation”, where the acceptable range of heights within the population is wide enough that should the environment change to favor a new radically different height, there are already individuals within the extant population with a selective advantage. Thus drift can, in principle, contribute to the speed of adaptation.