I read once (maybe in a Dawkins book) that large gaps also help preserve genes during sexual reproduction when chromosome crossover happens. Basically if the crossover point happens in the middle of a gene, that gene doesn’t survive the meiosis… having large gaps increases the odds that crossover happens in junk DNA. I’m not sure how true/oversimplified this is though.
What I read was from “The Selfish Gene”, I went and dug up the section:
> A gene is defined as any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection. In the words of the previous chapter, a gene is a replicator with high copying-fidelity. Copying-fidelity is another way of saying longevity-in-the-form-of-copies and I shall abbreviate this simply to longevity. The definition will take some justifying.
> On any definition, a gene has to be a portion of a chromosome. The question is, how big a portion—how much of the ticker tape? Imagine any sequence of adjacent code-letters on the tape. Call the sequence a genetic unit. It might be a sequence of only ten letters within one cistron; it might be a sequence of eight cistrons; it might start and end in mid-cistron. It will overlap with other genetic units. It will include smaller units, and it will form part of larger units. No matter how long or short it is, for the purposes of the present argument, this is what we are calling a genetic unit. It is just a length of chromosome, not physically differentiated from the rest of the chromosome in any way.
> Now comes the important point. The shorter a genetic unit is, the longer—in generations—it is likely to live. In particular, the less likely it is to be split by any one crossing-over. Suppose a whole chromosome is, on average, likely to undergo one cross-over every time a sperm or egg is made by meiotic division, and this cross-over can happen anywhere along its length. If we consider a very large genetic unit, say half the length of the chromosome, there is a 50 per cent chance that the unit will be split at each meiosis. If the genetic unit we are considering is only 1 per cent of the length of the chromosome, we can assume that it has only a 1 per cent chance of being split in any one meiotic division. This means that the unit can expect to survive for a large number of generations in the individual’s descendants. A single cistron is likely to be much less than 1 per cent of the length of a chromosome. Even a group of several neighbouring cistrons can expect to live many generations before being broken up by crossing over.
[end quote]
I think a reasonable extrapolation from this is that “genes” (things that are subject to natural selection in our genome) can survive more generations if they are surrounded by genetic material that are not “genes” (ie. Their copying fidelity is not subject to any selection pressure.)
If a gene in this sense is recombined in a way that makes it no longer the same gene, it most likely isn’t going to be beneficial to the organism (and thus the gene’s longevity.) Most genetic mutations aren’t.
Funny, I read that same section two weeks ago. Dawkins’ definition is not what geneticists typical consider a gene. His “gene” is more like what I would call a haplotype in that he divorces “gene” from “protein-coding” region. But he has a good point and I like this section. But he wrote it in the genetic dark ages and we now know quite precisely where and how often recombinations occurs, and for how long haplotypes are preserved as a function of generation numbers.
Highly recommend David Reich’s book for are good overview of the math of recombination in humans.
Thanks for the recommendation! I’ll have to check it out.
I got into Dawkins’ books initially from The God Delusion (as I suspect many laypeople), and heard about The Selfish Gene from there, so evolutionary biology is not my area.
It makes sense that TSG is considered the dark ages as it’s such an old book. I was always curious to read more about the topic from other — and hopefully more recent — biologists, since Dawkins sometimes feels like he’s more of a communicator than a practicing biologist (and one with a particularly anti-religious chip on his shoulder, not that he’s wrong.)
