The previous post in this series can be found here.
In my last post in this series I described some of the ways in which gene therapy is beginning to help in the treatment of genetic disorders. A caveat of this (which was discussed further in the comments section of that post) is that currently available gene therapies do not remove the genetic disorder from the germline cells (i.e. sperm or eggs) of the patient and so do not protect that person's children against inheriting the disease. This could be a problem in the long run as it may allow genetic disorders to become more common within the population. The reason for this is that natural selection would normally remove these faulty genes from the gene pool as their carriers would be less likely to survive and reproduce. If we remove this selection pressure by treating carriers so that they no longer die young, then the faulty gene can spread more widely through the population. If something then happened to disrupt the supply to gene therapeutics - conflict, disaster, etc. - then a larger number of people would be adversely affected and could even die.
Although this is a significant problem to be considered, it is one that is fairly simply avoidable by screening or treating the germline cells of people undergoing gene therapy in order to remove the faulty genes from the gene pool. This is currently beyond our resources on a large scale, but will almost certainly become standard practice in the future.
All of this got me thinking: are there any other genes that might be becoming more or less prevalent in the population as a result of medical science and/or civilisation in general? If so, can we prevent/encourage/direct this process and at what point do we draw the line between this and full-blown genetic engineering of human populations? This is the subject of this post, but before we get into this, I want to first give a little extra detail about how evolution works on a genetic scale.
Evolution by natural selection, as I'm sure you're aware, is simply the selection of traits within organisms based on the way in which those traits affect that organism's fitness. An organism with an advantageous trait is more likely to survive and reproduce and so that trait becomes more and more common within the population. Conversely, traits that disadvantage the organism are quickly lost through negative selection as the organism is less likely to reproduce. The strength of selection in each case is linked to how strongly positive or negative that trait is - i.e. a mutation that reduces an animal's strength by 5% might be lost only slowly from a population, whereas one that reduces it by 90% will probably not make it past one generation. In turn, the strength of that trait is determined by the precise genetic change that has occurred to generate it.