Well, I had a great time at the Junior Cafe in Tauranga yesterday :-) I always like to talk about things that interest me with other equally interested people, & the questions afterwards were really stimulating. As I've said before, I enjoy challenging questions because they stimulate my own learning :-)
Anyway, there was a question about mutations & whether they drive evolution. My answer was basically, no, but they provide some of the raw material (the genetic variation underlying the phenotype, which is the thing actually 'visible' to natural selection) that selection pressures can work on. And then came the deep question - when does something stop being a mutation & become 'normal'? What is 'normal', anyway?
I think the answer has a lot to do with how common the mutation is in a population's gene pool. Suppose (as a thought experiment) that an error in the DNA involved in coding for hair colour produced someone with bright purple hair. (OK, I know you can get that effect using something out of a bottle, but bear with me!) At first, the rare individuals expressing that allele would probably be viewed as a bit freakish, different; 'mutants' perhaps. But if the allele had no negative impact on the reproductive success of people expressing it & in fact enhanced their ability to produce offspring, over time it would become more common in the population as a whole. And at some point it would cease to be regarded as a mutation & become viewed as part of the general broad spectrum of normal human variation.
And perhaps we are a bit too ready to label things that are different, to call them mutations & - because the word 'mutations' can carry negative connotations in many people's minds - to ascribe less value to individuals with those features. This is something that really annoys me in regard to some of the more extreme 'anti-vaxxers', in the US in particular.
There is an on-going, essentially manufactured controversy in the States over a purported (but non-existent) link between vaccination & what is known as autism-spectrum-disorder (ASD). That's bad enough, because there is the real possibility that concern over this will lead people to avoid vaccinations & lead to the resurgence of vaccine-preventable diseases. But with regard to my initial discussion of 'normal', what really really annoys me is the way that some parents describe severely autistic children as 'damaged' or 'lost' or 'destroyed'. While in some cases use of these terms could simply reflect the parents' frustration about being unable to find some cause to blame for their child's autism, it still comes across (to me, anyway) as wrong. The children are different, yes; disabled - in some cases, yes. But lost, destroyed???
By the way, the fact that ASD can run in families suggests that there is some genetic component. And there appears to be general agreement that perhaps 1 in 150 individuals is affected by ASD to some degree. That's actually fairly common. Common enough to be viewed as part of normal human variation??
(Oliver Sacks' book, An anthropologist on Mars, contains a superb study of an inidividual with Asperger's syndrome, part of the complex of ASD.)