Here is a story that discusses the changing diagnosis of autism, a hot topic this week in the science news. The doctor interviewed, Dr Bryan King, has spent the last 5 years working with a committee charged with revising the diagnosis of autism for DSM-5, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, the diagnostic standard for psychiatric illnesses. Reports of a dramatic increase in the prevalence of autism, along with genetic findings revealing autism's complexity (which we've posted about), are in the news. So Dr King, involved in setting the standard diagnostic criteria for autism or autism spectrum disorder (ASD), is interviewed about the process.
Obviously, neither environmental nor genetic factors cause 'autism' per se, if the very meaning of the term changes. ASD is in some ways a cultural trait, since it's we who define it. If we change our definition, the risks associated with specific genes or environments necessarily change as well--yet, in physical terms, they have clearly not changed at all! If a genetic variant conferred a risk of, say, 0.5% of autism 10 years ago, then today on average it would confer nearly 1.0% (twice as much as before).
This is one problem with doing the genetics of 'autism' when the trait you're doing the genetics of is a moveable target. The politics and other aspects of the diagnostic criteria may or may not be proper, but certainly the behavioral cutoff is cultural both in the sense of its manifestation in a given cultural setting, but also in the way that setting sets diagnostic criteria.
Relevant to MT is that Nature probably works the same way. Here the key issue is natural selection. Natural selection is a screen of organisms for traits that are more, or less, compatible with local circumstances. But those circumstances change, sometimes rapidly. Thus, like cultural definitions, the criteria that determine the relative fitness--reproductive success--are changing. This means that here, too, the fitness of particular genetic variants is context-dependent, not fixed or absolute.
This is one of the challenging aspects of evolutionary biology, because it is tempting to view a genotype as inherently good or bad, inherently likely to succeed or not. That makes theory and modeling of natural selection, evolution, and species formation tractable.
But Nature may not be like that. If fitness is a shifting phenomenon, which it certainly is to at least some extent, then everything is context-dependent, and relative to circumstances, all the time. So many of the scenarios proposed to account for what we see today may have a degree of the arbitrariness of the definition of a given trait, like autism.