And what if it's true for a short period of time until a new mutation arises that eliminates the double edged sword? Most of our adaptations are really really old. They've had enough time to "perfect" themselves.
What if evolution isn't clean or pretty? What if it's a messy business where a gene arises that gives a small edge, but brings with it a host of problems. It'll still spread because of that edge and it may take millions of years to sort out and eliminate the side-effects.
Because it doesn't. That sort of pleiotropy just is not common, empirically, in humans. Hundreds or thousands of genetic correlations tend to confirm this: there are loads of free lunches. Why is that? A lot of it looks like relaxed selection (eg my other submission shows that our ancestors thousands of years ago appear to have had substantially lower genetic risk of disease because it was decreasing over time, but has reversed), the slowness of selection given the small effect sizes vs the constant incoming stream of new mutations, and the demographic history of humanity of massive expansion from a small starting population based on migration of also tiny groups.
If I understood you correctly, environment pressure relaxed for former traits. But then the new human environment caused the presure for other set of mutations to kick in, even if they're counterproductive for old environment...
There really is no 'hiding' in evolution; if a trait has deleterious effects, even only in specific genetic or environmental contexts, then they are felt. It is this brutally simple force that permits such complex systems to function effectively.
I see it as almost axiomatically true, but I think the difference is you are taking a narrow definition of what constitutes a "bad" trait. Examples:
- Immune system: protects us from microbes but contributes to atherosclerosis, autoimmunity, and age-associated chronic inflammation
- Mitochondria: Generates ATP but also oxidative stress
- Infant brain size: makes humans as smart as we are but increases maternal mortality
- Body mass/volume: gives greater protection against predation but increases energy requirements
- Intelligence & brain volume: improves on-the-fly adaptability but costs a lot of energy to run
In the most general case, any gain-of-function adaptation which adds a new protein/organelle/limb/etc has a built-in bad trait which is that it costs resources to produce, DNA to maintain, and also adds another potential point of failure. The equilibrium point between benefit and costs is where evolution will land.
So I would argue that the reason we have X IQ points or Y neurons is exactly the same reason we have Z mitochondria: any more and the costs would have exceeded the benefits in the ancestral environment. The fact that we can seemingly get more benefits than costs from increased intelligence now merely reflects the fact that our environment has changed.
Also, JBS Haldane wrote an anti-eugenics book many years ago which used exactly this sort of argument.
if it were true, we would see all sorts of genetic
correlations where one good trait systematically
boosts a bad trait