Natural selection doesn't "select" for anything except the survival and continuation of species. Everything else that it tends to favor is merely a correlation with that - so you have to expect a certain level of variation and exception.

> It may not try to reduce dependencies.

This one's pretty straightforward; natural selection has two very common approaches to dependencies:

1. If the presence of something is not a reliable feature of the environment, dependence on that something will be ruthlessly weeded out.

2. If the presence of something is a reliable feature of the environment, dependence on that something will be created and grow to permeate most existing systems.

The first effect has a lot of popular awareness; the second doesn't. They're both important.

Counterargument to 2: the air is largely nitrogen, but humans do not utilize it during respiration. Thus, this reliable feature of the environment has not been selected as a dependency.

I'm not necessarily arguing it's untrue, just not quite the hard-and-fast rule that 1 is.

> I'm not necessarily arguing it's untrue, just not quite the hard-and-fast rule that 1 is.

This is pretty subjective. If you grow up with sufficient dietary iodine, that's worth 10-15 IQ points (a gigantic amount) compared to the counterfactual. Iodine supplementation is highly effective in populations that have presumably been going without for, at the very least, many hundreds of years. This is the reason we iodize salt.

With that in mind, how much of a hard-and-fast rule is #1?

The other side of this argument is that dependencies can be surprisingly subtle. This is pure speculation, but the thing that leaps out at me about atmospheric nitrogen is that oxygen is extremely corrosive. If the atmosphere were more oxygen and less nitrogen, lungs of today might experience a lot more wear and tear than they are currently designed for. (Or not! I really have no idea. But I think the hypothetical has some thought-experiment value anyway.)

How should we use nitrogen gas in the air? Oxygen is useful to us for energy production. Nitrogen we get from easier sources in different forms.

Humans are systemically dependent on nitrogen in the air, in that it is required for nitrogen fixation.

Yes indeed, there are examples of dependencies that have been removed by evolution.

For example cobalt and nickel were both essential for the first living beings. However, on the continents they are much less accessible than in the oceans.

Because of that, most terrestrial plants have lost the dependency on cobalt (which is why they cannot provide vitamin B12 to those who eat them) and most have also lost the dependency on nickel (except that for many terrestrial plants nickel, while no longer strictly required, can still be useful by enabling them to use urea as a source of nitrogen, when other sources of nitrogen are scarce).

> 1. If the presence of something is not a reliable feature of the environment, dependence on that something will be ruthlessly weeded out.

Something is only weeded out if it is sufficiently disadvantageous. There are lots of examples in biology where mutations are neutral and allowed to accumulate. There is also a lot of junk DNA, and even though we've found that some junk DNA was misclassified, there still is a lot of junk DNA in each genome

Dependence on something that isn't there is disadvantageous by the definition of "dependence". I don't see the point you're trying to make.

In my opinion, the second is just not true.

If a reliable feature is evolutionary advantageous, then new systems will come about to take advantage of that (and may out-compete old systems, indeed, to the point of extinction), I agree.

But, if a reliable feature isn't evolutionary advantageous, it can be ignored or even removed.

Negative features may need to be managed, rather than taken advantage of. Examples: ionizing radiation, landfill.

Interesting. Any examples of the 2nd?

Honeybees, sunlight, water, oxygen, etc.

Edit: Also - https://en.wikipedia.org/wiki/Radiotrophic_fungus

Another awesome example:

https://en.wikipedia.org/wiki/Nylon-eating_bacteria

In mammals, the dependance on a certain body temperature.

The fact that it is regulated to a margin of 0.3% shows quite how important it is. Increase or decrease the body temperature by 5% (on an absolute scale, so Kelvin), and you will be a dead human...

> The fact that it is regulated to a margin of 0.3% shows quite how important it is.

This is an excellent point. It can be applied to a huge number of biological phenomena, but it almost never is.

An example where this idea actually has made it into the mainstream is body weight. I read an article that pointed out that, yes, we're fatter than we used to be, but regulating body weight by measuring energy intake and expenditure is doomed to fail. Articles making this claim are a very rich genre, but I found the particular argument fascinating:

People routinely maintain the same body weight, within say 5 pounds, for years at a time. This is a level of accuracy that we do not have the technology to achieve by using measurements of energy inflow and outflow -- we can't measure dietary energy that precisely! (And modern technology is, by most standards, very precise.)

Thus, there is a very strong suggestion, based on this incredible stability, that your body is regulating itself to a certain weight. If you adjust energy intake, or outflow, other things will adjust accordingly; those are unlikely to be primary determinants of the system's behavior.

A question from a layman: Could changing body's temperature by a few percent, for a small amount of time, be used to heal from some diseases, like cancer or diabetes?

Some diseases, yes; people do this all the time and it is called "running a fever". Fevers are not induced by the disease; they are a strategy you use to kill the disease.

Cancer or diabetes, no. Diabetes is not even a "disease" in the sense of an organism that can be alive or dead.

Thanks for the answer!

Inevitable becomes necessity.