Whether a thing can be selected-out depends on the shape of the fitness landscape in the environment.
For example, appendix-bursts are clearly rare enough and treatable-enough that they cannot be selected-out in modern humans. (But almost nothing can if almost everyone is able to reproduce, and any selection effects will be driven by the number of children which is largely cultural.)
If a thing hasn't been selected out, you can roughly conclude either that:
1. The selection pressure to do so isn't strong. Either few appendix bursts occur in an ancestral env, or they don't disrupt reproduction bc they happen later in life, or are treatable, or other causes of death kick in before the appendix matters.
2. Or, if the selection pressure is strong, there is "nowhere to go" in gene-space that improves this aspect of fitness, within the search-radius. (Which is really equivalent to 1: the selection pressure isn't strong enough to search widely enough)
3. Or there is a stronger selection pressure for it, even if you can't figure out what it is, like the "backup gut bacteria" thing for the appendix. (Which is actually equivalent to 1/2 also: the selection pressure isn't strong enough to find a way to separate the upside from the downside)
For example, appendix-bursts are clearly rare enough and treatable-enough that they cannot be selected-out in modern humans. (But almost nothing can if almost everyone is able to reproduce, and any selection effects will be driven by the number of children which is largely cultural.)
If a thing hasn't been selected out, you can roughly conclude either that:
1. The selection pressure to do so isn't strong. Either few appendix bursts occur in an ancestral env, or they don't disrupt reproduction bc they happen later in life, or are treatable, or other causes of death kick in before the appendix matters.
2. Or, if the selection pressure is strong, there is "nowhere to go" in gene-space that improves this aspect of fitness, within the search-radius. (Which is really equivalent to 1: the selection pressure isn't strong enough to search widely enough)
3. Or there is a stronger selection pressure for it, even if you can't figure out what it is, like the "backup gut bacteria" thing for the appendix. (Which is actually equivalent to 1/2 also: the selection pressure isn't strong enough to find a way to separate the upside from the downside)