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It follows that any morphological change that isn't neutral, which your proposition is not since it effects survival, must be equal to, or greater than, the morphological benefits of a normal, fit organism of the same species it is in competition with. Anything less than equivalence would immediately infer a disadvantage, and grounds for falsification that it would dominate a population.

You rely on "the survival of the fittest" and therefore only something better can survive. But that is not true at all. Why? Very easy to explain.

A population is not located in only one place where all are always competing with each other. In fact populations are dislocated.

If the environmental pressure is lowered then more or less handicapped individuals can survive. And diversity even is needed to adapt to changing environments.

For me it's not "survival of the fittest" it's "the dead before breeding".

Human examples:
A woman died due to breast cancer with 45 years but she got 5 children. A physical healthy woman died with 89 year but without children.

Quarterback and a cheerleader died during a car accident because one of them drove drunken and the other one was an idiot driving with the drunken one. The biggest computer nerd would have driven safely.

You'll will never know on what field you'll need to fit to survive. Maybe you must run very fast maybe you have to survive a long time without meal. The advantage useful to your father must not be useful for you.

Although it's possible that the trait would dominate a population, I simply don't see any selective advantage that it would do so.

Because there is no "selective advantage" for just one trait! Your reasoning is monocausal and therefore not applicable to living matter.

To my understanding, survival of the fittest is not the same thing as natural selection, in the context of modern evolutionary theory, or synthesis theory.

Since there may be numerous individuals within the same population that are equally fit for different reasons, you would have to explain the dominance of a given trait within this seeming disparity...and natural selection, to my knowledge is the only mechanism that can account for this.

Furthermore, I was under the impression that we're not talking about convergent evolution, i.e., the spread of similar traits within isolated, or dislocated populations.

That said, I'm not sure how your scenarios would account for the pervasiveness of any given trait, let alone the one we're talking about here. It seems completely incoherent to me and disassociated with the principles of evolutionary theory.

I could be wrong, but that's just my opinion

Just in terms of your first example (the breast cancer) - that is exactly why cancer is one of the leading killers in developed countries today. Almost all cancers begin in middle or old age. Why? Because there was no selective advantage to staying alive to that age. After women go through menopause, they can't reproduce. Evolution picks traits that are good for continued reproduction.

If there was a gene for dying at age 50 in women, evolution would essentially ignore that gene. Because by age 50, women have had all the children they're going to have. If there was a gene for 'living past age one,' then that gene would be overrepresented in the gene pool, as people who did not have that gene would not have children. However, the gene for 'living past age 50' is not going to be overrepresented in the gene pool, as women can't have children past that age.

So it's not really a coincidence that most cancers start striking once people have reached the end of their reproductive life span. Evolution has made it that way.