Regarding chromosomes and persistance of traits. My late husband and I visited ancestral property in southern Illinois a few years before he died in 2005. We found a distant relative on adjacent land. It turned out they had a common ancestor in the late 1700’s. There were certain physical similarities, around the eyes, and they both had similar long fleshy ears. In my husband’s case it was the great grandfather. For the other man it was the great, great grandfather. His line married at a younger age.
Regarding fitness, I think it is survival of the more fit, but as conditions often change, what fit is also tends to change. Vive la diferance.
[snip]
In mere impressionism we take our stand. We have no positive tests nor standards. Realism in art: realism in science—they pass away. In 1859, the thing to do was to accept Darwinism; now many biologists are revolting and trying to conceive of something else. The thing to do was to accept it in its day, but Darwinism of course was never proved:
The fittest survive.
What is meant by the fittest?
Not the strongest; not the cleverest—
Weakness and stupidity everywhere survive.
There is no way of determining fitness except in that a thing does survive.
“Fitness,” then, is only another name for “survival.”
Darwinism:
That survivors survive.
(Charles Fort, “Book of the Damned”, pp. 23-24)