alternative strategies should not persist in a population

[keannu]

Does the "should" in "alternative strategies (should) not persist in a population" mean presumption or original meaning of "responsibility"? I can't understand the whole underlined part. Is the writer saying that they, the fish should stop alternating in eating either on the left or right?

ex)A scale-eating cichlid fish has two forms, one that eats scales off the right side of its victims and one that eats scales off the left side. Individual fish inherit the strategy and mouthparts twisted in the appropriate direction; they cannot switch between the alternative strategies by feeding sometimes on the left and sometimes on the right. Ordinarily, alternative strategies (should) not persist in a population, because if one strategy bestows even slightly higher fitness, it should increase in frequency relative to the other until only the better strategy remains.

[Raymott]

Does the "should" in "alternative strategies (should) not persist in a population" mean presumption or original meaning of "responsibility"? I can't understand the whole underlined part. Is the writer saying that they, the fish should stop alternating in eating either on the left or right?

ex)A scale-eating cichlid fish has two forms, one that eats scales off the right side of its victims and one that eats scales off the left side. Individual fish inherit the strategy and mouthparts twisted in the appropriate direction; they cannot switch between the alternative strategies by feeding sometimes on the left and sometimes on the right. Ordinarily, alternative strategies (should) not persist in a population, because if one strategy bestows even slightly higher fitness, it should increase in frequency relative to the other until only the better strategy remains.

"Ordinarily, alternative strategies (should) not persist" means "Ordinarily, one would not expect alternative strategies to persist (according to the theory of Natural Selection)"

[keannu]

"Ordinarily, alternative strategies (should) not persist" means "Ordinarily, one would not expect alternative strategies to persist (according to the theory of Natural Selection)"

So, it's expectation not obligation. So the second should is the same expectation, I guess, right?
it should increase in frequency relative to the other until
Does alternative strategy mean "you can choose left-feeder or right-feeder whenever you want?" or "just stick to one side"?

[5jj]

Yes