In the book written by John Endler Natural Selection in the Wild p. 4 
it says that even if condition a, b and c are met, evolution by natural selection might occur,
[...] , but not necessarily, [...]
See last paragraph of the image. Endler, J. A. 1986. Natural Selection in the Wild. Princeton University Press.
I’m wondering in which case it’s possible that everything is in place for natural selection to occur, but is not having any effect.
It seems to me that if every condition of natural selection are met, there will absolutely a change in the phenotypic distribution? Why does the text say evolution is not guaranteed to occur?
Answer
What does the sentence below mean?
As a result of this process, but not necessarily, the trait distribution may change in a predictable way.
To my understanding, but not necessarily means that other processes than natural selection can affect the trait distribution in a predictable way.
Other than natural selection, what affects the trait distribution in a predictable way?
I am not trying to make an exhaustive list but I am just providing here two obvious examples.
An intense gene flow (migration) from a nearby population will also cause the trait distribution in a local population to change in a predictable way.
Genetic drift, will reduce genetic variance and therefore reduce the variance of the trait distribution. While the change of the mean of the trait distribution is not predictable under genetic drift, its change in variance is predictable.
Note that the book has been published in 1986 and the vocabulary and science is probably slightly outdated. If you are learning about evolutionary biology, you can probably find better source of knowledge. For example Understanding Evolution is a free online introduction to evolutionary biology. There are many textbooks that offer very good introductions to evolutionary biology.
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