Nietzsche uses the word Selektion.
dict.leo.org/ende?lp=ende&p=hPXz … =Selektion
We can dismiss the meaning of “screening”, of course. Now let us see what Wikipedia says about selection:
“Natural selection is the most familiar type of selection by name. The breeding of dogs, cows and horses, however, represents “artificial selection.” Subcategories of natural selection are also sometimes distinguished. These include sexual selection, ecological selection, stabilizing selection, disruptive selection and directional selection (more on these below).”
en.wikipedia.org/wiki/Selection
As you can see, this includes all meanings given by LEO except for the one we dismissed.

As you can see from this image, sexual selection is a form of natural selection. This means there are broadly two kinds of selection: natural and artificial. Wikipedia puts the phrase “artificial selection” between quotation marks, I think, because human beings and the things made by them are still a part of nature, of course. What then is artificial selection as opposed to natural selection? As the article on artificial selection puts it, the latter is intentional. In the article on sexual selection, something called “mate choice” is mentioned. This is a natural preference a member of one sex may have for certain members of the opposite sex (members with certain traits). Though this is a matter of “choice”, it is unintentional because it is instinctive. I think therefore that “intentional”, in the article on artificial selection, means so much as “rational”: man has reasons to breed certain types of cows, cows with certain traits.
Now Nietzsche says pity crosses the law of selection. This can only happen by means of articificial selection. Pity crosses natural selection by influencing or even determining artificial selection. When this happens, what happens is that emotion overpowers reason. Nietzsche uses the word Mitleid, which is literally “compassion” (both mean literally “suffering with”: see etymonline.com/index.php?term=compassion). I think “pity” is a better translation, though. But the word “compassion” is instructive as it includes the word “passion”. And a passion is basically a strong emotion. The emotion of Mitleid, therefore, is so strong that it tends to overpower reason. This is why Nietzsche says somewhere (I quote this from heart) that he recognised Mitleid as “more dangerous than any vice”.
Nietzsche uses the Latinism Selektion rather than the Germanic Auslese. These words are more or less interchangeable, as they both derive from words meaning “to read” (see dict.leo.org/ende?lp=ende&p=hPXz…&search=Lesen). I think he used the Latinism because it is a more technical term (as you can see at LEO, Auslese is translated as “selection” whereas Selektion is translated as “selection [biol.]”. Auslese may mean any form of selection, whereas Selektion means specific, especially biological, forms of selection. But why then did he not use the Latinism Evolution rather than the Germanic Entwicklung? I think because he meant both natural development (id est, “evolution” in the Darwinian sense) as well as artificial development, even design (which LEO provides as a possible translation of Entwicklung). Pity does not “just” cross natural development. More primarily, it crosses rational artificial development - that development by which man tries to perfect nature through art (in the broadest sense of the word). Thus the Spartans sought to perfect nature by breeding and disciplining a certain type of man. As one Moody Lawless once wrote:
“The Spartans themselves were art;- they lived art, they embodied art, they personified art - so much so, that there was no distinction bewteen art and life in Sparta.”