When to use 選る/選ぶ?

Question

When might 選る be used over 選ぶ?

Also is there an additional fossilised morpheme deriving one from the other (i.e. is 選ぶ er-ab-u), and if so, what does it mean?

Answer 1

The verbs 選{えら}ぶ, 選{えら}む and 選{よ}る are thought to be derived from the original verb 選{え}る.

Here's what 日本国語大辞典 has to say about the etymology of 選ぶ:

動詞「える(選)」の未然形に、継続を表わす「ふ」の付いたもの。万葉仮名により、上代では、「ふ」は清音であったと考えられる。

Of course, this dictionary is using the traditional analysis where words are segmented on kana boundaries, in which the continuative/repetitive auxiliary ふ attached to the irrealis form えら. That would mean it was e-ra-pu.

Less traditionally, we can segment it er-ap-u, with the continuative auxiliary -ap- attached to the verb er-. At some point the /p/ voiced to /b/, so it became er-ab-u, as you wrote above. This must have happened early on, as although /p/ is indicated in the earliest writing, it was already indicated as /b/ in 栄花物語 (circa 1028). The form with /m/ is recorded in 観智院本『類聚名義抄』 (1241).

In most other words containing this auxiliary the /p/ didn't voice, and so they underwent regular sound change, for example in 戦う tatak-a(w)-u < 戦ふ tatak-ap-u, which is built on the root 叩く tatak-u.

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In Modern Japanese, 選{えら}ぶ is much more common than 選{よ}る／選{え}る. I don't think there's a really big difference in meaning today despite the presence of the continuative morpheme in 選ぶ. In general, you should probably use 選ぶ rather than 選る.

One exception might be compound forms like 選り好む, where you'll use 選る and not 選ぶ. In this case, it's possible to read 選り好む as より or えり, but in general I think えり sounds more archaic and より more modern. But this sort of usage isn't very common.

Another exception is in set phrases like よりによって, which is usually written in kana but actually contains the verb we're discussing. In kanji, it would be 選りに選って. In this sort of use, 選る is still a common word.

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An unrelated verb, すぐる, can also be written 選る. What's more, none of すぐる・える・よる are included as readings on the 常用漢字表. So to avoid confusion, you may want to write all of these in kana. Otherwise, people might not be able to figure out what 選り選る is :-)