OK, and I guess we can live without 最も新しい alone.

evolutionary biology or phylogenetics, yeah.  but i mean i only get 7 google hits for "もっとも新しい共通祖先" (0 ngrams).  最も近い共通祖先 (for which we already have an entry) at least gets 25 ngrams.  the meaning is clear, and short of a solid reference, i think this can be omitted

Apologies for not responding sooner. I don't 
think we need "もっとも新しい" as an entry 
anymore than we need any random もっとも+adj-i 
combination. もっとも新しい共通祖先 on the other 
hand isn't A+B as it refers to a very specific 
concept in evolutionary biology (or whatever 
the specific branch of science might be 
called).

OK. Here goes with my suggested change. Maybe "exp".

@@ -5 +5 @@
-<keb>最も新しい共通祖先</keb>
+<keb>最も新しい</keb>
@@ -8 +8 @@
-<reb>もっともあたらしいきょうつうそせん</reb>
+<reb>もっともあたらしい</reb>
@@ -11,4 +11,4 @@
-<pos>&exp;</pos>
-<pos>&n;</pos>
-<xref type="see" seq="2689860">最も近い共通祖先</xref>
-<gloss>most recent common ancestor</gloss>
+<pos>&adj-i;</pos>
+<gloss>newest</gloss>
+<gloss>up-to-the-minute</gloss>
+<gloss>most recent</gloss>

最も新しい共通祖先	< 20
最も新しい	50840
最も近い	254693
最も近い共通祖先	25
Eijiro: 最も新しいレポート : up-to-the-minute report
世界で最も新しい国 : the world's newest nation

While I think it's harmless, I think a much better inclusion would be 最も新しい => newest/up-to-the-minute/most recent/etc. Then we can happily put this aside as a rare A+B.