What is I-S21825? It is all of I-P37 except I-M26 and I-M423. That means I-S21825 includes the rare I-L233 "Western" branch and the much rarer I-Y11949 "Alpine", I-L1294 "France", and I-L880 "Northern France" branches
Here is a simple tree showing the major groups of I-P37
Note: P37, M26, S21825, M423, L621 and L161 each have dozens of equivalent SNPs, but CTS595 is the only known SNP at its level. Therefore I-P37 split into three branches almost simultaneously (into the I-M26, I-S21825 and I-M423 branches).
And here is the new I-S21825 draft tree:
click here to download a pdf version of the tree.