The relationships of the major branches of I-P37.2 are now known. These five branches and their nicknames:
M26 (Sardinian)
M423 (Dinaric, Disles, Isles)
L1286 (Alpine and L233 Western)
L880 (Northern France)
L1294 (France)
The key SNPs that show the history of these branches are:
CTS595, S21825, and L1286. The importance of each of these was learned in a different way:
CTS595 was included in the Geno 2.0 test
S21825 was included in the Chromo2 test at www.britainsdna.com and their chief scientist Jim Wilson released 2000 anonymous results in January. Ken Nordtvedt compared these results. See the most recent Chromo2P37 tree at Ken's website.
Finally, L1286 was discovered back in 2012 in a WTY (Walk Thorough the Y) by Thomas and Astrid Krahn back when they were at Family Tree DNA. For a long time we thought that only the Alpine and the I-L233 Western groups were L1286+, but then Zdenko Markovic noticed that not all I-P37.2 groups had tested it, and it turned out that the I-L880 "Northern France" group is also L1286+.
We used the nicknames "Sardinian" "Dinaric" etc. for many years because in many cases no SNPs were known to define the branches. For example, there were a few men with P37.2+ M26- M42- L233- results who all had a distinctive pattern of STR markers including DYS388=9. Many of the men had paternal origins from France (but others from Germany, Britain etc), so haplogroup I expert Ken Nordtvedt called the group P37.2*-"France" just to have an easy way to refer to the group.
Now all of the major groups have their own defining SNP and most of the subgroups do too. But very few of these SNPs are on the ISOGG tree. Zdenko Markovic joined the FTDNA I2a Project as co-administrator in January, and began coordinating the P37.2 part of the ISOGG tree at the same time. Thanks to his efforts several SNPs in the M423 area have been added to the ISOGG tree, and so has L880, and he's working on many more additions.