If the rules say that an eidolon gains X ability, but doesn't mention the evolutions, does X ability duplicate the relevant evolution?
Take the serpentine eidolon base form for example; it has a tail slap attack.
Does this mean it also has the tail and tail slap evolutions? Or just a natural attack? If it is the former, then there are skill bonuses towards Acrobatics; if it is the latter, then only the natural attack is gained.
This might also make a difference for things that effect evolutions (as opposed to just free abilities) or for evolutions that limit how many times you take them.
They are evolutions.
Base Form: Eidolons of some subtypes are restricted in their choice of base form. Restrictions are spelled out here. The evolutions listed here are gained automatically, and do not cost points from the eidolon's evolution pool.
So when an eidolon takes a serpentine base form, they look at the subtype in question and see a line like this:
serpentine (bite, reach [bite], reach [sting], sting, tail)
Daemon serpentines would gain those evolutions for free, but not tail slap.
serpentine (bite, improved natural armor, reach [bite], tail, tail slap).
Elemental serpentines would get tail slap.
I think they included the abilities in the base form descriptions for ease of reading, or something.
you'll find the answer in the eidolon subtypes
Each subtype that allows serpentine form lists the specific evolutions you get that go along with that tail slap, and they all mention "tail"
For example the daemon subtype for serpentine gives you "serpentine (bite, reach [bite], reach [sting], sting, tail)"