In article ,
Stewart Robert Hinsley wrote:

The Rosaceae is divided into subfamilies. The peach belongs to one
subfamily, Amygdaloideae. Within this family it belongs to the genus
Prunus - it being Prunus persica. The most closely related fruit to the
peach is the nectarine, which is a variety (i.e. within the same
species) of peach without the surface fuzz. I'd guess the most closely
related species is the apricot, Prunus armeniaca, but as there's several
hundred species of Prunus there's ample opportunity for me to be wrong
on this point.

I'd guess that peach and almond are most closely related of the
domesticated species, based on the similarity of that thing which
surrounds the seed which we were just told is not part of it, no matter
what it looks like. Also, peach and almond have been crossed to
introduce more cold-hardiness into the almond. (And, as you say, peach
is in the subfamily Amygdaloideae ;-))

Apricot crosses easily with Japanese plums (P.salicina). The crosses
are sometimes called plumcots (or apriums), but some plum cultivars
like Early Golden, which derives from a chance seedling and is a
popular commercial variety in Ontario, are thought to be natural
hybrids. Japanese and several American species of 'plum' Prunus have
the same chromosome number and cross readily to produce many cold hardy
cultivars which resemble the Japanese parent in edible quality, but are
much smaller. The European plum has a different chromosome number and
doesn't cross with any of these northeast Asian and North American species.
There's also been a lot of crossing of Canadian prairie and Siberian
species of 'cherry' Prunus with 'plum' Prunus to produce 'chums' or
just cherry-like Prunus with much better eating quality than the wild
species, that are also hardy under really awful conditions of cold
and drought.

What happened to that splitter movement a little while back that was
going to raise up Persica, Armeniaca and other genera in a desperate
effort to make sense out of this very diverse yet closely interrelated
lot of stone fruits?

Btw, if you Know Your Prunus, you can appall passers-by by grazing on
some of the ornamental ones, like teh Nanking cherry, which is quite
good flavoured, though rather big on pit and small on flesh. The white
ones in particular freak people out when they see you eating them. I
don't recommend any of the native chokecherries, though: even the best
of them, the black cherries (P.serotina), are awfully astringent. All
the Amelanchier spp I've tried are very good, too.