It seems to me that such speciation supports the assumption of endurance. Over time you'd see selection for species which are patient, diligent, fecund star colonizers. Just like medieval Europeans spent decades or centuries building a cathedral, over time you'd select for species which spend centuries working on their next starshot.

"Over time you'd see selection for species which are patient, diligent, fecund star colonizers."

Or for species that excel at command-deck politics.

Depends on if the explorers are the ones doing the bulk of the breeding. On planet earth, educated people actually tend to have fewer kids. There is therefore selective pressure against intelligence on earth.

I'd expect the first exploration to be done by machines, and digital transfer of the controlling instance would remove almost all drift.

Machines suffer mutation rate too. Cosmic ray induced bitflips could be possible. Although since we are all spitballing anyhow maybe you can handwave a cosmic shield along with your cosmic explorer.

It would also be interesting if the host system collapsed. That would be some interesting scifi fodder: advanced civilization sends out probes but by the time FTL visitors show up, their civilization already collapsed to the stone age.