Anya compares the evolution of a bookstore to a coffee-bookstore to a coffee shop with biological evolution, only without the "getting better" part. But that is a common misconception of Darwin's theory of evolution, which imparts no value judgment on adaptations.
When Dawn is possessed by the demon that "hitched a ride" with Buffy when she was brought back from the dead, Dawn breathes fire and a pipe is clearly visible in one of the shots with the fire coming out of it.
When Spike assures Summers sisters he too had to dig with bare hands out of his grave, it's just a merciful lie: Spike never had a "proper" funeral ( Not Fade Away (2004) ). Dru probably just put his body some inches underground, as she did with Darla ( Reunion (1995) ).