Heh, of course, in reallife, a species differs from a culture.
But in the Players Handbook, the differences between races look almost entirely cultural.
For example, among the elven subraces, the sea elf can breathe water and the avariel elves can fly by means of wings. The elves seem to have acquired their biological gills or wings − by means of shapeshifting magic. In other words, even these physical features are cultural after all.
That’s a wild leap.
They gained those feature because they are biologically prone to rapid adaptation with pretty extreme potentials. They didn’t cast spells to get wings or gills.
Actually, I think either can work . . . .
Did
sea elves evolve over time, magically or genetically, to breathe water? Or perhaps they were changed instantaneously in the ancient past due to some magical event (wizard, god) to adapt to life under the sea, but then passed down that trait genetically/magically? This idea works fine, and
kinda-sorta matches our modern scientific ideas about biological evolution . . . .
Or maybe it's a mutant ability activating, X-Men (X-Person?) style (for an entire population) . . . throw a group of elves into the sea without a boat, and the trauma might activate a situation-dependent mutation to allow them to survive . . . . and then that ability is passed down generation after generation . . .
But . . . perhaps all elves have an innate ability to adapt to extreme environments, but must be trained to adapt to a specific environment. Perhaps
sea elves are born in air-filled caves under the sea, and must be taught to adapt to underwater breathing. Or, perhaps elf babies can be taught when still in the womb . . . If you have a
wood elf raised by
sea elves, perhaps they too can learn to adapt to water-breathing! Perhaps even adult elves can learn to adapt to new environments, although it would be difficult and thought near impossible (much like language acquisition by adults IRL).
Even more extreme adaptations, like growing fins for
sea elves and wings for
avariel . . . could be described as learned adaptations, using the elven magical adaptability to grow fins or wings when young . . .
EDIT: Forgot to address @doctorbadwolf and his pushback against mortal elves casting spells to adapt to their environments . . . why not? Elves are described as being very magical beings, both innate magic and the ability to use high levels of wizardry. In D&D fiction, elven high mages cast powerful spells to adapt their environments to the elves' needs, why not powerful spells to adapt their own bodies? Once the high magic has been cast and an elven population now has fins and can breathe underwater, or has wings and can fly through the air . . . these traits are passed down to the generations. Or maybe the spells need to be cast for each new generation! Maybe the high magic is lost, and the elves are now dependent on powerful magical items to accomplish the adaptation (mythals?).
Something to keep in mind . . . we are talking about changing the mechanics of race in D&D, but we can also change the story behind those mechanics . . . how much we change the story depends on our own personal preferences, but there isn't anything stopping us!