The most common and most likely answer to this crossword clue is the 8 letter word ECOTYPES
I've seen this crossword clue in The Guardian
E
C
O
T
Y
P
E
S
Buddy explains!

The straight part
We know that "these races adapted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case these races adapted = ecotypes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ECOTYPES mean?
ECOTYPES noun- A phenotype that is adapted to a specific environment.