( ( ( ( ( ( ( Horses and ) donkeys ) ( are ( considered ( separate species ) ) ) ) , ) but ) ( ( if ( ( ( a horse ) and ) ( a ( donkey mate ) ) ) ) ( they ( can ( produce ( ( a ( surviving offspring ) ) ( called ( a mule ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NNS Horses) (CC and) (NNS donkeys)) (VP (VBP are) (VP (VBN considered) (NP (JJ separate) (NNS species))))) (, ,) (CC but) (S (FRAG (IN if) (NP (NP (DT a) (NN horse)) (CC and) (NP (DT a) (JJ donkey) (NN mate)))) (NP (PRP they)) (VP (MD can) (VP (VB produce) (NP (NP (DT a) (VBG surviving) (NN offspring)) (VP (VBN called) (NP (DT a) (NN mule))))))) (. .)))
Horses and donkeys are considered separate species, but if a horse and a donkey mate they can produce a surviving offspring called a mule.
(ROOT (S (SBAR (IN If) (S (NP (CD two) (NNS populations)) (VP (VBP do) (RB not) (VP (VB mate) (CC and) (VB produce) (NP (ADJP (JJ fertile)) (NN offspring)))))) (, ,) (NP (JJ separate) (NNS species)) (VP (VBP result)) (. .)))