( ( ( The members ) ( of ( a ( unique species ) ) ) ) ( ( can ( mate ( with ( each ( ( other ( to ( produce fertile ) ) ) offspring ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNS members)) (PP (IN of) (NP (DT a) (JJ unique) (NN species)))) (VP (MD can) (VP (VB mate) (PP (IN with) (NP (DT each) (ADJP (JJ other) (S (VP (TO to) (VP (VB produce) (ADJP (JJ fertile)))))) (NN offspring))))) (. .)))
The members of a unique species can mate with each other to produce fertile offspring.
(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)) (. .)))