( ( A species ) ( ( is ( ( a population ) ( of ( ( living organisms ) ( that ( can ( ( ( mate and ) reproduce ) ( fertile offspring ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NNS species)) (VP (VBZ is) (NP (NP (DT a) (NN population)) (PP (IN of) (NP (NP (VBG living) (NNS organisms)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (VP (VB mate) (CC and) (VB reproduce) (NP (ADJP (JJ fertile)) (NN offspring)))))))))) (. .)))
A species is a population of living organisms that can mate and reproduce 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)) (. .)))