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