( ( ( ( species ( ( ( A population ) ( of ( similar individuals ) ) ) ( that breed ) ) ) and ) ( ( produce ( fertile offspring ) ) ( under ( natural conditions ) ) ) ) . )
(ROOT (S (VP (VP (VB species) (NP (NP (DT A) (NN population)) (PP (IN of) (NP (JJ similar) (NNS individuals))) (SBAR (WHNP (WDT that)) (S (VP (VBP breed)))))) (CC and) (VP (VB produce) (NP (ADJP (JJ fertile)) (NN offspring)) (PP (IN under) (NP (JJ natural) (NNS conditions))))) (. .)))
species A population of similar individuals that breed and produce fertile offspring under natural conditions.
(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)) (. .)))