( ( ( The concept ) ( of ( a species ) ) ) ( ( ( ( ( often ( refers ( to ( ( ( a population ) ( of ( ( physically similar ) individuals ) ) ) ( that ( ( can successfully ) ( mate ( between ( each other ) ) ) ) ) ) ) ) ) , ) but ) ( ( can not ) ( ( produce ( fertile offspring ) ) ( with ( other organisms ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN concept)) (PP (IN of) (NP (DT a) (NN species)))) (VP (VP (ADVP (RB often)) (VBZ refers) (PP (TO to) (NP (NP (DT a) (NN population)) (PP (IN of) (NP (ADJP (RB physically) (JJ similar)) (NNS individuals))) (SBAR (WHNP (WDT that)) (S (VP (MD can) (ADVP (RB successfully)) (VP (VB mate) (PP (IN between) (NP (DT each) (JJ other)))))))))) (, ,) (CC but) (VP (MD can) (RB not) (VP (VB produce) (NP (ADJP (JJ fertile)) (NN offspring)) (PP (IN with) (NP (JJ other) (NNS organisms)))))) (. .)))
The concept of a species often refers to a population of physically similar individuals that can successfully mate between each other, but cannot produce fertile offspring with other organisms.
(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)) (. .)))