( ( Species ( ( ( A population ) ( of ( ( morphologically similar ) organisms ) ) ) ( that ( ( ( can ( ( reproduce sexually ) ( among themselves ) ) ) but ) ( ( can not ) ( ( produce ( fertile offspring ) ) ( when ( mated ( with ( other organisms ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB Species) (NP (NP (DT A) (NN population)) (PP (IN of) (NP (ADJP (RB morphologically) (JJ similar)) (NNS organisms))) (SBAR (WHNP (WDT that)) (S (VP (VP (MD can) (VP (VB reproduce) (ADVP (RB sexually)) (PP (IN among) (NP (PRP themselves))))) (CC but) (VP (MD can) (RB not) (VP (VB produce) (NP (ADJP (JJ fertile)) (NN offspring)) (SBAR (WHADVP (WRB when)) (S (VP (VBN mated) (PP (IN with) (NP (JJ other) (NNS organisms))))))))))))) (. .)))
Species A population of morphologically similar organisms that can reproduce sexually among themselves but cannot produce fertile offspring when mated 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)) (. .)))