( There ( ( ( are also ) ( ( ( cases ( where ( ( two ( different species ) ) ( ( ( mate and ) produce ) offspring ) ) ) ) , ) ( thought ( ( their offspring ) ( is ( unable ( to ( produce offspring ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBP are) (ADVP (RB also)) (NP (NP (NNS cases)) (SBAR (WHADVP (WRB where)) (S (NP (CD two) (JJ different) (NNS species)) (VP (VB mate) (CC and) (VB produce) (NP (NN offspring))))) (, ,) (SBAR (S (VP (VBD thought) (SBAR (S (NP (PRP$ their) (NN offspring)) (VP (VBZ is) (ADJP (JJ unable) (S (VP (TO to) (VP (VB produce) (NP (NN offspring)))))))))))))) (. .)))
There are also cases where two different species mate and produce offspring, thought their offspring is unable to produce 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)) (. .)))