( Scientists ( ( ( classify ( living organisms ) ) ( into ( ( separate species ) ( ( based ( upon ( ( ( their ( physical form ) ) and ) ( the potential ) ) ) ) ( to ( ( ( interbreed and ) produce ) ( ( viable ( , fertile ) ) offspring ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Scientists)) (VP (VB classify) (NP (VBG living) (NNS organisms)) (PP (IN into) (NP (NP (JJ separate) (NNS species)) (VP (VBN based) (PP (IN upon) (NP (NP (PRP$ their) (JJ physical) (NN form)) (CC and) (NP (DT the) (NN potential)))) (S (VP (TO to) (VP (VB interbreed) (CC and) (VB produce) (NP (ADJP (JJ viable) (, ,) (JJ fertile)) (NN offspring))))))))) (. .)))
Scientists classify living organisms into separate species based upon their physical form and the potential to interbreed and produce viable, 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)) (. .)))