( ( ( ( ( ( ( The ( neutral theory ) ) ( of evolution ) ) ( can ( explain ( the clock ) ) ) ) , ) but ) ( it ( is ( ( a challenge ) ( to ( ( the theory ) ( of ( natural selection ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (JJ neutral) (NN theory)) (PP (IN of) (NP (NN evolution)))) (VP (MD can) (VP (VB explain) (NP (DT the) (NN clock))))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT a) (NN challenge)) (PP (TO to) (NP (NP (DT the) (NN theory)) (PP (IN of) (NP (JJ natural) (NN selection)))))))) (. .)))
The neutral theory of evolution can explain the clock, but it is a challenge to the theory of natural selection.
(ROOT (S (NP (NP (NNP Theevolution)) (PP (IN by) (NP (JJ natural) (NN selection) (NN theory)))) (VP (VBZ explains) (SBAR (WHADVP (WRB how)) (S (NP (NP (NNS populations)) (PP (IN of) (NP (NNS organisms)))) (VP (MD can) (VP (VB change) (PP (IN over) (NP (NN time)))))))) (. .)))