( Darwin ( ( claimed ( that ( ( populations ( of organisms ) ) ( ( ( change gradually ) ( through time ) ) ( as ( ( their members ) ( are ( ( ( weeded out ) , ) ( which ( is ( ( his ( basic idea ) ) ( of ( evolution ( through ( natural selection ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Darwin)) (VP (VBD claimed) (SBAR (IN that) (S (NP (NP (NNS populations)) (PP (IN of) (NP (NNS organisms)))) (VP (VBP change) (ADVP (RB gradually)) (PP (IN through) (NP (NN time))) (SBAR (IN as) (S (NP (PRP$ their) (NNS members)) (VP (VBP are) (VP (VBN weeded) (PRT (RP out)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ is) (NP (NP (PRP$ his) (JJ basic) (NN idea)) (PP (IN of) (NP (NP (NN evolution)) (PP (IN through) (NP (JJ natural) (NN selection))))))))))))))))) (. .)))
Darwin claimed that populations of organisms change gradually through time as their members are weeded out, which is his basic idea of evolution through 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)))))))) (. .)))