( ( If ( evolution happened ) ) ( , ( ( natural selection ) ( ( ( was not ) ( ( ( the ( principal mechanism ) ) , ) ( until ( someone ( can ( ( ( come up ) ( with ( a ( valid mechanism ) ) ) ) ( to ( explain ( such changes ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (NN evolution)) (VP (VBD happened)))) (, ,) (NP (JJ natural) (NN selection)) (VP (VBD was) (RB not) (NP (NP (DT the) (JJ principal) (NN mechanism)) (, ,) (SBAR (IN until) (S (NP (NN someone)) (VP (MD can) (VP (VB come) (PRT (RP up)) (PP (IN with) (NP (DT a) (JJ valid) (NN mechanism))) (S (VP (TO to) (VP (VB explain) (NP (JJ such) (NNS changes))))))))))) (. .)))
If evolution happened, natural selection was not the principal mechanism, until someone can come up with a valid mechanism to explain such changes.
(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)))))))) (. .)))