( ( ( ( ( ( The reason ) ( is ( that ( ( sexually ( reproducing organisms ) ) ( have ( ( different sets ) ( of genes ) ) ) ) ) ) ) , ) so ) ( they ( ( are ( in ( competition ( with ( one another ) ) ) ) ) reproductively ) ) ) . )
(ROOT (S (S (NP (DT The) (NN reason)) (VP (VBZ is) (SBAR (IN that) (S (NP (RB sexually) (JJ reproducing) (NNS organisms)) (VP (VBP have) (NP (NP (JJ different) (NNS sets)) (PP (IN of) (NP (NNS genes))))))))) (, ,) (IN so) (S (NP (PRP they)) (VP (VBP are) (PP (IN in) (NP (NP (NN competition)) (PP (IN with) (NP (CD one) (DT another))))) (ADVP (RB reproductively)))) (. .)))
The reason is that sexually reproducing organisms have different sets of genes, so they are in competition with one another reproductively.
(ROOT (S (NP (JJS Most) (NN fish)) (VP (VB reproduce) (PP (IN with) (NP (CD one) (DT another))) (ADVP (RB sexually))) (. .)))