( ( Allopatric speciation ) ( ( occurs ( when ( ( ( some members ) ( of ( a species ) ) ) ( become ( geographically separated ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Allopatric) (NN speciation)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT some) (NNS members)) (PP (IN of) (NP (DT a) (NN species)))) (VP (VBP become) (ADJP (RB geographically) (JJ separated)))))) (. .)))
Allopatric speciation occurs when some members of  a species become geographically separated.
(ROOT (S (NP (JJ Sympatric) (NN speciation)) (VP (VBZ is) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (JJ new) (NN species)) (VP (VBZ arises) (PP (IN without) (NP (JJ geographic) (NN separation))))))) (. .)))