Vertex Pharmaceuticals is one of two companies that will join the S&P 500, along with Ametek Inc., a Pennsylvania electronic instruments firm. The pair will switch spots with AMD Inc. and SAIC Inc., which will join the MidCap 400. S&P said the moves will better align the businesses with indices more representative of their market capitalizations. Both Vertex and Ametek have market caps of more than $10 billion, the S&P said. The switch will take place after trading closes Sept. 20.