That's addressed in the novels. During Mass Effect Retribution, David Anderson (as either the Human Council member or the Human Ambassador - it is never mentioned which) orders an attack on Cerberus that goes badly. During the political fall-out, he resigns from his post (whichever it is) and goes off with Kahlee Sanders (to stop Cerberus and have sex with Kahlee, in that order).jensenthejman said:5. Udina as the human councilor: Thanks Bioware for making my choice from ME 1 mean absolutely nothing. There is a point when it is discovered that Udina is an undercover Cerberus agent. (Wow, who saw that coming, except everyone.) And there is an assassination attempt on the council. During this whole sequence, I just kept repeating: "This shit wouldn't have happened if Anderson was councilor, like I picked him to be". It just feels contrived.
So basically, whichever you chose in ME1, Anderson quits between ME2 and ME3.