Defecting MEPs defy the logic of party lists

http://www.theguardian.com/politics/2015/feb/09/defecting-meps-defy-logic-of-party-lists

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Given that members of the European parliament are elected by proportional representation on the basis of regional closed lists, how can it be that an MEP who changes party retains his or her seat? Amjad Bashir was elected as one of three successful Ukip candidates after the party gained 31.1% of the vote in Yorkshire and the Humber. Labour won two seats and the Conservatives one. But following the defection of Bashir to the Conservatives, they now have two members representing the constituency. Why hasn’t Bashir lost his seat to the fourth-named candidate on the Ukip list? Can anyone explain why the will of the voters, in a party list PR system, can be overridden by an elected representative switching parties?Brian Ross-MeeringNewark, Nottinghamshire