isotope/_includes/option-buttons.js

      // switches selected class on buttons
      $('#options').find('.option-set a').click(function(){
        var $this = $(this);

        // don't proceed if already selected
        if ( !$this.hasClass('selected') ) {
          $this.parents('.option-set').find('.selected').removeClass('selected');
          $this.addClass('selected');
        }

      });