$(window).scroll(function () {
  var inview = '#' + $('#wrap > .wrap > .block > h1:in-viewport:first').parent().attr('id'),
      $link = $('nav a').filter('[hash=' + inview + ']');
      
  if ($link.length && !$link.is('.selected')) {
    $('nav a').removeClass('selected');
    $link.addClass('selected');    
  }
});