It looks like you need to use some JavaScript. What you will want to do is add an onChange command to your dropdown, for example:
<form name="search" action="./" method="post">
<select name="lastname" onChange="movePage()">
<option value="Jones">Jones</option>
</select>
</form>
in the html you will need to add anchors, for example
<a name="Jones"></a>Jones, Fred<br>
Jones, Martha<br>
<a name="Henitoot"></a>Henitoot, Fred<br>
then all you need to do is the JavaScript function which would be something like this:
function movePage() {
var item = document.forms['search'].elements['lastname'].selectedIndex;
var choice = document.forms['search'].elements['lastname'].options[item].value;
document.location="./#"+choice;
not sure of the last line, you may need the absolute URL