Sort dropdown alphabetically - how to sort by Last name and not First?

Sort dropdown alphabetically - how to sort by Last name and not First?

Is there a way to have a dropdown list sort by a customer's last name and not the first?  The order for the dropdown list is:  last name, first name, date-of-birth.   Keeps sorting by First Name - which works, but doesn't work as well as sorting by last name.

Thanks!