How to define a menu with and without url arguments in hook_menu?
You have two options:

1) Define both paths, jobs and jobs/%.

2) Define only the first path

$menu['jobs'] = array(
  'page callback' =&gt; 'my_page_callback',
  ...


Then make the argument optional:

function my_page_callback($job_id = FALSE) {
  ...   


This callback will be called for both paths.