Wordpress query not changing with onchange form submit - php

Question

I have a page that displays all the staff members, which are grouped by company. Each staff member is a custom post type. I have a dropdown menu and a submit button to filter the staff by company, which works fine.
<div id="choose">
<form name="selectCompany" method="post" action="">
<span>Select Company: </span>
<select name="company" >
<option value="All Staff">All Staff</option>
<option value="Company 1" <?php echo $yourgroup; ?>>Company 1</option>
<option value="Company 2" <?php echo $yourpower; ?>>Company 2</option>
<option value="Company 3" <?php echo $yourelectrical; ?>>Company 3/option>
<option value="Company 4" <?php echo $newmills; ?>>Company 4"</option>
</select>
<input type="submit" value="Go" name="submit" />
</form>
</div>
The Wordpress query I'm using is (where $company is the value from the dropdown menu):
if ($company != "All Staff") {
$mypost = array( 'post_type' => 'staff_members',
'orderby' => 'title',
'order' => 'ASC',
'meta_query' => array(
array(
'key' => 'company',
'value' => $company,
'compare' => '=',
),
),
);
}
else {
$mypost = array( 'post_type' => 'staff_members',
'orderby' => 'title',
'order' => 'ASC',);
}
if (empty($_POST['submit'])) { //Set default value for displaying staff
$mypost = array( 'post_type' => 'staff_members',
'orderby' => 'title',
'order' => 'ASC',);
$company = "All Staff";
}
?>
<p><span class="display">Displaying Staff for <?php echo $company; ?></span></p>
<?php
$loop = new WP_Query( $mypost );
while ( $loop->have_posts() ) : $loop->the_post();?>
<div class="staff">
<div class="photo">
<a href="<?php the_permalink(); ?>" rel="shadowbox[staff-1]">
<?php
if ( '' != get_the_post_thumbnail() ) { //Recommended way of doing it
the_post_thumbnail();
}
else {
echo '<img src="' . get_bloginfo( 'stylesheet_directory' ) . '/staff-default.gif" />';
}
?>
</a>
</div>
<div class="name"> <?php the_title(); ?></div>
<div class="title">
<?php echo esc_html( get_post_meta( get_the_ID(), 'job_title', true ) ); ?>
</div>
<?php // echo esc_html( get_post_meta( get_the_ID(), 'company', true ) ); ?>
</div>
<?php endwhile; ?>
<?php wp_reset_query(); ?>
<?php get_footer(); ?>
This all works fine, however I want to remove the submit button and just do it when the list changes. I changed it to <select name="company" onchange="this.form.submit()">. It submits the form and the page reloads but it doesn't actually filter the staff. I don't know why using a submit button works but but using onchange doesn't.