jQuery Validation Plugin - Only validate form that's showing...

In the attached script there's a form with a dropdown box and two sections.  When they select "option one," section one should display (section one should display automatically).  If the select "option two," it should display.  I am using the jQuery validation plugin (http://bassistance.de/jquery-plugins/jquery-plugin-validation/) and I don't want it to validate the fields that aren't showing.  I only want it to validate the fields that are currently displayed according the the selection the user made.

<script type="text/javascript">

$().ready(function() {
	
	// validate signup form on keyup and submit
	$("#signupForm").validate({
		rules: {
			firstname: "required",
			lastname: "required",
			username: {
				required: true,
				minlength: 2
			}
		},
		messages: {
			firstname: "Please enter your firstname",
			lastname: "Please enter your lastname",
			username: {
				required: "Please enter a username",
				minlength: "Your username must consist of at least 2 characters"
			},
		}
	});

});

</script>
<form action="" method="post">

<!-- 
WHEN THE USER HITS OPTION ONE OR TWO, IT WILL REVEAL THE CORRESPONDING FORM FIELDS.  I ONLY WANT THE VALIDATION PLUGIN TO VALIDATE THE FIELDS THAT ARE DISPLAYING.  I DON'T WANT IT TO TRY TO VALIDATE THE FIELDS THAT AREN'T SHOWING. 
-->

<select>
  <option value="volvo">Option one</option>
  <option value="saab">Option two</option>
</select>

<!-- BEGIN OPTION ONE -->

		<p>
			<label for="firstname">Firstname</label>
			<input id="firstname" name="firstname" />
		</p>
		<p>
			<label for="lastname">Lastname</label>
			<input id="lastname" name="lastname" />
		</p>

<!-- END OPTION ONE -->

<!-- BEGIN OPTION TWO -->

		<p>
			<label for="firstname">Firstname</label>
			<input id="firstname" name="firstname" />
		</p>
		<p>
			<label for="lastname">Lastname</label>
			<input id="lastname" name="lastname" />
		</p>

<!-- END OPTION TWO -->

</form>

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