def _join_alignments(align_dict):
    """
    Take a dictionary of multiple sequence alignments, and join according to
    dictionary key order (generally position in sequence).

    Args:
        align_dict (dict): A dictionary containing single-site multiple sequence
            alignment objects accessed by position in original alignment.

    Returns:
        MultipleSequenceAlignment: A multiple sequence alignment object
            containing all polymorphic sites.
    """
    output = None
    for key in sorted(align_dict):
        if not output:
            output = align_dict[key]
        else:
            output = output + align_dict[key]
    return output
