def _in_species_list(species, species_list=None):
    """
    Return True if species_list is None or if species is in species_list.

    >>> _in_species_list('homo_sapiens')
    True
    >>> _in_species_list('homo_sapiens', ['homo_sapiens', 'mus_musculus'])
    True
    >>> _in_species_list('pan_paniscus', ['homo_sapiens', 'mus_musculus'])
    False
    """
    if species_list is None:
        return True
    return species in species_list