If a causal relationship exists between a signifier and a signified, its directionality depends on the type of sign considered.
In the case of indexes, the signifier is caused by the signified (smoke, the signifier, is caused by fire, the signified).
In the case of symbols, we have the inverse relationship, the signified is caused by the signifier (amino acid, the signified, is caused by the codon, the signifier).