What do the Clustal Alignment Symbols Mean?
From the FAQ for the Clustal-W2 program:

An * (asterisk) indicates positions which have a single, fully conserved residue.
A : (colon) indicates conservation between groups of strongly similar properties - scoring &gt; 0.5 in the Gonnet PAM 250 matrix.
A . (period) indicates conservation between groups of weakly similar properties - scoring =&lt; 0.5 in the Gonnet PAM 250 matrix.

Therefore the hierchy of conservation using these symbols is * (identical) &gt; : (colon) &gt; . (period).