Level of selection

Question

[ibasan]

Dear community,

i performed codon based z-test for two genes to compare selective constraints between species groups ( purifying selection (dN < dS) as alternative hypotheses to the null hypotheses of strict neutrality (dN = dS)). Now i have Z statistic values which are slightly different between the groups. See example

                  Z statistic   p value
 gene A    group 1  3.262         0.001
           group 2  2.964         0.002

 gene B    group 1  3.062         0.005
           group 2  4.964         0.001

My question regards the strength of selection constraints acting on the different groups. Is it that in group 1 for gene A selection acts stronger? When we compare ka/ks ratios, i know that lower values would indicate stronger selection, like in this example for Group 2:

Gene A      ka/ks          p value
Group1      0.033     <0.001
Group2      0.027     <0.001

I am happy about any help!