Applying for a postdoc when you don't have all the necessary skills I have recently completed my PhD and now looking for a postdoc/research assistant position. I found a few positions that look attractive - but I don't have all the skills listed in the job ad. I have some and I know I can learn the rest quickly; I also know from talking to other postdocs that their current research is not related to their PhD projects. 
The question is - should I apply for the jobs I'm not completely qualified for since it's normal to learn on the job, or is it a total waste of time? And how should I answer to selection criteria in this case?
Assuming you believe you can do the job and can make a compelling case in your application, yes you should apply. A person with the exact set of skills listed may not be available, in which case they will have to chose whomever they believe is the best fit of those who apply. Also, the skills you have may be beneficial for the position but may not have been thought of for the job ad.

In answering the selection criteria naturally emphasize the skills that you have which match the job ad. Additionally, highlight the skills which you have that are not listed but you believe would be beneficial and why. Finally, you can also make a case for why you can quickly (and independently) learn the additional skills that you do not already have.