Occupation and IQ

Tod shared a link with me yesterday that got me thinking. It is a chart listing the IQs of people in various professions. I looked at it a good long time and it made me ask myself a lot of questions.
Why are the highest IQs in professions like doctor, lawyer, professor, scientist?
Why aren’t there more high-IQ people being janitors and metalworkers?
Which occupations are appealing to a broad range of IQs? Why them and not the others?
It was food for thought while I cut out Morsbags.