This histogram clearly shows the primary Kirkwood gaps in the asteroid main-belt. These gaps (labeled "3:1", "5:2", "7:3", "2:1") are caused by mean-motion resonances between an asteroid and Jupiter. For example, the 3:1 Kirkwood gap is located where the ratio of an asteroid's orbital period to that of Jupiter is 3/1 (the asteroid completes 3 orbits for every 1 orbit of Jupiter). The effect of these mean-motion resonances is a change in the asteroid's orbital elements (particularly semimajor axis) sufficient to create these gaps in semimajor axis space.
<ul><li><a href="//commons.wikimedia.org/wiki/File:Kirkwood_Gaps.svg" title="File:Kirkwood Gaps.svg">Kirkwood_Gaps.svg</a>: based on plot by Alan Chamberlain, JPL/Caltech</li></ul>
<ul><li><a href="//commons.wikimedia.org/wiki/File:Kirkwood_Gaps.svg" title="File:Kirkwood Gaps.svg">Kirkwood_Gaps.svg</a></li></ul>