1. A computer-implemented method, comprising: identifying, by a system of one or more computers, a geographic map for presentation on an electronic display of a computing device; identifying, by the system, a set of content items that are candidates for presentation on the geographic map and that are each associated with a respective geographic location that is located within a geographic area represented by the geographic map; identifying, by the system and for each content item in the set of content items, a base value that represents an initial weight for selecting the content item for presentation on the geographic map; for each content item in the set of content items, adjusting the base value of the content item to determine an adjusted value for the content item based at least on a distance between the respective geographic location of the content item and the respective geographic location of another content item in the set of content items; selecting, by the system and based on the adjusted values of content items in the set of content items, one or more content items to present on the geographic map; and providing, by the system, over a network and to the computing device, data that is usable by the computing device to present the selected one or more content items on the geographic map.