The r and K classification was originally proposed by the biologists Robert McArthur and E. O. Wilson (1967).
A good way of thinking about r selected species is, again, to think of a weed. Weeds exist in disturbed habitats, such as gaps in a forest canopy where tress have blown down, allowing light to penetrate to the forest floor. Weeds grow quickly and reach reproductive age early, devoting much energy to a large number of seeds that disperse widely, often to more light gaps in the forest. Weedy species remain small and have a brief life span, perhaps passing a few generations in the light gap before it closes.