Debate over the mechanisms responsible for the phylogenetic and genomic distribution of introns has proceeded largely without consideration of the population-genetic forces influencing the establishment and retention of novel genetic elements. However, a simple model incorporating random genetic drift and weak mutation pressure against intron-containing alleles yields predictions consistent with a diversity of observations: (i) the rarity of introns in unicellular organisms with large population sizes, and their expansion after the origin of multicellular organisms with reduced population sizes; (ii) the relationship between intron abundance and the stringency of splice-site requirements; (iii) the tendency for introns to be more numerous and longer in regions of low recombination; and (iv) the bias toward phase-0 introns. This study provides a second example of a mechanism whereby genomic complexity originates passively as a "pathological" response to small population size, and raises difficulties for the idea that ancient introns played a major role in the origin of genes by exon shuffling.