By combining a TCRbeta transgene with a TCRalpha minilocus comprised of a single V and two J gene segments, we engineered a mouse line exhibiting ample but focused TCR diversity, restricted to CDR3alpha. Using single-cell PCR and high-throughput sequencing, we have exploited this system to scrutinize T cell repertoire selection and evolution. Some striking observations emerged: (1) thymic selection produces a repertoire that is very "bumpy," with marked overrepresentation of a subset of sequences; (2) MHC class I- and class II-restricted TCRs can be distinguished by minute, single-residue changes in CDR3alpha; and (3) homeostatic expansion and survival in the periphery can markedly remold the postselection repertoire, likely reflecting variability in the potential of cells displaying different TCRs to respond to homeostatic cues.