Clonal selection is about lymphocytes. They activate if they find particular antigene in organism. Effect this identifications cells started to generate antibodies and lymphocytes T for example and cells make mitotic division. This mitosis help them rise in numbers and make resistance for particular pathogene. After that cells goes to apoptosis, and only "few" survives. Those few cells have immune memory for that pathogene. In future these "few" cells make organizmal response for that pathogene much quicker.