By collecting seeds from only certain plants, you are in the process of Selection. You can select from plants that grew well for you in your conditions (climate, soil type, etc), as well as production, size, taste, or any other plant characteristic. Over several years, you will have "weeded" out undesirable traits. Whether you save your own seeds or buy seed every year, you don't want plants that don't produce for you, so you don't grow these. Alex