Abstract
This chapter looks at a range of factors over both time and geography that can affect the levels of genetic diversity found in crop plants. In particular, the chapter focuses on factors that can cause genetic bottlenecks or loss of useful genetic diversity to occur. The authors recognize that genetic diversity is lost as a result of anthropogenic or natural selection and do not equate such purging/purifying selection (elimination of deleterious or undesirable alleles) with the largely undefined concept of genetic erosion.