Mutations and genetic recombination (in sexually reproducing organisms) are major sources of variations in natural populations, In this section we will briefly mention the various types of mutations, their rates and how they are caused.

The term mutation is used to designate the process by which changes arise in the genetic material and, the end products of such processes. Mritations form a separate category of sources of variability, distinct' from variability arising from genetic recombination or independent assortment of chromosomes, characteristic of sexually reproducing individuals. There are two categories of mutations. '(1) Chromosomal mutations which affect the'number of chromosomes and the number or the arrangement of gencs in a chromosome; (2) gene mutations which affect only one or a few nucleotides in a gcne. We shall now tiiscuss these two aspects of mutaion.