The ultimate cause of all genetic variation is mutation. It occurs either during replication or under the action of a mutagenic agent, i.e. a chemical or radiation.
They can happen in somatic cells and in germ -line cells, but only mutation in germ-like cells have an evolutionary consequence.
There are a larger number of cell divisions involved in the male sperm line than in the female germ line, therefore more new mutations are transmitted through sperms than eggs.
The different types of mutation can be distinguished:
a) Point mutation: one base mutates to another
b) Frameshift mutation: insertion/deletion of a single base/short stretch of nucleotides, which shifts the whole reading frame
c) Duplications/deletions: “in-dels”, possible due to slippage or unequal crossing-over
d) Inversions: reversal of DNA order
e) Translocation: exchange of segments among non-homologous chromosomes
f) Fissions/Fusions: chromosomes merge or one becomes two
g) Polyploidy: change in the number of whole sets of chromosomes
Measuring mutation is difficult, it can though be estimated from the rate at which new mutations are detected on phenotypic level. But there are 2 problems:
a) Only small part of mutations have a visible effect on phenotypes
b) Bearers of deleterious mutations may die before they are detected