Let’s look at 3 broad categories of source of genetic variation: gene mutation, chromosome mutation and sexual reproduction.
DNA mutation occurs spontaneously and creates a direct change in the genetic material. Single bases may be changed, added or removed, or even large chunks of chromosomes can be swapped between them or duplicated. Whole chromosomes, too, can shift around.
Mutations are a random occurrence during DNA replication and the rate of mutation is influenced by external factors such as UV radiation. There are different types of mutation:
1. Deletion where one or more nucleotide bases are deleted. AGTCA becomes AGCA
2. Substitution where one or more nucleotide bases are replaced by others. AGTCA becomes AGTCG
3. Addition where one or more nucleotide bases are added as extra. AGTCA becomes ATGTCA
4. Inversion where a section of DNA is inverted e.g. AGTTCATTCCAGG becomes AGTTCCCTTAAGG
5. Duplication where one or more nucleotide bases are repeated e.g. AAGTCG becomes AAGTCGAAGTCG
6. Translocation where one or more nucleotide bases are moved between non-homologous chromosomes e.g. AAGCTT on human chromosome 1 is moved and becomes AAGCTT on human chromosome 3
Meiosis as part of sexual reproduction adds to genetic diversity via independent assortment of homologous chromosomes and crossing over. This creates gamete cells that are each different from the next, with millions of possible combinations of chromosomes.
Independent fertilisation means that of these millions of different possible cells, only 2 actually take part in fertilisation. This event creates an additional level of genetic diversity.
Cross-fertilisation in plants also maintains genetic diversity. Each flower has both male and female gametes and can self-fertilise, but cross-fertilisation between separate plants enhances genetic diversity. This is why plants have evolve sophisticated mechanisms to promote cross-fertilisation. These include attracting pollinating insects and making pollen that is adapted to being carried long distances by wind.