Mutations in DNA
Although the double helix structure of DNA allows for remarkably faithful and accurate replication, mistakes do take place. So called mutations occur when DNA subunits are replaced with other subunits (ATTCGG changes to ATTAGG), extra subunits are added (ATTCCCCGG) or subunits are lost (ATTGG). This can alter the function of proteins by changing the sequence of amino acids.
DNA and Evolution
These changes in DNA sequence provides variation in function, which is utilized by Natural selection to select and preserve those variations best suited to the environment.
Similarities and differences in DNA can be compared within species and between species (the sequence of subunits in DNA can be read).
Imagine Joe, Bob and Simon are at the blue sweet shop and each buys 5 blue sweets. Joe continues to buy blue sweets while Bob and Simon visit the red sweet shop and buy 5 red sweets each. Then Simon visits the yellow sweet shop and buys 5 yellow sweets. At the end of the shopping trip Joe has 15 blue sweets; Bob has 5 blue and 10 red sweets; Simon has 5 blue, 5 red and 5 yellow sweets. The only colour sweets they all share is blue; therefore, they must have started in the blue sweet shop. Similarly, if DNA is compared in humans from Africa, Europe and Asia, all individuals share some of the variations found in the African population; therefore, all humans originated in Africa.
If DNA from different species is compared, those species with a more recent common ancestor have lots of identical sequences while distantly related species have lots of differences. For example, we share 99% of our DNA with chimpanzees but only 50% with bananas; we are genetically more like chimps than bananas!
See Nature 421 395-453