Deoxyribonucleic acid (DNA) is the blueprint for all inherited characteristics in living things. It is a very long sequence, written in code, that needs to be transcribed and translated before a cell can make the proteins that are essential for life. Any sort of changes in the DNA sequence can lead to changes in those proteins, and, in turn, they can translate into changes in the traits those proteins control. Changes at a molecular level lead to microevolution of species.
The Universal Genetic Code
The DNA in living things is highly conserved. DNA has only four nitrogenous bases that code for all differences in living things on Earth. Adenine, cytosine, guanine, and thymine line up in a specific order and a group of three, or a codon, code for one of 20 amino acids found on Earth. The order of those amino acids determines what protein is made.
Remarkably enough, only four nitrogenous bases that make only 20 amino acids account for all diversity of life on Earth. There has not been any other code or system found in any living (or once living) organism on Earth. Organisms from bacteria to humans to dinosaurs all have the same DNA system as a genetic code. This may point to evidence that all life evolved from a single common ancestor.
Changes in DNA
All cells are pretty well-equipped with a way to check a DNA sequence for mistakes before and after cell division, or mitosis. Most mutations, or changes in DNA, are caught before copies are made and those cells are destroyed. However, there are times when small changes do not make that much of a difference and will pass through the checkpoints. These mutations may add up over time and change some of the functions of that organism.
If these mutations happen in somatic cells, in other words, normal adult body cells, then these changes do not affect future offspring. If the mutations happen in gametes, or sex cells, those mutations do get passed down to the next generation and may affect the function of the offspring. These gamete mutations lead to microevolution.
Evidence for Evolution
DNA has only come to be understood over the last century. The technology has been improving and has allowed scientists to not only map out entire genomes of many species, but they also use computers to compare those maps. By entering genetic information of different species, it is easy to see where they overlap and where there are differences.
The more closely species are related on the phylogenetic tree of life, the more closely their DNA sequences will overlap. Even very distantly related species will have some degree of DNA sequence overlap. Certain proteins are needed for even the most basic processes of life, so those selected parts of the sequence that codes for those proteins will be conserved in all species on Earth.
DNA Sequencing and Divergence
Now that DNA fingerprinting has become easier, cost-effective, and efficient, the DNA sequences of a wide variety of species can be compared. In fact, it is possible to estimate when the two species diverged or branched off through speciation. The larger the percentage of differences in the DNA between two species, the greater the amount of time the two species have been separate.
These "molecular clocks" can be used to help fill in the gaps of the fossil record. Even if there are missing links within the timeline of history on Earth, the DNA evidence can give clues as to what happened during those time periods. While random mutation events may throw off the molecular clock data at some points, it is still a pretty accurate measure of when species diverged and became new species.