Genes are sequences of DNA that code for polypeptides (which constitute proteins)
With the sequencing of a variety of organismal genomes, it has been found that many organisms that are not closely related, still retain particular genes in common
These genes are said to be conserved and typically serve a critical role in controlling organismal development
Examples of such genes that are ultraconserved include:
Regulatory genes (e.g. coding for protein-based hormones)
Genes encoding for tRNA (needed for translation to occur)
Genes involved in ATP production (e.g. coding for the cytochrome proteins in the electron transport chain)
The number of conserved genes can be used to show evolutionary relationships between organismsOver the course of millions of years, mutations will accumulate within any given segment of DNA
The number of differences between comparable base sequences can be used to demonstrate the degree of evolutionary divergence
Non-coding sequences of DNA provide the best means of comparison as mutations will occur more readily in these sequences
Gene sequences mutate at a slower rate, as changes to base sequence can potentially have detrimental effects on protein structure and function
Protein similarity (based on amino acid sequence) can also be used for comparison, but will have the slowest rate of change due to codon degeneracy
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. a 1% change per million years)
If this rate of change is reliable, scientists can calculate the time of divergence based on the number of differences between two species
This concept is called the molecular clock, and is limited by a number of factors:
The rate of change can differ for different groups of organisms (e.g. animals and plants)
The rate of change can vary between different genes and different proteins
Over long periods, earlier changes may be reversed by later changesMitochondrial DNA (mtDNA) is an important tool for tracing evolutionary relationships within a species
Mitochondria DNA is structurally identical to nuclear DNA, but differs in a number of important ways:
Descent via maternal line: mtDNA is inherited from the mother only, providing a much more direct genetic lineage
Lack of recombination: As mtDNA is passed directly from the mother, no recombination occurs, maintaining fidelity of the maternal sequence
Higher mutation rate: Mitochondria produce reactive oxygen species which cause non-conserved sequences to mutate at a faster rate
High copy number: As every cell has multiple mitochondria, larger amounts of mtDNA can be gathered for sampling
Because of these differences, mtDNA sequencing is ideal for comparing organisms within a species or those who have diverged in a relatively short time (~20 million years)
Distinctive human mtDNA sequences have been found in different global populations and have been used to divide the human population into distinct haplogroups
All humans are thought to be able to trace their origins back to one of seven super-haplogroups derived from an original haplogroup - the 'mitochondrial Eve'The term phylogeny describes the evolutionary development of a species
This evolutionary divergence can be represented in a branching diagram called a phylogeny tree (or cladogram)
These diagrams can be used distinguish between, and trace the evolutionary history of, biological taxa or even groups of taxa (clades)
The branching of the phylogeny tree can be based on structural features, protein similarities or, more commonly, genome comparisons
Species that have fewer branching points between them are more closely related to one another
