MOLECULAR EVOLUTION

Many evolutionists study changes in animal structure and function
that are observable on a large scale—for example, changes in
the shape of a bird’s bill or in the length of an animal’s neck. All
evolutionary change, however, results from changes in the base sequences
in DNA and amino acids in proteins. Molecular evolutionists
investigate evolutionary relationships among organisms
by studying DNA and proteins. For example, cytochrome c is a
protein present in the cellular respiration pathways in all eukaryotic
organisms. Organisms that other research has
shown to be closely related have similar cytochrome c molecules.
That cytochrome c has changed so little during hundreds of millions
of years suggests that mutations of the cytochrome c gene are
nearly always detrimental, and are selected against. Because it has
changed so little, cytochrome c is said to have been conserved
evolutionarily.
Not all proteins are conserved as rigorously as cytochrome c.
Although variations in highly conserved proteins can help establish
evolutionary relationships among distantly related organisms,
less conserved proteins are useful for looking at relationships
among more closely related animals. Because some proteins are
conserved and others are not, the best information regarding evolutionary
relationships requires comparing as many proteins as
possible in any two species.