Understanding:
• Evidence for which species are part of a clade can be obtained from the base sequence of a gene or the
corresponding amino acid sequence of a protein
All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesised is (almost) universal
- This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness
Over the course of millions of years, mutations will accumulate within any given segment of DNA
- The number of differences between comparable base sequences demonstrates the degree of evolutionary divergence
- A greater number of differences between comparable base sequences suggests more time has past since two species diverged
- Hence, the more similar the base sequences of two species are, the more closely related the two species are expected to be
When comparing molecular sequences, scientists may use non-coding DNA, gene sequences or amino acid sequences
- Non-coding DNA provides 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 may potentially affect protein structure and function
- Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy
Amino acid sequences are typically used to compare distantly related species (i.e. different taxa), while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups)
Comparison of the Haemoglobin Beta Chain in Different Species
Understanding:
• Sequence differences accumulate gradually so there is a positive correlation between the number of differences
between two species and the time since they diverged from a common ancestor
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)
If this rate of change is reliable, scientists can calculate the time of divergence according to the number of differences
- E.g. If a gene which mutates at a rate of 1 bp per 100,000 years has 6 bp different, divergence occurred 600,000 years ago
This concept is called the molecular clock and is limited by a number of factors:
- Different genes or proteins may change at different rates (e.g. haemoglobin mutates more rapidly than cytochrome c)
- The rate of change for a particular gene may differ between different groups of organisms
- Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions
Molecular Clocks
