Molecular evolution and neutral theory - What does the genetic evidence show?
Testing the theories on rates of change
The question whether natural selection can work on silent DNA can be tested by comparing the rates of change in the silent sites of different genes, to see whether evolutionary rates are constant in silent sites. Initially, it was thought that the silent rates were indeed constant.
Recent evidence has suggested otherwise:
• Evolutionary rates are less variable in silent than in replacement sites, suggesting a lower degree of constraint, but they do still vary.
• Evidence shows that evolutionary rates for a single gene can vary between different lineages, contradicting the idea of pan-neutral evolution.
Direct evidence of selective constraints in silent base sites:
• The relative rates of evolution in pseudogenes and silent positions. A pseudogene is an example of non-coding DNA. It is a sequence of bases in the DNA that clearly resembles the sequence of a known gene, but differs from it in some crucial respect and has no function. Research suggests that pseudogenes evolve at the same rate as silent base changes: this means that evolution is constrained even in pseudogenes. The argument works because we have independent evidence, from biases in codon usages, that evolution is constrained in silent sites.