Chapter 20.6, Problem 14SAQ

Summary Introduction

To explain:

How the time of divergence of two species can be estimated by a molecular clock.

Concept introduction:

All the living organisms have had a common ancestor. They all diverged from it with time forming new species. Thus as they diverged their genes changed too, this also the main reason for them to change phenotypically too. This change in DNA helps in the estimation of when a species diverged from another. This is called a molecular clock.

Explanation of Solution

To know when two species diverged, their nucleotide sequences or may be amino acids sequence for a protein must be compared. The more the difference amongst them, earlier they diverged. This is because they were isolated from each other for a period that allowed the mutation and fixation of new genes.

However, to compare them a standard has to be set. This is done by dating the fossils. For example: Carbon dating proved that Old World monkeys and New World monkeys separated about 30 million years ago (mya). The comparison of the DNA of humans and chimpanzees showed that their difference were a fifth of the difference in the DNA of Old World monkeys and New World monkeys. Hence, humans and chimpanzees diverged from each other about a fifth of 30 million years ago which is exactly about 6 million years ago and proved to be right.

Conclusion

Comparison of the nucleotides or amino acids of a protein to find the differences in them and calibrated that according to fossil dating will give an insight about the time the species in question diverged from each other. This is referred to as the molecular clock.