How do researchers figure out when the common ancestor of different organisms lived? How do they know the chronological order of evolutionary events?
The molecular clock is a method that uses biomolecular data (generally mutation rates) to estimate the amount of time needed for a certain amount of evolutionary change to occur. The molecular clock hypothesis has helped researchers answer these questions as well as fill in gaps in the fossil record.
Fig. 1. The graph above is a very schematic representation of a molecular clock. It represents the number of mutations that occurred to the gene at different points in time, expressed as millions of years in the past.
In this article, we will discuss the origin of the molecular clock hypothesis, the definition of the molecular clock, some examples of how a molecular clock can be used in constructing phylogeny, and the limitations of using the molecular clock.
Molecular Clock Concept
The origin of the molecular clock hypothesis
In 1965, the proponents of the molecular clock hypothesis Zuckerkandl and Pauling observed that the constant accumulation of amino acid substitutions in haemoglobin was similar to the regular ‘ticks’ of a clock.
From this observation, they thought that it was possible that a molecular evolutionary clock–which describes changes in amino acids over time since the divergence of species– also existed.
The molecular clock hypothesis argues that DNA and protein sequences mutate at a constant rate over time among different organisms and that the number of genetic differences between organisms can give us an estimation of when they last shared a common ancestor.
Mutation: changes in the sequence of genes.
Molecular clock definition
Today, the molecular clock is a method used to estimate the amount of time needed for a certain amount of evolutionary change. This is done by analyzing biomolecular data, such as the number of changes or substitutions in nucleotide sequences of DNA and RNA, or the amino acid sequence of proteins. Substitution is a type of mutation where one nucleotide is replaced by another.
Assuming that the nucleotide or amino acid sequences mutate at a constant rate, the number of substitutions over time is equivalent to the evolutionary rate. For this reason, the molecular clock is also known as the gene clock or the evolutionary clock.
Example of a molecular clock diagram
Below is an example of a molecular clock diagram. It shows how quickly CCDC92, a protein-coding gene, changes by graphing the number of amino acid substitutions per millions of years. As points of comparison, it also shows the rate of change in Fibrinogen (a protein with a higher mutation rate) and Cytochrome C (a protein with a lower rate of change).
Fig 2. Molecular clock diagram showing the amino acid substitutions per millions of years to show the rate at which the gene CCDC92 changes.
How are DNA mutations used in molecular clocks?
Mutations may be harmful, beneficial, or neutral. Harmful mutations have a negative impact on an organism's evolutionary fitness, or its ability to survive and reproduce. On the contrary, beneficial mutations have a positive impact on an organism's evolutionary fitness. Most mutations are neutral: they have no effect on an organism’s evolutionary fitness.
Because neutral mutations have no effect on evolutionary fitness, their frequency in the succeeding generations of the population is determined by chance rather than natural selection. This means that all neutral mutations have an equal chance of undergoing substitution. As such, the substitution rate for neutral mutations is equal to the mutation rate.
Neutral mutations are used for molecular clocks because they tend to accumulate at a constant rate over time.
If a gene's specific amino acid sequence is necessary for survival, the majority of mutations will be harmful, with only a few neutral; such genes take a long time to change. On the other hand, if a gene's amino acid sequence is not as essential, fewer mutations will be harmful, and more will be neutral; such genes change at a faster rate. The molecular clock of a gene can be calibrated by corroborating the number of substitutions with dates from the fossil record that are known as diverging points.
The process of calculating a molecular clock can be summed up as follows:
Estimate the number of substitutions in the nucleotide or amino acid sequences.
Using the fossil record, determine the date when the organisms being studied last shared a common ancestor.
Estimate the number of substitutions in the nucleotide or amino acid sequences per unit of time. This will be our evolutionary rate.
Using the evolutionary rate, calculate the time of divergence for the new sequences.
Let’s say the evolutionary rate of a species is 2 mutations every million years. If there are 10 mutations in the nucleotide or amino acid sequence being studied, then the sequences must have diverged 5 million years ago.
Evolutionary rate: the number of evolutionary changes over a period of time.
Fossil record: the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary rock layers.
Example of how molecular clocks are used
Molecular clocks can be used to determine when different species last shared a common ancestor and to put evolutionary events in chronological order, both of which are essential to the construction of phylogenetic trees.
Phylogenetic trees are branching diagrams showing the evolutionary history and relationship of organisms or groups of organisms.
Fig. 3 is a phylogenetic tree that was reconstructed using the 16S rDNA of one member of each of the major clades belonging to the genus Rickettsia, which consists of bacteria that include disease-causing bacteria in lice, ticks, and mites. Notice that there is a scale at the top-left corner indicating the number of substitutions per site. This is because a molecular clock was used to infer the times of divergence, and the branches of the phylogenetic tree were scaled accordingly.
Fig. 3 Phylogenetic tree constructed using a molecular clock.
The phylogenetic tree shown in Fig. 3 tells us that the common ancestor Pelagibacter, which are free-living bacteria, existed over 750 million years ago.
Around 525 to 775 million years ago, there was a transition to living inside cells and, at around 425 to 525 million years ago, split into Holospora and a clade that primarily infests arthropods. The genus Rickettsia emerged approximately 150 million years ago. It is important to note that not all phylogenetic trees indicate the date of divergence of the organisms being studied; such is made possible by the use of a molecular clock.
In addition to dating evolutionary changes, molecular clocks are also useful for studying species that do not fossilize well. For example, using molecular clock analyses, researchers found that animals and fungi last shared a common ancestor more than a billion years ago. This kind of information is difficult to obtain from the fossil record because the oldest fossils of fungi–which do not fossilize well because they are soft–can be dated only as far back as about 460 million years ago.
Limitations of molecular clocks
As previously mentioned, molecular clocks work under the assumption that genetic changes (in DNA, RNA, or protein sequences) occur at a fixed rate. Limitations of molecular clocks include:
DNA, RNA, or protein sequences may change at irregular bursts instead of at a constant rate.
Some DNA, RNA, or protein sequences may appear to change at a smooth average rate but actually have some deviations from that average rate.
As a result of natural selection, some genetic changes are favoured over others.
The same DNA, RNA, or protein sequence substitutions may be occurring at different rates in different organisms.
Some DNA, RNA, or protein sequences evolve faster than others.
Evidence suggests that nearly half of the amino acid differences in Drosophila simulans and D. yakuba protein are not selectively neutral so they are affected by natural selection, leading to irregular mutation rates. However, the direction of natural selection can change several times over a long period, so these differences can average out.
In addition, estimates may be contested when molecular clocks are used to date evolutionary divergences that took place beyond what is documented by the fossil record. Molecular clocks have been used to estimate dates of evolutionary divergence that took place billions of years ago, but the fossil record extends back to only around 550 million years ago.
These limitations can be resolved in some calibrating molecular clocks using data on the evolutionary rate of genes in various taxa. In other circumstances, it is helpful to use a large number of genes rather than just one or two. Natural selection or other circumstances may cause fluctuations in evolutionary rate, but by studying multiple genes, these fluctuations may be averaged out. As such, despite its limitations, molecular clocks can still be useful in determining evolutionary relationships when used carefully.
Molecular Clock - Key takeaways
- The molecular clock hypothesis states that the divergence of species can be estimated using the constant accumulation of amino acid substitutions in a protein sequence which is similar to the regular ‘ticks’ of a clock.
- The molecular clock is a method used to estimate the amount of time needed for a certain amount of evolutionary change using biomolecular data such as nucleotide sequences in DNA and RNA or amino acid sequences in protein.
- The molecular clock is useful for: determining when different species last shared a common ancestor, putting evolutionary events in chronological order, and studying the evolutionary history of organisms that do not easily fossilize.
- A key assumption in using a molecular clock is that the nucleotide or amino acid sequences mutate at a constant rate.
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