Probing question: What is a molecular clock?

November 20, 2008 By Solmaz Barazesh
Probing question: What is a molecular clock?
Credit: Chris Limb

It doesn't tick, it doesn't have hands, and it doesn't tell you what time of day it is. But a molecular clock does tell time -- on an epoch scale. The molecular clock, explained S. Blair Hedges, is a tool used to calculate the timing of evolutionary events.

Instead of measuring seconds, minutes and hours, said Hedges, Penn State professor of biology, the molecular clock measures the number of changes, or mutations, which accumulate in the gene sequences of different species over time.

Evolutionary biologists can use this information to deduce how species evolve, and to fix the date when two species diverged on the evolutionary timeline. "Unlike a wristwatch, which measures time from regular changes (ticks), a molecular clock measures time from random changes (mutations) in DNA," Hedges noted.

The concept of a molecular clock was first put forward in 1962 by chemist Linus Pauling and biologist Emile Zuckerkandl, and is based on the observation that genetic mutations, although random, occur at a relatively constant rate. Thus, the theory goes, the number of differences between any two gene sequences increases over time. As Hedges explained, this thinking led to the idea that the number of mutations in a given stretch of DNA could be used as a measure of time.

But before any clock can work, it has to be calibrated, he added. Setting a molecular clock “begins with a known, like the fossil record,” for a specific species. Then, once the rate of mutation is determined, calculating the time of divergence of that species becomes relatively easy. “If the rate is 5 mutations every million years, and you count 25 mutations in your DNA sequence, then your sequences diverged 5 million years ago.”

“A nice aspect of molecular clocks is that different genes evolve at different rates, which gives us flexibility to date events throughout the history of life” Hedges pointed out. Broadly speaking, the evolution of important genes occurs more slowly than that of genes with less vital functions. More rapidly changing genes are used to date more recent evolutionary events, and slower evolving genes are used to map more ancient divergences, he explains.

“The molecular clock is useful for obtaining evolutionary information when you have little or no fossil record,” said Hedges. “For example, fungi, which are soft and squishy, don’t make fossils well. But we can take the rate of change of genes from vertebrates or plants, which have a decent fossil record, and apply it to the unknown group.”

The molecular clock also can be used for putting a series of evolutionary events into chronological order. This is done by comparing sequences from different species to determine when they last shared a common ancestor, in effect drawing the family tree. “It’s often difficult to do find common ancestors between species using fossils, no matter what the organism,” said Hedges.

Though the molecular clock is still regarded as somewhat controversial, said Hedges, it is gaining acceptance as our understanding of genome sequences improves. “As more researchers opt to use the technique,” he concluded, “the molecular clock is itself evolving into a more accurate timepiece.”

Source: By Solmaz Barazesh, Research Penn State

Explore further: A 100-million-year partnership on the brink of extinction

Related Stories

A 100-million-year partnership on the brink of extinction

May 24, 2016

A relationship that has lasted for 100 million years is at serious risk of ending, due to the effects of environmental and climate change. A species of spiny crayfish native to Australia and the tiny flatworms that depend ...

Explainer: What is the molecular clock?

September 15, 2015

In the 150 years since Charles Darwin recognised the kinship of all life, scientists have worked to fulfil his dream of a complete Tree of Life. Today, the methods used to trace the evolutionary branches back through time ...

Recommended for you

Previously unknown global ecological disaster discovered

June 28, 2016

There have been several mass extinctions in the history of the earth with adverse consequences for the environment. Researchers from the University of Zurich have now uncovered another disaster that took place around 250 ...

Opal discovered in Antarctic meteorite

June 28, 2016

Planetary scientists have discovered pieces of opal in a meteorite found in Antarctica, a result that demonstrates that meteorites delivered water ice to asteroids early in the history of the solar system. Led by Professor ...

Clandestine black hole may represent new population

June 28, 2016

Astronomers have combined data from NASA's Chandra X-ray Observatory, the Hubble Space Telescope and the National Science Foundation's Karl G. Jansky Very Large Array (VLA) to conclude that a peculiar source of radio waves ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.