Summer is a good time in Oregon. I like going on a hike in my neighborhood park and watch for birds. Ah, An American robin! I love listening to their musical whistles. They stop by in my front yard year round.
Medium zoom - Attu + bird animation
Ah! Another songbird, a house finch! A common visitor at my park too here in Oregon.
Zoom in - Attu + 2 birds animation
I wonder how closely related these songbirds are. Or how far apart.
Zoom in - Attu + 2nd voice over
Ever heard of a molecular clock?
Exterior Scene zooming out showing Ashwin talking outside with an American Robin animation or real shot in front and the sound of a Robin whistle.
Close up shot - Attu + animation
I know what a molecule is and what a clock is, but what exactly is a molecular clock? And how does it help answer the question about how closely related these birds are? Let's explore
We know that all living things store their genetic information in DNA. All of us have almost exactly the same basic genes, with some small differences. And the same goes for other species such as birds. But every so often, a small error or a typo called a mutation creeps into the genome.Sometimes this mutation is dangerous because it can cause the body to malfunction, but a lot of times, it is benign. The parent then passes this mutation on to their children.
Ashwin walking and talking outside with a house finch real shot or animation.
Attu voice only with animation
Closeup of Ashu talking with birds animation fluttering in front
Attu voice only with animation
The cool thing is that evolutionary biologists for the past several decades have been investigating the possibility that these evolutionary changes or mutations occur in a clock-like fashion.
A different voice or Attu clone asking a question while Attu is pondering. Just a plain white background will do
Attu talking with graphics / animation
In any given stretch of DNA, over the course of millions of years, these mutations may build up at a rate that is relatively constant over time and among a group of organisms.
A molecule appears on one stretched hand when Attu talks about the molecule and a clock appears on another hand when Attu talks about the clock.
Clock ticking animation on an evolutionary timeline with Attu voice over. Different animals and mammals appear on the evolutionary clock as it keeps ticking. Also can do neanderthal, caveman, modern man sequence.
Clock ticking animation on an evoluationary timeline w Attu voice over. Genetic code with one mutation at a time appear on each tick of the clock.
Whiteboard animation with voice over
Let’s suppose that the DNA sequences of 2 bird species have a content difference of 5%. But did that divergence happen at 1% every million years over a period of 5 million years, or 5% every million years over just a period of 1 million years or some other combination?
To figure that out, we need to understand how the molecular clock is calibrated. Now let's suppose the common ancestor of these 2 bird species occupied an island. And let's suppose that 2 million years ago, there was a geological event such as the formation of a mountain barrier that separated this species into its east and west populations. Over time, let's assume that each of these population accumulated its own set of mutations and individuals in the 2 populations are reproductively isolated leading to the formation of 2 separate species.
So, if we determined from the geological record that a mountain formed 2.5 million years ago in this location and that was likely the cause for the two populations to be separated, then we can conclude that a 5% content difference must have happened over a period of about 2.5 million years or at approximately a rate of 2% content difference for every million years or 1% change in a given species over a million years.
Since the clock is now calibrated with this geological event, we now know that the 2 bird species with a 5% genetic divergence must have diverged about 2.5 million years ago. In addition to geological events, scientists also use fossil records as evidence to double check and improve the accuracy of their timing estimates for evolutionary events.
[We need to find something in the geological record that we could connect to the possible separation of these 2 species.]
[We know that speciation often occurs when a population is divided into two by some physical barrier,like the formation of a mountain, or sea level rise that cuts off an isthmus and creates two separate land masses, and so on.]
One question: you assumed a mutation rate of 1% in1 Million years for birds. How aboutother organisms? Can you compare a mountain gorilla in Central Africa to a birdof paradise in the island of new guinea using the same rate of mutation? Goodcatch. Different organisms may have different rates of changes. Scientists haverefined the molecular clock since its original proposal so that molecular rates are allowed to vary among lineages, in a limitedmanner. The rate of molecular evolutionvaries significantly even among different bird species. Many bird lineages haveevolved at a speed relatively close to the average rate of 1% per 1 millionyears, but some birds were found to be evolving more than four times fasterthan others.
Conversation between Attu1 and Attu2
One last thing. Why doyou need a molecular clock? Can’t we find lineage divergence data from fossil records? Not always. What if wewant to study organisms that have left few traces of their biological historyin the fossil record, such as flatworms and viruses. What if the fossil record isincomplete?
If applied correctly, the molecular clock can yield enlightening date estimates for evolutionary events that would otherwise be difficult to study from the fossil record alone.Maybe the species was divided by a physical barrier that formed -- such as a mountain