Is this all that surprising? If mammals tend to have genome sizes of around 1-10 pg, and a mutation has a chance of around 10^-8 per base, then you would expect around 10-100 mutations per generation. So if you were sampling random members of a species at close periods in time, you would probably see a lot of genetic variation, but over wider stretches of time, the later samples are only descendants of the old genes that reached fixation and survived, plus whatever new mutations centered around the later time. Fixation is slow and depends on the mutation's positive selection benefits and the overall population count, but some mutations happen every generation and most aren't positive... With viruses their mutation chance is even higher... This is just from references in the 90s, so is Ho's result just a nice math model to better take into account time scales with samples?
Agreed, I was surprised this was surprising. I saw some study where some moths can evolve from white to black in a couple generations if they go from white bark forests to black bark forests. But that doesn't mean they'll evolve into something unrecognizable in a hundred generations.
I was just looking at Taylor expansions today (guid uniqueness...birthday problem), so, this kind of reminds me of that. First you evolve, then you evolve the ability to evolve, then you evolve the ability to evolve the ability to evolve, etc. So maybe there's a formula.
I think the issue is local optima. The Cambrian explosion is known for a huge diversity of forms, but diversity generally means a lot of sub optimal results which mostly get killed off.
Beyond that, what worked for trilobites largely works for horseshoe crabs.
Consider if something is getting larger by say 1% per thousand years, well over 1 million years that's simply unsustainable without becoming ~21,000x as large and thus a very different organism. Further conditions may change promoting periods of shrinking which means the average is going to trend to ever smaller changes per period of time.
Sure, a fox is not the size of a wolf because both are useful sizes to be. If you follow the evolution of foxes getting slightly larger or slightly smaller both provide advantages so it's really a balance which may change over time that determines what happens.
AKA, fox sizes might drift up and down some over time, the average is going to stay near their current size unless say wolves died out. At which point they may quickly get larger and replace wolves, but they would not keep growing indefinitely and end up the size of great whales as they are now constrained by the 'wolf' niche. Further, foxes may end up spiting, you have foxes that look about the same, and foxes that look a lot like wolves.
Now if you chart sizes as say 10,11,12,11,10,9 then the transition between those stages is a 'fast' + or - 1, but the net result is only -1 over 6 steps or -1/6th.
The same thing is likely going on with less obvious features, but they are less intuitive.
This seems entirely in harmony with Stephen Jay Gould's theory of punctured equilibrium. [1] "Punctuated equilibrium (also called punctuated equilibria) is a theory in evolutionary biology which proposes that once species appear in the fossil record they will become stable, showing little evolutionary change for most of their geological history. This state is called stasis. When significant evolutionary change occurs, the theory proposes that it is generally restricted to rare and geologically rapid events of branching speciation called cladogenesis."
Species experience perturbations in their genetic expression constantly but these only evolve into stable changes when the environment changes to require it.
As others have said, I found it surprising that the change in rate with time scale compared was surprising. Simon Ho's paper is downloadable (link in OA) and it has a mathematical framework for his discussion. I'm not sure if the argument is accessible to non-specialists but I'll have a read.
Rabbit hole warning: Björn Kurtén the Finnish paleontologist whose work on horse fossils was cited early in the OA turns out to be a very interesting character. He wrote what he described as a 'paleonovel' about encounters between CroMagnon and Neandertal people...
This strikes me as a very hyped article. "It’s like Einstein’s theory of relativity, but for viruses" - seriously?! While the proposed explanation for differing rates of evolution is doubtlessly interesting from a scientific perspective, it is definitely not as big as this article makes it out to be.
Also the dig at the molecular clock is pointless - we have always known that there are many unknowns involved when using a "standard mutation rate" to calculate the time since the divergence between two species. That is nothing new, and the estimates we have are consequently constantly being revised. The time-dependent rate is a good addition to our methodologies, but it isn't the revolution advertised here.
(Oh and by the way, the original paper is from 2005.)
Does not seem to me to be like that. Seems to me what this is saying is that on shorter time scales it tries out many different combinations that when you look at them on longer time scales have settled into whatever wins that process. It sort of settles on longer scales of time to the optimal solution that survives, in shorter periods of time it's experimenting with the search space. In longer periods of time it's decided on the answer so it converges on that space. Putting it this way, evolution is just a search to find something that works or works better, the results being tested continuously by the environment.
This makes sense. If a species is thriving and not under much selective pressure, it does not have to be optimized for its environment to survive. This allows for a larger amount of genetic variance. Most mutations either are maladaptions, don't cause any significant change or lead to death while relatively few confer an advantage. In addition, some adaptations may take many generations of mutation to actually provide an advantage or may need time to find a niche where a mutation that is a maladaptation in one environment may be an advantage in others (Sickle cell anemia gives resistance to malaria, Tay Sachs does the same for tuberculosis).
When selective pressures/shocks arise, there is a wider amount of genetic variance that can adapt to the new environment... a larger menu of options for survival, and this pressure acts as a feedback loop, further promoting that more mutations in that direction. Think of the pressures that led to the growth of the giraffe's neck. Perhaps it is this combination... allowing time for a species to thrive and have genetic diversity and shocks which select for and promote the evolutionary path
Does genetic variation rise in response to selective pressure?
It seems to me that this is a two stage process.
1. Variation is introduced at a fairly constant rate by mutation.
2. Variation is filtered out by natural selection based on how well it fits the environment.
The shocks occur when the environment changes so that the criteria for fit change and new variations survive for the long term.
So when we look at evolution we see a rate of change driven by the rate of mutation.
When we look at the long term we see a rate of change driven by the environment.
I agree. When I look at the long term though, the environment and mutation are just different parts of the same whole. Both are necessary for phenotypic changes and speciation. Even that is simplistic. It doesn't take into account the significance of viral transfer of dna and other epigenetic factors, how an environment can cause normally dormant genes to express themselves, sexual selection where attractiveness != most environmentally fit, etc.
See also the link to "This might be the coolest visualization of evolution ever" and comments re: "Wolf359" and "Microcosmic God" by Theodore Sturgeon at...
