Name: The evolutionary logic of survival and death, in 54 minutes | Sean B. Carroll: Full Interview
Uploaded: 2025-12-19T17:59:50.000Z
Duration: 54 min 34 s
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How can all the diversity and sort of seeming order that's out there in the world emerge from a process dependent upon chance?

Chance invents and natural selection propagates that chance invention.

Our immune system works on the very same principles of mutation and selection as evolution at large does in the big world.

Cancer is truly an evolutionary disease, meaning that it is a disease that results from the exact same evolutionary process we're describing.

So while Darwin is the most famous figure associated with the theory of evolution, Alfred Russel Wallace played a key part, and

in their day, this was referred to as the Darwin Wallace theory.

Carroll, evolutionary biologist and author of several books, including my most recent, A Series of Fortunate Events, Chance and the Making of Planet Life and you.

So evolution, what it really means is change over time.

So we want to know how that change occurs over time.

And there's two dimensions to this process.

And one process is mutation, and that's the rise in the staircase.

If mutation didn't happen, all things would be identical.

So you need mutation to make individuals different from one another.

Those mutations are genetic changes, changes in their DNA.

If by chance, that changes a property that favors reproduction, survival of the individuals, it will spread.

And that's the selection process, and that's sort of the run in the staircase.

Chance invents, and natural selection propagates that chance invention.

And once that process has happened, new mutations can then be added on top of that, maybe even changing the character a little bit further.

And that's another rise in the staircase.

And if that's working better, that's going to spread.

And so it's the cumulative cell of mutations in the cumulative process of selection that takes us up that staircase.

So that staircase could have goodness knows how many stairs, but

it has to go through this stepwise process of individual mutations arising, sweeping through the population, new ones arising, sweeping through the populations.

It doesn't go from the base of the stairs to the top of the stairs in one jump.

And this is hard for people to get their heads around because they may think about that first step.

And it's sort of hard to imagine how do you get something as complicated as individual organs like an eye or a wing of a bird or things like that.

But time, immense time, the speed with which some new adaptation spreads through a population depends upon the magnitude of the advantage it conveys.

If it's about a 3% advantage, meaning that about 103 individuals survive for every 100 that don't have it.

Well, that will take about 1,000 generations to spread through the population.

Now I'm saying generations because it depends upon the generation time of the creature.

So if the generation time, for example, of humans is 25 years, it'll take 25,000 years for that to spread through the human population.

But if the Generation time is 20 minutes, you can work it out.

It'll take, you know, 20,000 minutes to spread through, which is much shorter.

It's a matter of generations because it's reproduction as a generation by generation process.

It's really important to underscore that these mutations occur at random without any consideration of whether they're good or bad for the organism.

It's the external conditions that are going to sort this out.

So some things can be good for one creature and bad for another.

Okay, so for example, a color change might make a creature more invisible in some settings, more visible in another.

Some mutation may make it better adapted to warmer climates, less well adapted to colder climates.

Right, so it depends on these external circumstances.

Well, what about those external circumstances?

What we'd say the abiotic conditions, whether the physical world that the creature is living in, well, that's also generated a lot at random.

Tectonic plates moving across the earth, you know, volcanism, all the things that shape the conditions of life are often due to physical processes of the earth.

Of all the many thousands and millions of things that creatures have come up with, I have a few of my favorites that I think really exemplify this process of adaptation.

And one of my favorite sets of creatures are some fish called ice fish.

They live in the Southern Ocean around the Antarctic, and

they live in water that is actually below the freezing temperature of fresh water.

They're right around about 29 degrees Fahrenheit is the ocean around the Antarctic.

So challenging, for example, that you may know that there's ice flows around there.

And if little ice crystals just get into the bodies of these fish, they'll nucleate freezing.

So they need to have a mechanism that prevents them from freezing at that cold temperature.

And what they've done is they've evolved antifreeze, certain proteins in their bloodstream made in very large quantities suppress the ability of ice crystals to grow inside their bodies.

And so they're able to tolerate that sub freezing water of the Antarctic and other fish aren't.

So what's happened over the last 30 or 40 million years is that a lot of fish that once swam in those oceans, sharks and rays and all that, they're all gone, they're extinct from those waters.

We find them in more temperate waters, but

it's the ice fish and the antifreeze bearing relatives that exploit those waters.

The ice fish have also come up with something that's incredibly nifty and shocked naturalists when they first discovered it, which is if you open any fish, you know, slice it open, you're going to see red blood because that's something that animals with backbones have and have had for almost 500 million years on this planet.

But you slice open an ice fish, their blood is colorless because they've gotten rid of red blood cells.

So they don't even have a mechanism for carrying oxygen actively in their bloodstream like we do.

And the reason they've ditched red blood cells is that those low temperatures, it makes their blood too viscous.

And it's, that's a disadvantage to the ice fish.

It was an advantage to get rid of red blood cells.

So it's a good illustration of just how conditional these adaptations are that to exploit the resources of the Southern Ocean you got to make antifreeze and get rid of your red blood cells, where the other parts of the world antifreeze would be irrelevant.

And your red blood cells are necessary for every second of life.

Let's talk about speciation, that process of variations helping to adapt.

But speciation is the generation of two species from one.

What is, what does that take for there to be two to two species to form?

this is where the island biology of Wallace and Darwin help is that they were seeing species on different islands.

Well, islands provide some isolation and that isolation means that those populations aren't exchanging genesis.

Over time, each of those populations will accumulate mutations that exist in one population and not the other.

Well, those can become genetically distinct enough that if those things were ever to come back again in contact, they may not be compatible with one another and they'll be species.

Well, it's been estimated for big animals like mammals and birds, that works out to roughly be about 2 million years.

But 2 million years is still a long time.

And you've probably heard, for example, that there's now very strong evidence that Homo sapiens are species mated with Neanderthals.

And what's really happened in the last few decades is that for biologists, that species barrier has become much more porous than we first thought it was in the old days.

In Darwin's day, species were characterized as really distinct things, and

we didn't think there was any kind of shenanigans going on between them.

But we now understand that it's a much more porous situation, that for some period of time, as populations are diverging, they can get back together, and

things that we humans might call distinct species can often interbreed.

Another common question or idea is that, for example, if humans evolve from apes, why are apes still around?

Well, the poor thing is to understand that evolution is a splitting process.

So the human part of that tree has had now its own separate history from the ape part of that tree.

We shared a common ancestor about 6 million years ago.

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The evolutionary logic of survival and death, in 54 minutes | Sean B. Carroll: Full Interview

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