คำบรรยาย (319)
0:00- Humans sort of think like engineers.
0:02We think of what we want as an outcome
0:05and how we can get there sort
of in the shortest distance.
0:08And we may use some trial and error,
0:10but we really try to minimize the error.
0:13these evolutionary
processes start with error.
0:15They start with a random change
0:17and then try those things out.
0:19And it's really hard
to get our heads around
0:21how can all the diversity
0:23and sort of seeming order
that's out there in the world
0:26emerge from a process
dependent upon chance.
0:31evolutionary biologist and
author of several books,
0:34including my most recent,
0:35"A series of Fortunate Events:
0:37Chance and the Making of
the Planet, Life, and You."
0:44[Narrator] How life works,
the staircase of evolution.
0:48- So evolution, what it really
means is change over time.
0:52So we wanna know how that
change occurs over time.
0:55And there's two dimensions to this process
0:58and it kind of works like a staircase.
1:00And one process is mutation
1:03and that's the rise in the staircase.
1:06Those occur by chance.
1:08If mutation didn't happen,
all things would be identical.
1:12to make individuals
different from one another.
1:14Those mutations are genetic
changes, changes in their DNA.
1:20that changes the property
that favors reproduction,
1:23survival of the
individuals, it will spread,
1:26and that's the selection process
1:28and that's sort of the
run in the staircase.
1:31Chance invents and natural
selection propagates
1:34that chance invention.
1:35And once that process has happened,
1:38new mutations can then
be added on top of that,
1:41maybe even changing the
character a little bit further,
1:43and that's another rise in the staircase.
1:46And if that's working
better, that's gonna spread.
1:48And so it's the cumulative
set of mutations
1:51and the cumulative process of selection
1:54that takes us up that staircase.
1:57could have goodness knows how many stairs,
1:59but it has to go through
this stepwise process
2:02of individual mutations arising,
2:05sweeping through the population,
2:06new ones arising, sweeping
through the populations.
2:09It doesn't go from the base of the stairs
2:11to the top of the stairs in one jump.
2:13And this is hard for people
to get their heads around
2:15because they may think
about that first step,
2:16and it's sort of hard
to imagine, you know,
2:18how do you get something as complicated,
2:21as individual organs like
an eye or a wing of a bird
2:25or things like that, but,
2:30The speed with which some
new adaptation spreads
2:33through a population
depends upon the magnitude
2:36of the advantage it conveys.
2:37If it's about a 3% advantage,
2:39meaning that about 103 individuals survive
2:42for every 100 that don't have it,
2:44well that will take
about 1,000 generations
2:47to spread through the population.
2:49Now I'm saying generations
2:50because it depends upon the
generation time of the creature.
2:52So if the generation time,
2:54for example, if humans is 25 years,
2:56it'll take 25,000 years
2:57for that to spread through
the human population.
3:00But if the generation time is 20 minutes,
3:03It'll take, you know, 20,000
minutes to spread through,
3:05which is much shorter.
3:06It's a matter of generations
3:07because it's reproduction
3:11is a generation by generation process.
3:14It's really important to underscore
3:16that these mutations occur at random
3:18without any consideration
of whether they're good
3:20or bad for the organism.
3:22It's the external conditions
that are gonna sort this out.
3:25So some things can be
good for one creature
3:28and bad for another, okay?
3:30So for example, a color change might make
3:33a creature more invisible
in some settings,
3:35more visible in another.
3:38Some mutation may make it better
adapted to warmer climates,
3:42less well adapted to
colder climates, right?
3:45So it depends on these
external circumstances.
3:47So the mutations arise at random.
3:50Well what about those
external circumstances?
3:52What we'd say the abiotic conditions,
3:54whether the physical world
that the creature is living in,
3:57well that's also
generated a lot at random,
4:00tectonic plates moving across the earth.
4:05all the things that shape
the conditions of life
4:08are often due to physical
processes of the earth.
4:11Of all the many thousands
and millions of things
4:14that creatures have come up with,
4:15I have a few of my favorites
4:17that I think really exemplify
this process of adaptation.
4:21And one of my favorite sets of creatures,
4:22there's some fish called Icefish.
4:25They live in the southern
ocean around the Antarctic,
4:28and they live in water
that is actually below
4:31the freezing temperature of fresh water.
4:34They're right around about
29 degrees Fahrenheit,
4:37is the ocean around the Antarctic.
4:39And that's a challenging environment.
4:43So challenging for example,
4:45that you may know that there's
ice flows around there,
4:47and if little ice crystals
4:49just get into the bodies of these fish,
4:51they'll nucleate freezing,
4:54they'll freeze like fish sticks.
4:56So they need to have a mechanism
4:58that prevents them from freezing
at that cold temperature.
5:02And what they've done is
they've evolved antifreeze.
5:05Certain proteins in their bloodstream made
5:08in very large quantities
suppress the ability
5:11of ice crystals to grow
inside their bodies.
5:14And so they're able to
tolerate that subfreezing water
5:17of the Antarctic and other fish aren't.
5:20So what's happened over the
last 30 or 40 million years
5:23is that a lot of fish that
were once swam in those oceans,
5:26sharks and rays and all
that, they're all gone.
5:28They're extinct from those waters,
5:29we find them in more temperate waters.
5:31But it's the Icefish and the
antifreeze bearing relatives
5:35that exploit those waters.
5:38And the Icefish have also
come up with something
5:39that's incredibly nifty
and shocked naturalists
5:42when they first discovered it,
5:44which is if you open,
which any fish, you know,
5:47slice it open, you're gonna see red blood
5:49because that's something
that animals with backbones
5:51have and have had for
almost 500 million years
5:55But you slice open an Icefish,
their blood is colorless
5:59'cause they've gotten
rid of red blood cells.
6:01So they don't even have a mechanism
6:03for carrying oxygen actively
in their bloodstream
6:07They've ditched red blood cells.
6:09And the reason they've
ditched red blood cells
6:11is that those low temperatures,
6:12it makes their blood too viscous.
6:15And that's a disadvantage.
6:19it was an advantage to get
rid of red blood cells.
6:23To the rest of us, it's instantly fatal.
6:26So it's a good illustration
6:27of just how conditional
these adaptations are,
6:30that to exploit the resources
of the southern ocean,
6:33you gotta make antifreeze
6:34and get rid of your red blood cells,
6:36where the other parts
of the world antifreeze
6:38would be irrelevant and your
red blood cells are necessary
6:41for every second of life.
6:43Let's talk about speciation.
6:45That process of variations
helping to adapt,
6:48that's the process of adaptation.
6:50But speciation is the generation
of two species from one.
6:54What that take for there
to be two species to form?
6:59And we know, and this is
where in the island biology
7:01of Wallace and Darwin help,
7:03is that they were seeing
species on different islands.
7:07Well, islands provide some isolation,
7:10and that isolation means
that those populations
7:12aren't exchanging genes.
7:16each of those populations
will accumulate mutations
7:19that exist in one population
and not the other vice versa.
7:23Well, those could become
genetically distinct enough
7:27were ever to come back again in contact,
7:29they may not be compatible
with one another,
7:32and they'll be species.
7:34So how long does that take?
7:36Well, it's been estimated
7:37for big animals like mammals and birds,
7:40that works out to roughly
be about 2 million years.
7:43But 2 million years is still a long time.
7:46And you've probably heard, for example,
7:48that there's now very strong evidence
7:50that homo sapiens are species mated
7:53with Neanderthals a distinct species.
7:57And what's really happened
in the last few decades
7:59is that for biologists,
that species barrier
8:01has become much more porous
than we first thought it was.
8:05In the old days, in Darwin's
day, species were characterized
8:08as really distinct things
and we didn't think
8:09there was any kinda shenanigans
going on between them.
8:13But we now understand that it's
a much more porous situation
8:17that for some period of time
as populations are diverging,
8:20they can get back together.
8:21And things that we humans
might call distinct species
8:25can't often interbreed.
8:27Another common question or idea
8:29is that, you know, for example,
if humans evolve from apes,
8:32why are apes still around?
8:34Well, the important thing is to understand
8:34that evolution is a splitting process.
8:36So the family trees keep splitting.
8:38So the human part of that tree
8:40has had now its own separate history
8:43from the eight part of that tree.
8:44We shared a common ancestor
about 6 million years ago.
8:48But apes have gone on living
as they do, for example,
8:54And there's a great diversity
of apes, of course still here.
8:56Baboons, orangutans, gorillas,
chimpanzees, et cetera.
8:59While the human branch has gone on,
9:01and has its own evolutionary trajectory,
9:06it's not that evolution is linear
9:07and that everything new replaces the old,
9:09it's a splitting process.
9:11And so that's why the Tree of Life
9:14has just split into
enormous numbers of branches
9:17from common ancestors.
9:19Two of our main records of
evolution are the fossil record
9:25The DNA record is largely
only accessible to us
9:28for living creatures
and maybe some creatures
9:31that have lived over
the past million years.
9:33You know, do we have every brick?
9:35Do we have every intermediate?
9:37No, 'cause you know,
extinction takes away 99.9%
9:40So if there was no extinction,
9:41we'd have a perfect record
of evolution, right?
9:44We don't have the DNA record
of dinosaurs, for example,
9:46but we've got the fossil
record of dinosaurs.
9:49So we use these two records
to sort of reconstruct
9:53That allows us to reconstruct
the evolution of things
9:55that are fairly complex.
9:56Let's take something like a
walking limb from a fish fin.
10:01This evolutionary process transpired
10:03over 20, 30 million years,
10:06about 380, 390 million years ago.
10:10To do that, we have to get fossils
10:12that represent the various
stages of evolution
10:14and see how did all the bones change?
10:16So you went from a swimming
fin to a walking limb,
10:20and the fossil record by
now is pretty darn good.
10:24Then we can go to creatures
that have fins, fish,
10:27and we go to creatures like
amphibians that have limbs
10:30and we can figure out how do
those genetic programs work
10:33and where are the differences.
10:36Now we don't have every detail sorted out
10:39and that would be
incredibly time consuming
10:40and expensive to do,
10:42but we certainly have the general picture
10:46that we understand how the
bones changed in history
10:50and we understand how the
development of program changed
10:54to generate a limb in the place of a fin.
10:58And that's not something Darwin ever had.
10:59That's something that we
really only had for the last 20
11:04So evolutionary science keeps building
11:06on this huge foundation
11:07and what we get is an ever more detailed
11:10and ever more confident
record of what's happened.
11:13But what's at no doubt
is that this process,
11:16mutation and selection,
mutation and selection
11:19is the universal, going
on in every population
11:22of every living thing every day.
11:26In fact, this process of mutation
11:28and selection seems so universal
11:30that we think it must
have played a vital role
11:32at the origin of life.
11:35where life exists in the
universe it's operating.
11:38So evolution is this vast and rich process
11:41and you know, we've been trying
11:42to understand it for 160 years.
11:44And in the course of that,
there's some misunderstanding
11:46in just how it's communicated,
11:48where I think there's some conflict
11:49between the terms we scientists use
11:51and sort of their common understanding.
11:54And one of those is theory.
11:57We talk of scientific theories.
11:59Theory is much higher on the
hierarchy than, for example,
12:02a fact or just an observation.
12:04A theory is assembled from
lots and lots of facts
12:08and independent lines of
evidence that sort of cohere.
12:11So a theory is really
sort of the top category
12:16Where of course, you know, in
the street theory might mean
12:19that's my best guess
12:20or that's just something
that I'm conjecturing
12:23The way we talk about
those kind of conjectures,
12:25we call those hypotheses.
12:26And that when hypotheses
have been rigorously tested,
12:30that's what can contribute
to making a theory.
12:33So folks, when we speak of theory,
12:37think that we're still very tentative
12:39about the truth of evolution.
12:42That's not all the case.
12:44The theory of evolution,
which has grown enormously
12:48in the last 160 years
12:50is a huge body of observations,
evidence, and facts
12:54that are consistent with one another,
12:56that come from completely
different sources of science.
13:01And that's what gives it its power.
13:03Of course wish that maybe
that connotation of theory
13:07that's, you know, more of
the everyday connotation
13:10would be better understood.
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