Hey! Kallie here. Todayâs episode was made in collaboration
with the Smithsonian National Museum of Natural History. Blake and I were lucky enough to visit the
museum's Deep Time Hall during its renovation and thatâs not all! On Friday June 15th, I will be back in the
Deep Time Hall, hosting a livestream Weâll be getting a behind-the-scenes tour
of the exhibit and an expert from the Smithsonian will be answering your questions. More information on where to watch the stream
will be in the description. Now letâs talk about evolution. The story of life on Earth is a story of change. Living things have changed the atmosphere
and the climate. Theyâve endured the movements of the continents,
and the rise and fall of the seas. And theyâve responded to these changes,
over the long course of Earthâs history, through a process that still continues today:
evolution. This powerful force is, in the simplest terms,
just change over time. And itâs responsible for the shape of the
tree of life, and for generating the diversity that we see in the fossil record as well as
in modern ecosystems. Itâs the very foundation of our understanding
of biology, and it continues to help us make sense of the world around us. As a scientific concept, evolution was revolutionary
when it was first introduced. Charles Darwin and Alfred Russell Wallace
were the first to put all of the pieces together into a unified explanation that would radically
alter our understanding of life on our planet. But our understanding of evolutionary theory
didnât stop there. Weâve learned a lot -- and weâre still
learning. In the last 160 years, weâve learned what
Darwin and Wallace didnât know, and weâve figured out a lot about how evolution actually
works - like how it can produce the incredible array of animals you see here, and how we
know theyâre all related. When the Smithsonian opened its first exhibit
of fossil animals in the early 1900s, it was called the Hall of Extinct Monsters. Today, Kallie and I are in a space thatâs completely renovated and re-imagined: the David H. Koch Hall of Fossils: Deep Time,
also known as the Deep Time Hall. And it depicts, more vividly than ever, the
staggering variety of animal forms that have arisen and disappeared over time. A monument to the fact that, as Darwin himself
put it, âendless forms most beautiful and most wonderful have been, and are being, evolved.â Charles Darwin and Alfred Russell Wallace
were both British naturalists whose thinking about the natural world was deeply shaped
by long voyages of exploration. Darwin famously traveled to South America
and the Galapagos Islands, and Wallace went to South America and Southeast Asia. And together they observed an incredible diversity
of life. They saw first-hand how very similar organisms
seemed to be slightly modified in ways that made them ideally suited to their environments. For Darwin, he saw this in the different shapes
in the beaks of finches on the different islands of the Galapagos archipelago. For Wallace, it was the differences between
monkeys living on different riverbanks in the Amazon. And they both realized that the patterns they
observed meant that these species all probably arose from the same place - a common ancestor. The bodies of these animals, they realized,
had been shaped over time by the conditions in their environments, resulting in the different
forms they found on different islands and riverbanks. But, in addition to being well-traveled, Darwin
and Wallace were also well-read. And their ideas were deeply influenced by
other, earlier thinkers, in natural history, geology, and even economics. Scholars like Georges Cuvier, Charles Lyell,
Jean-Baptiste Lamarck, and Thomas Malthus helped establish the ideas that were critical
to evolutionary thinking -- namely, that the Earth was very old, that species seemed to
change and go extinct over time, and that individuals competed over limited resources. Darwin and Wallace used these insights -- along
with their own observations -- to both arrive at the same mechanism by which species evolve:
natural selection. In a paper read to a meeting of scientists
in London in 1858, they presented their theory of natural selection based on a series of
principles: The first key idea was that, in a population
of living things, natural variations will occur, and as a result of those changes, some
members of the population will survive and reproduce more than others. Then, they posited that those that survive
and reproduce will pass on their traits to their offspring. And this meant that traits that give individuals
an advantage in a certain environment will get passed on more often. And as a result, more members of the population
will have that trait. Therefore, gradually and over time, this will
result in certain traits showing up more or less often in a population. Today, when this series of events happens
within a species, we call it microevolution. Itâs how a single species responds to changes
in the environment. On a broader scale, we call it macroevolution. This is how these changes accumulate over
long periods of time to produce entirely new body plans, new species, and the grander patterns
of diversity in the tree of life. And one of the most incredible things about
the development of the theory of evolution by natural selection was that Darwin and Wallace
didnât have a good explanation for how traits were passed from parent to offspring. Genetics as a field was still a long way off,
and neither of them were aware of the experiments that were being done on pea plants at the
time, by a Czech monk named Gregor Mendel. In the 1850s, while Darwin and Wallace were
putting all the puzzle pieces of natural selection together, Mendel was breeding peas at his
monastery to try to figure out how heredity worked. And he figured out that traits didnât simply
blend together when living things reproduced. Instead, only some were inherited as discrete
traits by different numbers of offspring. Mendelâs results were rediscovered around
the turn of the 20th century, when a new generation of biologists was investigating genetics. And it was this new wave of researchers that
brought our understanding of evolution to the next level. One of these scientists was American biologist
Thomas Hunt Morgan. Instead of peas, he bred flies, and in 1910,
he bred a fly with an odd trait. It had eyes that were white, instead of red Whatâs more, he was able to breed that white-eyed
trait back into the parent population. Morgan had discovered another key driver of
evolution by natural selection: mutation. He realized that the fly had undergone a random
change in its genes that made it different from the rest. So Morgan theorized that mutations were a
source of variation in living things - and that it was the source of the variation that
natural selection acted on. Beneficial mutations would be passed on, he
thought, and detrimental ones would eventually disappear. So by the early 1900s, weâd already recognized
two of the four major forces of evolution: Darwin and Wallace gave us natural selection,
and Morgan brought mutation into the mix. It wasnât until the 1920s that things would
really start to come together through the work of three of the founders of the field
of population genetics: Ronald Aylmer Fisher, John Burdon Sanderson Haldane, and Sewall
Wright. Fisher and Haldane both looked at natural
selection mathematically, especially in large populations, using Mendelâs ideas about
inheritance to figure out how often and how fast natural selection worked on variations. It was Haldane who did the math that explained
the transition of Englandâs famous peppered moths, in which a gene for dark color spread
quickly, as pollution darkened the bark of the trees they lived on. Studies like this led Fisher and Haldane to
conclude that natural selection acted slowly, but also uniformly, in large populations. Meanwhile, in the US, a geneticist named Sewall
Wright was thinking about how evolution worked in smaller, more isolated populations. He did some research breeding animals like
cattle and guinea pigs. But it was his mathematical studies of genetics
that led him to uncover another key idea: genetic drift. This is the idea that the frequency at which
certain genes appear will sometimes change, totally by chance, and randomly, and Sewell
found that this has a greater effect in smaller populations than in larger ones. Another, somewhat related, idea that came
up around this time, in the late 1930s, is gene flow - the movement of genes between
populations, by way of migration. So, when members of one population of a species
-- say, panthers from Texas -- breed with members of another population -- like panthers
in Florida -- that will change the makeup of the gene pool in the Florida population. And this, too, is a driving force of evolutionary
change. Together, the work of Fisher, Haldane, and
Wright showed that natural selection acting on genes was the most likely explanation for how evolution works And in 1937, another biologist brought together
all of the evidence from genetics and natural history to show how evolution by natural selection
could produce new species. And this enabled us to make the enormous conceptual
jump from microevolution to macroevolution. His name was Theodosius Dobzhansky, and he
had worked in Huntâs fly lab. Heâd found that fly populations from different
countries seemed to be genetically different, even though they were considered to be the
same species. But, these flies werenât so good at reproducing
with each other. So he wondered if they were actually different
species. And this took the scientific conversation
all the way back to the 1800s, and the once-novel idea that evolution could eventually, gradually
produce new species. From his experiments, Dobzhansky produced
a theory about how new species originate. Mutations happen naturally in populations,
creating variations that can stick around if theyâre beneficial or just neutral. And if populations are isolated, these variations
can remain within a single group, with new mutations popping up. But none of these would spread to the rest
of the species. Over time, this would make one group genetically
distinct from others, potentially causing problems if it tried to interbreed with others. And given enough time, it would lose the ability
to interbreed with other populations entirely. It would become a new species. This was the beginning of âthe Modern Synthesis,â
a collaboration by many evolutionary biologists of the time to explain large-scale patterns
of evolution. And while the Modern Synthesis has changed
over time, itâs still the framework for our current understanding of how evolution
works. In 1953, we added a better understanding of
how genetics works, through the discovery of the structure of DNA and how it functions. So, now we know that mutations randomly happen
when DNA is copied incorrectly during replication - something Morgan didnât know. Now we also know that natural selection
is only one of the mechanisms of evolution, along with mutation, genetic drift, and gene
flow. And itâs this knowledge that allows us to
witness -- first hand! -- microevolution taking place today -- say, in studies of bacteria
that develop resistance to antibiotics. And it also helps us understand the story
that the specimens around us right now are trying to tell us, the story of macroevolution. With this understanding, we can see how the
aquatic mammal behind me, Pezosiren, is related to the more recent Metaxytherium. Separated by some 30 million years, these
mammals lost their hind limbs and acquired flippers instead of feet. And today, their closest living relatives
are the manatees and dugongs. Places like the Deep Time Hall show us evolution
at work, with specimens that take us from the most ancient fossil ancestors, to transitional
forms, all the way to the organisms we know today. In less than 200 years, scientific study has
completely revolutionized our understanding of the constantly changing nature of life. Weâve been able to see how organisms are
shaped by their environments to better survive and reproduce there. And weâve learned that time is a key component
of evolution. This is why a deep look into the deep past
is critical for our understanding how life has changed over millions of years. And, itâs why weâre so excited about the
opening of the new Deep Time Hall in the Smithsonian National Museum
of Natural History in Washington, D.C. If you can make it in person, pay them a visit! And, if you canât, follow them on social
media to see some of the amazing specimens now on display And be sure to watch a new special about the
evolutionary history of some of our planetâs most fascinating animals. When Whales Walked: Journeys in Deep Time
premieres on PBS and Smithsonian Channel June 19th at 9 PM Eastern. Check out the links in the description. Thanks for joining me today in the Konstantin
Haase studio. And an extra big thanks to our current Eontologists:
Jake Hart, Jon Ivy, John Davison Ng, and Steve
I cringed a little at "micro/macro-evolution" to be honest.
And how sex enabled complexity...
This is a wonderful channel.
First, what exactly is wrong about Darwin's ideas?
Secondly, what was Darwin's definition of species?
Thirdly, this PBS vid was about to explain evolution, not the ideas of Darwin.