Professor Dave here, let’s talk about Darwin. For millennia, humans have wondered just how
we got here. Throughout most of human history, any answer
to this question was the exclusive domain of religion, but when modern science came
about, it wasn’t bashful in having a go at this question, too. Remarkably, science came to answer it so well,
that any discussion of the answer requires many separate disciplines and years of study. We need cosmology to discuss the origin of
the universe. Astrophysics is necessary to talk about the
formation of the Milky Way galaxy, the solar system, and our little earth. We need chemistry and biochemistry to explain
the assembly of self-replicating molecules and the first living cell. And finally, biology allows us to show how
the first unicellular organisms eventually gave rise to all the incredible species we
see on earth today. Many people remain understandably incredulous
to all of this at face value, since they do not personally possess this mountain of knowledge,
but that’s why education is so important, along with cultivating a societal attitude
that values knowledge and growth, rather than adherance to ancient dogma. As far as biological evolution goes, the evidence
for the descent of all life from a single common ancestor is as firm and compelling
as the evidence that the earth goes around the sun, so let’s talk about the history
of how this understanding came about, starting with a man named Charles Darwin. Charles Darwin was a biologist in the 19th
century, and as a child he was fascinated by nature and living things. In his early twenties, he spent five years
aboard the HMS Beagle, traveling all around the world. In his travels, he made some profound observations. He noticed that organisms tend to be suited
for their specific environments. Whether ocean or land, jungle or desert, organisms
have features and behaviors that make them especially suited for their surroundings. Also, he noticed that there is an incredible
diversity of life on our planet, what he called life’s “endless forms most beautiful”. There are millions of different species, some
of which are unrecognizably different from one another. Even so, there are characteristics that are
shared by many forms of life. A lot of animals have the same organs, similar
appendages, brains, and behaviors. Darwin wondered how this could be, and he
published a hypothesis in his book, The Origin of Species, in 1859. This book almost single-handedly created the
field of evolutionary biology, and it remains one of the greatest accomplishments of scientific
thought. In it, he describes how descent with modification
is responsible for the wide variety of species as well as the similarities between them,
as all life is descended from an ancient common ancestor. This kind of modification was later dubbed
evolution, and the process of evolution can now be discussed on the genetic level, with
mutations that occur in the genome of a particular organism that alter the products of gene expression,
which can then be passed on to offspring. The specifics of this require a modest understanding
of biochemistry, so a viewing of that playlist will offer a fuller appreciation of what will
be discussed here. But on a basic level, the end result is that
if a mutation is favorable, increasing the fitness of the organism, the organism is more
likely to proliferate and pass on this mutation. If the mutation decreases the fitness of the
organism, the organism is more likely to die, and that genome is never heard from again. This is easily observable in bacterial cultures,
where a chance mutation results in a bacterium that is resistant to an antibiotic, which
then divides while the others die, until the whole culture has descended from that one
mutant. This concept, which we will discuss in greater
detail later, is called natural selection. We can also discuss evolution in a more historical
context, by looking at fossils and other types of data. In this way, the theory of evolution by natural
selection is a powerful model that correlates an incredible array of observations. In this case, as with all others, the word
theory does not imply that we are not sure about its validity. In science, the word theory does not imply
uncertainty at all; theories are not guesses. They are models that correlate data. If you like, click on this card to watch a
clip with more information about exactly what theories are. As we just said, we can watch bacteria evolve
in a Petri dish, so evolution itself is not a theory. It’s just something that happens. Natural selection is the theory that explains
how evolution has produced the vast array of life on earth. Now that we have a few basic concepts straightened
out, let’s look at Darwin’s work in a bit more detail. The field of paleontology began to develop
around the turn of the 19th century, and the fossil record offered some peculiar data. Fossils found deeper in the ground were understood
to be older, and there was a distinct pattern in how the older the organism, the less similar
it was to current living organisms. Some geologists of the time proposed that
very gradual mechanisms like erosion from wind and water could produce stunning geological
features like canyons and valleys over huge time spans, and this led the community to
believe that the earth must be much older than previously thought. Indeed, prior conceptions of the age of the
earth came only from a literal interpretation of the bible, but this thinking came to be
completely incompatible with the evidence at hand. Biologists of the time realized that if the
earth is indeed billions of years old, changing incredibly slowly through these subtle processes,
then the same idea could be applied to biological life. Miniscule changes to a species, propagated
thousands or millions of times, could eventually produce large-scale transformations in living
organisms. Jean-Baptiste de Lamarck was the first to
suggest this kind of evolution, coincidentally in 1809, the very year that Darwin was born. However, he incorrectly proposed that these
changes occurred in individual organisms, where limbs and other features grew or atrophied
according to their level of use in the lifetime of that organism. We now understand that no such change occurs
within the lifetime of one organism, as individuals do not evolve. It is only over many generations that any
such changes can occur. But Lamarck did set the stage for Darwin to
take the idea of evolution much further. This accomplishment was catalyzed by his time
on the HMS Beagle, which found Darwin exploring South America and collecting thousands of
plants and animals. He gathered his most important evidence on
the Galapagos Islands, studying several kinds of mockingbirds and finches. Some types were unique to a particular island,
some lived on two or more islands, and most of these were not found anywhere on earth
but here. He reasoned that ancestors of these birds
must have arrived from the mainland and then slowly diversified over time, giving rise
to these new species. He also reasoned that these species showed
examples of adaptation, whereby they possessed characteristics that made them well suited
for their environments. Each species had a beak shape that was adapted
to the food source available on their particular island. He understood that a small change in an organism’s
characteristics may impact its chances of survival, and this is how he arrived at the
notion of natural selection. If a longer beak helps a bird eat more food,
then the birds with the longest beaks will be more likely to eat enough to survive, and
thus pass on their genes to new offspring. Gradually, over many generations, the beak
length of a species will grow until it has become differentiated enough from its ancestors
that it merits the distinction of being a completely new species. We must understand that in this way, evolution
is completely blind, and has no particular trait or form in mind. It is simply that any physical change that
results in enhanced fitness for the organism will result in a higher likelihood of survival
and therefore reproduction, and thus those traits are more likely to be passed down,
whether this is the ability to better evade predators, eat more food, or survive the elements. Over millions of years, a time span now understood
to be feasible thanks to advancements in geology, one can see how this process could result
in the emergence of dramatically different species all over the world, as organisms responded
to the completely different sets of stimuli present in various earthly environments. Darwin was reluctant to publish these findings
because he knew that they would cause a huge backlash. Evolution is still strongly contested by religious
communities today, so imagine back then, when the science was brand new. He delayed publication, and another naturalist
named Alfred Russel Wallace put forth a paper presenting conclusions very similar to Darwin’s. But then the following year, Darwin published his masterpiece, The
Origin Of Species. Its logic, evidence, and mechanism were undeniable,
and the scientific community quickly accepted that the diversity of life on earth is indeed
the product of evolution by natural selection over millions of years. There are those who find this hard to grasp,
arguing that species are too dissimilar for one to have evolved from the other, but they
neglect to realize that when two species have a common ancestor, there are so many organisms
that have been lost to extinction in between the two species being compared. Tiny gradations of change over a huge time
span eventually result in visibly different species, so if we take away everything in
the middle, the gap becomes highly pronounced. But this is just the way that one color gradually
turns into another on a color wheel. If we see the whole wheel, it makes sense
how one color turns into the other. But if we take away all the shades in between,
all we see is these two distinct and seemingly unrelated colors. The perils of nature have taken away the vast
majority of all species, over 99 percent of them in fact, and we must keep this in mind
so that we can allow ourselves to use the fossil record to fill in the gaps when understanding
exactly how animals like the elephant and the manatee really did evolve from a common
ancestor. It’s not that an animal gave birth to sibling
manatees and elephants, nor did a chimpanzee ever give birth to a human, as is popularly
suggested by critics of evolution. Instead, an animal gave birth to two nearly
identical organisms that initiated divergent paths of evolution, which over millions of
years and many thousands of generations, produced completely divergent sets of characteristics,
due to differing environmental stimuli. Natural selection is quite easily demonstrable
in an analogous but accelerated version, which we call artificial selection. Dogs used to all look kind of like wolves,
but once we started selectively breeding specific dogs with other specific dogs to deliberately
get the tiniest dogs possible, or fastest, or most colorful, or any other arbitrary characteristic,
we eventually produced an incredible variety of dog breeds, which are now so numerous only
because of human intervention. If a future biologist were to study fossils
of all these breeds, they would undoubtedly label them as separate species. We have done this extensively with plants
as well. Kale, Brussels sprouts, cabbage, and broccoli
are all direct descendants of the wild mustard plant, resulting from artificial selection
based on certain traits of the plant. A similar process has produced modern corn,
which once looked very different from this familiar image. Darwin realized that this happens in nature
at a much slower rate, since there is no sentience to guide it. As inherited traits must vary slightly, and
many offspring fail to survive, nature rewards the individuals best suited for survival with
a higher chance of reproducing and thus passing on their traits. This results in the gradual accumulation of
favorable traits within a population, which is what creates the illusion that an organism
was somehow designed for its surroundings. It’s incredible to think that so much evidence
was compiled in favor of evolution by natural selection even before we had any understanding
of the chromosomal theory of inheritance or gene expression. Now we understand how the genome can be altered
through routine errors in the cellular machinery or exogenous mutagens, and how this can act
as a source of novel phenotypes. These mutations can be subtle, but some can
be significant, as there are individual mutations observed in fruit flies that cause the fly
to grow legs where antennas should be. So we now have a firm understanding of the
genetic basis with which variation comes to pass. When we combine this with natural selection,
and an understanding of how characteristics that enhance chances for survival will slowly
mold life into new forms over time, just like water slowly erodes rock to form a canyon,
we arrive at a powerful comprehension of the origin of all life on earth. Over the century and a half since Darwin’s
work, the entire field of biology has become inextricably intertwined with evolution, such
that biology makes no sense whatsoever without evolution for contextual basis. So let’s move forward and look at some of
the evidence that has cropped up to make evolutionary biology consistent far beyond reasonable doubt.
