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Evolution
∗
David Rintoul
Robert Bear
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0†
Abstract
An introduction to evolutionary theory, summarizing some of the key lines of evidence in support of
the theory.
1 Evolution
How stupid of me not to have thought of that
." Thomas Huxley, after reading Darwin's Origin (On the Origin of Species by Means of Natural Selection,
or the Preservation of Favoured Races in the Struggle for Life ).
2 Introduction
What is Evolution? Surely everyone has heard the word, and perhaps a lot of other words to describe it,
but do you really know what that word means, in the context of biology? Here are a few common notions
about evolution. How many do you agree with?
1. Evolution has never been observed directly.
2. Evolution is only a theory, and has not been shown to be a fact.
3. Evolution means that life originated, and living things change, randomly.
4. Evolution is progress; organisms get better and more complicated whenever evolution occurs.
5. Evolution means that individual organisms change.
6. In order for evolution to occur, the ospring of some organisms will have to be radically dierent from
the parental organisms.
If you said that all of these statements are false, then you have a good understanding of evolution. They
are indeed all untrue. However, this is a list of some fairly common misconceptions about evolution, and
many people in the world (and particularly in the USA) share one or more of these misconceptions. It is
likely that you think that some or all of these statements are true. One of the hardest parts of learning is
to undo a well-established misconception, so if you do think that one (or more) of those statements is true,
this chapter might be a bit harder for you. But it will be worth the eort, since, as you will learn below,
evolution is the guiding framework for modern biological science. Once you have a good understanding of
evolution, and the mechanisms that drive it, you will be well-poised to learn and understand the biology
that comes in the rest of this course.
∗ Version
1.4: Jul 28, 2014 9:24 pm -0500
† http://creativecommons.org/licenses/by/4.0/
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3 Evolution what is it?
The biological world is extremely diverse. In fact, that is one of the most powerful realizations that come
from the study of biology, or even just from being an observant person in the world. Living things range
from the microscopic bacteria to the immense blue whale. They have a diversity of life styles and metabolic
capacities, from photosynthetic creatures who can make their own food from carbon dioxide gas, to predatory
creatures, all the way to parasitic creatures who have some of the most complicated life styles of all. Within
any one of these groups, there is also astounding diversity. Open any eld guide, whether for birds, mammals,
owering plants, or mushrooms, and you will be confronted with an abundance of colors, sizes, shapes and
behaviors. Even within a single species, say
Homo
sapiens, there is diversity. Look around your classroom
and you will see people with a wide variety of skin colors, hair colors, eye colors, heights and weights. This
diversity is a fact, and for many millennia, human beings have been trying to come up with explanations for
that well-observed fact.
You are probably familiar with some of these explanations.
Many ancient peoples imagined that the
world was created in the form that exists now, and that blue whales, pythons, and lilac bushes were created
and unchanged since that creation.
This is known as
typology ;
every species conforms to an ideal and
unchanging type, and all members of the species are true to that type. Wolves are a type, and all wolves
(within certain parameters) were considered to be similar to all other wolves, but not similar to foxes, and
even less similar to lions. And all of these creatures had ancestors who were also true to the type. Once it
became clear that there had been creatures, preserved in the fossil record, unlike any creatures seen today,
other explanations were needed to account for these new observations. When it became clear, from geology,
that the earth was very ancient, and had been in existence for millions and even billions of years, other
explanations became even more satisfactory. When it became clear, from studies of comparative anatomy,
that many creatures shared anatomical and developmental similarities, even though they were of dierent
types, other explanations became obvious.
We won't go through the many explanations for the diversity of life that have been proposed and been
discarded over the centuries. There are lots of places where you can read about that historical progression,
and it is interesting, for sure.
Rather we will get to the explanation that is the most widely accepted
scientic explanation today, and show how this explanation is supported by evidence, and also leads to
predictive hypotheses that can serve as a further test of the explanation. That explanation is known as the
Theory of Evolution.
As discussed in the previous chapter, theories are powerful frameworks for explaining observations, and
for making new predictions about the natural world. The theory of evolution is no exception. In fact, it
is the most powerful explanatory framework in biology today. Theodosius Dobzhansky, a famous biologist,
expressed this sentiment quite well when he wrote in 1973, Nothing in biology makes sense except in the
light of evolution. On a daily basis, scientists around the world are using the theory of evolution to generate
hypotheses, to interpret conclusions, and to make contributions to scientic knowledge. So let's look at that
powerful explanation in more detail.
At its simplest, evolution is dened as descent with modication. That is joined to another concept,
natural selection, to give us the rst expression of the theory of evolution, published by Charles Darwin
in 1859.
Darwin's genius was in recognizing, and thoroughly explaining, that descent with modication
was a common phenomenon, and that selection, whether natural or articial (e.g. animal breeding) was an
explanation for life's diversity. So let's look more closely at natural selection, since Darwin identied it as
the engine that drives the process of evolution.
Natural selection (aka adaptive evolution) is, as Darwin pointed out 150+ years ago, analogous to the
process by which animal breeders produce animals with novel traits (aka articial selection). For example,
a pigeon breeder might notice that one of his pigeons has an unusually large ru of feathers around its neck.
He breeds this pigeon with another pigeon, and selects the pigeons with the biggest rus from among the
ospring to be the parents of the next generation. After a few cycles of this, some of the pigeon ospring will
have very unusual and pronounced neck rus, and will look nothing at all like the original pigeon ancestor in
that regard (gure 2.1, below). This common practice gets its name from the fact that the breeder
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selects,
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or chooses, specic animals to be the parents of the next generation. And it works; there are many examples
where substantial changes in animal appearance, or behavior, can be brought about in just a few generations
by applying this articial method of selection.
Darwin's pigeons (Original line drawings from Darwin's "Variation in Animals and Plants
under Domestication", 1868). The common ancestor for all of these fancy pigeons was the Rock Pigeon
(center). By selecting for unusual morphological characteristics, pigeon breeders are able to develop all
of these unusual pigeons, and many more.
Figure 1:
Darwin's genius, and the source of Huxley's self-disparaging statement at the top of this chapter, was to
Natural
selection, the idea for which Darwin is so famous, simply recognizes three well-known observations and puts
recognize that this process could also occur in the absence of an individual who did the selecting.
them into a context that generates evolutionary change. Let's look briey at each of these three observations.
The rst thing that Darwin postulated is that the variations seen in living things are due, to a greater
or lesser degree, to heritable factors. In other words, there are
heritable variations
among the individuals in
a population of organisms. Let's break down that term a bit, and look at each of the words, using examples
mostly from human populations.
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Firstly, we know that there are
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variations among individuals in a population.
Look around your classroom,
or at your family picture album. You probably don't look exactly like your brother or sister, and your mom
and dad don't look exactly like your uncles or aunts. So even in situations where the parents are the same,
variation occurs among the ospring.
Variation is even greater in a population of individuals who don't
share the same parents. Variation is normal, and easily observed.
What about that other word,
heritable ?
Again we now know that many of those variable traits are
heritable, i.e. they are passed from one generation to the next. In humans, eye color, hair color, height, etc.
are all characteristics that might be the same in you and your parents. If you have a dog or cat, and that
dog or cat has ospring, you can often see aspects of the ospring (e.g. coat color, size) that are identical to
those in the parental animal. One likely explanation for that observation is that you and your parents have
some shared molecule or molecules that determine each of those traits. We now know (but Darwin didn't)
that the molecule is DNA, about which you will learn more later. On the other hand, some conditions are
not heritable. For example, if you have a cat that lost its tail in a horrible and noisy accident involving a
rocking chair and your 300-lb great-aunt, and if the cat has kittens, those kittens will have normal tails.
The rocking chair might damage the cat's tail, but not its DNA. At the time of Darwin, the mechanisms of
heritability were not known (he knew nothing about genes), but everyone understood that some traits were
heritable, and others were not. So again, the heritable variation that is necessary for evolution to occur is
easily observed in the natural world.
The second thing that Darwin observed, and that was a huge factor in his synthesis of these observations
into his theory, is that not all of the individuals in a given generation will survive and reproduce to the
same degree. Simple mathematics corroborates that. If all of the fruit ies from a single pair of fruit ies
survived and produced a maximal number of ospring, after a mere 25 generations (which can take just a
single year in this species) that population of ies would ll a ball 96 million miles in diameter, or more than
the distance from the earth to the sun. Fruit ies have been around for lots longer than a year, and you can
still see the sun, so obviously fruit ies do not all survive and reproduce.
Finally, the third condition necessitates that these heritable variations can result in dierences in survival
or reproductive success. Again, there is abundant evidence for that. Inherited human conditions that result
in mental retardation, or physical deformation, often mean that the aected individual will not survive or
reproduce.
Medical intervention has, in some cases, been able to counteract those disabilities and allow
individuals with some inherited conditions to survive and reproduce, but in previous generations, or in populations of organisms that do not have access to medical care, many heritable variations were not represented
in the next generation because the individuals with those variations simply did not reproduce.
So the model Darwin proposed is quite simple.
If all of those conditions were true, organisms with
heritable variations that enhanced their chances for survival and reproduction would be more likely to be
among the parents of the next generation, and the frequency of those organisms with those particular heritable
variations would increase in the next generation. This is a simple idea, but it has many ramications for the
study of biology.
It seems clear, just from observations we all have made, that these three conditions do pertain in the
natural world. If that is the case, then the process of natural selection could operate, and variations that
resulted in reproductive success would become more common in the population. It is important to understand
that this process is the result of an interaction between the organisms and their environment. Over time,
organisms that t better into that environment will become more abundant in the population, and may
eventually be the only organisms in the population. The term
tness, in this context, simply is a measure of
how well individuals with certain traits survive and reproduce in a particular environment. The environment
is an incredibly important aspect of this process. If the environment changes, organisms which were t for
the previous environment may suddenly nd themselves less well-adapted, and rare organisms that were
ill-adapted in the previous environment may suddenly become more t to that new environment. Fitness is
relative, and the environment is a major player in the determination of tness.
In addition, consideration of these processes in the real world leads to a better understanding of the
questions in the introduction to this chapter.
As you can see, the process of evolution is NOT random;
the interaction of the organism and its environment leads to selection, and selection, by the very nature of
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the word, is not random. Just as an animal breeder selects specic organisms as the parents of the next
generation, the process of evolution selects specic organisms as the parents of the next generation. There
are some important dierences, however. In articial selection, the breeder has a goal (e.g. to get a goat that
produces more milk), and designs the breeding program with that goal in mind. In natural selection, there
is no ultimate goal, and no plan; organisms are selected for their adaptation in a particular environment,
which can (and often does) change. The process is unguided, in the sense that there is no goal in mind, but
unguided is not the same thing as random.
Secondly, careful consideration of this process also disproves the notion that evolution equals progress
toward a better organism. An organism that is better adapted to one environment can be very ill-adapted
if the environment changes. In that situation, a worse organism, one that is rare in the rst environment, is
now the better one in the new environment. That is not progress, it is just change. In fact, some organisms
become so well-adapted to their environments that they lose some of the complex structures or pathways
that their ancestors had.
Cave sh have no eyes, even though their ancestors did.
Whales have no legs,
even though their ancestors did. Some parasites, living in a rich sea of nutrients, have lost organelles such as
mitochondria, even though their ancestors had those organelles and all of the metabolic pathways associated
with them. These highly-adapted organisms are actually less complex than the ancestors from which they
evolved. Evolution clearly is not a synonym for progress!
Finally, it should be clear that evolution is a change at the level of the population, and not at the level of
the organism. Natural selection acts on organisms, but the result of selection is seen in the next generation.
And this change is usually very gradual; there is no need to invoke absurd situations where a cat gives birth
to a dog, or vice-versa.
Darwin correctly pointed out the analogies between this process of natural selection and articial selection,
the well-known process that animal breeders used to select for interesting or useful variants in animal species.
In other words, natural processes can generate the diversity we nd in the natural world if all of those
conditions are true, and if there is sucient time to produce many generations.
You will learn more in
the studio exercises about how even small dierences in reproductive success can, over time, lead to large
changes in the characteristics of organisms in a population. Small changes (one or two genes in organisms
micro-evolution. Larger changes that
macro-evolution. This is an articial distinction,
that still are members of the same species) are sometimes described as
result in dierent species, for example, are described as
actually.
Macro-evolution is merely micro-evolution that has proceeded for a longer time.
graphical illustration of that, see gure 2.2 below.
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For a clever
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Figure 2:
example .
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How very gradual changes can, over time, result in signicant changes: A textual
In summary, natural selection is a powerful agent, and recognition of this process was a powerful insight.
Darwin proposed his theory in 1859, and elaborated on it in other books and other editions of the
Species.
Origin of
Since Darwin's time, other scientists have identied other agents, in addition to natural selection,
that result in changes in the characteristics of a population, and you will learn more about those in later
chapters. Additionally, other scientists made many predictions based on this explanatory framework, and did
many experiments to test those predictions. Scientists are still engaged in that process today, and Darwin's
ideas have been conrmed many times over, and even extended so that we understand how the process works
in much more detail than Darwin did. That is, as you learned in the previous chapter, one hallmark of a
great theory.
4 Evidence for Evolution
There are multiple lines of evidence, many of which were unimagined in the time of Darwin, that support
his explanation for the diversity of life. The following is not meant to be an exhaustive cataloguing of that
evidence. Indeed, more evidence accumulates every day, making it impossible to point out all of the threads
in that fabric.
It is also important to recognize that the evidence doesn't come just from biology.
For
example, as noted above, Darwin's explanation would require a lot of time and many generations. If the
earth was too young, none of this could have happened. The sciences of physics and geology conrm that
the earth is over 4.5 billion years old, which is plenty long enough for evolution to occur. The fossil record,
the research subject of geology and paleontology, also provides substantial supporting evidence for Darwin's
big idea. The discovery of continental drift, and the development of plate tectonic theory, made sense of a
lot of observations about both the fossil record and about populations of living organisms. Let's look at a
few of the lines of evidence, and see how they all weave together to make the coherent and elegant fabric
that is the hallmark of a good scientic theory.
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4.1 The fossil record
In science, radically new explanations can only be successful when the conventional explanations no longer
explain all the observations.
In the history of biology, this was the situation in the early part of the
nineteenth century, when many interesting fossils were being discovered and carefully scrutinized. It soon
became apparent that fossils were indeed the remains of once-living organisms, and that fossils in geologically
younger strata seemed to be both similar and dierent from those in older strata. The fossil record showed
that whole groups of organisms appeared and disappeared during the history of the earth. Others seemed
to be much the same in rocks of dierent ages. Familiar organisms, particularly marine mollusks such as
clams and snails, could be found in older rocks, but in many cases these organisms were not identical to
the current organisms. Plant fossils told the same story. The reigning explanation for the diversity of life,
creation of all these creatures at the same time and place, clearly did not explain these new observations.
Evolutionary theory was a much more satisfying scientic explanation, and the development of that theory
by Darwin and others started at that time.
Since Darwin's time the fossil record has become much more extensive, and the evidence for this explanation has become much more well-supported. Gaps in the fossil record that were pointed out by Darwin's
contemporaries have been gradually lled in. Indeed, the sciences of geology and paleontology, in combination with biology, have allowed scientists to make predictions about where, exactly, particular fossils in
particular gaps should be found.
The most recent (and spectacular) example of this was the discovery of fossilized remains of a creature
that bridges the gap between sh and amphibians, which the rst four-legged creatures (aka tetrapods) to
move onto land.
The fossil record, coupled with genetic evidence from modern-day amphibians and sh,
indicated that this transition to land occurred about 375-400 million years ago.
amphibian fossils, had ever been found.
But only sh fossils, or
Logic dictates that there should be a transitional creature, or
missing link in popular jargon, which had characteristics of both sh and amphibians.
It was reasoned
that creatures such as this, if they existed, would probably live in shallow areas at the edge of seas or bays.
Geologists knew which particular rock formations resulted from those sort of environments of that age, and
so expeditions were dispatched to search for such fossils in one of those geological formations. These rocks
were deposited in warm shallow tropical seas 375 million years ago, but are now, as a result of continental
drift, located on Ellesmere Island in the Canadian Arctic.
elegantly t that prediction; the creature was named
In 2004 fossils were found in those rocks that
Tiktaalik roseae.
(gure 2.3, below). The genus name
for this shapod comes from the name of a sh and was suggested by local Inuits on Ellesmere Island, and
the specic epithet roseae honors an anonymous donor who helped fund these grueling expeditions to the
high Arctic.
Tiktaalik
ns have basic wrist bones, but no digits, or ngers. It is truly a missing link, and
its discovery stems directly from predictions made on the basis of previous scientic observations, in a classic
example of the power of the explanatory framework known as the theory of evolution. Descriptions of the
expeditions, and lots more about the incredible insights that have come from those fossils, can be found in
a charming book called Your Inner Fish, written by Neil Shubin and published in 2008.
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Figure 3: Fishapod evolution (By Maija Karala (Own work) [CC-BY-SA-3.0], via Wikimedia
Commons. A cladogram showing the evolution of tetrapods, using the best-known transitional fossils.
From bottom to top: Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, Pederpes.
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4.2 Comparative anatomy and embryology
At the same time that the fossil record was making some scientists scratch their heads and question the
creation explanation for the diversity of life, other scientists were looking more closely at these fossils and
at the bones of existing organisms. These comparative anatomists also made observations which were much
more easily explained by the theory of evolution.
The dierent bones in fossil skulls, for example, could
be compared to the bones in modern skulls, allowing anatomists to discern that the fossil skulls and the
modern skulls had remarkable similarities in the number and the position of the individual bones in the skull.
Most of the bones in a fossil sh skull have counterparts not only in modern sh skulls, but in fossil and
modern amphibian skulls, or fossil and modern reptile skulls, and even fossil and modern mammal skulls.
Occasionally the fossil record shows us when a skull bone is added or one is lost, and also allows us to track
progressive modications in the positions of these bones on the skull surface. We can only understand these
observations in the light of evolutionary theory if we conclude that the bones reect the fact that each
kind of organism is descended from some other. Descent with modication is the most satisfying scientic
explanation for these observations.
The anatomy of modern organisms also reects this common ancestry. The limbs of all tetrapods contain
a similar number and arrangement of bones, even though the size and shape of the bones can vary greatly in
dierent organisms. For example, the two bones in your forearm, the radius and the ulna, have counterparts
in other mammals (gure 2.4), in reptiles, in birds, and even in fossil dinosaurs and pleisiosaurs . If all of
these structures were specically created for moving around in a dierent environment (e.g. water for the
plesiosaur and air for the bird or bat), simple engineering principles would dictate that dierent structures
would be more ecient in those dierent situations. Yet the same structures, endlessly modied, are found
in all of them. The simplest explanation for this is that the organisms share a common ancestor where that
structure originated, and evolutionary mechanisms resulted in the modications in size and shape that we
see today. This phenomenon is known as
homology ; structures are said to be homologous structures
occupy similar positions and arise from a common ancestral structure.
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Homologous bones in the forelimbs of four vertebrates (By Petter Bøckman, via
Wikimedia Commons). A-human, B-dog, C-bird, D-whale. The various colors indicate bones of various
groups (e.g. dark brown = bones of the ngers,yellw = bones of the wrist, red = ulna, beige = radius, and
light brown = humerus). The various bones in the forelimbs of four vertebrates dier in size and shape,
resulting in very dierent morphologies of the forelimbs of these organisms. But both the number of
bones, and their position relative to each other, are quite similar, as is their embryological development.
These homologous parts provided one of Darwin's arguments in support of his theory of evolution.
Figure 4:
Embryologists also made predictions based on this evolutionary explanation. They predicted that homologous bones would arise from similar structures during the development of the embryo. For example,
the forearm bone that we call the radius, which looks radically dierent in the forearms of a bat or a human
or a mouse or a bird, would come from similar structures in the embryos of bats, humans, mice or birds.
Those predictions also were found to be correct. So homology argues strongly for an explanation that invokes
descent with modication.
In contrast, the wings of insects and the wings of bats or birds do not have similar structures, although
they have similar functions (to propel the organism through the air).
analogous
These structures are said to be
rather than homologous; they share a function but do not arise from a structure that is found in
a common ancestor. Indeed, if organisms predominantly had analogous structures, which would be dierent
engineering solutions to a common problem, that evidence would be more consistent with the explanation
of independent creation of those organisms. But homologous structures seem to be the much more common
observation, making descent with modication a much more scientically satisfying explanation.
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4.3 Comparative biochemistry
One of the biggest surprises of modern biology came from the eld of science known as biochemistry. Once
biochemists started to unravel the mysteries of metabolism, the unity of life on this planet became quite
obvious. Creatures with incredibly dierent morphologies, habitats, and lifestyles all seem to have incredibly similar metabolic pathways.
Bacteria, bonobos, bats and bananas all use a molecule known as ATP
(adenosine triphosphate) to store and provide energy within their cells, for example. The metabolic pathway
known as glycolysis, which you will learn about in subsequent chapters, is found in all the organisms on
the planet, and the enzymes that are used in that pathway are quite similar in these diverse organisms.
Again, this argues strongly for common ancestry, which is a strong prediction that arises from a descent
with modication explanation. Once an ancient cell developed these metabolic pathways, there was no need
to re-invent that wheel. It is somewhat ironic that some of the best evidence for a particular explanation for
the diversity of life comes from the discovery of the unity of life at the molecular level.
4.4 Genetics and genomics
Besides ATP, another molecule common to all life forms on the planet is DNA (deoxyribonucleic acid). This
molecule stores genetic information, so it is the molecule of heredity. Its role in heredity also means that
it can be modied under some circumstances, thus giving rise to the variations described above.
Darwin
knew nothing about DNA when he proposed his theory in 1859; his ideas about mechanisms of heredity
were, in fact, spectacularly wrong. But the discovery of the mechanisms of heredity, starting with Mendel
in 1866, and extended by many others in the early part of the 20
th
century, made it possible to nally
propose mechanisms by which heritable variations arise and are transmitted between generations. In fact,
th
the rst 4 decades of the 20
century were the years when the two seemingly unrelated elds of genetics and
evolution were united. This Neo-Darwinian synthesis, starring Theodosius Dobzhansky, Ernst Mayr, and
George Gaylord Simpson, resulted in modern evolutionary theory, and allowed scientists from both genetics
and evolutionary backgrounds to work together to make and test predictions.
The elucidation of the structure of DNA by Watson and Crick in 1953, followed soon by the breaking
of the genetic code, provided even more evidence for descent with modication.
DNA, as you will learn
later, functions as a repository of information. In order for the information to be used to build a cell or an
organism, it must be read and translated into dierent molecules. The processes, and the enzymes, that do
this work of reading and translating are virtually identical in all living creatures on the planet. The genetic
code was, perhaps prematurely, called the universal genetic code for precisely that reason; it is translated
identically in almost all organisms that have been discovered to date. Once again, this is a strong argument
for common ancestry and descent with modication.
But the really impressive outcome of this fusion of molecular knowledge and organismal knowledge
comes from the study of the structure of genes, and genomes, at a detailed level. Incredibly, scientists have
discovered molecular fossils of a sort stretches of DNA which are not used in modern organisms, but which
remain in the genome as a record of functions in the past. For example, chickens don't have teeth, but they
have genes for tooth proteins, turned o long ago, still lurking in their genomes. Those genes can be turned
on under the right conditions, producing toothy structures, which were last seen in dinosaurs, the extinct
ancestors of modern chickens. There is no good explanation for these observations, other than descent with
modication. Similarly, detailed analysis of the DNA of organisms, including now some long-dead organisms
like mammoths and Neanderthals, allows scientists to test predictions about common ancestry, and gain
insights into the course of evolutionary change in all organisms. In fact, evidence from analysis of DNA, and
other molecules, has allowed us to ne-tune our hypotheses about ancestry and relationships throughout the
biological world, as explained in the next chapter on Taxonomy and Phylogeny.
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