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Introduction to Evolution

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Evidence for Evolution
Reading: Freeman, Chapter 23, 26
The Fact of Evolution
– Evolution-the progressive change of organisms as
they descend from ancestral species-is a fact. By
now, the evidence for it is overwhelming and
• It is of such obvious clinical significance in medicine that
to deny it is irresponsible.
• That said, any explanation for its existence and mode of
action is a scientific theory, which must be testable and, in
theory, falsifiable.
• Darwin’s theory of natural selection, combined with other
mechanisms of evolution discovered since Darwin, form
what is known as the “modern synthesis”, the current
scientific paradigm in the biological sciences.
– It provides a central explanation for phenomena in such diverse
fields as paleontology and developmental biology, medicine and
– The existence of evolution has been proposed several times
in history. For instance, the ancient Greek scientist,
Animaxander, proposed a theory of evolution.
– In terms of modern science, it was first advanced proposed
in the late 1700’s and early 1800s by several scientists
including Compte de Buffon and Erasmus Darwin.
– The idea of evolution remained controversial for a long time,
partially because it ran contrary to contemporary religious
ideas and partially because no mechanism for evolution
was known.
– Darwin and Wallace’s theory of evolution by natural selection
was the first plausible, widely-accepted mechanism for
evolutionary change.
– By now it is well-tested, supported by hundreds of
independent scientific investigations.
– It is also falsifiable-aspects of Darwin’s theory of evolution
have been successfully challenged, others supported. This
is the case for the other mechanisms of evolution as well.
Examples of the clinical significance of
evolutionary biology to medicine
HIV. HIV is a retrovirus of enormous medical concern.
Because of evolutionary studies, we know that two separate
lineages of this retrovirus passed into the human population
from African Apes in the mid 20th century.
This knowledge has alerted us to the danger of emergent
diseases from other animal hosts, a reason for our concern
about SARS and bird flu.
In addition, it is an understanding of evolutionary biology
that has enabled us to develop a therapy for HIV.
The so-called “triple therapy” HIV treatment is an example
of evolutionary medicine.
– A single drug will not work against the disease because the virus
evolves so quickly, it attains resistance to every drug we have within
a few months.
– By using three drugs simultaneously, we subvert the evolution of the
virus…evolving resistance to one drug means loosing resistance to
Antibiotic resistance is an evolutionary phenomenon
of tremendous clinical significance.
Early in the 20th century, a variety of antibiotics, used
to treat bacterial diseases, were developed.
– An understanding of evolution is helpful to understand
where these antibiotics come from to begin with…many,
such as penicillin, were evolved by fungi, over millions of
years, to kill off their bacterial competitors.
– Humans have co-opted them for our own purposes.
Since the 20th century, the bacterial pathogens have
evolved resistance to our antibiotics, because
extensive use of these drugs has caused very strong
natural selection in favor of mutations which favor
antibiotic resistance.
– For instance, various strains of Neisseria gonorrheae have
evolved resistance to penicillins, tetracyclines,
spectinomycin and floroquinolones.
Natural Selection as the Mechanism of
– Scientific understanding of evolution came out of
its infancy in 1859, when theories of evolution
by natural selection by Charles Darwin and
Alfred Russel Wallace became widely known.
– We now know of other mechanisms of evolution,
including genetic drift and mutation, but natural
selection is the only mechanism capable of
producing adaptation.
– Natural Selection was not immediately
accepted-it took until the1930s for Darwin’s
ideas to be synthesized with a modern
understanding of genetics for widespread
Intellectual stepping-stones to
developing a theory of evolution
Linnaeus and Taxonomy
Malthus and the Principle of Population
Lyell and Uniformitarianism
Lamarck and the fist comprehensive
theory of evolution
The Voyage of the Beagle
Wallace and Darwin
Linneus and Taxonomy
Carolus Linnaeus was a sixteenth-century Swedish
physician and Botanist.
He founded the science of taxonomy, the branch
of biology concerned with naming and classifying
living things.
He developed the two part system of binomial
nomenclature we use today.
His genera were clustered into increasingly broader
categories; families, classes, phyla, and
kingdoms…although he did not believe in evolution
by descent, this pattern does provide a framework
for thinking about evolution from a common
Thomas Malthus, an eighteenth
century economist, published “An
Essay on the Principle of
Population” in 1798.
This document had profound
Simply stated: people tend to have more children
than can possibly survive, and human populations
have historically been kept in check by famine,
starvation, and disease.
Darwin read this essay and was strongly
influenced: he noted that every species has more
offspring than can be expected to survive.
How Old is the Earth?
From a scientific standpoint, the age of the Earth
was essentially unknown until the 19th century.
Early ideas varied greatly, some cultures, such as
classical Hindu society, thought of the Earth as
incredibly old.
Christian theology limited the age of the Earth to a
few thousand years, because of the biblical
account of creation as lasting seven days, and the
geneologies included in the book of numbers..
Based upon the old testament, the Archbishop
James Usher calculated that God created the
Earth in 4004BC. This left little time for incredibly
slow, gradual processes like evolution...
Hutton, Lyell and Uniformitarianism
The English geologist, James Hutton
proposed that it was possible to explain
geological land formations by processes
that are currently in operation, such as
erosion and sedimentation.
Canyons were cut by the erosion of
streams, layers of sediment were deposited
at the edge of river deltas, these processes
occurred slowly over a very long time-this
idea was called gradualism.
The English geologist, Charles Lyell was a
contemporary of Darwin’s.
– He was a proponent of Hutton, and went a bit
farther, embracing the principal
uniformitarianism-the idea that geological
processes in operation now operated similarly in
the past, at about the same rate.
The Uniformitarian view of
nature, requires vast
amounts of time to explain
the present state of the
Jean Baptiste Lamarck
Jean Baptiste de Lamark developed the
first comprehensive model of evolution.
Lamarck was a French Zoologist, curator
of the invertebrate collection at the Paris
Lamarck saw many different lines of
descent among the fossil invertebrates he
encountered: instead of Aristotle’s single
scala natura, there were many.
He proposed that organisms increased in
complexity through time because of an
innate tendency.
Lamarck proposed that interactions of
organisms and environment drove the
process of evolution.
He followed the widely accepted notion that
characteristics acquired during an
individual’s lifetime could be passed to one’s
He proposed that patterns of use and disuse
drove the evolution of adaptations. In
stretching their necks to reach leaves high in
the treetop, giraffes acquired slightly longer
necks, and passed these longer necks to
their offspring.
According to Lamarck, every organism was
continually striving for greater complexity, a
clam strove to be a better clam, etc.
Lamarckian evolution can be disproved by
experiment, specifically, we now know that
acquired characters cannot be passed to
offspring, also, evolution carries no innate
tendency toward increasing complexity, but
Lamarck’s theory was an important prelude
to Darwin’s, it opened the door to thinking
that organisms can and do change over the
course of time.
The Voyage of the Beagle
Much of Charles Darwin’s inspiration for his theory
of evolution by natural selection came from his
voyage on the HMS Beagle, in 1831.
He saw an incredible diversity of species, with
adaptations to a wide variety of environments;
Brazilian rainforests, Chilean deserts, oceanic
islands, etc.
The Galapagos islands particularly impressed him;
most of the species there live nowhere else in the
world, yet their closest living relative is on the
mainland a few hundred miles away.
He was to spend the next 27 years developing a
theory to explain what he saw.
Darwin and Wallace
Alfred Russell Wallace, a nineteenth century naturalist
and explorer, an expert on collecting specimens for
resale in Europe, developed essentially the same
theory of evolution by natural selection as Darwin.
– An active man, he sat down to write it recovering
from a bout of malaria, when he was unable to go
out and explore.
The two shared credit for the discovery, a rare
example of diplomacy in 19th century science.
• Darwin is better known today, because he amassed a
considerable amount of evidence to support his ideas.
Wallace’s arguments were more intuitive and contained a
less-extensive battery of examples.
Darwin had spent much of his life amassing the
evidence he needed to support his model of
• He was finally goaded into publishing when he came
across a manuscript by Wallace which contained many of
the same ideas.
– Both theories had very broad implications, forcing
European intellectuals to re-examine their place in
• By proposing a mechanism for the evolution of the human
species, its mind, and its achievements, that is not
supernatural, it removed the need for a divine “prime
mover” from science.
– Such a creator, or “prime mover” had been an element of
Western science, since Roman times or earlier, and had been
removed from physics and astronomy centuries earlier.
1859: The Origin of Species
Darwin’s manuscript contained several new
ideas, ideas not found in earlier notions of
– All species evolved from earlier species.
– The mechanism is natural selection; members of
a species possessing more desirable traits will
have more offspring and survive to reproductive
– Evolution occurs over a very long span of time.
The Origin of Species makes this argument,
structured logically…
– All organisms produce more offspring than can possibly
– All organisms vary for a wide variety of different
attributes and features-they also vary in reproductive
success: some have more offspring than others.
– Some variation is heritable.
– Some of this variation must influence reproductive
– Given that the above are true…desirable characteristics
will thus be preferentially passed to offspring
• This is a logical conclusion of the first four points
Darwin concluded, based upon intuitive grounds, that, over
vast spans of time, present day species have descended
from a common ancestor. The book contained no
mechanism for speciation, however.
Evidence for Evolution
The gradual evolution of life on the planet, and
their descent from a common ancestor, is a
– Darwin’s theory of evolution is a comprehensive
body of evolution that attempts to explain how this
One of the hallmarks of a truly revolutionary
scientific theory is that it brings together many
previously unexplained patterns under a single
body of theory.
– Like Newton’s theory of universal gravitation,
Darwin’s theory of evolution created a new
scientific paradigm.
Some of the original evidence for
Vestigial and Homologous structures
The Fossil Record
Closely related species go through similar stages
of development, although the adults may not
resemble each other very closely.
For instance, all vertebrate embryos develop gill
pouches at some stage, even though in many
species, they are lost later. This is suggestive of a
common origin for vertebrates.
Embryological development is often suggestive of
evolution: birds have many developmental features
in common with reptilian ancestors, land vertebrate
embryos have many features suggestive of an
aquatic existence (gill pouches, a notochord,
blocks of segmented muscle).
Snake Chicken
Vestigial Structures
Many species retain structures that only make sense in
light of their ancestry.
These structures are typically reduced and nonfunctional,
but they are inherited from ancestors, in whom they were
important to survival or reproduction
Comparative Development and Embryology
If members of a taxonomic unit share a
common ancestry, it is reflected in their
Two of the many examples:
– limb bud development in whales
– extraembryonic membranes of the amniote egg
Homologous Structures
Closely related species frequently have
homologous structures: structures that are similar
in their fundamental layout and construction,
although they may serve very different purposes.
– For example, the forelimbs of mammals are
constructed from the same skeletal elements:
The wings of a bat, a whale, a human, a dog,
etc. all contain the same bones, despite their
different uses.
This suggests that common ancestry, rather than
design, plays a role in the construction of species.
The Fossil Record
The succession of forms in the fossil record
clearly suggests that organisms change
through time, and have descended from a
common ancestor.
Different groups appear in the fossil record at
different times, with a general trend toward the
simplest organisms appearing the earliest..this
is at odds with the view that they were all
created at the same time.
Many forms have gone extinct, another
observation that is at odds with the view that
each species was specially created for a
In some cases, a direct line of descent, and
change through time, can be observed in
fossils. Foraminifera, small oceanic
protozoans, leave a continuous fossil record
in oceanic sediments. It is possible to trace
their gradual evolution over millions of years.
Since Darwin’s day, our knowledge fossil
record has improved tremendously, we can
trace the evolution of many different groups
through fossils: horses, for instance, have a
superb fossil record, showing many instances
of speciation and many intervals of
evolutionary change.
Example-Whales have
an excellent fossil record-showing
transitional forms
The distribution of living plants and animals
suggests that organisms adapted to one
environment can invade a new environment, and
develop specific adaptations to the new
conditions. On the HMS Beagle, Darwin noted
that in South America, temperate species tended
to resemble their South American tropical
relatives, rather than temperate species in
Europe. On the Galapagos, most species had a
recognizable ancestor from the coast of Ecuador,
but species there had numerous adaptations
specific to the climate of the Islands.
Wallace observed the same pattern in many
different parts of the world.
Modern Evidence
Since Darwin’s time, there have been
hundreds of studies of evolution.
Natural selection has been measured in
many organisms in the field, and in
laboratory populations.
An understanding of evolution has also
become important to combating
Example-DDT resistance in
The misuse of DDT, and the reemergence of malaria as an important
human pathogen, is perhaps one of the
greatest public health failures of the
– it could possibly have been prevented if the
evolution of mosquitoes had been taken
into account
In nonresistant insects, DDT is a very
effective insecticide-causing massive
mortality and very strong selective
pressure in favor of any mutation that
might lead to resistance
Indiscriminate spraying (when there was
no particular need to control the
organism) led to the rapid evolution of
pesticide resistance. Five Anopheles
species were resistant by 1956 and 38 by
Resistance takes many forms-some of
this genetic variation was probably
present in the mosquito population
before the use of DDT, but in the
absence of DDT, these variants are
selected against.
1) Chemical adaptation: enzymes
evolve that break down the pesticide.
2) Behavioral adaptation: They evolved
to move from inner, sprayed walls to
outer, unsprayed walls. They evolved
sensitivity and avoid the pesticide.
These data are from Bangkok-the R allele is resistant,
the + allele is not.
Note that the + allele becomes more common in the
absence of DDT spraying
Natural Selection as the mechanism for
adaptation was Darwin’s most important
There are other forces of evolution
(most of which were discovered after
Darwin), but natural selection is the only
evolutionary mechanism that can
produce adaptation.
Some examples of adaptation are very
Find the mantis in this picture
The Variation Problem
For Natural Selection to be effective, there
must be genetic variation upon which
selection acts.
Darwin discussed the origin of variations
extensively in “On the Origin of Species..”, but
he did not know how variation persists.
Although he was a contemporary of Mendel’s,
Darwin did not know Mendelian genetics (his
work was not well understood at the time).
The current theory of genetics, blending
inheritance, suggested that useful
adaptations would blend into the population
and become diluted.
The synthesis of Darwin’s theory with
Mendelian genetics led to our modern
understanding of Evolution.
Several early twentieth century evolutionary
biologists are widely credited with developing
our modern understanding:
R.A. Fisher
J.B.S. Haldane
Sewall Wright
Theodosius Dobzhanski
Thomas Hunt Morgan
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