Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
•
Describe the major ideas on evolution
and related topics that had a significant
influence on Charles Darwin as he
developed the concept of evolution by
natural selection.
.
Historical perspective

divine design and perfection model

fossils

acquired traits

modern geology – Lyell, uniformitarianism, and
the ancient Earth

artificial selection

population limits
.
.
Historical perspective
divine design and perfection model


from Aristotle, whose ideas dominated most thinking on biology until the
renaissance

species were viewed on a scale from simple to complex (which was considered
more perfect)

all organisms were seen as moving toward perfection

based on divine intervention and design (thus supernatural, outside the true realm
of science)

now discredited in biology, but still part of the social consciousness
.
Historical perspective
fossils


fossils were known for centuries before Darwin

fossils reveal organisms unlike any living today

the idea that some fossils represent species that had
become extinct was recognized even as early as
Leonardo da Vinci (1452-1519)
.
Figure 1.4 A mylodon.
Drawing of a giant ground sloth. From A Naturalist’s Voyage Around the World:
The Voyage of the H.M.S. Beagle by Charles Darwin (D. Appleton and Co.,
New York, 1890).
.
Figure 1.1 “I was in many ways a naughty boy.”
Portrait of young Charles and his sister Catherine. Charles later wrote in his
autobiography, “I was much slower in learning than my younger sister Catherine, and
I believe that I was in many ways a naughty boy.” From More Letters of Charles
Darwin: A Record of His Work in a Series of Hitherto Unpublished Letters edited by
F. Darwin and A. Seward (D. Appleton and Co., New York, 1903).
.
Figure 1.2 Charles’ list of his father’s objections to the Beagle voyage.
Reproduced by kind permission of the Syndics of Cambridge University Library.
.
Figure 1.3 The HMS Beagle and Darwin’s quarters.
Based on a drawing by shipmate Philip King, with whom Darwin shared his
quarters. From Journal of Researches into Geology and Natural History of the Various
Countries Visited by H.M.S. Beagle by Charles Darwin (facsimile edition of 1839
First Edition, Hafner Publishing Company, New York, 1952).
.
Figure 1.6 HMS Beagle in the Strait of Magellan.
Drawing from A Naturalist’s Voyage Around the World: The Voyage of the
H.M.S. Beagle by Charles Darwin (D. Appleton and Co., New York, 1890).
.
Figure 1.7 Map of the Voyage of the HMS Beagle, 1831–1836.
Drawn by Leanne Olds.
.
Historical perspective
birth of modern geology – Lyell, uniformitarianism, and
the ancient Earth


prior to the early 1800s, the world view of most was that the Earth
is very young (around 6000 years old)

in the early 1800s, geologists began to apply scientific reasoning
to studies of geological processes, and quickly recognized that
these processes require that the Earth be very old (billions of
years) to occur naturally
.
Historical perspective

this “uniformitarian” model of geological processes was
made famous by Lyell’s Principles of Geology, which
influenced Charles Darwin

the uniformitarian model is essentially the basis of
geology today; confirming tests of this model include
dating rocks using radioisotope ratios (more on that later)
.
Figure 1.7 Map of the Voyage of the HMS Beagle, 1831–1836.
Drawn by Leanne Olds.
.
Figure 1.9 Galapagos finches.
Drawing from A Naturalist’s Voyage Around the World: The Voyage of the
H.M.S. Beagle by Charles Darwin (D. Appleton and Co., New York, 1890).
.
Figure 1.8 A Galapagos tortoise.
Drawing from A Naturalist’s Voyage Around the World: The Voyage of the H.M.S.
Beagle by Charles Darwin (D. Appleton and Co., New York, 1890).
.
1831:
Leaves on Beagle (age 22)
1836:
Beagle returns to England;
soon afterwards writes his
story of the trip and begins his
“secret notebooks”
Darwin’s voyage
Figure 1.7 Map of the Voyage of the HMS Beagle,
1831–1836.
Drawn by Leanne Olds.
Figure 1.10 The tree of life.
Page from notebook “B,” where Darwin recorded his idea that life is
connected like the branches of a tree, with ancestors at the bottom.
Reproduced by kind permission of the Syndics of Cambridge
University Library.
.
.
Darwin’s voyage
1831:
Leaves on Beagle (age 22)
1836:
Beagle returns to England; soon
afterwards writes his story of the trip
and begins his “secret notebooks”
1838:
Reads Malthus’ Essay on the
Principle of Populations
1839:
Voyage of the Beagle published
1842:
Makes 35-page sketch of theory
1844:
Expands sketch to 230 pages
1859:
Publishes On the Origin of Species
by Means of Natural Selection
1871:
Publishes The Descent of Man and
Selection in Relation to Sex
.
Darwin’s voyage
There was much discussion by immediate predecessors
and contemporaries of Darwin about:


how the divine design model did not mesh well with observation of
the extremes of variation among species

the idea of extinct species represented in the fossil record

functional similarities between the anatomy of extremely divergent
species

the idea that evolution occurs thus was “in the air” at the
time

attempts to find a convincing mechanism fell short (such
as Lamarck’s acquired characteristics model)
.
Historical perspective
acquired traits


mostly associated with Lamarck
(1744-1829)

still focused on a model of organisms
driven toward complexity, but involved
an explanation with natural causes

postulated that changes or “acquired
characteristics” during an organism’s
life could be passed on to offspring

famous example was Lamarck’s
model for how giraffes developed long
necks – he claimed that stretching of
the neck in one generation would lead
to offspring with longer necks

understanding of genetic inheritance
has led to rejection of acquired traits
models
.
Darwin’s theory: evolution occurs by
natural selection
Darwin’s theory of evolution was based on four
general observations:



overproduction

variation

competition

differential reproductive success
natural selection will produce a population of individuals
more suited to their environment through time
.
Historical perspective
population limits that would allow selection
to act naturally were recognized


Malthus (1766-1834)
wrote the most influential
works on this subject

mathematically,
populations will grow
geometrically if
unchecked

food supplies rarely can
be expected to grow
faster than arithmetically,
thus putting a limit on
population growth
.
Historical
perspective
artificial selection

it was well known that domesticated animals and plants had been
breed over centuries by humans to produce different varieties


indicates that the characteristics of a species can be modified by
selection
some examples:


different breeds of dogs

“wild cabbage” lineage of cabbage, broccoli, cauliflower, Brussels
sprouts, collards, kale, etc.

many more
.
Figure 1.9 Galapagos finches.
Drawing from A Naturalist’s Voyage Around the World: The Voyage of the
H.M.S. Beagle by Charles Darwin (D. Appleton and Co., New York, 1890).
.
Darwin’s voyage
Recall Darwin’s theological training –


Darwin was well aware of the impact that a workable, testable theory of evolution
would have

…and the intense controversy and scrutiny it would draw…

so, although he worked out most of his theory of evolution shortly after his trip on
the Beagle

…he spent 20 years accumulating evidence and doing experiments before finally
publishing the idea!
.
Darwin’s voyage

Darwin was spurred on to
publish when Alfred
Russel Wallace shared
his independent work
where he had reached
similar conclusions to
Darwin

they first presented the
theory of evolution by
natural selection together
in 1858
.
Figure 3.1. The massive Amazon River system.
The main river and its tributaries span more than fifteen thousand miles. Henry
Walter Bates spent most of his eleven years in the Amazon on the main river, while
Alfred Russel Wallace ventured far up the Rio Negro. Bates found more than
550 species of butterflies at Ega (now Tefé). Drawn by Leanne Olds.
.
Figure 2.1 Sketch salvaged from fire and shipwreck of the Helen.
This drawing of an Amazonian angelfish was one of the few sketches Wallace
managed to save out of all of his notes and specimens on his doomed voyage home. It
displays one of the important talents for naturalists before the age of photography —
that of being a good artist. Drawing from the autobiography of Alfred Russel Wallace,
My Life (New York: Dodd, Mead, and Co., 1905).
.
Figure 2.2 The Malay Archipelago.
Map by Leanne Olds.
.
Figure 2.3 The Golden Birdwing butterfly.
Wallace discovered this form (Ornithoptera
croesus lydius) on the island of Batjan.
Photograph by Barbara Strnadova.
.
Figure 2.4 The Wallace Line.
.
Darwin’s voyage

Darwin published his first
version of the book On the
Origin of Species by Means
of Natural Selection in 1859

in it, he laid out the entire
argument with all of the
evidence that he had been
gathering ever since his
voyage on the Beagle
.
Darwin’s voyage
Darwin’s book had immediate and
dramatic impact


the force of his argument and evidence
convinced many scientists quickly

of course it stirred tremendous controversy
as well

Darwin made several revisions of his work
in response to some of the most
reasonable criticisms

He also focused on human evolution in The
Descent of Man (1871)
.
•
Describe the major ideas on evolution
and related topics that had a significant
influence on Charles Darwin as he
developed the concept of evolution by
natural selection.
.
Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
•
Describe the logical reasoning behind
Darwin’s concept of natural selection.
.
Darwin’s theory: evolution occurs by
natural selection
Darwin’s theory of evolution was based on four general observations:



overproduction – each species produces more offspring than will survive to
maturity

variation – individuals in a population vary, and some of the variation is
heritable (this was expanded by others later, as genetics came to be
understood)

competition – there is competition among the individuals of a population for
limited resources (struggle for existence)

differential reproductive success – individuals that possess more favorable
characteristics (in the pool of variation) are more likely to survive and
reproduce; those with less favorable characteristics are less likely to survive
and reproduce
thus, natural selection will produce a population of individuals more suited
to their environment through time
.
Darwin’s theory: evolution occurs by
natural selection

when populations are separated (such as the
geographic separation of islands from each other
and a nearby continent), natural selection on two
separate populations can produce two distinct
populations with different characteristics –
resulting in two separate species

note that for this theory to explain the current
variety of species on Earth, there is a need for a
long amount of time for natural selection to
produce the variety observed; thus, the idea of an
ancient Earth hundreds of millions to billions of
years old is crucial
.
•
Describe the logical reasoning behind
Darwin’s concept of natural selection.
.
•
Explain the terms microevolution and
macroevolution (in their true scientific
meanings), and describe how
microevolution can lead to
macroevolution.
.
Darwin’s theory: evolution occurs by
natural selection

two major branches: microevolution, or
changes of a population over time, and
macroevolution, or the formation of
species

http://evolution.berkeley.edu
.
•
Explain the terms microevolution and
macroevolution (in their true scientific
meanings), and describe how
microevolution can lead to
macroevolution.
.
Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
• Discuss these major lines of evidence for
evolution:
– fossil record
– anatomical evidence (comparisons, vestigial
structures, “design” flaws)
– distribution of organisms
– developmental comparisons
– molecular comparisons
.
Figure 3.1. The massive Amazon River system.
The main river and its tributaries span more than fifteen thousand miles. Henry
Walter Bates spent most of his eleven years in the Amazon on the main river, while
Alfred Russel Wallace ventured far up the Rio Negro. Bates found more than
550 species of butterflies at Ega (now Tefé). Drawn by Leanne Olds.
.
Figure 3.2 Mimicry in butterflies.
This is an original plate from Bates’ 1862 paper reporting the discovery of mimicry.
The butterfly at the center (5) is Leptalis nehemia, the typical butterfly of the family.
The other Leptalis butterflies (1–8) deviate greatly from this pattern, as they are
mimics of other species. Each pair (3/3a, 4/4a, 6/6a, 7/7a, 8/8a) illustrates mimicry
between Leptalis and species of other families. Specimens 3a, 4a, and 6a are members
of the genus Ithomia that mimic varieties of Leptalis theonoe found in the area of Sao
Paulo. Specimens 7a and 8a are members of the Mechanitis and Methona genera that
mimic Leptalis amphione and Leptalis orise.
.
Figure 3.3 Caterpillar mimic of snake head.
First discovered by Bates, a number of species mimic the appearance of snake
heads. This is the Spicebush Swallowtail caterpillar (Papilio trollus).
Photo by Mary Jo Fackler.
.
Figure 3.4 Mimicry in snakes.
Arizona Mountain kingsnake (top) and Arizona coral snake (bottom).
Photos by Gary Nafis.
.
Evidence supporting the theory of evolution

the fossil record

comparative anatomy of related species

distribution of plants and animals

related species have similar patterns of
development

molecular comparisons among organisms
.
Evidence supporting the theory of evolution
the fossil record

fossils provide direct
evidence for change over
time


fossils range from mineralized
casts or imprints (most
commonly of bone, teeth, and
shells, but sometimes of softer
tissues) to actual body parts
preserved in bogs, tar, amber,
or ice

fossils provide evidence of
intermediates between extant
and extinct forms
.
Evidence supporting the theory of evolution
fossils provide direct evidence for
change over time


many relatively complete examples of
transitions in body forms are known, such
as the evolutionary lineage of horses and
the transition of terrestrial species to
modern whales

the fossil record provides tests of
evolution as an explanation for the history
of life on Earth – fossils can be dated, and
the age of fossils invariably matches the
predicted place of those body forms in the
history of life on Earth
.
fossils provide direct evidence for change over time
Basilosaurus, a fossil whale
.
fossils provide direct evidence for change over time
.
fossils provide direct evidence for change over time
McFadden, Bruce. 2005. “Fossil Horses – Evidence of
Evolution.” Science Vol. 307. no. 5716, pp. 1728 – 1730
.
Evidence supporting the theory of evolution
the fossil record

fossils most commonly form in sedimentary
rocks in aquatic environments


the fossil record is biased toward organisms
with hard parts that lived in aquatic or arid
environments, where decay is slow and
incorporation in rocks can occur with
reasonable speed

organisms that lived in places of rapid decay
are thus biased against in the fossil record
.
•
Explain how fossils are dated.
.
dating fossils
Youngest
relative position in rock
layers


sedimentary layers most
commonly have the youngest
layers nearer the surface, and
are progressively older as you
go deeper

large-scale geological events
can be used to correlate rock
strata from different sites; other
dating methods are also used
to correlate rock strata
Oldest
.
dating fossils

association with
index fossils that
have been dated by
other means from
other locations

radiometric dating

each radioisotope
has characteristic,
constant rates of
decay

some allow for
measurement of
when a rock was
formed or when an
organism died
.
dating fossils
radiometric dating

example: potassium-40 decaying to argon-40


when magma cools to solid rock, no argon is in the rock
(escapes as the rock forms)

once the rock hardens, the radioactive clock begins –
potassium-40 in the rock decays to argon-40

measurement of the amounts of potassium-40 and argon-40 in
the rock today are used to determine an age range for when the
rock could have been formed

half-life of 1.3 billion years: used for fossils tens of millions to
billions of years old
.
dating fossils
radiometric dating

another example: carbon-14 decaying to
nitrogen-14



half-life of 5730 years

used for organic remains hundreds to tens of
thousands of years old
there are hundreds of well-studied sites with
fossils that have been dated in some way;
no truly incongruous fossils have been
found
.
•
Explain how fossils are dated.
.
comparative anatomy of related species
organs or structures that have similar form due to a common
evolutionary origin are called homologous features


example: the similarity between the human arm, the dolphin's
flipper, the bat's wing, and the bird's wing

example: plant leaves, cactus needles, flower sepals and petals
.
comparative anatomy of related species
vestigial structures – a feature that
once had a role in the evolutionary
history of a species but that no longer
functions


natural selection will logically lead to
degeneration of unused features

however, it is not easy to completely
remove by natural selection – thus,
vestiges are left behind
.
comparative anatomy of related species
not all organs or structures with functional similarity have a common
origin


such cases are called homoplastic features, or analogous features

resemblance between homoplastic features is superficial – consider an
insect's wing and a bird's wing

independent evolution of similar features in distantly related organisms is
called convergent evolution
.
distribution of plants and animals

biogeography – the study of the past
and present geographical distribution of
organisms

organisms on islands are most closely
related (in form and genetically) to
those from the nearest mainland, not
those from similar islands in different
parts of the world
.
distribution of plants and animals
biogeography – the modern theory of plate tectonics and
reconstruction of the history of land masses on Earth explains:


much of the observed fossil distributions

timings of geographic isolations that would be expected for some
modern distributions of species (example: the dominance of
marsupials in Australia)
.
Figure 2.4 The Wallace Line.
Wallace discovered that the narrow strait between Bali and Lombok marked a boundary between Asiatic fauna
(with tigers, rhinoceri, and orangutans) and Australia-type fauna (with kangaroos, cuscus, and other marsupials).
Bali was once connected to the Asiatic continental shelf, but not to Lombok. The boundary line extends
throughout the archipelago as shown. Drawn by Leanne Olds.
.
related species have similar patterns of development

very young embryos of reptiles, birds,
mammals, and humans are indistinguishable

studies of developmental biology are revealing
the common genetic basis for such similarities
– “devo/evo” study is one of the hottest fields in
biology today
.
• Discuss how DNA sequence comparisons
and “molecular clocks” work.
.
molecular comparisons between organisms
the “big test” for the theory of evolution
by natural selection was this:


evolution by natural selection on inherited
traits predicts that genetic sequence
information will provide a record of
evolutionary change and evolutionary
relationships

these genetic records should correlate with
evolutionary relationships that have been
established by other means, such as
biogeography and comparative anatomy
.
molecular comparisons between organisms
evolution has passed this test
with flying colors



the virtual universality of the
genetic code is compelling
evidence of a common
ancestor

changes in proteins and
nucleic acids provide a record
of evolutionary change
detailed molecular studies
have clearly documented
microevolution in many cases,
such as the emergence of
antibiotic-resistant bacteria
.
molecular comparisons between organisms
changes in proteins and nucleic acids provide a record of
evolutionary change


DNA sequencing provides a
means to measure genetic
similarities and differences
between species

sequences of amino acids
in proteins can also be used
– these provide an indirect
comparison of DNA
sequences

DNA and protein
sequencing can be used to
create a phylogenetic tree,
a diagram showing the
relatedness between
species and lines of descent
.
molecular comparisons between organisms
changes in proteins and nucleic acids
provide a record of evolutionary change

DNA sequencing can also be used in some
cases as a molecular clock to make some
inference about when any two species
diverged from each other (last shared a
common ancestor)


example – humans and chimpanzees: ~98%
sequence identity, diverged about 6 million
years ago
.
Base Sequence Comparisons

Divergence (difference)
in nucleotide base
sequence allows us to
draw relationships
between different
organisms.

Here, differences in
nucleotide base
sequence of humans
and some other
primates compared.

Of the organisms with
data given here, chimp
DNA is the most like
human DNA, and spider
monkeys DNA is the
least like human DNA.
.
http://www.pnas.org/content/88/20/9
051.full.pdf+html
http://www.youtube.com/watch?v=8
FGYzZOZxMw
Human Chromosome 2 and its analogs in the apes from Yunis, J. J., Prakash, O., The origin of man: a
chromosomal pictorial legacy. Science, Vol 215, 19
March 1982, pp. 1525 - 1530
.
• Discuss how DNA sequence comparisons
and “molecular clocks” work.
.
• Discuss these major lines of evidence for
evolution:
– fossil record
– anatomical evidence (comparisons, vestigial
structures, “design” flaws)
– distribution of organisms
– developmental comparisons
– molecular comparisons
.
Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
•
What is the “modern synthesis”?
.
The modern synthesis of evolutionary theory combines
Darwin’s concept of natural selection with genetics

although Mendel was a contemporary of Darwin,
remember that his work was largely
unrecognized until around 1900

how traits are inherited was central to Darwin’s
theory of evolution

thus Darwin (and others) were keenly interested in
finding working models of inheritance
.
The modern synthesis of evolutionary theory combines
Darwin’s concept of natural selection with genetics
when genetic mechanisms came to be widely
understood, they were quickly combined with
Darwin’s model in the modern synthesis, also
called Neo-Darwinism or the synthetic theory of
evolution


this model emphasizes the genetics of populations

evolution is seen as working by natural selection on
individuals to change the genetic makeup of
populations over successive generations
.
The modern synthesis of evolutionary theory combines
Darwin’s concept of natural selection with genetics
mutations play a key role in providing a source of
genetic variation


without genetic variation, evolution cannot occur

mutations are necessary to produce genetic variation

while many mutations have no impact and many others
are harmful, it is critical to recognize that some
mutations are advantageous
.
•
What is the “modern synthesis”?
.
Chapter 22: Evolution

Historical perspective on evolution

Darwin’s voyage

Darwin’s theory: evolution occurs by natural
selection

Evidence supporting the theory of evolution

The Modern Synthesis

The central role of evolution in modern biology
.
The central role of evolution in
modern biology
the modern synthetic theory of evolution is accepted today
by most biologists as:



a robust and well-supported model

the central framework for the study of life
nearly all biologists today agree with the famous statement
by the evolutionary geneticist T. Dobzhansky:
“Nothing in biology makes sense except in the light of evolution.”
.
The central role of evolution in
modern biology
studies of evolution itself today focus largely on:

the causal processes of evolution, such as:


the speed of evolutionary change

the role of chance in evolution
molecular comparisons between and within species by


comparing DNA sequences

comparing genomes

comparing proteins and proteomes
.