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chapter 22
Descent With Modification:
A Darwinian View of Life
The Fossil
Inheritance of
Uniformitarianism Descent with
Match… Distinguish… Cite Evidence… Differentiate…
Critique… Connect… Create…
Who? What? Where? When? Why? Which? How?
What is evolution?
• Discuss @ your table.
• Provide examples of the evolutionary process.
– 1) WHO? Name the organism
– 2) WHAT? summarize the physical or behavioral traits that
make it unique (adaptations)
– 3) WHY? correlate the cause-effect (trait  advantage)
– 4) How? Hypothesize… construct a scenario that would
have led to the evolution of that trait.
What is evolution?
Organismic evolution means changes in populations,
species, or groups of species.
It occurs because:
1. Populations vary by the frequency of heritable traits
that appear from one generation to the next.
2. These traits are represented by alleles for genes that
modify morphology (form/structure), physiology, or
3. There is a struggle for survival and most organisms
perish. Only those that survive pass their genes on.
Evolution is changes in allele
frequencies over time.
• Discuss what this statement means.
Can individuals Evolve?
• NO. Individuals can BE different and can survive and
pass those genes on or be killed and not.
• For an organism to change itself to suit its environment it
would need to change the genes (DNA) inside every cell
of it’s body.
• Most importantly, they would need to change the DNA
in their gametes- so the “change” could be passed on to
the next generation.
• Some individuals are better suited for their environment.
They leave more offspring. Over generations, the genetic
composition of a population reflects more of their
“beneficial” genes.
Two areas of evolutionary study:
1. Microevolution describes the details of how
populations of organisms change from
generation to generation and how new species
originate. (next chapter’s focus)
2. Macroevolution describes patterns of changes
in groups of related species over broad periods
of geologic time. The patterns determine
phylogeny, the evolutionary relationships among
species or groups of species. (fossil record)
Evolution of insecticide resistance in insect populations
DDT resistance of insects
Phylogenetic trees are diagrams that show
evolutionary relationships between groups
of extinct and extant organisms
Ex. Fossil Record
The historical context of Darwin’s life and ideas.
Historical Context of
Evolutionary Theory
Paradigm shift from organism are STATIC for to
the idea that organisms can CHANGE FORM.
• Plato (427-347 B.C.) Two worlds: real/ideal
world that is eternal and world of imperfection we
perceive through senses. Living things were
created in their perfect, static form by the gods.
• Aristotle (384-322 B.C.) “scale of nature” all
living forms could be arranged on. Each form
assigned a rung, all rungs were taken.
nonliving beings- lower plants-higher plants sponges, jellyfish
shellfish - insects -- crustacea cephalopoda -- ovipara - whales •
ovoviviparous quadrupeds -- humankind. These are the "steps
of nature," or the "hierarchy of nature”… LADDER OF LIFE
Tree of Life /
Bush of Life
instead of a
ladder or
tips of branches
Natural theology
A philosophy dedicated to discovering the
Creator’s plan by studying nature- the
earth and it’s inhabitants.
1. Adapations were evidence that the Creator
had designed each and every species for a
particular purpose.
2. Classifying species was a major objective.
Carolus Linnaeus (1707-1778)
• Specialized in taxonomy- naming and classifying the
diverse forms of life.
• Developed binomial nomenclature- a two part naming
• Developed a system of grouping similar species into a
hierarchy of increasingly general categories. (speciesgenus- family)
• He saw no evolutionary relationships in his groupings.
Georges Cuvier (1769-1832)
largely developed paleontology- the study of fossils.
• Big idea: species go extinct
• Observed a pattern of many strata separated by
• Explanation catastrophism
• Each boundary between strata corresponded to a
catastrophe (drought, flood,supervolcano) destroyed
many of the local species.
• Area repopulated by immigration.
Figure 22.4 Strata of sedimentary rock at the Grand Canyon
Figure 22.3 Formation of sedimentary rock and deposition of fossils from different
time periods
Trilobite fossils- lived in the seas hundreds of millions of years ago
ex. 65mya the meteorite impact in the Gulf of Mexico brought on the
extinction of the dinosaurs.
Fathers of Modern Geology
Hutton & Lyell
1795 James Hutton, Scottish geologist
Explained Earth’s geologic features by the theory of
Gradualism: Speciation occurs gradually, profound change
is the cumulative product of slow but continuous processes.
Charles Lyell (1797-1875) geologist
incorporated gradualism into the theory of
Uniformitarianism: Geologic processes have not changed
throughout Earth’s history (ie. forces that build and erode
mountains occur at a steady rate) thus Earth is much older
than previously believed.
Figure 22.4 Strata of sedimentary rock at the Grand Canyon
Fossil Record Interpretation
Punctuated equilibrium
• Argues that evolution occurs by the gradual
accumulation of small changes.
• Individual speciation events or major changes in
lineages occur over long periods of geologic time.
• Fossil evidence provides snapshots of the
evolutionary process, revealing only major
changes in groups of organisms.
• Intermediate stages of evolution not represented
by fossils is due to the incompleteness of the fossil
Punctuated Equilibrium
• Argues that evolutionary history consists of
geologically long periods of stasis with little or no
• interrupted, or “punctuated” by geologically short
periods of rapid evolution.
• The fossil record should consist of fossils mostly
from the extended periods of stasis with few if any
fossils from the short bursts of evolution.
Phylogenetic trees are diagrams that show
evolutionary relationships between groups
of extinct and extant organisms
Ex. Fossil Record
• Earth must be very old much older than six
thousand years (it is 4.5 billion years old)
• Very slow and subtle processes persisting
over a long period of time can add up to
substantial change.
• Slow but significant changes in
environments caused slow but significant
changes in species over “geologic time.”
By the end of the 18th century several
naturalists, including Darwin’s grandfather
Erasmus Darwin, suggested that life had
evolved as environments changed.
No one had suggested a mechanism.
THEORY… 2 parts
• Evolution, change in species over geologic
time, does exist.
• Theory to explain WHY/HOW… the
mechanism causing this change.
The Inheritance of Acquired Characteristics
Jean Baptiste de Lamarck proposed the first explanation
which was widely accepted for years.
Three parts:
Use and disuse described how body parts of organisms can develop
with increased usage, while unused parts weaken. Correct.
Inheritance of acquired characteristics described how body
features acquired during the lifetime of an organism (such as muscle
bulk) could be passed on to offspring. Incorrect.
Natural transformation of species described how organisms
produced offspring with changes, transforming each generation into
a slightly different form that is more complex. Species did not
become extinct nor did they split and change into two or more
species. Incorrect.
The giraffe
Short neck.
Food scarce.
Stretch neck.
Longer neck.
• Inheritance of Acquired Characteristics or Lamarckism
• With a Partner: Create a Lamarkian explanation for the evolution
of one of your creatures.
Descent with Modification via
Natural Selection by DARWIN
Charles Darwin 1809-1882
Med school dropout
BA Natural Theology
Captain Fitzroy- HMS Beagle
Galapagos, islands of relatively
recent volcanic origin, 900 km
west of SA coast.
• Gathered mountains of evidence
• Waited 20 years to write and
publish a groundbreaking book.
Beaks adapted to specific foods available on their home islands.
On the Origin of Species
By Means of Natural
Selection November 24, 1859
Darwins’s groundbreaking
book explained what had once
seemed a bewildering array
of unrelated facts.
He focused on:
• Diversity of organisms
• Similarities & differences
• Geographic distribution
• Adaptations to surrounding
Main Ideas of Natural Selection
1) Overproduction of offspring… populations
posses an enormous reproductive potential.
ex. Darwin calculated that two elephants would
produce a population of 19 million individuals
after 750 years if all offspring survived to
reproductive maturity and fostered their normal
number of offspring.
2) Members of a population compete for survival.
• Population sizes remain stable- populations generally
fluctuate around a constant size.
• Resources are limited. Resources such as food, water, light
don’t increase as populations grow larger.
• Eventually, the needs of a growing population will exceed
the available resources.
• As a result, individuals must compete for resources.
Influenced by Thomas Malthus’ 1798 essay on human
population… “much of human suffering- disease, famine,
homelessness, and war- was the inescapable consequence of
the potential for the human population to increase faster
than food supplies and other resources.”
3) Genetic variability exists among
individuals in a population.
Most traits reveal considerable variety in
their form.
Ex. Musculature, height, fur color, hair color,
skin color, eye color, etc.
Figure 22.9 A few of the color variations in a population of Asian lady beetles
4) Some individuals will be better suited for
their environment. Only the most fit
individuals survive. “survival of the fittest”
Individuals with traits best adapted for
survival in the environment are able to outcompete other individuals for resources.
5) Individuals with the better adaptations survive and
leave offspring who inherit the traits of their parents.
In turn, the best adapted of these offspring leave the most
Over time, traits best adapted for survival in the
environment and the alleles that generate them
accumulate in the population.
Evolution occurs as advantageous traits accumulate.
Figure 22.10 Camouflage as an example of evolutionary adaptation
Weedy/ Leafy Sea Dragon
Key Points of Natural Selection
1. Populations (groups of interbreeding individuals
belonging to a particular species and sharing a
common geographic area) are the smallest unit that
can evolve. Evolution is measurable over successive
2. Natural selection acts only on heritable
variations/phenotypes. (not acquired ones)
3. Natural selection is situational as environmental
factors vary from place to place and from time to
time. An adaptation in one situation may be useless or
even detrimental in another situation.
(five scientific disciplines agree)
1. Paleontology provides fossils that reveal the
prehistoric existence of extinct species. As a
result, changes in species and the formation of
new species can be studied.
• Fossil deposits are often among sediment layers.
• Law of superposition states that the deepest
layer of earth contains the oldest specimens.
2. Biogeography uses
geography to describe the
distribution of species.
• Has revealed that
unrelated species in
different regions of the
world look alike when
found in similar
• Provides strong evidence
for the role of natural
selection in evolution.
• Ex. Wallaby (marsupial)
and Rabbit (placental)
• flying squirrel-sugar
3. Embryology is the study
of embryos and their
• Reveals similar stages in
development (ontogeny)
among related species.
• The similarities help
establish evolutionary
relationships (phylogeny).
• Ex. Gill slits and tails are
found in fish, chicken, pig,
and human embryos. These
species are all vertebrates.
Haeckel exaggerated these
Sketches. Not as similar as shown.
“Ontogeny recapitulates
Phylogeny” is not correct.
4. Comparative anatomy
describes three kinds of
structures that contribute
to the identification of
evolutionary relationships
among species.
• Compares external
morphology and internal
Ex. Homologous
structures: are body parts
that are structurally
similar in related species.
Serve different functions
have been modified by
different natural selection
from the common
These homologous structures
are anatomical evidence of
descent with modification.
Modified for grasping, walking,
Swimming, flight…
homologous structures reveal
common ancestry and a pattern of
evolution called DIVERGENT
Ex) Vestigial organs: organs with
no apparent function or purpose
imply evolutionary relationships
to primitive ancestors.
May still be around as an
evolutionary relic.
The structure served a purpose in an
ancient ancestor but no longer
Example: pelvic girdle, hind leg
bones in whales
Tail bone in humans.
Figure 22.17 A transitional fossil linking past and present
Ex) Analagous
Structures are body
parts similar in
function but not in
Not because they have
evolved from a
common ancestor
Instead, because they
evolved these similar
because they evolved
in similar
environments with the
same selection
For example: the finned,
streamlined, fat insulated
bodies of Sharks, Penguins,
& Porpoises are all adaptations
for a life of swimming in the seas.
Humming bird
Hummingbird Moth
5) Molecular Biology examines
the nucleotide and amino acid
sequences of DNA and proteins
from different species.
Closely related species share
higher percentages of sequences
than distantly related species.
All living things share the same
genetic code.
Favors evolution of different
species through modification of
ancestral genetic information.
For example, more than 98% of
the nucleotide sequences in
humans and chimpanzees are
The incorporation of genetics into
evolutionary thinking created a more
comprehensive view of evolution called
neo-Darwinism or the modern synthesis.
You will study evolution from a genetic point
of view in the next chapter.
Historical examples that support
the theory of natural selection
Pesticide resistance
Antibiotic resistance
Industrial Melanism & the peppered moth
Sickle Cell Anemia- Malaria
Humans make breeding
choices among livestock,
crops, dogs, horses, etc.
“drive” evolution.
Cattle breeders, ancient africa
Cabbage, broccoli, brussel sprouts, cauliflower
The Origin of Species was truly
radical for its time
• Darwin waited more than 20 years
to publish.
• Challenged prevailing scientific
• Shook the deepest roots of Western
• Contrasted with the conventional
paradigm of an Earth only a few
thousand years old, populated by
unchanging forms of life which had
been individually made during a
single week by the Creator (along w/
entire universe).
The Origin of Species
• Darwin continued to gather evidence for nearly 20 years.
• Lyell cautioned Darwin to publish before someone beat him to it.
• 1858 Alfred Wallace developed a theory of natural selection and sent
it in a letter to Darwin- it was presented at a conference that year.
• Darwin quickly finished Origin of Species and published it the
following year- 50 years after Lamarck published his ideas .
• Darwin’s theory that natural selection, or “survival
of the fittest,” was the driving force of evolution is
now called Darwinism.
• Descent with modification was used instead of the
word evolution in his book (until the very last page).
Descent with Modification
• Unity in life
• All organisms related through descent from some unknown
ancestor that lived in the remote past
• As the descendants of those ancestral organisms spilled
into various habitats over millions of years, they
accumulated diverse modification, or adaptations, that fit
them to specific ways of life.
Descent With Modification
means evolution