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Principles of Evolution
Chapter 14
What is Evolution?
• Evolution is the change in inheritable
traits in a population over generations.
• Change in traits is caused by changes in
the genes (in DNA) that code for those
traits.
• Natural selection tends to increase
favorable traits in a population and
decrease unfavorable traits.
What Evolution is NOT
• Many people argue against evolution
based on statements they have heard.
Yet many of those statements are ideas
that are not part of evolutionary theory.
Evolution is NOT about things
getting “better and better” or
more and more complex.
Selection is about survival and reproduction.
Organisms with traits that help them survive long
enough to reproduce will pass their genes on.
Evolution is NOT about
organisms “trying” to adapt.
“Trying” does not change genes. Only inheritable
traits already in a population can be passed on.
Evolution does NOT give
organisms what they “need.”
If that were true, there would never be deaths or
species extinctions. If individuals have traits that
help them survive environmental change, they
may pass those traits on while others die.
Evolution does NOT mean
that individuals in a
population slowly change.
Change happens to populations over generations as those
with unfavorable traits are weeded out and those with
favorable traits are more likely to reproduce.
Evolution is NOT a theory
of how life began.
Evolution explains how living
organisms change over generations.
Evolution is a Scientific Theory
• Theories are evidencesupported explanations for
natural phenomenon.
• To exist, theories must have
abundant evidence from many
well-tested hypotheses.
• Theories grow from
evidence! Theorizing
without evidence is not
science!
Early Ideas
• While many people associate evolution
with Darwin, other scientists were talking
about organic change long before
Darwin published his theory of natural
selection.
Aristotle (384-322 B.C.)
viewed the living world as
fixed and unchanging. All
living things could be
arranged in a ladder from
inferior to superior, with
humans on top. Aristotle’s
views influenced thought for
over 2000 years.
Curiosity about fossils around the 16th century led to
new questions.
Were fossils the
remains of
organisms?
If they were, what
did this say about
species being
unchanging?
George Louis LeClerk, Compte de
Buffon (1707 - 1788)
sought a naturalistic explanation for
fossils. Buffon devised one of the first
theories of organic change. However,
Buffon’s ideas were grounded in logic
rather than in evidence from the
natural world, as much science was at
the time.
Buffon suggested that organisms had a basic form, the
“interior mold,” which could be shaped by the
environment.
Buffon thought that the “internal mold” was caused by particles in
an organism that shaped the organism while it was an embryo. All
animals, he said, had a basic form: horse-like, dog-like, cat-like,
and so on.
In Europe, horses are brown
and horse-shaped because of
the environment.
Move the horses to Africa, and
the environment rearranges
particles in embryos when the
horses are bred.
Over many generations, the
particles reshape horses
into zebras.
James Hutton
(1726 - 1797)
Charles Lyell
(1797 - 1875)
These two naturalists are best known for the theory of
Uniformitarianism, proposed by Hutton and expanded by
Lyell. Uniformitarianism states that the physical laws that
now shape the earth have always done so, and that the
past can be understood by studying the present. Both
suggested that the Earth is very old.
Georges Cuvier (1769 - 1832)
was a student of Buffon, but
disagreed about organic change.
Cuvier was an anatomist. He pointed
out that animals had features that
helped them survive. If those
features changed, wouldn’t that be
harmful?
Cuvier studied fossils and identified
several new animals based on
fossils. However, he had his own
explanation for fossils.
Cuvier doubted
Uniformitarianism and used
Catastrophism to explain the
fossils of the Paris basin. He
proposed one natural origin
of life, followed by
unpredictable catastrophes -perhaps beyond known
physical laws. After each
catastrophe, organisms from
other places moved in.
Jean-Baptiste de Lamarck (1744 1829)
was also a student of Buffon. Like Buffon,
Lamarck sought a naturalistic explanation
for the diversity of modern organisms and
the animals seen in the fossil record.
Lamarck used the theory of Inheritance of
Acquired Characteristics, a widely-held
belief at the time, to explain change in
organisms. Blacksmiths, for example, were
thought to pass their well-developed right
arms on to their sons.
Lamarck’s Organic Theory of Development
Organisms are shaped by their
environment.
Change is goal-directed -organisms have an internal drive
to become more and more
complex.
Use and disuse of parts causes
change that is passed on from
generation to generation.
Charles Darwin
(1809 - 1882)
and
Alfred Russell
Wallace
(1823 - 1913)
Darwin and Wallace, working separately, devised a model
for organic change that was based on their observations of
living and fossil organisms over many years. Natural
Selection was the first evidence-based mechanism for
evolution that was proposed.
Darwin and Wallace observed that change did not
happen equally to all members of a population, as
Buffon and Lamarck thought.
Instead, Darwin and Wallace proposed that selection
happens each generation. Some individuals have traits that
help them survive and reproduce, while others have traits
that put them at a disadvantage.
Over generations, the number of individuals with favorable
traits increases while those with unfavorable traits
decrease.
Evidence for Evolution
• Both Darwin and Wallace were convinced
that evolution -- change in the traits of a
population over time -- did happen.
• Darwin spent over 20 years collecting
evidence and reading research by others
before he wrote On the Origin of Species. He
found many features of organisms that
suggested they were related by ancestry.
Fossils
Fossils showed that
species existed in
the past that no
longer exist today,
and species exist
today that did not
exist in the past.
Fossils
Smith
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•
•
Rocks found in layers
Fossils in layers are consistent
Law of superposition
Lyell
•
•
•
•
Natural forces produce layers of rock
Slow repeated process
Uniformitarianism
The Earth is very old
Fossils
- Slow natural geologic processes
- Dating of rocks by elemental
analysis
Fossils
In many groups of organisms, fossils demonstrated
change from one form to another over long periods
of time.
Homologous Structures
Four-limbed vertebrate animals all have the same bones in
the forelimbs, but the bones are shaped differently. In
modern terms, this suggests a shared set of genes (for the
bones) arising from common ancestry.
Vestigial Structures
Why do big snakes,
like boas, have tiny
little hind limbs?
Why do whales have
hip bones?
Vestigial Structures
Can you wiggle your ears?
(Without using your hands!)
Vestigial Structures
Hips in whales, snakes.
Ear-wiggling
muscles in
humans.
Vestigial organs are greatly reduced in size and function
compared with other members of a group.
Analogous Structures
The influence of the environment is seen in analogous
structures, where less-related species living in similar
environments have similar body forms, though often
involving different body parts. From variations in a
population, the environment selects those that best help
an organisms survive in that environment.
Analogous Structures
Echidna
(Australia)
Porcupine
(North America)
Spiny Euphorbia
(South Africa)
Hedgehog
(Europe)
Golden Barrel Cactus
(North America)
Embryological Similarities
Are these organisms very different looking?
How about now?
Lemur
Pig
Human
Similar structures and similar developmental processes in
embryos demonstrate a shared set of genes between
groups of organisms, suggesting common ancestry.
Note: Humans do not become fish or grow gills during
development. We do share several embryological
features with other vertebrate animals.
Biochemical and Genetic Analysis
All cells use:
• DNA as heritable genetic information
• RNA and ribosomes to make proteins
• roughly same 20 amino acids to build
proteins
• ATP to carry energy
DNA to Expressed genes
Of course, molecular evidence wasn’t
available to Darwin. Publication of The
Origin inspired scientists to figure out how
heredity worked!
Biochemical and Genetic Analysis
Darwin inferred
shared ancestry from
shared traits.
Modern analysis of
DNA demonstrates
shared genes for
many traits between
related species,
suggesting descent
from a common
ancestor.
Mice and humans both have the gene
for the protein Cytochrome c. Small
differences show changes over time
as the two groups diverged.
Natural Selection
• Evidence from the natural world showed
that change happens to organisms over
time.
• The important question was how?
Darwin and Wallace’s Theory of Natural
Selection provided a natural mechanism
for change.
Natural Selection
Variation
Within any population,
individuals differ from
one another in many
ways.
Natural Selection
Inheritance
Some differences
between
individuals are
inheritable, and
can be passed
from parent to
offspring.
X
Natural Selection
Differences in
survival and
reproduction
In most populations,
more young are born
than can survive. Many
of the young will die. A
few will live long
enough to reproduce.
Natural Selection
Success is not
random
While random accidents
do happen, an
individual’s survival
depends mainly on the
individual’s traits. Those
that live and reproduce
may pass their traits to
the next generation.
Natural Selection
Selection favors organisms that
are best adapted to the
environment.
Selection does not produce
new traits, nor does it make
organisms “perfect.” Selection
is all about survival, and those
organisms with traits that help
them survive are more likely to
reproduce.
Evidence for Selection
• In order to construct his theory, Darwin
had to gather and examine evidence to
discover whether selection happens in
nature.
• Research continues to refine our ideas of
how selection happens and what effects
it has on species.
Artificial Selection
Darwin knew from his studies of pigeon breeding that
selection can change organisms, sometimes rapidly.
Human selection created many breeds of dogs from wild
dogs or wolves as humans chose to breed those
animals with traits that the humans wanted.
Selection acts on variations
Studies have shown that
guppies living in ponds
with few predators are
more brightly colored than
those living with many
predators.
In both populations there are guppies with genes for
bright colors. Why are dull colors favored in ponds with
many predators? Why are bright colors favored in ponds
with few predators?
Selection does not cause new variations.
Studies on Anole lizards showed
that long, fast-moving legs were
favored where trees had thicker
branches, while shorter, more
agile legs were favored where
trees were flimsier.
If lizards from places where trees had thin branches were
moved to a place where trees had thick branches, those
with the longest legs survived better. Each generation had
more individuals with longer legs because the genes for
longer legs were already in the population.
Recap
• Evolution is the change in inherited traits
in a population over generations.
• Fossils, molecular data, and other
evidence shows that the genetics of a
population do change over time.
• Natural Selection, one mechanism for
change, is well-supported by evidence.