Natural Selection and
Evolution
CHAPTER 16: DARWIN’S THEORY
The eight characteristics of living things?
1. Living things are based on a universal genetic code
2.
3.
4.
5.
6.
7.
8.
(DNA)
Living things grow and develop
Living things respond to their environment (stimulus)
Living things reproduce
Living things maintain a stable internal environment
(homeostasis)
Living things obtain and use material and energy
(metabolism)
Living things are made up of CELLS
Taken as a group, living things evolve over time
Making Inferences and
Forming Hypotheses:
Inference
 A logical
interpretation based
on what scientists
already know.
Hypothesis
 A scientific explanation
for a set of observations
that can be tested in ways
that support or reject it.
 A possible explanation for
what you expect to
happen
 Ex. Plants will grow
toward a source of light.
Scientific Theory
 A well-tested explanation that unifies a
broad range of observations and hypotheses



Explains a broad set of observations about the natural world.
Is supported by evidence and data
Provides a set of principles that can be used to make testable
predictions and hypothesis.
The meaning of the word theory in daily life is different from its
meaning in science
 A scientific theory is NOT a hunch

Cell Division and Reproduction
Asexual Reproduction
 A single parent
Sexual Reproduction
 Fusion of two sex cells – one
from each of two parents
offspring
 Genetically diverse offspring
 Advantage: Simple,
 Advantage: Genetic
efficient, & effective way for
diversity helps ensure
an organism to produce a
survival of species when
large number of offspring.
environment changes
 Disadvantage: may not be  Disadvantage: Process
able to adapt to change
takes a long time
 Genetically identical
Genes and Alleles
Each of the traits Mendel studied was controlled by one gene that
occurred in two contrasting varieties.
 These gene variations produced different expressions, or forms, of
each trait.
 The different forms of a gene are called alleles.
 Dominant = Form of a gene that is expressed even if
present with a contrasting recessive allele.
 Example: BB or Bb
A = dominant allele
 Recessive = Form of a gene (allele) that is only expressed
in the homozygous state.


Example: bb
Homozygous (Purebred) vs.
Heterozygous (Hybrid):
 Homozygous = Organism that has two identical
alleles for a particular trait. (Ex: AA, bb, CC, dd)
 Heterozygous = Organism that has two different
alleles for the same gene. (Ex: Aa, Bb, Cc, Dd)
Genotype vs. Phenotype:
 Genotype: Genetic make-up of an organism (Ex:
Aa, BB, cc)
 Phenotype: Physical characteristics of an
organism (Ex: flower color, eye color)
Types of Cells Affected
 Germ Mutation - affects a reproductive cell
(gamete or sperm/egg)
Does not affect the organism
 Passed to offspring

 Somatic Mutation – affects body cells
(all cells except gametes)

Not passed to offspring
Polygenic Traits
Traits controlled by two or more genes are said to
be polygenic traits. Polygenic means “many
genes.”
 Polygenic traits often show a wide range of
phenotypes.
 The variety of skin color in humans comes about
partly because more than four different genes
probably control this trait.

Segregation

During gamete formation, the alleles for each gene
segregate from each other, so that each gamete carries only
one allele for each gene.
Synapsis and Tetrads
 During
synapsis, the homologuous
chromosomes pair up, forming a structure
called a tetrad, which contains four
chromatids.
Crossing Over
 As homologous chromosomes pair up and form
tetrads, they undergo a process called crossing-over.
 First, the chromatids of the homologous chromosomes
cross over one another.
 Then, the crossed sections of the chromatids are
exchanged.
 Crossing-over is important because it produces new
combinations of alleles in the cell.
Darwin’s Epic Journey




Charles Darwin was born in England on February 12, 1809. He
grew up at a time when the scientific view of the natural world
was shifting dramatically.
Geologists were suggesting that Earth was ancient and had
changed over time, and biologists were suggesting that life on
Earth had also changed.
The process of change over time is called evolution.
Darwin developed a scientific theory of biological evolution
that explains how modern organisms evolved over long periods
of time through descent from common ancestors.
Darwin’s Epic Journey

Darwin was invited to sail on the HMS Beagle’s five-year
voyage mapping the coastline of South America.


Darwin was the ship’s naturalist – he planned to collect
specimens of plants and animals on the voyage.


Left from England, sailed around South America, across Pacific,
around Australia and Africa, and back to England
Most famous for observations made at Galapagos Islands
No one knew it, but this would be one of the most important
scientific voyages in history.
16-1 Darwin’s Voyage of Discovery
Darwin noted 3 patterns of biodiversity:
1. Species vary globally
2. Species vary locally
3. Species vary over time
1. Species vary globally
Different, yet ecologically similar animal species
inhabited separate, but ecologically similar,
habitats around the globe
Ex) flightless birds: rheas, ostriches and emus
Rhea
Native to S.America
Ostrich
Native to Africa
Largest of 3
Emu
Native to Australia
Slightly Larger than Rhea
2. Species vary locally
Different, yet related animal species occupied different
habitats within a local area
(tortoise shell shape - Galapagos)
3. Species Vary Over Time


Darwin also collected fossils, which are the preserved remains
or traces of ancient organisms.
Darwin noticed that some fossils of extinct animals were similar
to living species.
Fossils found in Towamencin
 While construction crews from Allan A. Myers (AAM) have
been building stormwater basins and doing excavations by
Exit 31 in Lansdale as part of the ongoing Northeast
Extension road-widening project, geologists from Kutztown
University have uncovered something even bigger hidden
among the massive slabs of rock.
 Fossils Found in Turnpike Excavation article on The
Reporter
 Fossils Found in Turnpike Excavation pictures
Lamarck’s Theory of Evolution
 Inheritance of Acquired Traits
 Acquired traits could be inherited
 For example: If you spent your life lifting weights, then
your children would inherit large muscles.
Lamarck’s Theory of Evolution
 Evaluating Lamarck’s
Theory



He was wrong in several
ways
He didn’t know how traits
were inherited
He did realize that
organisms adapt to their
environment.
Artificial Selection

To find an explanation for change in nature, Darwin studied
change produced by plant and animal breeders.

Breeders knew that individual organisms vary, and that some
of this variation could be passed from parents to offspring and
used to improve crops and livestock – selective breeding.


For example, farmers would select for breeding only trees that
produced the largest fruit or cows that produced the most milk.
Over time, this selective breeding would produce trees with
even bigger fruit and cows that gave even more milk.
Artificial Selection


Darwin called this selective breeding process artificial
selection, a process in which nature provides the variations,
and humans select those they find useful.
Darwin put artificial selection to the test by raising and breeding
plants and fancy pigeon varieties.
Darwin's pigeons described by Randal Keynes (7:49)
BBC Darwin's Pigeons (2:18)
Artificial Selection

Darwin had no idea how heredity worked or what caused
heritable variation, but he did know that variation occurs in wild
species as well as in domesticated plants and animals.

Before Darwin, scientists thought variations among individuals in
nature were simply minor defects.

Darwin recognized that natural variation was very important
because it provided the raw material for evolution.


Natural variation = differences among individuals of a species
When Darwin published his scientific explanation for evolution,
it changed the way people understood the living world.
Population Growth
 Malthus observed that babies were
being born faster than people were
dying.
 If population continued to grow
there would be insufficient food
and space.
 In plants or animals most of the
offspring die
 Only those who survive can
reproduce
 What determines which ones will
survive and reproduce?
The Struggle for Existence

After reading Malthus, Darwin realized that if more individuals
are produced than can survive, members of a population must
compete to obtain food, living space, and other limited
necessities of life.

Darwin described this as the struggle for existence.
Variation and Adaptation

Darwin knew that individuals have natural variations among
their heritable traits, and he hypothesized that some of those
variants are better suited to life in their environment than
others.

Any heritable characteristic that
increases an organism’s ability to
survive and reproduce in its
environment is called an
adaptation.
Three types of adaptations that involve
body parts or structures :
1. Structural – physical features of an
organism
•Ex: long tongue to get food, sharp
teeth, mimicry, camouflage
2. Behavioral – actions an organism
takes
•Ex: migration, tracking prey,
storing nuts, growing towards light
3. Physiological – functioning or
biochemical processes
•Ex: venom, ink of octopus, protein
in web, respiration rate, digestive
enzyme, blood clotting
Mimicry
 Scarlet king snake exhibits mimicry - an adaptation in which
an organism copies, or mimics, a more dangerous organism.
 Although the scarlet king snake is harmless, it looks like the
poisonous eastern coral snake, so predators avoid it, too.
 Non poisonous snakes will also rattle their tale and flatten
their head to look poisonous to a predator.
Mimicry
 Monarch butterfly - is
toxic and very nasty to eat.

Its bright orange coloration is a
warning to birds to leave it alone.
 Non-toxic viceroy
butterfly has developed
colors and wing patterns that
are very similar to those of
the monarch

most birds won’t take a chance by
taste-testing it
Camouflage
 An adaptation that allows an organism to
blend into its background and avoid
predation.
Variations vs. Adaptations
• Variations – differences that exist within a population
that may have no effect on fitness
• Ex: length of your thumb
• Adaptations – a variation that all members of a
population have inherited because that trait improves
fitness
• Ex: an opposable thumbs
Sources of variation
• Sources of variation:
•
Mutations – individual genes change
• Ex: ATC  AGC
•
Events during meiosis – during
meiosis, chromosomes get mixed up
(crossing over) in making eggs &
sperm
•
Random fusion of gametes –
which sperm fertilizes which egg –
determined by chance
Survival of the Fittest
 According to Darwin, differences in adaptations affect an
individual’s fitness.
 Fitness describes how well an organism can survive and
reproduce in its environment.


Individuals with adaptations that are well-suited to their environment
can survive and reproduce and are said to have high fitness.
Individuals with characteristics that are not well-suited to their
environment either die without reproducing or leave few offspring and
are said to have low fitness.
 This difference in rates of survival and reproduction is called
survival of the fittest.

In evolutionary terms,
survival means
reproducing and passing
adaptations on to the next
generation.
Natural Selection

Darwin named his mechanism for evolution natural selection
because of its similarities to artificial selection.

Natural selection is the process by which organisms with
variations most suited to their local environment survive and
leave more offspring.

In natural selection, the environment — not a farmer or animal
breeder — influences fitness.
Natural Selection
 Well-adapted individuals survive and reproduce.
 From generation to generation, populations continue to change as
they become better adapted, or as their environment changes.
 Natural selection acts only on inherited traits because those are
the only characteristics that parents can pass on to their offspring.
Natural Selection

Natural selection does not make organisms “better.”

Adaptations don’t have to be perfect—just good enough to enable
an organism to pass its genes to the next generation.

Natural selection also doesn’t move in a fixed
direction. There is no one, perfect way of doing
something.

Natural selection is simply a process that enables
organisms to survive and reproduce in a local
environment.
Natural Selection

If local environmental conditions change, some traits that were
once adaptive may no longer be useful, and different traits may
become adaptive.

If environmental conditions change faster than a species can
adapt to those changes, the species may become extinct.
Common Descent
 Natural selection depends on the ability of organisms to
reproduce and leave descendants. Every organism alive
today is descended from parents who survived and
reproduced.
 Darwin proposed that, over many generations, adaptation
could cause successful species to evolve into new species.
 He also proposed that living species are descended, with
modification, from common ancestors—an idea called
descent with modification.
 According to the principle of common descent, all
species—living and extinct—are descended from ancient
common ancestors.
16-3 Darwin Presents His Case
•Darwin published his findings in 1859 in a book
entitled The Origin of Species by Means of Natural
Selection.
•He was motivated to publish his book in 1859 because
Alfred Wallace had independently come up with the
same conclusions
The Age of Earth

Evolution takes a long time. If life has evolved, then Earth
must be very old.

Hutton and Lyell argued that Earth was indeed very old, but
technology in their day couldn’t determine just how old.

Geologists now use radioactivity to establish the age of certain
rocks and fossils. Radioactive dating indicates that Earth is
~4.5 billion years old - plenty of time for evolution by
natural selection to take place.
16.4 Evidence of Evolution
1. Fossils
2. Anatomy
• Analogous structures
• Homologous structures
• Vestigial structures
3. Embryology
4. Biochemistry
5. Direct observation
1. Fossil Evidence
• Fossils - the remains of past
life
•
Ex: shells, bones, teeth,
imprints
•
Tell us - age, habitat, diet, &
lifestyle of organisms.
•
Record is incomplete – many
organisms leave no fossils
behind
2. Anatomical Evidence –
Homologous Structures
 Darwin proposed that animals with similar structures evolved from
a common ancestor with a basic version of that structure.
 Structures that are shared by related species and that have been
inherited from a common ancestor are called homologous
structures.
 Homologous structures are similar in structure because
they develop from same tissues early in development

may or may not have different functions.
Bones of
vertebrate
forelimbs
Homologous Structures
2. Anatomical Evidence –
Analogous Structures


The clue to common descent is common structure, not
common function. A bird’s wing and a horse’s front limb have
different functions but similar structures (homologous
structures).
Body parts that share a common function, but not structure,
are called analogous structures. Analogous structures are
used for the same purpose but are not due to a common
ancestor. The wing of a bee and the wing of a bird are
analogous structures.
Analogous vs. Homologous Structures
Vestigial Structures


Not all homologous structures have
important functions.
Vestigial structures are inherited from
ancestors, but have lost much or all of their
original function due to different selection
pressures acting on the descendant.


The hipbones of bottlenose dolphins are
vestigial structures. In their ancestors,
hipbones played a role in terrestrial
locomotion. However, as the dolphin lineage
adapted to life at sea, this function was lost.
The human tailbone and appendix are
vestigial structures.
Cormorant –
flightless bird
Vestigial Structures
Vestigial Structures
Vesitgial Structure Video
in dropbox
Evolution Evidence
3. Embryology

Similar patterns of embryological development provide further
evidence that organisms have descended from a common
ancestor.

the more similar the embryos are at certain stages of
development, the more closely related they are thought to
be.
Which one is a…
Chicken, Fish, Human, Rabbit, Salamander, Turtle??????
Embryology - the more similar the embryos are at certain stages
of development, the more closely related they are thought to be
4. Biochemical Evidence
• Universal genetic code – organisms use the same
triplet code and the same 20 amino acids in
proteins
• All organisms have certain organic molecules in
common.
1. Hemoglobin - carries oxygen in blood
2. Cytochrome c - protein for cell respiration
found in almost all living cells
3. HOX genes – control development
Genes Control Body Segmentation PBS - 13:45
Significance of biochemical
differences
5. Direct Observation
• Direct observation – we’ve
seen evolution occur in cases
like:
• The Grant’s studies of
finches on Daphne Major in
the Galapagos (35 yrs)
• Bacteria becoming resistant
to antibiotics
• Insects that become
resistant to pesticides