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Evolution Review
DNA Pioneers
• Hershey-Chase
• Two American scientists: Alfred Hershey
and Martha Chase
• Collaborated on studying viruses that
infect living organisms
• Bacteriophage: type of virus that infects
bacteria
Hershey-Chase Experiments
DNA Pioneers
• Erwin Chargaff, American
biochemist
• Puzzled by relationship between DNA’s
nucleotides
• Discovered that guanine and cytosine are
nearly equal in any DNA sample; same
went for adenine and thymine
Chargaff’s Rules
Nucleotides
• Are made of
– A simple sugar (Deoxyribose)
– A phosphate group
– A nitrogen base
DNA Structure
• Each side of the double helix is made of
alternating sugar (deoxyribose) and
phosphates
• Each strand is linked to the other by
nitrogen bases
DNA Nitrogen Bases
• Four nitrogen bases make up the “stairs”
of the DNA double helix
• Adenine
• Guanine
• Cytosine
• Thymine
DNA Nitrogen Bases
• Adenine and Guanine are purines
– Made of two rings of carbon and nitrogen
atoms
• Cytosine and Thymine are pyrimidines
– Made of one carbon and nitrogen ring
DNA to RNA to Proteins
• RNA takes the genetic code from the
nucleus to the ribosomes in a process
called transcription.
• RNA differs from DNA in 3 ways:
– It is a single strand (alpha helix)
– It contains the sugar ribose
– It has a different nitrogen base: uracil
• Uracil replaces thymine
A sea voyage helped Darwin frame his
theory of evolution
• Darwin was particularly intrigued by the
geographic distribution of organisms on
the Galápagos Islands, including
– marine iguanas
– giant tortoises.
– finches
– Their features were very like the animals on
the mainland
A sea voyage helped Darwin frame his theory of
evolution
• By the early 1840s, Darwin had composed a long
essay describing the major features of his theory
of evolution by natural selection.
• But he delayed publishing his essay, continued to
compile evidence in support of his hypothesis,
and finally released his essay to the scientific
community when learning of the work of another
British naturalist, Alfred Wallace, who had a
nearly identical hypothesis.
The study of fossils provides strong
evidence for evolution
• Some fossils are not the actual remnants of
organisms.
• The 375-million-year-old fossils in
Figure 13.2B are casts of ammonites,
shelled marine animals related to the
present-day nautilus.
Figure 13.2b
The study of fossils provides strong evidence for
evolution
• The fossil record is the chronicle of evolution
over millions of years of geologic time engraved
in the order in which fossils appear in rock strata.
SCIENTIFIC THINKING: Fossils of transitional
forms support Darwin’s theory of evolution
• Thousands of fossil discoveries have since shed
light on the evolutionary origins of many groups
of plants and animals, including
– the transition of fish to amphibian,
– the origin of birds from a lineage of dinosaurs,
and
– the evolution of mammals from a reptilian
ancestor.
SCIENTIFIC THINKING: Fossils of transitional
forms support Darwin’s theory of evolution
• Whales are cetaceans, a group that also
includes dolphins and porpoises.
– They have forelimbs in the form of flippers but
lack hind limbs.
– If cetaceans evolved from four-legged land
animals, then transitional forms should have
reduced hind limb and pelvic bones.
SCIENTIFIC THINKING: Fossils of transitional
forms support Darwin’s theory of evolution
• Beginning in the late 1970s, paleontologists
unearthed an extraordinary series of transitional
fossils in Pakistan and Egypt.
• The new fossil discoveries were consistent with
the earlier hypothesis, and paleontologists
became more firmly convinced that whales did
indeed arise from a wolf-like carnivore.
Figure 13.3b
Pakicetus
Rodhocetus
Dorudon
Living
cetaceans
Key
Pelvis
Femur
Tibia
Foot
Homologies provide strong evidence for evolution
• Evolution is a process of descent with modification.
– Characteristics present in an ancestral organism are
altered over time by natural selection as its
descendants face different environmental conditions.
– Evolution is a remodeling process.
– Related species can have characteristics that have an
underlying similarity yet function differently.
– Similarity resulting from common ancestry is known
as homology.
13.4 Homologies provide strong evidence for
evolution
• Darwin cited the anatomical similarities among
vertebrate forelimbs as evidence of common
ancestry.
• As Figure 13.4A shows, the same skeletal
elements make up the forelimbs of humans, cats,
whales, and bats, but the functions of these
forelimbs differ.
• Biologists call such anatomical similarities in
different organisms homologous structures.
Figure 13.4a
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
13.5 Homologies indicate patterns of descent that
can be shown on an evolutionary tree
• Homologous structures can be used to
determine the branching sequence of an
evolutionary tree.
• These homologies can include
– anatomical structure and/or
– molecular structure.
Figure 13.5
Each branch point represents the
common ancestor of the lineages
beginning there and to the right of it
Lungfishes
Amniotes
Mammals
2
Tetrapod
limbs
Amnion
Lizards
and snakes
3
4
5
Ostriches
6
Feathers
Hawks and
other birds
Birds
A hatch mark represents
a homologous character
shared by all the groups
to the right of the mark
Crocodiles
Tetrapods
Amphibians
1
Figure 13.14
Frequency of
individuals
Original population
Original
population
Evolved
population
Stabilizing selection
Phenotypes (fur color)
Directional selection
Disruptive selection
The origin of species is the source of
biological diversity
• Microevolution is the change in the gene
pool of a population from one generation to
the next.
• Speciation is the process by which one
species splits into two or more species.
• Each time speciation occurs, the diversity
of life increases.
There are several ways to define a
species
• The word species is from the Latin for “kind” or
“appearance.”
• Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal
definition is not easy and raises questions.
• In many cases, the differences between two
species are obvious. In other cases, the
differences between two species are not so
obvious.
Figure 14.2a-0
There are several ways to
define a species
• How similar are members of the same
species?
– Whereas the individuals of many species
exhibit fairly limited variation in physical
appearance, certain other species—our own,
for example—seem extremely varied.
Figure 14.2b
There are several ways to define a
species
• Reproductive isolation
– prevents genetic exchange (gene flow) and
– maintains a boundary between species.
• But there are some pairs of clearly distinct
species that do occasionally interbreed.
– The resulting offspring are called hybrids.
– An example is the grizzly bear (Ursus arctos)
and the polar bear (Ursus maritimus), whose
hybrid offspring have been called “grolar
bears or pizzly bears.”
Figure 14.2c-0
Grizzly bear
Polar bear
Hybrid “grolar” or “pizzly” bear
VISUALIZING THE CONCEPT:
Reproductive barriers keep species
separate
Reproductive barriers
– serve to isolate the gene pools of species and
– prevent interbreeding.
• Depending on whether they function before or
after zygotes form, reproductive barriers are
categorized as
– prezygotic or
– postzygotic.
Reproductive barriers keep species
separate
• Five types of prezygotic barriers prevent
mating or fertilization between species.
1. In habitat isolation, there is a lack of
opportunity for mates to encounter each
other.
2. In temporal isolation, there is breeding at
different times or seasons.
Figure 14.3-0
PREZYGOTIC BARRIERS
Habitat isolation
(different habitats)
Temporal isolation
(breeding at different times)
Mechanical isolation
(incompatible reproductive parts)
Behavioral isolation
(different courtship rituals)
Gametic isolation
(incompatible gametes)
POSTZYGOTIC BARRIERS
Reduced hybrid vitality
(short-lived hybrids)
Reduced hybrid fertility
(sterile hybrids)
Hybrid breakdown
(fertile hybrids with
sterile offspring)
Reproductive barriers keep species
separate
3. In behavioral isolation, there is failure to send
or receive appropriate signals.
4. In mechanical isolation, there is physical
incompatibility of reproductive parts.
5. In gametic isolation, there is molecular
incompatibility of eggs and sperm or pollen
and stigma.
Figure 14.3-3
Behavioral isolation
(different courtship rituals)
The blue-footed booby
(Sula nebouxii) performs an
elaborate courtship dance.
The masked booby
(Sula dactylatra) performs
a different courtship ritual.
Figure 14.3-4
Mechanical isolation
(physical incompatibility of reproductive parts)
Heliconia latispatha is pollinated
by hummingbirds with short,
straight bills.
Heliconia pogonantha is
pollinated by hummingbirds
with long, curved bills.
Figure 14.3-5
Gametic isolation
(molecular incompatibility of eggs and sperm
or pollen and stigma)
Purple sea urchin
(Strongylocentrotus
purpuratus)
Red sea urchin
(Strongylocentrotus
franciscanus)
Reproductive barriers keep species
separate
• Three types of postzygotic barriers operate
after hybrid zygotes have formed.
1. In reduced hybrid viability, interaction of
parental genes impairs the hybrid’s
development or survival.
2. In reduced hybrid fertility, hybrids are
vigorous but cannot produce viable
offspring.
3. In hybrid breakdown, hybrids are viable and
fertile, but their offspring are feeble or sterile.
Reproductive barriers keep species
separate
• Three types of postzygotic barriers operate
after hybrid zygotes have formed.
1. In reduced hybrid viability, interaction of
parental genes impairs the hybrid’s
development or survival.
2. In reduced hybrid fertility, hybrids are
vigorous but cannot produce viable
offspring.
3. In hybrid breakdown, hybrids are viable and
fertile, but their offspring are feeble or sterile.
Figure 14.3-7
Reduced hybrid fertility
(vigorous hybrids that cannot
produce viable offspring)
A mule is the sterile hybrid
offspring of a horse and a donkey.
In allopatric speciation, geographic
isolation leads to speciation
• A key event in the origin of a new species
is the separation of a population from
other populations of the same species.
– With its gene pool isolated, the splinter
population can follow its own evolutionary
course.
– Changes in allele frequencies caused by
natural selection, genetic drift, and mutation
will not be diluted by alleles entering from
other populations (gene flow).
In allopatric speciation, geographic
isolation leads to speciation
• In allopatric speciation, the initial block to
gene flow may come from a geographic
barrier that isolates a population.
In allopatric speciation, geographic
isolation leads to speciation
• Several geologic processes can isolate populations.
– A mountain range may emerge and gradually split a
population of organisms that can inhabit only
lowlands.
– A large lake may subside until there are several
smaller lakes, isolating certain fish populations.
– Continents themselves can split and move apart.
– Allopatric speciation can also occur when individuals
colonize a remote area and become geographically
isolated from the parent population.
In allopatric speciation, geographic
isolation leads to speciation
• How large must a geographic barrier be to
keep allopatric populations apart?
– The answer depends on the ability of the
organisms to move.
– Birds, mountain lions, and coyotes can easily
cross mountain ranges.
– In contrast, small rodents may find a canyon
or a wide river a formidable barrier. The
Grand Canyon and Colorado River separate
two species of antelope squirrels.
Sympatric speciation takes place
without geographic isolation
• Sympatric speciation occurs when a new species
arises within the same geographic area as its
parent species.
• How can reproductive isolation develop when
members of sympatric populations remain in
contact with each other?
• Gene flow between populations may be reduced
by
– polyploidy,
– habitat differentiation, or
– sexual selection.
Sympatric speciation takes place
without geographic isolation
• Many plant species have originated from
sympatric speciation that occurs when
accidents during cell division result in extra
sets of chromosomes.
• New species formed in this way are
polyploid, in that their cells have more
than two complete sets of chromosomes.
Sympatric speciation takes place
without geographic isolation
• Sympatric speciation can result from
polyploidy
– within a species (by self-fertilization) or
– between two species (by hybridization).