Chapter 27: Evolution of Life
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Evidence of Evolution
Evolution is all the changes that have
occurred in living things since life
began.
The earth is 4.5 billion years old, and
prokaryotes evolved 3.5 billion years
ago.
The eukaryotic cell arose 2.1 billion years
ago, but there was no multicellularity
until 700 million years ago.
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Evolution encompasses common
descent and adaptation to the
environment.
Due to common descent, all living things
share common characteristics: they are
made of cells, take chemicals and
energy from the environment, respond
to external stimuli, reproduce, and
evolve.
Many fields of biology give evidence that
evolution has occurred.
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Fossil Evidence
Fossils are the remains of past life,
usually consisting of hard parts, such
as shells, bones, or teeth.
Most fossils are found embedded in
sedimentary rock.
Sedimentation causes rock formation as
particles accumulate in layers; any
given stratum (layer) is older than the
one above it, and younger than those
below.
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Paleontologists are biologists who study
fossils.
Certain fossils serve as transitional links
between groups.
Such fossils allow paleontologists to
deduce the sequence in which certain
groups evolved (i.e., fishes evolved
before amphibians, which came before
reptiles, which evolved before both
birds and mammals).
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Transitional fossils
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Geological Time Scale
As a result of studying strata across the
earth, scientists have divided earth’s
history into eras, periods, and epochs.
There are two ways to date fossils:
Relative dating provides an approximate
age based on position of the fossil
within rock strata.
Absolute dating uses radioactive isotopes
to measure the amount of radiation left
in a fossil, yielding an actual age.
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Carbon 14 (14C) is the only radioactive
isotope in organic matter.
The amount of radioactivity remaining in
a fossil can be compared with that of a
modern sample to determine the age of
a fossil.
Radioactive isotopes decay at a known
rate; the half-life of a radioactive
isotope is the length of time it takes for
half of the radioactive isotope to
change into another stable element.
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Mass Extinctions
Extinction refers to the death of every
member of a species.
During a mass extinction, a large
percentage of species become extinct
within a relatively short period of time.
Mass extinctions occurred at the ends of
the Ordovician, Devonian, Permean,
Triassic, and Cretaceous periods.
The Cretaceous mass extinction that led
to the demise of dinosaurs was likely
caused by an meteorite hitting the
earth.
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Dinosaurs
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Biogeographical Evidence
Biogeography is the study of the
distribution of plants and animals
throughout the world.
The world’s six biogeographical regions
have their own distinct mix of living
things.
Continental drift refers to the changing
positions of the continents over time.
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Two hundred twenty-five million years ago,
all the present land masses belonged to
one continent (Pangaea).
The distribution of plants and animals is
consistent with continental drift.
Organisms, such as certain seed plant
groups or reptiles, are widely distributed
throughout the world.
Other groups, such as mammals that arose
after the continents broke up, have great
differences in species on different
continents.
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Continental drift
27-13
Anatomical Evidence
Despite dissimilar functions, all
vertebrate forelimbs contain the same
sets of bones – this strongly suggests
common descent.
Structures that are similar because they
are inherited from a common ancestor
are homologous structures.
Analogous structures are used for the
same purpose but are not due to a
common ancestor.
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Bones of vertebrate forelimbs
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Vestigial structures are anatomical
features that are fully developed in one
group but reduced or nonfunctional in
other, similar groups.
Vestigial structures can be explained by
common descent.
The homology shared by vertebrates
extends to their embryological
development; all vertebrates have a
dorsal notochord and paired
pharyngeal pouches at some point.
Evolution modifies existing structures.
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Significance of developmental
similarities
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Biochemical Evidence
All organisms have certain biochemicals
in common.
All use DNA, ATP, and many identical or
nearly identical enzymes.
Organisms use the same triplet code and
the same 20 amino acids in proteins.
This similarity is not necessary, but can
be explained by common descent.
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Significance of biochemical
differences
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Origin of Life
Under conditions present on the
primitive earth, it is possible that a
chemical evolution produced the first
cells.
Chemical evolution refers to the reaction
of inorganic chemicals to produce
simple organic chemicals, that would
later polymerize into macromolecules.
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Once a plasma membrane formed, a
protocell could have come into
existence.
Energy for the chemical reactions could
have come from ultraviolet radiation,
volcanoes, bombardment by comets, or
from oceanic hydrothermal vents.
The early atmosphere lacked oxygen and
also a shield of ozone; it was not until
photosynthesis evolved that oxygen
was present in earth’s atmosphere.
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Origin of the first cell(s)
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Evolution of Small Organic
Molecules
Experiments by Stanley Miller in 1953
tested the hypothesis that small organic
molecules were formed at the ocean’s
surface.
The first atmospheric gases (methane,
ammonia, and hydrogen) were placed
into a closed system, heated, and
circulated past an electric spark to
simulate lightning.
A variety of amino acids and organic
acids formed.
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Miller’s experiment
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Chemical evolution at
hydrothermal vents
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Macromolecules
There are three hypotheses concerning
how small organic molecules could
give rise to macromolecules:
The RNA-first hypothesis suggests that
only the macromolecule RNA was
needed to progress toward the first
cell.
RNA molecules (as ribozymes) can
sometimes be both substrates and
enzymes.
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The protein-first hypothesis, by Sidney
Fox, suggested that amino acids
collected in small puddles, and heat
from the sun caused them to form
proteinoids; when proteinoids were
returned to water, they formed
microspheres and had many properties
of cells.
This hypothesis assumes that DNA came
after proteins.
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The third hypothesis, by Graham CairnsSmith, suggests that clay was helpful in
causing polymerization of both
proteins and nucleic acids at the same
time.
Clay attracts small organic molecules
and contains iron and zinc, which may
have served as inorganic catalysts for
polypeptide formation.
This hypothesis suggests that RNA and
polypeptides arose at the same time.
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The Protocell
Before the first true cell, there would have
been a protocell that had a lipid-protein
membrane and used energy metabolism.
Fox has shown that if lipids are available to
microspheres, the two form a lipidprotein membrane.
Other work by Alexandr Oparin has shown
that concentrated mixtures of
macromolecules form coacervate
droplets that a semipermeable boundary
may form around.
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Protocell anatomy
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The Heterotroph Hypothesis
The protocell was likely a heterotroph,
absorbing small organic molecules
from its environment.
Natural selection would favor cells able
to extract energy from carbohydrates to
transform ADP to ATP.
Fox has shown that microspheres have
some catalytic ability, and Oparin found
coacervates incorporate available
enzymes.
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The True Cell
A true cell is a membrane-bounded
structure that can carry on protein
synthesis to produce the enzymes that
allow DNA to replicate.
It is possible that the sequence of DNA to
RNA to protein developed in stages.
Once the protocells acquired genes that
could replicate, they became cells
capable of reproducing, and evolution
began.
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Process of Evolution
Individuals do not evolve.
As evolution occurs, genetic changes
occur within a population, and these
lead to phenotypic changes that are
commonly seen in that population.
Changes in gene frequencies in
populations over time constitute
microevolution.
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Population Genetics
A population is all the members of a
species occupying a particular area at
the same time; members of a
population reproduce with each other
to produce the next generation.
The various alleles of all the gene loci in
all the members make up the gene pool
for the population.
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Hardy and Weinberg used a binomial
expression to calculate the genotypic
and phenotypic frequencies of a
population:
p2 + 2pq + q2 = 1
This expression is used to determine
gene frequencies at a given time and to
predict gene frequencies in the future.
If reproduction is completely random, the
Hardy-Weinberg equation predicts the
same gene pool frequencies generation
after generation.
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Using the Hardy-Weinberg equation
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The Hardy-Weinberg Law
The Hardy-Weinberg law states that gene
frequencies will stay the same in a
large population over time provided:
1) There are no mutations or mutations
are balanced.
2) There is no genetic drift; changes in
allele frequencies due to chance alone
are insignificant.
3) There is no gene flow – no migration
of individuals in or out of the
population.
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4) Mating is random – individuals pair by
chance and not by choice.
5) There is no selection – no selective
force favors one genotype over
another.
In real life, these conditions are rarely
met, and microevolution, as seen by
changing gene frequencies in HardyWeinberg equilibrium, occurs.
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Microevolution
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Five Agents of Evolutionary
Change
Mutations
Mutations provide new alleles and
therefore underlie all other mechanisms
that produce variation.
Mutations alone are unlikely to cause
evolution; selective agents acting on
heritable variation cause evolution.
The adaptive value of a mutation depends
on the environmental conditions.
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Genetic Drift
Genetic drift refers to changes in allele
frequencies of a gene pool due to
chance; genetic drift has a much larger
effect in a small population.
The founder effect occurs when a few
individuals leave the original population
and begin a new population.
A bottleneck effect is seen when much of a
population is killed due to a natural
disaster, and only a few remaining
individuals are left to begin a new
population.
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Genetic drift
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Founder effect
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Gene Flow
Gene flow is the movement of alleles
between populations, such as when
individuals migrate from one
population to another.
Gene flow between two populations
keeps their gene pools similar and
prevents close adaptation to a local
environment.
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Nonrandom Mating
Nonrandom mating occurs when
individuals pair up, not by chance, but
according to genotypes and
phenotypes.
Inbreeding is an example of nonrandom
mating.
In a human population, inbreeding
increases the frequency of recessive
abnormalities.
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Natural Selection
Natural selection is the process by which
populations become adapted to their
environment.
Evolution by natural selection requires:
Variation
Inheritance of the genetic difference
Differential adaptedness
Differential reproduction.
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Three types of natural selection are
known:
Stabilizing selection – an intermediate
phenotype is favored.
Directional selection – one extreme
phenotype is favored.
Disruptive selection – both extreme
phenotypes are favored over an
intermediate phenotype.
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Stabilizing selection
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Directional selection
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Disruptive selection
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Maintenance of Variation
An example of sickle-cell disease shows
how genetic variation is sometimes
maintained within a population.
Persons with sickle cell disease have
sickle-shaped blood cells, which can
lead to hemorrhage and death.
Persons without a sickle-cell gene are
susceptible to malaria in parts of Africa.
But heterozygotes, with one sickle-cell
gene and one normal gene, have only
minor problems with blood cells and are
resistant to malaria.
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Speciation
A species is a group of interbreeding
subpopulations that share a gene pool
and are isolated reproductively from
other species.
Reproductive isolation can occur due to
premating isolating mechanisms, in
which reproduction is not attempted, or
postmating isolating mechanisms that
do not produce fertile offspring.
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Process of Speciation
Whenever reproductive isolation develops,
speciation has occurred.
Allopatric speciation occurs when a
geographic barrier isolates two
subpopulations from each other; when the
barrier is removed, the two groups are no
longer able to reproduce.
Sympatric speciation occurs when a single
population suddenly becomes two
reproductively isolated groups without
geographic separation.
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Allopatric speciation
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Adaptive Radiation
The evolution of several species of
finches on the Galapagos Islands is an
example of adaptive radiation because
each one has a different way of life.
Adaptive radiation occurs when a few
individuals migrate to a new area, then
natural selection promotes different
feeding habits in different ecological
habitats.
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The Galapagos finches
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The Pace of Speciation
Two hypotheses concern the pace of
speciation:
Phyletic gradualism – suggests that
change is slow and steady within a
lineage before and after a divergence;
few transitional links would exist.
Punctuated equilibrium – suggests that a
period of no change is punctuated by
period of rapid speciation.
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Phyletic gradualism versus
punctuated equilibrium
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Classification
Classification involves the assignment of
species to a hierarchy of categories:
species, genus, family, order, class,
phylum, kingdom, and domain.
Each species has a binomial scientific
name including the genus and species.
Humans are Homo sapiens.
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Five-Kingdom System
The five-kingdom system of classification
is based on structural differences and
also on modes of nutrition among the
eukaryotes.
The five kingdoms include:
Monera (prokaryotes)
Eukaryotic kingdoms of Protista, Fungi,
Plantae, and Animalia.
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Five-kingdom system of
classification
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Three-Domain System
The three-domain system recognizes
three domains: Bacteria, Archaea, and
Eukarya.
This system of classification is based on
biochemical differences that show
there are three vastly different groups
of organisms.
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Three-domain system of
classification
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The three domains of life
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Chapter Summary
The fossil record and biogeography, as
well as comparative anatomy,
development, and biochemistry all give
evidence for evolution.
All organisms have certain biochemicals
in common, and chemical similarities
indicate the degree of relatedness.
The fossil record shows that mass
extinctions occurred several times.
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Chemical evolution likely resulted in the
first cells.
Inorganic chemicals derived from the
primitive atmosphere reacted to form
simple organic molecules.
The RNA-first and protein-first
hypotheses seek to explain how the
first protocell arose.
Eventually, the DNA → RNA → protein
self-replicating system evolved, as did
the first true cell.
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Evolution is a process that involves
changes in gene frequencies in a
population according to HardyWeinberg equilibrium.
Equilibrium is maintained unless
disrupted by mutations, genetic drift,
gene flow, nonrandom mating, or
natural selection.
Speciation requires geographic isolation
followed by reproductive isolation.
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There are two hypotheses regarding the
pace of evolution – phyletic gradualism
and punctuated equilibrium.
Classification involves the assignment of
species to a hierarchy of categories:
species, genus, family, order, class,
phylum, kingdom, and domain.
The three-domain system recognizes
three domains: Bacteria, Archaea, and
Eukarya.
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