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Evolution
What is evolution?
• The process by which modern organisms have
descended from ancient organisms.
• In other words, species are not constant, they change
over time.
Paradoxophyla palmata, a narrow-headed frog native to
Madagascar. The frog's brown and yellow coloring, as well
as its rough texture, allow it to blend in with the mud and
tree trunks in its environment.
tartan hawkfish: The fish's striking coloration allows it to
blend in with these bright gorgonian fans.
cryptic frog - This species has
developed a coloring, texture and form
that are similar to the leaves found in
its environment.
Evidence for evolution
• Fossils
• Embryo similarities
• Body structure similarities
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Homologous Structures
Vestigial Structures
Convergent Evolution
Analogous Structures
• Molecular evidence
I. Fossils
• Fossils are the preserved remains of ancient
organisms.
How are fossils formed?
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Preservation in sap
Mineral replacement
Preservation in ice
Traces e.g. footprints
Molds
• Fossils demonstrate the existence of intermediate
forms of species, thus demonstrating evolution.
• The fossil record
demonstrates that
more primitive
species preceded
more recent
species
• e.g. fish precede
amphibians,
which precede
reptiles, which
precede
mammals.
Dating Fossils
• Relative dating is the
method of dating fossil
specimens relative to
other fossils in in
different rock layers.
• Older fossil are in
deeper rock strata.
Radiometric Dating
• Radiometric dating is
the method of dating
fossils based upon rates
of the decay of
radioactive elements.
Radioactive Parent
Stable Daughter
Potassium 40
Argon 40
Rubidium 87
Strontium 87
Thorium 232
Lead 208
Uranium 235
Lead 207
Uranium 238
Lead 206
Carbon 14
Nitrogen 14
Half-Life’s
• Half-life’s determine how long it takes for a radioactive
element to decay into a stable daughter element.
• A half-life is defined as the amount of time it takes for ½
of the atoms in a radioactive element to decay.
• For example, the half-life of uranium238 is 4.5 BY.
• The half-life of potassium40 is 1.3 BY
• And the half-life of carbon14 is about 5’600 years.
II. Embryo Similarities
• Although certain adult organisms may be very
different from each other, a comparison of the
early stages of their embryonic development may
show similarities that suggest a common ancestry.
• For example, the early embryos of fish, birds, pigs
and humans closely resemble one another.
Note the presence of gill slits and a fish-like tail in all
of the embryos.
III. Body Structure Similarities
• Anatomical similarities between different species
suggest evolution from a common ancestor with
modification.
• If organisms in an area evolve to have
structures that are best suited for
survival in that environment, then it
makes sense that different species in
that area will have similar structures.
Convergent Evolution: two unrelated species have
similar form
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Example: a whale and a shark.
Example : different types of trees in the rainforest will have
similar shaped leaves (they come to a point) to allow water to
flow off of them.
Analogous structures: Structurally different, but
used for similar functions.
-Example: the wings of a fly and the wings of a
bird
Homologous structures: Body parts in different
organisms that have similar bones and similar
arrangements of muscles, but do not necessarily
serve the same function.
• Vestigial organs are remains of a structure that was
functional in some ancestor but is no longer
functional in the organism in question.
• For example, humans have a tail bone (the coccyx)
but no tail. What else do humans have that we don’t
need?
A vestigial structure in the whale is the pelvic bone
Whale Evolution
A reconstruction of an early close cousin of whales.
IV. Molecular Evidence
• DNA, RNA, amino acids and proteins have all
been used to determine evolutionary relationships
between organisms.
• For example, differences between amino acid
sequences in the protein hemoglobin in various
species can be used to construct an “evolutionary
tree”.
• The greater the differences in amino acids, the
more distant two species are.
• The numbers represent
the number of amino
acid differences in the
hemoglobin of humans
and the hemoglobin’s of
the other species.
Human Hemoglobin (beta)
Gorilla
Gibbon
0
1
2
Rhesus monkey
Dog
Horse, cow
8
15
25
Mouse
Gray kangaroo
Chicken
27
38
45
Frog
Lamprey
67
125
Sea slug (a mollusk)
127
Soybean (leghemoglobin) 124
A phylogenetic "tree of life"
Theories of Evolution
• The first comprehensive
theory of evolution was
proposed by Jean Lamarck
in 1809.
• Lamarck believed that
evolution was driven by an
innate tendency toward greater
and greater complexity as
organisms became perfectly Jean Lamarck (1744-1829)
adapted to their environments.
Lamarcks two part theory:
• A. Theory of use and
disuse, which stated
that those parts of the
body that were used
extensively would
become larger and
stronger, whereas those
parts not used would
waste away.
• B. The second part of
Lamarck’s theory was
called inheritance by
acquired characteristics.
• According to this
concept, the modification
an organism acquired in
its lifetime could be
passed on to its offspring.
• The classic example of
this is the giraffes long
neck.
• Can you think of any reasons why Lamarck's
theory is flawed?
Weismann’s experiment
• In the 1880’s Weismann
challenged Lamarck’s
theory.
• Weismann cut the tails
off 22 consecutive
generations of mice.
• Even though 22
generations of mice could
not use their tails, what
was true about the 23rd
generation?
August Weissmann
• Who was responsible for
articulating a theory that
explained how evolution
actually worked?
• Charles Darwin
• In 1859, Darwin published a
book entitled The Origin of
Species by Means of Natural
Selection.
• Two themes in this famous book were:
– A. Common descent: which meant that all species
have descended from a single common ancestor.
– B. Adaptation: which refers to modifications that
enable organisms to be better suited to their
environment, that is better able to survive and
reproduce.
The Darwinian Revolution
• Charles Darwin, born in
1809 in Shrewsbury
England, came from a
family of doctors. Having
studied medicine from
1825 to 1828 at
Edinburgh University he
abandoned his medical
pursuit to study theology
instead. A mediocre
student, he was more
interested in botany and
Charles Darwin
geology.
(1809-1882)
• One of the most
important influences
that helped shape
Darwin’s view of life
was his voyage
aboard the HMS
Beagle (1831-1836).
One of the most important visits Darwin made on
his voyage was to the Galapagos Islands.
• Although, many of the
animals on the
Galapagos resembled
mainland species, they
were unique.
• Darwin puzzled over how
such species could colonize the
islands and become different
from the mainland species.
Darwin’s Finches
Among the thousands of specimens Darwin collected,
were 13 species of finches. The finches were unique in
their beak adaptations.
• Darwin would only
later propose that the
Galapagos finches
evolved on the islands
from a single species of
finch from mainland
South America.
Charles Lyell’s influence
• While on board the Beagle,
Darwin read Charles Lyell’s
Principles of Geology.
•In the book, Lyell made the
argument that the Earth was
very old and that it’s
landscape changed slowly
and gradually.
• By acknowledging the Earth was indeed very
old and constantly changing, Darwin recognized
that organisms had to slowly adapt to these
changes. That is organisms had to evolve.
The influence of Thomas Malthus
• In An Essay on the Principle of
Population, written in 1798,
Thomas Malthus predicted that the
human demand for food would
inevitably surpass its supply and
many people would die of
starvation.
• This prediction was based on the
idea that human population
increased at a exponential rate ( 24-8-16…)while the food supply
grew at the slower linear rate (1-23-4-5…).
• Darwin applied Malthus’ idea to animals in
nature and argued that since most animals overreproduce, limited food supplies causes a
“struggle for existence”.
• That is, there is competition for resources, so
that only the best adapted survive to reproduce.
The long delay…
• Upon returning to Great Britain in 1836, Darwin
began to ponder over how the many unique
species adaptations he had observed could have
originated.
• By the early 1840’s, Darwin had worked out the
major features of his theory of natural selection
as the mechanism of evolution.
• Chronic illness and perhaps fear of criticism
from the clergy however, had prevented Darwin
from publishing.
Competition from another naturalist…….
• Then in 1858, Darwin
received a letter from a
young naturalist called
Alfred Wallace. The letter
was accompanied by a
theory of evolution.
Alfred Russel Wallace (1823-1913)
• This encouraged Darwin to publish his book,
The Origin of Species.
• Darwin published in 1859.
• In his book, Darwin avoids using the word
evolution until the last page.
The only illustration in Darwin’s book
• The theory outlined in Darwin’s book was called
natural selection.
• Natural selection occurs when there are differences
in individuals abilities to survive and reproduce
based upon inheritable traits.
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How does evolution work according to Darwin?
The Theory of Natural Selection:
More offspring are produced than actually survive
due to limited resources (Malthus).
This causes a “struggle for existence”.
Survival is not random, but depends on hereditary
factors. Those individuals with favorable inheritable
traits will survive and reproduce. Those with less
favorable inheritable traits will be eliminated.
This will lead to a gradual change in the population,
with favorable hereditary variations accumulating
over time i.e. the species will change.
“I have called this principle, by which each slight
variation, if useful, is preserved, by the term
Natural Selection”.
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What have we learned since Darwin?
The modern theory of evolution recognizes gene
mutations (point and frame-shift mutations) and
how they introduce inheritable variations into a
population.
Occasionally, a mutation is advantageous and will
be favored by natural selection.
An individual born with an advantageous mutation
will survive and reproduce better than its
competitors.
In time, the new mutation will spread through the
population and the species will evolve.
Global warming threatens
millions of species
• A major international study has warned that
global warming may drive 25% of land
animals and plants to the edge of extinction
by 2050 (NewScientist).
• Why can’t species adapt to increased global
temperatures?
Observed cases of natural selection
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Peppered moth (Industrial Melanism)
Insects resistance to insecticides
Bacterial resistance to antibiotics
Increased frequency of sickle cell anemia in
Africans.
Case Study I: Industrial Melanism
• In England, there are two varieties of the peppered
moth (Biston betularia), a dark variety and a light
variety.
• Prior to the industrial revolution, there was a much
higher frequency of the light variety of the peppered
moth, which, were adapted to the light colored lichen on
tree bark.
• However, industrial pollution in the 1800’s began to kill
the lichen, turning the tree bark into a dark color.
• Now, the number of dark variety of peppered moth
increased i.e. were naturally selected.
Case study II: Resistance to Insecticides
Suppose that there is a crop infested by a large
population of insects. Note the genetic variation that
exists.
Insecticides are sprayed in order to protect the crop.
Note how some insects have the inherent ability to be
resistant to the insecticide.
Those insects with inheritable traits that enable them to
be resistant to the insecticide survive and reproduce.
In time, the insecticide is no longer effective. The
insects have evolved by natural selection.
Case study III: Bacterial resistance to antibiotics
Staphylococcus aureus
• In 1943, penicillin was introduced as an antibiotic to
protect against Staphylococcus infections.
• By 1946, a number of strains of Staphylococcus
demonstrated resistance to penicillin.
• Today, as many as 80% of Staphylococcus aureus
are resistant to penicillin.
a. Gene for antibiotic resistance in
plasmid
b. The antibiotic
gene/plasmid is replicated
and transferred to another
bacterial thereby
conferring antibiotic
resistance to it.
• In a population of bacteria, there may be a few
individuals with mutant genes that provide antibiotic
resistance, these bacteria will survive and reproduce.
• Natural selection will favor these bacteria.
Case study IV: Sickle Cell Anemia
• Read: Recall that sickle cell
anemia is a fatal disease
resulting from homozygous
recessive alleles which code
for a part of hemoglobin, the
oxygen transporting molecule
in human blood
• Despite the lethality of
the allele, it occurs at
frequencies as high as
40% in some parts of
tropical Africa. (By
contrast it occurs at less
than 5% in African
Americans and 0.1% in
Caucasian Americans)
• Since those people who are
heterozygous for sickle cell are
protected against malaria, natural
selection has favored those who are
carriers in areas where there is
malaria. (like Africa)
Speciation
Speciation
• How are new species formed?
• In order for a new species to form, one population
must break away from another and be
reproductively isolated.
• In other words, genes can’t be exchanged between
both populations.
• Over time, the populations will experience different
mutations and be subject to different kinds of natural
selection.
• When this happens, the populations can no longer
interbreed. A new species has formed.
• How are two populations reproductively isolated?
• Two populations may be isolated by the formation
of mountains, rivers, or land masses separating
(Pangea).
Continental Drift
• The idea was thought
up by Alfred Wegener
in the early 1900’s.
• It was based upon the
observation that
continental coastlines
often match up.
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BIOL 1010 – Ch 25
Continental Drift and Evolution
New World and Old
World monkeys are
• New World
and Old
classified
into two
World monkeys are
distinct groups.
taxonomically
groups;
The different
ancestors
to these
• These
lineages
two
groups
separated
afterevolved
Pangeaseparately
broke up.
on different
continents after
Pangea broke up
Fig 34.35
24
BIOL 1010 – Ch 25
• When the Grand Canyon formed, two related
species of squirrels evolved.
Can you explain how different species of Darwin’s
Finches evolved now?
Chance events can affect the
evolution of species.
65 MYA a meteor five miles wide struck the earth
28
BIOL 1010 – Ch 25
• As a result of this impact, all the dinosaurs became
extinct as well as many other species including 50%
of all marine organisms.
Would primates have evolved?
• The mammals were a small,
specialized group of
organisms during the
Cretaceous
• The big mammal divergence
occurred after the dinosaurs
went extinct
• Mammals probably would
not have diverged, producing
primates (and humans!) if the
meteor had not struck the
planet!
29
BIOL 1010 – Ch 25
Fig 25.19
The Evolution of Populations
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Populations evolve, individuals don’t!
Although natural selection acts on individuals, by
way of differential survival and reproduction,
individuals do not evolve.
Biological evolution is only evident by examining
the impact of natural selection on a population of
individuals.
Technically, evolution results from the change in
gene frequencies within a population overtime.
Therefore, in order to understand evolution, it
would be important to describe those events that
change gene frequencies in a population.
Darwin’s dilemma
• Although Mendel had written to Darwin explaining
his discoveries for heredity, Darwin apparently
never read Mendel’s thesis.
• Indeed, natural selection requires a hereditary
process that Darwin could not explain.
Mr. Darwin
still hasn’t
written back!
A major flaw in my
theory is explaining
how adaptive
inheritable traits
are passed on.
Population Genetics
• When Mendel’s laws were rediscovered in the early
20th century, geneticists were perplexed about how
Mendel’s discrete “either-or” traits could be used to
explain the continuous variation within populations
that natural selection acted on.
• The birth of population genetics, which emphasizes
extensive genetic variation within populations and
recognizes the significance of quantitative traits,
helped to improve this uncertainty.
The modern synthesis:
A comprehensive theory of evolution
• By the early 1940’s, the discoveries and ideas from
many scientific fields including paleontology,
taxonomy, biogeography and population genetics
were integrated into what became known as the
modern synthesis.
• The modern synthesis emphasized the importance of
populations as the units of evolution, the central role
of natural selection as the main mechanism of
evolution and the idea that evolution is a slow and
gradual process (as we will see later, some aspects
of this model are now challenged).
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The genetic structure of a population. Some basic
definitions:
A population is a localized group of individuals
belonging to the same species.
A species is a group of populations whose
individuals can potentially interbreed.
Although members of a species may be
geographically isolated from one another, the
integrity of the species is maintained by
interbreeding members of adjacent populations.
A gene pool is all of the genes in a population at any
one time. It consists of all alleles at all gene loci in
all individuals of the population.
Measuring gene frequencies in populations: the first step
to understanding generation-to- generational change in a
population’s allele and genotype frequencies
• We will not get into the math… it’s
complicated!
What causes gene frequencies to change in a
population i.e. evolution?
• Ironically, two scientists (Hardy and Weinberg)
addressed this question by first describing how gene
frequencies could remain constant over time,
essentially describing the circumstances necessary
for a non-evolving population.
• The Hardy-Weinberg theorem states that the
frequencies of alleles and genotypes in a
population’s gene pool remain constant over the
generations unless acted upon by agents other than
sexual recombination.
• In other words, the sexual reshuffling of alleles
due to meiosis and random fertilization has no
effect on the overall genetic structure of a
population.
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Microevolution
What conditions are required for gene frequencies to
remain constant and for the Hardy-Weinberg
equilibrium to be maintained in a population?
1. Very large population size.
2. No gene flow (population isolation).
3. No mutations.
4. Mating must be random.
5. No natural selection.
• Are each of these condition met?
Each one of the conditions required to maintain
Hardy-Weinberg equilibrium is violated!
• Are all populations large?
– Although many populations are, some are small enough
for random events to change gene frequencies.
• This microevolutionary phenomena is called genetic
drift.
• Bottleneck Effect:
• In this case, disasters such as floods and fires can
drastically reduce the size of the population, leaving
by chance, individuals that are not necessarily
representative of the original population.
• Founder Effect
• This occurs whenever a few individuals colonize
a new habitat.
• The founding population is usually small.
• Their gene pool may not be representative of the
entire gene pool they left.
• In the Amish, in fact, Ellis-
van Creveld syndrome has
been traced back to one
couple, Samuel King and his
wife, who came to the area in
1744. The mutated gene that
causes the syndrome was
passed along from the Kings
and their offspring, and today
it is many times more
common in the Amish
population than in the
American population at large.
Polydactyly
Gene flow occurs
• Populations may gain or lose alleles by the
migration or immigration of individuals, seeds,
pollen etc.
• For example, a wind storm may blow pollen from
an aa population into a population consisting of just
AA individuals.
Mutations occur……
• A new mutation that is transmitted in gametes
can immediately change a gene pool of a
population.
• In fact, rates of one mutation per locus per 105 to
106 gametes is typical for most species.
Mating is nonrandom
• Inbreeding, especially among plants (i.e. selfpollination) increases the frequency of
homozygous genotypes at the expense of
heterozygous genotypes.
• Another type of non-random mating is assortative
mating, in which individuals select mates that are
phenotypically similar.
• For example, tall women prefer tall men.
• In fruit flies, studies have demonstrated although
4% of all females fail to mate successfully, 20% of
all males fail to mate.
Snow geese demonstrate assortative mating
Differential reproductive success (natural
selection) occurs in nature.
• See the study of finch beak size.
Three modes of natural selection
• Stabilizing Selection
• Directional Selection
• Diversifying Selection
• Stabilizing selection
– Favors the intermediate phenotype out of a range of
phenotypes.
– The extremes in variation are selected against.
– For example, infants weighing significantly less or more than
7.5 pounds have higher rates of infant mortality.
– Selection works against both extremes.
• Directional selection
– Favors phenotypes at one extreme of the range of variation.
– Insecticide resistance is an example. DDT was a widely used
insecticide. After a few years of extensive use, DDT lost its
effectiveness on insects. Resistance to DDT is a genetic trait
that the presence of DDT in the environment made into a
favored trait. Only those insects resistant to DDT survived,
leading over time to populations largely resistant to DDT.
• Diversifying selection
– favors individuals at both extremes of variation: selection is
against the middle of the curve.
– This causes a discontinuity of the variations, causing two or
more morphs or distinct phenotypes.
– The African swallowtail butterfly (Papilo dardanus) produces
two distinct morphs, both of which resemble brightly colored
but distasteful butterflies of other species. Each morph gains
protection from predation although it is in fact quite edible.
Are Humans Exempt from Natural Selection?
• It has been argued that advances in medicine,
sanitation, etc. have removed humans from the
rigors of natural selection. There is probably some
truth to this, but consider:
– Of all the human eggs that are fertilized, only one-third
of so will ever reproduce themselves. The others are
eliminated as follows:
• Mortality selection
– Some 30% of pregnancies end by spontaneous abortion of
embryos and fetuses.
– 5% by stillbirths and infant deaths.
– 3% by childhood deaths.
• Sexual selection
– Another 20% will survive to adulthood but never marry.
• Fecundity selection
– Of those that do marry, 10% will have no children.