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Evolution
Myths & Misconceptions
https://www.youtube.com/watch?v=92oHNd8vFwo
Evolution
• Evolution: the
theory that species
change over time; it
is the process of
change that
produces new
species from preexisting species.
• Evolution is an important scientific concept
because it attempts to explain why living
organisms, so similar in their biochemistry
and molecular biology, are so different in
form and function.
Start of the
Evolutionary Theory
• The world during the mid-eighteenth
century was quite different from the
world we know today. The religious
belief that every type of organism had
been separately created influenced
many biologists. Many tried to explain
the diversity and complexity of all living
things but none formulated a definite
theory.
• None could present enough supportive
data to suggest a general mechanism
by which evolution might occur.
Carolus Linnaeus (1707-1778)
• Swedish botanist, developed a
classification system to organize all
known organisms
• System is still used today
• Grouped organisms based on
similarities, rather than evolution, but his
classification also reflects evolutionary
relationships
George-Louis Leclerc, Comte
de Buffon (1707-1788)
• French naturalist, first to propose that
species did not develop separately but
instead shared ancestors
• Proposed that animals developed in one
area and spread to another as a result
of climate change (which accounted for
some of the fossils found during
Buffon’s lifetime.)
• As species moved to new habitats, they
changed in response to new
environmental conditions (though
Buffon did not think that this could be a
very radical change.)
• Also suggested that the Earth was
70,000 years old, which was
significantly older than the accepted age
of the Earth (6000 years) at the time.
Jean Baptiste de Lamarck
• French naturalist who
proposed the first coherent
theory of evolution in 1809.
• Stated clearly that more
complex organisms
descended from less
complex organisms
• Suggested that this demonstrated a
built-in drive towards perfection
(represented by humans, it was thought
that all organisms were on their way to
achieving this perfection.)
• Accounted for the continued presence
of less complex organisms by
suggesting that spontaneous generation
was always producing a fresh supply of
these organisms.
• Lamarck believed that plants and
animals adapted to their environments,
and that environmental factors caused
changes in organisms. He proposed 2
basic theories:
1. Use and disuse
• Individuals develop and keep
characteristics that are useful, and lose
characteristics that they do not require
to survive in their environments
2. Inheritance of Acquired
Characteristics
• Individuals develop traits throughout
their lives that can then be passed on to
their offspring. Similarly, any
characteristics lost during an individual’s
lifetime would not be passed on to the
offspring.
Geologic Change
• Since Buffon’s time, the age of the
Earth was a key problem in evolutionary
theory.
• It was widely accepted that the Earth
was roughly 6000 years old, and that
during that time, the Earth and the
species that lived on it had not changed.
• Early geologists, however, began to
question the Earths age and the idea
that it had remained constant
throughout time.
Georges Cuvier (1769-1832)
• Observed that each stratum (rock
layer) in the Earth contained
different types of fossils. Fossils in
deep layers were very different
from those in the upper layer. The
deeper the layer, the older the layer
and the fossils it contained.
• Established the idea of extinction
as a scientific fact by demonstrating
that many older fossils did not
represent living organisms.
• Proposed the catastrophism theory,
which suggested that catastrophic
events (volcanoes, floods, earthquakes,
etc.,) were responsible for mass
extinctions and the formation of
landforms.
• Cuvier believed that the appearance of
new species in each rock layer meant
that these organisms moved into the
area after a catastrophic event. He did
not believe that species could change.
James Hutton (1726-1797)
• Scottish geologist, proposed that
landforms changed slowly over long
periods of time. His theory is called
gradualism.
• According to his theory, soil deposits
and creation of landforms (for example,
a canyon) were not the result of largescale events but slow processes that
happened in the past.
• Using the example of a
canyon, Hutton argued
that these were
created by rivers
gradually wearing
down rock over a very
long period of time.
• His theory is important
to evolution because
evolution is also a slow
process that requires a
lot of time.
Charles Lyell (17971875)
• English geologist who expanded upon
Hutton’s gradualism theory
• Proposed the theory of
uniformitarianism, which stated that
geologic processes that shape Earth
are uniform throughout time (in other
words, rock strata are formed by the
constant laying down of sediment.)
• Lyell proposed that geologic processes
did not just occur in the past; rather, that
they are still occurring today.
• Geological changes, over a long period
of time, add up to cause great change.
• Uniformitarianism eventually replaced
catastrophism as the favoured theory of
geologic change.
Comparing and Contrasting
Theories of Geologic Change:
Catastrophism
Gradualism
Catastrophic
events form new
landforms or
change old
landforms
Changes to the
Earth occurred
in the past, over
a long period of
time
Relatively fast
process
Slow process
Uniformitarianism
Geologic
processes are
still occurring
today, uniform,
add up over
time (sediment)
to cause great
change
Slow process
Charles Darwin (1809 -1882)
• Darwin was influenced
by Lyell’s view of
gradual geologic
change
• Found evidence
supporting Lyell’s
theory during his 1831
voyage to South
America aboard the
HMS Beagle.
• Noticed the variation of traits among
similar species found on the South
American coast and Pacific Islands,
most notably the Galapagos Islands.
• Darwin observed that species found on
one island looked different from those
on other islands, and that many of the
islands’ species looked different than
those on the mainland.
• Some differences seemed suited to specific
environments and diets.
• Also observed fossils that supported Lyell’s
theories, leading Darwin to include an old
Earth and slow, gradual change in his ideas
of organic evolution.
• Described his theory and the evidence
he observed in his famous book On the
Origin of Species.
• Darwin’s proposed mechanism of
evolution is called natural selection (as
opposed to artificial selection in
domesticated animals, in which humans
change a species by breeding it for
certain traits.)
Alfred Russell Wallace
(1823-1913)
• Came to many of the same conclusions
as Darwin, but did so by observing
organisms in Europe.
• Contacted Darwin, and published his
theory jointly with Darwin’s in 1858.
Natural Selection
• Darwin’s theory of natural selection may
be summarized as follows:
• 1) Overproduction - in a population, there
are many more individuals produced each
generation than can survive and reproduce.
• 2) Struggle for Existence
(Competition) - due to
overproduction, organisms must
compete for limited resources
(food, water and habitat)
• 3) Variation (Change by Chance)
- No two individuals are exactly
alike. These differences in traits
were believed to be passed on to
offspring by parents. Darwin did
not understand what caused
these variations among different
organisms.
• 4) Adaptation (Survival of the fittest) individuals with adaptive characteristics are
more likely to successfully reproduce than
those with other characteristics.
5) Descent with
modification Adaptive
characteristics are
inherited by offspring
to give them a better
chance of survival.
Each generation will
have more individuals
with this trait, as long
as it is beneficial.
• 6) Speciation – (Origin of New Species) - As
generations pass, new species arise from
the accumulation of inherited variations.
When an organism appears that is much
different from the original organism, it
becomes a new species.
Pre-Darwinian Concepts Post-Darwinian Concepts
A creator previously
decided the structure and
function of each type of
organism.
Adaptation to environment
explains the structure and
function of each type of
organism.
Observation and
experimentation are
unnecessary to
substantiate self-evident
truth.
Observation and
experimentation are used
to substantiate theories of
evolution.
Pre-Darwinian
Concepts
Post-Darwinian
Concepts
Earth is relatively young - Earth is relatively old age was measured in
age is now measured in
thousands of years
billions of years.
Fixity of species Organic Evolution organisms do not change organisms change and
and the number of
several new species can
species has remained the
arise from a single
same.
ancestor.
Natural Selection
https://www.youtube.com/watch?v=0SCjhI86grU
Post-Darwinian Evolution and
Genetics
• Darwin did not understand how traits
were passed on from parents to
offspring. An Austrian monk, working in
isolation in a monastery, discovered the
mechanisms for heredity (though he
was also unaware of Darwin’s
evolutionary theory.) Genetics and
evolution were combined to form
modern evolutionary theory (called the
Modern Synthesis Theory to distinguish
it from Darwin’s original theory.)
Gregor Mendel
(1822-1884)
• Working with pea plants, Mendel was
able to recognize patterns of
inheritance.
• He noticed that certain versions of
characteristics showed up more often in
generations of plants, while others
seemed to disappear and reappear in
subsequent generations.
• He called these characteristics
“dominant” and “recessive”,
respectively, and suggested that
invisible factors (genes) controlled these
traits.
• Lived and worked at the same time as
Darwin, but neither were aware of the
other’s work.
Friedrich Miescher
(1844-1895)
• Isolated and discovered nucleic acids
(ex: DNA) in 1869
• First person to theorize that nucleic
acids were involved in heredity
Hugo De Vries (1848-1935)
• Proposed the mutation
theory, which stated that
genetic mutations, not
chance, were
responsible for the
variations that Darwin
observed.
Evidence of Evolution
A) Fossil Evidence
1. The fossil record is the history of life
recorded by remains of the past.
• 2. Fossils include skeletons, shells,
seeds, insects trapped in amber,
imprints of leaves, and footprints.
• 3. Transitional forms reveal links
between groups.
• Archaopteryx is an intermediate
between reptiles and birds.
• Eustheopteron is an amphibious fish
• Seymouria is a reptile-like amphibian.
• Therapsids were mammal-like reptiles.
• For example, the fossil record allows us
to trace the history of the modern-day
Eqqus.
B. Biogeographical
Evidence
1. Biogeography studies the distribution of
plants and animals worldwide.
• 2. Distribution of organisms is explained
by related forms evolving in one locale
and spreading to other accessible
areas.
a) Darwin observed South America
had no rabbits; he concluded rabbits
originated elsewhere.
b) Biogeography explains the
abundance of finch species on the
Galápagos Islands lacking on the
mainland.
• 3. Physical factors, such as the location
of continents, determine where a
population can spread.
a. Marsupials arose when South
America, Antarctica, and Australia were
joined; Australia separated before
placental mammals arose, so only
marsupials diversified in Australia
C. Anatomical
Evidence
1. Organisms have anatomical similarities
when they are closely related because of
common descent.
• Homologous
structures in
different
organisms are
inherited from a
common
ancestor.
• Analogous
structures are
inherited from
unique ancestors
and have come to
resemble each
other because
they serve a
similar function.
• Vertebrate
forelimbs are
examples of
homologous
structures: they
contain the same
sets of bones
organized in
similar ways,
despite their
dissimilar
functions.
• 2. Vestigial structures are remains of a
structure that was functional in some
ancestors but is no longer functional in
the organism in question. Examples:
remnants of hip bones in whales (not
needed because they don’t have legs),
humans have a tailbone but no tail.
3. Embryological
development – During
development, all vertebrates
have a post-anal tail and
paired pharyngeal pouches.
In fishes and amphibian
larvae, the pouches become
gills. In humans, the
pouches become the middle
ear, tonsils, and glands.
D. Biochemical Evidence
1. Almost all living organisms use the
same biochemical molecules, eg: DNA,
ATP, enzymes.
2. Organisms utilize the same DNA triplet
code and the same 20 amino acids in their
proteins.
3. These similarities can be explained by
descent from a common ancestor.
4. Life’s vast diversity has come about by
only a slight difference in the same genes.
Evidence of Evolution
https://www.youtube.com/watch?v=lIEoO5KdPvg
Pace of Evolution: How fast
does evolution proceed?
1. Phyletic gradualism – slow process with
many transitional forms
2. Punctuated equilibrium – speciation
occurs rapidly, transitional links not
evident, explains lack of fossils
3. Living fossils (horseshoe crab,
coelacanth) support punctuated
equilibrium.
Isolation and
Evolution
• As long as a population has the
opportunity to interbreed and exchange
genes, they remain one species. A
population of one species can only
evolve into more than one species if
groups within the population become
isolated from each other by barriers that
prevent exchange of genes.
• There are three types of isolating
barrier:
• Geographical – Features such as rivers
or mountain ranges isolate groups.
Movement of land masses by
continental drift led to geographical
isolation millions of years ago.
• Ecological – Although groups are not
geographically isolated from each other
they may be isolated by such things as
occupying different habitats or breeding
areas, pH, salinity.
• Reproductive – Breeding between
groups within a population may not be
possible because of differences in
courtship behavior, physical differences
which prevent mating, or failure of
gametes to fuse.
Microevolution
• Microevolution is evolution
that occurs within a species
or a small group of
organisms. May happen in
a relatively short amount of
time.
Genetic Mutations
• Many traits in organisms are
polymorphic, ie., two or more distinct
phenotypes are present in the
population due to mutated genes.
• Analysis of Drosophila enzymes
indicate that they have multiple alleles
at least at 30% of their gene loci.
• In humans, freckles are an example of
polymorphism, as are the ABO blood
types.
• Mutations can be beneficial, neutral, or
harmful; a seemingly harmful mutation
that requires Daphnia (a genus of small
planktonic crustaceans) to live at higher
temperatures becomes advantageous
when the environment changes.
Gene Flow
• Gene flow (gene migration) is the
movement of alleles among populations
by migrating or breeding individuals.
• Gene flow can increase variation within
a population by introducing novel alleles
• Continued gene flow decreases
diversity among populations, causing
gene pools to become similar.
• Gene flow among populations can
prevent speciation from occurring
Non-Random Mating
• Random mating involves individuals
pairing by chance, not according to
genotype or phenotype.
• Non-random mating involves inbreeding
and assortative mating.
• Inbreeding is mating between relatives
to a greater extent than by chance.
• Inbreeding does not change the allele
frequencies.
• However, inbreeding decreases the
proportion of heterozygotes.
• In human populations, inbreeding
increases the frequency of recessive
abnormalities.
• Assortative mating occurs when
individuals mate with those that have
the same phenotype.
• Sexual selection occurs when males
compete for the right to reproduce and
the female selects males of a particular
phenotype.
Genetic Drift
• Genetic drift refers to changes in allele
frequencies of a gene pool due to
chance, more often in small populations
• Genetic drift occurs when founders start
a new population, or after a genetic
bottleneck with interbreeding.
Natural Selection
• Natural selection requires:
• variation (i.e., the members of a
population differ from one another),
• inheritance (i.e., many of the differences
between individuals in a population are
heritable genetic differences)
• differential
adaptedness (i.e.,
some differences
affect how well an
organism is adapted
to its environment),
and
• differential
reproduction (i.e.,
better adapted
individuals are more
likely to reproduce).
• Fitness is the extent to which an
individual contributes fertile offspring to
the next generation.
• Relative fitness compares the fitness of
one phenotype to another.
Types of Selection
• 1. Directional
selection occurs
when an extreme
phenotype is
favoured; the
distribution curve
shifts in that direction.
•
•
•
•
Shift to dark-colored peppered moths from lightcoloured correlated with increasing pollution.
Drug-resistant strains of bacteria are a serious
health threat and represent this type of selection.
Increases in insecticide-resistant mosquitoes and
resistance of the malaria protozoan Plasmodium
to medications are also examples of directional
selection.
The gradual increase in the size of the modern
horse, Equus, correlates with a change in the
environment from forest-like conditions to
grassland conditions
2. Stabilizing
selection
• Occurs when
extreme
phenotypes are
eliminated and the
intermediate
phenotype is
favoured.
Examples:
• The average number of eggs laid by
Swiss starlings is four or five.
• If the female lays more or less than this
number, fewer survive.
• Genes determining the physiology of
yolk production and behavior are
involved in clutch size.
3. Disruptive selection
• Occurs when extreme phenotypes are favoured
and can lead to more than one distinct form.
• British snails (Cepaea nemoralis) vary
because a wide range causes natural
selection to vary.
• In forest areas, thrushes feed on
snails with light bands.
• In low-vegetation areas, thrushes feed
on snails with dark shells that lack
light bands.
Macroevolution
• Macroevolution refers to any
evolutionary change at or
above the species level.
• Speciation is the splitting of
one species into two or more
species, or the transformation
of one species into a new
species over time; speciation
is the final result of changes in
the gene pool allele and
genotypic frequencies.
What is a species?
• Linnaeus separated species based on
morphology (their different physical
traits.) Darwin saw that similar species
are related by common descent
(phylogeny).
• Ernst Mayr (1942) developed the
biological species concept: a species is
a group of actually or potentially
interbreeding populations that are
reproductively isolated from other such
groups.
• The biological definition of a species
says that the members of one species
interbreed and have a shared gene
pool, and each species is reproductively
isolated from every other species.
• Gene flow occurs between populations
of one species but not between
populations of different species.
• Biochemical genetics uses DNA
hybridization techniques to determine
relatedness of organisms; the
phylogenetic species concept uses
DNA/DNA comparisons.
Reproductive
Isolating Mechanisms
• For two species to be separate, gene
flow must not occur between them.
• A reproductive isolating mechanism is
any structural, functional, or behavioral
characteristic that prevents successful
reproduction from occurring.
Modes of Speciation
• Allopatric speciation occurs when new
species result from populations being
separated by a geographical barrier that
prevents their members from
reproducing with each other.
• First proposed by Ernst Mayr of
Harvard University.
• While geographically isolated, variations
accumulate until the populations are
reproductively isolated.
• Sympathetic speciation would occur
when members of a single population
develop a genetic difference (eg:
chromosome number) that prevents
them from reproducing with the parent
type.
• The main example of sympatric
speciation is in plants.
• Failure to reduce chromosome number
produces polyploidy plants that
reproduce successfully only with
polyploids.
• Backcrosses with diploids are sterile.
Adaptive Radiation
• Adaptive radiation is a rapid
development from a single ancestral
species of many new species.
• The case of Darwin’s finches illustrates
the adaptive radiation of 14 species
from one founder mainland finch.
Human Evolution