Option D: Evolution
IB Biology
I. Origin of Life on Earth (D.1)
A. Earth’s Early Beginnings
1. The Earth was formed approximately 4.6
billion years ago from a cloud of dust particles
orbiting the sun
2. This compacted due to gravity and
together with the decay of radioactive elements
generated heat that caused the interior to melt
and form a central dense core of iron and nickel
3. Lighter materials formed the mantle around the
core and the lightest silicates solidified into an
outer crust, the continents and the outer floors
4. The atmosphere was formed from gases
escaping through volcanoes and consisted of H2
H2O, CH4, NH3, N2, H2S, but no O2
5. The oceans formed by the condensation of
water vapour.
B. There are four processes that had to have
taken place in order to provide for the
spontaneous origin of life on Earth
1. The non-living synthesis of simple organic
molecules
2. The assembly of these molecules into
polymers
3. The origin of self-replicating molecules that
made inheritance possible
4. The packaging of these molecules into
membranes with an internal chemistry
different from their surroundings
C. Miller and Urey
1. Wanted to test the hypothesis that organic
molecules can form spontaneously under the
right conditions just as they might have at the
early on in earth’s history.
2. Gases used: ammonia, methane and
hydrogen, which created a reducing
atmosphere (could donate lots of hydrogen
and electrons).
3. It worked! Amino acids and other simple
organic molecules were formed by the
apparatus
4. However, this experiment was conducted
with sterile glass, distilled water, and enclosed
gas so it’s hard to say whether it is an accurate
representation of what might have happened
D. An alternate theory to that of the
spontaneous origin of organic materials on
earth is that comets may have delivered
the organic materials from space
1. This theory still begs the question of
how the organic compounds became
living organisms, however
E. Possible locations where conditions
would have allowed for the synthesis of
organic compounds
1. deep-sea hydrothermal vents
2. volcanoes
3. extraterrestrial locations (e.g. other
planets; follows along with the comet
theory)
F. Most scientists believe that RNA was the
first organic molecule that allowed for life
1. Reasons for this include two important
properties of RNA:
a. RNA is composed of a single
helix, versus DNA’s double helix.
- The bases are exposed and
ready to combine with a
complement, giving them the
ability to self-replicate.
b. RNA has the ability to catalyze
other reactions to occur (e.g. the
synthesizing of other organic
molecules such as DNA and
protein)
E. Possible origin of membranes and
prokaryotic cells
1. Living cells may have been
proceeded by protobionts, with an
internal chemical environment
different from their surroundings
a. protobionts = abiotic spheres
that are precursors to cells and
exhibits some of the properties
of life
1) Coacervates- small spheres
made of hydrocarbons that
contain nucleic acids in their
center (forerunners of
phospholipid membranes).
- can grow, shrink, and
split due to having a semipermeable membrane
2) Microspheres – spheres
formed upon the cooling of
thermal proteins
- considered more stable
than coacervates
F. Contribution of prokaryotes to the
creation of an oxygen-rich atmosphere
1. Cyanobacteria, which are
photosynthetic, converted Earth’s early
atmosphere from anoxia, to one
containing free oxgyen
2. Allowed for heterotrophs to evolve
3. Occurred about 2.7 to 2.2 billion
years ago
G. Endosymbiotic Theory
1. Describes the possible formation of
eukaryotes
2. According to the theory,
mitochondria were originally
independent prokaryotic organisms,
which were engulfed by another
independent prokaryotic organism
3. Instead of being dismantled for
nutritional purposes, the host found
it more beneficial to keep the
mitochondria intact
4. Similar circumstances are
believed to have occurred with
chloroplasts.
I. Species and Speciation (D.2)
A. Allele frequency and gene pools
1. allele frequency = the
percentage with which a particular
allele is found in a population
2. Gene pool = The sum total of all
alleles present in all populations of
a particular species
3. Evolution, at the genetic level,
involves a change in allele frequency in
a population’s gene pool over a
number of generations
B. Speciation
1. Speciation = formation of a new species
by the splitting of an existing species
2. New species result from the
accumulation of many advantageous
alleles over a long period of time
3. In other words, new species form as a
result of macroevolution
a. macroevolution = the accumulation
of multiple microevolutionary steps,
combined with reproductive isolation
(e.g. Darwin’s finches)
3. There are three types of reproductive
isolation (barriers between gene pools)
a. Geographical Isolation – occurs
when a population is physically
separated, usually due to a
natural disaster such as an
avalanche, fire, etc.
b. Temporal Isolation – due to
reproductive timing barriers (e.g.
different rainforest orchid species
blooming at different times of year)
c. Behavioral Isolation = Courtship
mating displays may only be
recognized by members of the same
species (e.g. bird songs)
4. Polyploidy can contribute to speciation
a. Polyploidy occurs when more than two
sets of homologous chromosomes are
present.
b. Examples such as triploidy (3x) and
tetraploidy (4x) are often due to a disruption
in the meiotic sequence.
c. Chromosomes replicate, but remain
together in the same cell.
d. Once polyploidy occurs, the individual is
often unable to mate with the original
species, causing immediate species
divergence.
5. Allopatric vs. Sympatric Speciation
a. Allopatric speciation – occurs in
different areas
1) due to geographical isolation
b. Sympatric speciation – occurs in the
same area
2) Due either to temporal or
behavioral isolation
7. Adaptive Radiation
a. As populations drift
or expand to different
geographical locales,
local environmental
conditions will favor
some traits over
others, causing
phenotypes in
different areas to
diverge
b. A classic example
is the finches of the
Galapagos Islands
(studied by Darwin)
C. Convergent and Divergent Evolution
1. Convergent Evolution
a. individuals of the same species develop
similar traits in response to living in the same
habitat
b. For example, many species of desert
plants develop thick cuticles (waxy outer
layer) to prevent water loss
2. Divergent Evolution
a. Occurs when different traits share a
common evolutionary origin
b. For example, vertebrate limbs have many
unique shapes, but their bone patterns trace
back to a common ancestral configuration
D. Pace of evolution
1. Gradualism = slow
changed from one form to
another
2. Punctuated equilibriumlong periods of no change
and short periods of rapid
evolution.
a. Some causes are
volcanic eruptions
and meteor impacts
on Earth.
3. Both theories are
debated by scientists
E. Transient vs. Balanced Polymorphism
1. Transient polymorphism- industrial melanism
and the peppered moth
a. Before 1850’s one homozygous
phenotype was favored by natural
selection
b. Once industrial revolution began natural
selection began to favor the other
homozygous phenotype
2.Balanced polymorphism- sickle cell anemia.
a. The heterozygous condition (carrier)
gives resistance to malaria, hence it is
also favored by natural selection.
III. Human Evolution (D.3)
A. Radioactive Dating of Fossils
1. Isotopes – Elements that have different
numbers of Neutrons.
Common Isotopes
- 12C
- 13C
- 14C
Carbon-12 is the most common form of
Carbon. Over 98% of carbon in the earth.
1. Over time, a certain percentage of Carbon
becomes an isotope
2. Living systems incorporate carbon, and
have the same % of Carbon isotopes as
the atmosphere
4. The half-life of C14 is 5730 years, and can be used
to date material up to 50,000 years old
5. The half-file of K40 is 1.3 billion years, and can be
used to date rocks over one million years old.
B. Major physical features that defines humans
as primates
1. Opposable thumbs
2. Large forward facing eyes for
stereovision and distance judgment
3. Color vision to identify other primates
and identify food
4. Large cranial capacity
5. Fingers with nails, not claws, and
fingertips sensitive to touch
D. The evolution of apes to humans can be seen
through the fossils of Ardipithecus ramidus,
Austrolopithicus, and Homo
1. A. ramidus
a. 5.8-5.2 million years ago
b. Oldest hominid
c. Large canines
d. Evidence of bipedalism is
inconclusive
2. A. afarensis
a. 3.9 – 2.9 million years ago
b. Bipedal
c. Reduced canines
3. A. africanus
a. 3.3-2.5 million years ago
b. Similar to A. afarensis, but slightly
larger brain
4. H. habilis
a. 2.6-1.4 million years ago
b. Used first simple stone tools
c. Protrusions in face starting to reduce
5. H. erectus
a. 1.8-1 million years ago
b. More advanced tool
c. Probably used fire
6. H. neanderthalis
a. 500,000 – 24,000 years ago
b. Short, thick bodies adapted to
cold climate
c. Largest cranial capacity
7. H. sapiens
a. 50,000 – present
b. Cranial capacity not as large as N.
neanderthalis, but better able to use
brains to develop agriculture and
hunting skills
8. At various
stages in hominid
evolution, several
species may have
coexisted
1) for example
H. sapiens and
N. neanderthalis
E. The fossil record is incomplete and can lead
to some uncertainties about human evolution
1. Many fossils, from Australopithecines
through the genus Homo, are incomplete
a. Usually partial skulls and just a few
bones are found because only a small
amount of organic matter is ever
fossilized
b. due to the fact that many body parts
do not fossilize
c. also, the environmental
circumstances needed for fossilization
to take place is rare
2. The fossils between males and females
differ and if only one is found it may not be
an accurate representation of the species as
a whole
3. Dating of fossils is approximate and not
always correct
2. By arranging extinct animals and plants
into some kind of geological sequence, it is
possible to suggest how one group may
have evolved into another
a. rarity of fossils and breaks in the
fossil record can make it hard to link
groups or may lead to scientists
making mistakes in assuming species
are related to each evolutionarily
F. Change in diet is correlated with an
increase in brain size during hominid
evolution
1. As brain size increased, the ability to
hunt and farm more efficiently
increased
2. This leads to better nutrition, which
in turn supported an even greater
cranial capacity
3. In essence, an evolutionary
feedback loop
G. Genetic Evolution vs. Cultural Evolution
1. Genetic Evolution- the random change of base
pair sequences
a. unit is the gene
b. results in changes in anatomy/physiology
2. Cultural Evolution- the change in practices and
traditions, communicated in some form from
generation to generation
a. unit is language or symbols
b. Examples: art /agriculture/ language
technology
3. Both types of evolution help humans to rise above
environmental limiting factors (e.g. food, water,
disease), but cultural evolution tends to occur faster
than genetic evolution
IV. The Hardy-Weinberg Principal (D.6)
A. Evolution occurs at the population level, not
the individual level
1. An allele that helps members of a species
to adapt best to their environment will most
likely to be passed on to the next generation
2. Over time the frequency of that particular
allele will increase while the other alleles for
that gene will decrease
3. The change in allele frequencies of a
population is what the Hardy-Weinberg
principal is based on
B. Hardy-Weinberg Equation
Used to determine how fast a
population is changing and in predicting
the outcomes of matings/crosses.
1. Assuming a STATIC population
2. A= Dominant allele
=p
3. a = Recessive allele = q:
–p + q = 1 (1.0 = 100%)
–Possible genotypes are: pp, pq, qp
qq
–p x p = p2, etc., therefore…
–p2 + 2pq + q2 = 1
• When calculating remember PEMDAS!!!
•
D. The Hardy-Weinberg principle can
also be used to calculate allele, genotype
and phenotype frequencies for genes with
two alleles
E. The HW equation only works if the following
assumptions hold true:
1. Large population (ideally infinite)
2. Random mating (autosomal only)
3. No natural selection
4. No allele-specific mortality
5. No mutation
6. No immigration or emmigration
C. Example: Cystic Fibrosis is a recessive
genetic disorder. In a certain population, 2 out of
every 2000 individuals have cystic fibrosis.
What are the values of p & q? What percentage
of the population are carriers?
•
•
•
•
•
•
q2 = 2/2000=.001
q = √.001 = .031
p + .031 = 1
p = .969
2pq = .06
Hence, 6% of the population are carriers.
Allele
Frequencies
t
q
T
p
Genotype
frequencies
tt
q2
Tt
2pq
TT
p2
• A homozygous recessive disease. Allele
frequency of recessive allele is 10%.
Calculate the percent of the dominant
allele.
• q = .1
• p + q = 1 so
• p = .9 or 90%
Allele
Frequencies
t
q
T
p
Genotype
frequencies
tt
q2
Tt
2pq
TT
p2
• Out of 989 organisms only 11 organisms
from the population had this disease.
Calculate the actual frequency of allele t.
• 989/11 = 0.011 = 1.1% tt organisms
• tt = q2 so t= square root of .011
• q = 0.105 or q = 10.5%
Allele
Frequencies
t
q
T
p
Genotype
frequencies
tt
q2
Tt
2pq
TT
p2
• Now calculate the frequency of carriers in
500 members of this population.
• q=.1 p = .9
• q2= .01 so 1% of population is tt
• 2pq = .18 so 18% of population is Tt
• p2= .81 so 81% of population is TT
• 500 organisms * 18% = 90 organisms
Allele
Frequencies
t
q
T
p
Genotype
frequencies
tt
q2
Tt
2pq
TT
p2
• Out of 500 organisms how many will be
healthy?
• TT or Tt = 18% + 81% = 99%
• 99% * 500 = 495 people will be healthy
V. Phylogeny and Systematics (D.5)
A. Value of classifying organisms
1. Organization of data assists in
identifying organisms
2. Suggests evolutionary links
3. Allows prediction of characteristics
shared by members of a group
B. Biochemical evidence of evolution
1. Universality of the genetic code
a. All amino acids are coded for
by mRNA codon sequences,
which are transcribed from DNA
codons
b. Codons are derived from the
same four bases regardless of
species: A,T,C,G
c. The universality of the code
points to a common evolutionary
ancestry
2. Hemoglobin
a. found in most animals, but the nucleotide
sequence can vary by species
b. Tracking and comparing these variations
can help species relative each other on the
phylogenetic tree
3. Mutations can be used as an
“evolutionary clock”
a. Mutations in a genome (due to
mistakes during replication) occur at a
predictable rate
b. Therefore, base pair sequences
in two populations can be compared,
and by looking at the number of
differences between the two, an
inference can be made as to how long
ago the two populations diverged
reproductively
c. The greater the number of
differences, the farther apart the two
groups are on the phylogenetic tree
C. Cladistics
1. Clade = A group of
organisms who share
common
characteristics
2. Cladistics = A
taxonomic system of
separating clades
based on the sharing
of derived
characteristics from
common ancestors
3. Analogous vs. Homolgous characteristics
a. Analogous characteristics - show similarity
without having a common ancestor
- develop due to evolving in
similar habitats or facing a similar
environmental challenge
- e.g. the wings of bats, birds, and
insects
b. Homolgous characteristics – show
similarity due to having a common
ancestory
- e.g. bone structure of a whale
flipper and a human hand
- used for developing cladograms
4. How to create a cladogram
a. Cladograms start with an “in group”,
which contain certain characteristics
b. Another group is then compared to
the “in group”
c. If it differs in any way it is placed in
its own clade
d. Clades are separated from each
other based on single differences, and
are then placed in sequence
e. Note that cladograms do not make
any assumptions about the time period
involved in an evolutionary change,
rather they indicate that one has
occurred
5. Relationship between cladograms
and the classification of living
organisms
a. Monophyletic – group that
shares a common ancestor
b. Paraphyletic – a group which
contains some, but not all
members associated with a
common ancestor
c. Polyphyletic – a group which
does not share a common
ancestor