EVOLUTION NOTES – PRINCIPLES OF BIOLOGY
Modules 63,64,65,66,
I.
Definitions
 biological evolution – the change over time in the allele frequencies or genetic makeup
of populations of organisms – Discuss – how long, genetics? , populations evolve !!!!
 Scientific theories – scientific explanations to complex phenomena that are supported
by evidence. The theory usually remains the same but the evidence changes, so the
theory becomes better understood.
II.
Natural Selection – a mechanism of evolution in which organisms possessing certain
genotypic characteristics that make them better adjusted to an environment tend to
survive, reproduce, increase in number or frequency, and therefore, are able to transmit
and perpetuate their essential genotypic qualities to subsequent generations.
 Natural selection has the following aspects:
i. Variation – individuals exhibit variation in a population, they have a unique set
of traits. Some of these traits improve their chances of survival while others are
less favorable.
ii. Overproduction – populations produce too many young, many must die.
iii. Struggle for existence – food, water and other resources are limited, organisms
compete with one another for these resources.
iv. Differential reproductive success – those individuals that have the most
favorable characteristics in an environment, has higher chance of survival and
reproduction.
v. Descent with modification – different reproduction and survival can result in
changes in the characteristics of organisms and the rise of new species.
 Charles Darwin and Alfred Wallace came up with the theory of evolution by natural
selection. They based their theory on observations and many other evidence but did
not know about molecular evidence.
III.
Evidence of Evolution -- data or information that is collected by scientists
 Direct observation (See examples of evolution)
 Molecular evidence
 Fossil evidence – radiometric dating,
http://www.sciencefriday.com/segment/08/22/2014/tar-noir.html , only 1 % of all
organisms ever lived left fossils in sedimentary rocks, tar pits, pitbugs
 Comparative anatomy – homology and analogy
 Vestigial organs
 Comparative embryology
 Biogeography
IV.
Examples of Evolution – specific evolutionary events that are/were observed
 Darwin’s finches
 Antibiotic resistance
 Herbicide and pesticide resistance
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V.
Human birth weight evolution
Industrial melanism
Adaptations and natural selection
 Adaptation is any change in heritable traits that increase an organism’s evolutionary
fitness
 Evolutionary fitness – shows the chance of an organism to survive and reproduce
Must be able to give examples of various adaptations – See Module 68,69
Making of the fittest: http://media.hhmi.org/fittest/birth_death_genes.html -- Start at min. 2
1. What is the scientific discovery?
2. Why is it surprising?
3. What is the adaptation?
4. What is the evolutionary advantage?
5. Guide this story through the concept and aspects of natural selection.
6. What is the scientific explanation of the existence of antifreeze protein?
Modules 68,69
VI.
Mechanisms of Evolution – Natural processes that can change the allele frequency in a
population and as a result cause evolution
 Natural Selection – According to the outcome of natural selection we can have the
following types:
i. Directional selection – Ex. Skin color gets darker in the tropics to protect against
the damaging effects
ii. Stabilizing selection – Ex. Human birth weight
iii. Disruptive selection – Ex. Finch beak sizes change to very thin or thick
depending on what type of seed they eat.
 Genetic Drift – fluctuations in the populations’ allele frequencies due to chance events
(weather conditions, geological phenomena, etc.). Especially influential in smaller
populations.
i. Bottleneck effect
ii. Founder effect
 Gene flow – mechanism of evolution in which alleles move between populations as a
result of migration. Gene flow into the population (immigration) increases genetic
variation, while gene flow out of the population (emigration) decreases genetic variation
(Figure 8 in Module 67 is a good representation of all three mechanisms)
Other mechanisms can include:
 Sexual Selection
 (Artificial Selection)
 Mutations
VII.
The Significance of Genetic Variation
 Genetic Variation – genetic differences among individuals within a population
 Genetic differences are frequently visible as phenotypic differences and may be passed
on to the next generation
Dutch study on epigenetics
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VIII.
IX.
Genetic variation drives evolution, because evolution can only work on existing genes.
More variations of those genes give evolutionary processes more “options”
Concerns about monocultures in our food supply
Genetic variation is affected by geographic variation, because different environmental
pressures are acting on the organisms of each population
Ex. Human skin color, industrial melanism, mice fur coloration -http://www.youtube.com/watch?v=AMtT5_AQmLg
Frequency-Dependent Selection
 The selection pressure on a phenotype is dependent on the frequency of that
phenotype.
Coevolution – result from beneficial or adverse interactions between species
Ex. Pollinators and flower structure
Lactose tolerance
Sickle cell disease and malaria
Lactose tolerance:
http://media.hhmi.org/fittest/Got_Lactase.html
EVOLUTION IN POPULATIONS – THE HARDY-WEINBERG THEOREM
I.
POPULATIONS THAT ARE NOT EVOLVING
 Because the allele frequency has to change for evolution, if the allele frequency remains
the same, populations are not evolving.
 The allele frequency can be changed by any mechanism of evolution. In a non-evolving
population none of the mechanisms of evolution are active.
 Non-evolving populations have the following five characteristics:
i. Large populations – no genetic drift
ii. Random mating – no sexual selection
iii. No mutations
iv. No natural selection
v. No gene flow
 There are really no natural populations that would have all these conditions fulfilled. So
we can conclude that evolution always takes place in natural populations. However, the
pace at which it takes place can change.
II.
CALCULATING ALLELE FREQUENCIES
A) Explanation of symbols used:
p = the frequency of the dominant allele in a population
q = the frequency of the recessive allele in a population
p2 = the frequency of the homozygous dominant genotype or individuals in the population
2pq = the frequency of the heterozygous individuals or genotype in the population
q2 = the frequency of the homozygous recessive individuals or genotype or recessive phenotype
in the population
p2 + 2pq = the frequency of the dominant phenotype in the population
B) Hardy-Weinberg Equations:
p+q = 1
P2 + 2pq + q2 = 1
These equations are used to determine the allele frequencies in various populations.
Practice Problems:
1. In Drosophila the allele for normal-length wings is dominant over the allele for vestigial wings
(vestigial wings are stubby little curls that cannot be used for flight). In a population of 1,000
individuals, 360 show the recessive phenotype. How many individuals would you expect to be
homozygous dominant and heterozygous for this trait?
2. In the United States about 16% of the population is Rh negative blood typed. The allele for Rh
negative is recessive to the allele for Rh positive. If the student population of Aurora High
School is 1,078, how many students would you expect for each of the three possible genotypes?
3. In a certain population, the dominant phenotype of a certain trait occurs 91% of the time. What
is the frequency of the dominant allele? The recessive allele? All three genotypes?
THE BIOLOGICAL SPECIES CONCEPT AND SPECIATION
I.
WHAT IS A SPECIES?
 An interbreeding group of organisms that are similar to each other but different from
other groups of organisms. Organisms of the same species are able to produce viable
and fertile offspring in nature.
 However, there are many other species definitions exist.
II.
REPRODUCTIVE ISOLATION
 Reproductive isolation occurs when two organisms from different populations cannot
produce viable and fertile offspring. If the organisms of two populations cannot
reproduce with each other, they will eventually form two separate species.
 There are two major kinds of barriers that prevent reproduction:
i. Prezygotic barriers – barriers that prevent the formation of a zygote
1. Gametic isolation
2. Mechanical isolation
3. Behavioral isolation
4. Temporal isolation
5. Habitat isolation
ii. Postzygotic barriers – gametes can unite and form a viable zygote but any time
after the formation of the zygote, failure can happen in the hybrid
1. Reduced hybrid viability
2. Reduced hybrid fertility
3. Hybrid breakdown
WHEN DO POPULATIONS BECOME NEW SPECIES?
 New species form when they become genetically isolated from the ancestor species.
 Usually genetic isolation means that one or many beneficial mutations accumulate in
one population.
 It may not be very clear when exactly a new species forms – See the case study
SPECIATION AND GEOGRAPHIC BARRIERS
 Geography influences species formation in two ways:
i. Allopatric speciation – occurs when two populations of the same species are
separated from each other by some geographic barrier. As a result, these
populations cannot reproduce with each other and over time become
genetically so different from each other that they will not be able to reproduce
with each other any more.
ii. Sympatric speciation – occurs, when two populations are living in the same
geographic area and have the potential to reproduce with each other but they
do not reproduce. The reasons for the lack of reproduction are:
1. Polyploidy – Figure 4, Module 74
2. Sexual selection – Figure 9. Module 74
3. Habitat differentiation – Case study
III.
IV.
V.
THE PACE OF SPECIATION
 Species that exist today evolved from once lived ancestors that were very different from
today’s species. This constant change of species is taking place today as well as millions
of years ago.
 There are two different ideas about at what pace evolution is taking place at any given
time:
i. Gradualism – slow, incremental changes take place all the time
ii. Punctuated equilibrium – species are mostly stable, but rare events apply
selective pressures that will result in fairly rapid chanes.
 Evidence indicates that a combination of these two processes take place.
What kinds of evidence would you use to prove this?
The Origin and History of Life on Earth
I.
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The Origin of Life and Evidence that Supports It
There is no coherent, widely accepted scientific theory about the origin of life today. But we
have several hypotheses that are supported by evidence.
A. The Origin of Simple Organic Molecules:
Review what an organic molecule is.
 The Earth, with the rest of the solar system formed 4.5 billion years ago. At this point, the
Earth was hot, but slowly cooled down by about 4.2-3.9 billion years ago. The atmosphere
at this point was mostly made up of CO2, N2, methane, ammonia, hydrogen sulfide and as
the planet cooled, water condensed.
o Evidence: air samples from rocks, radiometric dating of rocks, samples from other
planets
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Haldane and Oparin Hypothesis: The composition of the early atmosphere, UV rays, energy
from volcanic eruptions could create the simplest monomers of organic molecules, such as
amino acids, hydrocarbons, simple sugars, simple fats and nucleotides.
The Miller-Urey Experiment: Tried to replicate the conditions on the ancient earth under
laboratory conditions and could form simple organic molecules such as amino acids. Today,
we have hydrothermal vents on the ocean floor that still shows similar conditions to the
ancient atmosphere.
http://www.youtube.com/watch?v=BXGF3XS-yAI – hydrothermal vents
Alternative theory:
o Organic molecules arrived to the earth from outer space in meteorites. Scientists
find meteorites today with organic molecules in them. Some asteroids also had ice
in them which may be the source of water today on earth.
B. The Origin of Macromolecules
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According to one theory, macromolecules could have formed when simple monomers
attached to some substances with large surface areas. On these surfaces, the
monomers could have reacted with each other to form long chains and rings. These
surfaces are found on porous rocks and clay today.
RNA world hypothesis – It is hypothesized that the very first macromolecules were used
for multiple purposes. RNA was likely the first molecule that was responsible for being
both the molecule of inheritance that could self-replicate and the catalyst of chemical
reactions. Evidence of this hypothesis:
i. Polynucleotide chains are able to self-replicate today under proper conditions
ii. Ribozymes (enzymes that are made up of RNA not proteins) still exist today for
example in the form of ribosomes
iii. Some ribozymes can replicate with great accuracy
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Protocell formation: Membrane structures could develop from phospholipids that could
also form on the surface of porous rocks. Once phospholipids form, they self-assemble
into double layers in watery solutions. If the membranes become too large, they can
split into two (divide). Once protocells could self-replicate, they could also evolve
because their RNA or DNA could change over time.
Evidence:
i. Vague but it is possible to build protocells in ponds, lakes with varying
temperatures if the monomers are found in the water.
Figure 6 animation
C. The Early Evolution of Life
 http://www.pbs.org/wgbh/evolution/library/03/3/l_033_01.html -- how was the age of
the Earth determined.
 Radiometric dating – used to determine the age of fossils or rocks. Radioactive isotopes
decompose at a steady rate. As their concentration decreases from the level that it was
on during the formation of the rock or fossil, we can calculate the age of the rock or
fossil. – Use Figure 6 in Module 65
 By using radioactive isotopes, it was determined that the Earth is about 4.6 billion years
old.
 Life first formed about 3.5 billion years ago. Evidence: The first fossil evidence comes
from rocks called stromatolites that are about 3.5 billion years old. The first fossils were
printed remains of groups of prokaryotic cells.
 Oxygen levels dramatically rose at about 2.5 billion years ago. Evidence: iron oxides
increased in concentration in rocks. Significance: photosynthetic prokaryotes such as
cyanobacteria appeared. This destroyed most life forms that existed previously,
because all the previous life forms were anaerobic (poisoned by oxygen). Large areas
became empty, so newly evolved aerobic life forms could reproduce without
competition.
 First eukaryotes appeared 2.1 billion years ago. Evidence:
i. The earliest eukaryotic fossils were found from that time
ii. Available oxygen killed lots of organisms, so there were plenty of resources
available for punctuated equilibrium type of evolution
iii. The Endosymbiotic Theory – Mitochondria and chloroplasts evolved from free
living bacteria, when they were engulfed by simple but larger cells. Evidence:
1. These organelles have circular DNA
2. They reproduce by binary fission like bacteria do
3. Some of their metabolic enzymes are similar to bacterial enzymes
4. They are about the same size as bacteria
5. They have double membranes, one of which was the original bacterial
membrane, the other was the host’s cell membrane
Watch the animation on Figure 5 – Module 77
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Multicellular life evolved about 1.2 billion years ago. Evidence:
i.
Fossil record
ii.
DNA sequencing – scientists estimate that, based on the amount of change among
the DNA sequences of multicellular eukaryotes, they had a common ancestor about
1.5 billion years ago.
Major diversification of life – rapid speciation occurred about 500 million years ago. This is
also when life started on dry land. – Figure 7 – Module 77 introduces phylogenetic trees
Land organisms must have adaptations to avoid or survive drying out. A few of these:
i.
Thick skin, waxy outer covering
ii.
Inside transport system – blood vessels, vascular tubes
iii.
Root systems in plants
iv.
Unique respiratory systems
v.
Stronger support by having harder structures, bones, hard fibers
D. MAJOR FACTORS THAT CONTRIBUTE TO THE DIVERSIFICATION OF SPECIES
 Mass extinctions – Catastrophic changes in the environment can destroy very large, 75% or
more of species on the entire planet. These events are called mass extinctions. There were
5 of these in the history of life. These are very disadvantageous but can open up new
environments for surviving species and with that speed up speciation again. Figure 9,
Module 77
 Adaptive radiation – When major ecological niches open up because of previous mass
extinctions, surviving species can have major evolutionary changes that fill up all available
niches. This happened with dinosaurs, mammals etc.
 Continental drift – a continuous process throughout the history of the Earth. Continental
drift changes the climate and with that can radically change the environment. As a result, it
speeds up speciation
E. THE COMMON ORIGIN OF LIFE:
 All living organisms came from the same common ancestors and these evolved over the
billions of years to the great diversity that we see today.
 Evidence:
o Common genetic code
o Common material of inheritance
o Similar cellular processes
o Same structure of the phospholipid bilayer
o Same major types of macromolecules
o Similar structure of amino acids
PHYLOGENY
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EVOLUTION AND PHYLOGENY
Phylogenies represent the history of relationships among evolving lineages of populations.
Because anything that can reproduce and pass on their genes, over time will also evolve.
Lineage is a series of organisms connected by genetic information passed from one
generation to the next.
At one point in their history of a lineage some members may become isolated from the
original population. This isolation can be genetic or reproductive and can over time result in
a split in the lineage – speciation.
Phylogenetic trees can represent the history of a lineage in a simplified way. Nodes on
these phylogenetic trees show the split between two or more branches. The node
represents a common ancestor. These common ancestors are always historical, so they no
longer exist.
Diagram in Module 79
INTERPRETING PHYLOGENIES
The closer to the leaves the branching for two species are on a phylogenetic tree the more
recent common ancestors they have.
Guidelines to read phylogenetic trees:
o To understand relatedness, find the common ancestor
o A phylogeny can be flipped around at any node without changing the meaning of
the tree.
o Trees can be drawn in many different formats but the guidelines for reading them
don’t change
o All currently living species should end on the same level of a phylogenetic tree – this
rule is frequently not fulfilled, even in some AP materials, you will see an example
CLADISTICS
Sometimes homologous traits are used to determine common ancestry – cladistics, which
groups species and other taxa into groups called clades.
Clade includes one common ancestor and all of its potential descendants.
Cladistic analysis can be complicated because it can include many different groups.
o Monophyletic groups – include only one common ancestor and all of its
descendants
o Paraphyletic groups—include one common ancestor but not all of its descendants
o Polyphyletic groups – include many common ancestors with many descendants
Cladograms are charts that represent cladistics relationships
Parsimony -- the rule of parsimony is used to determine the most likely evolutionary
relationships in a cladogram. According to this rule, a tree with the fewest evolutionary
events is the most likely to show the correct evolutionary relationship.
PPT on how to build a cladogram is on Schoolpointe
Figure 3 – Module 81 – Using sequencing data to determine molecular relationships – used
in the lab