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Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
DARWIN IN HISTORICAL CONTEXT
Evolution
• Evolution: The changes that have
transformed life from single-celled organisms
of the past to complex organisms seen today
• Darwin proposed that populations of
organisms change over time in response to
environmental pressures
– These changes occur within a population due to
differences of reproductive success
– i.e. “Survival of the fittest”
Survival of the Fittest
• Organisms that are most fit to survive in their
environment are more likely to have offspring
and pass on their genes
Darwin
• In 1844, Darwin wrote an essay on his theory
of evolution
• He called the theory “Descent with
modification”
• He believed that all organisms were
descended from a single unknown prototype
• Over time, organisms have acquired
modifications that make each species unique
Natural Selection
• Darwin proposed that evolution occurs
through a process called “natural selection”
– There is variation within a population
– Individuals with advantageous traits produce
more offspring
– The unequal ability of organisms to survive and
reproduce leads to gradual changes in a
population
Constraints
• There are constraints to the process of natural
selection
– Only works on variations present in a population
– Only affects traits that are passed on to offspring
– Causes changes in a population, not an individual
Evidence to support evolution
• Evolution leaves observable signs as clues to
the past
– Fossils support the theory of evolution
• Help to establish the order of when organisms
appeared, even if now extinct
• Scientists can see how similar organisms have changed
over time
Evidence to support evolution
• Anatomical similarities between species also
supports evolution
– Example: Humans, whales, bats and all other
mammals have similar forelimbs
– Structures are similar, even though they perform
very different functions
– Some organisms possess vestigial structures
• An ancestral structure that has lost its use
• For example, some snakes have the remnants of a
pelvis and legs, suggesting that they evolved from
lizards
Evidence to support evolution
• Molecular evidence supports evolution
– Today scientists can compare the DNA and protein
sequences of organisms
– Closely related organisms often have similar
amino acid sequences between certain proteins
– Certain fundamental processes, like cell division,
have been conserved throughout evolution from
yeast, to plants to mammals
Evidence to support evolution
• While evolution is usually difficult to observe,
there are some examples that are easy to see
– Example: English peppered moth population
before and after the Industrial Revolution
– These moths spend much of their time on Birch
tree bark (normally have light colored bark)
– Before the Industrial Revolution, 99% of the
moths were light colored and were difficult for
predators to see and catch
Evolution of the English Pepper Moth
• The Industrial Revolution introduced many sootproducing factories
• The soot coated the birch trees, making them black
• Light colored moths became easy to see
• After the Industrial Revolution, 99% of the moth
population was dark colored
Artificial Selection
• Artificial selection is used in the selective
breeding of domesticated plants and animals
• Examples:
– Farmers breed cows to increase milk production and
generate leaner beef
– Crops are selected for higher yields and for taste
– Flowering plants are selected for their large, showy
flowers
– Often, characteristics of organisms can be changed in
just a few generations
• This suggests that the same process could occur
via natural selection
Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
MECHANISMS OF EVOLUTION
Definitions
• Macroevolution: Major change over very long
periods
• Microevolution: A change in the gene pool of
a population over a few generations.
– Smaller change
Population evolution
• Evolution: The change in the genetic makeup
of a population over time
• In order for a population to evolve:
– The population must have genetic variation
– Genetic variation: Genetic dissimilarity among
members of a population
– Mutations are a major source of genetic variation
– Also, sexual reproduction contributes to genetic
variation (independent assortment and crossing
over)
Population evolution
Sexual Reproduction
Independent assortment
Sexual Reproduction
Recombination
(aka crossing over)
Mutation
Genetic Diversity
Evolution
Individuals do not evolve
• Populations, not individuals, evolve
• Selection occurs at the level of the individual
• The reproductive success of individuals alters
the frequencies of certain traits within a
population, and the population evolves
Allele frequency
• How is the genetic makeup if a population
determined?
• Consider the allele frequency of a single gene
• Diploid organisms have two copies of each
gene (may or may not be identical)
• Allele: alternative form of the same gene
• In a population, the relative number of copies
of each allele may be different
• Allele frequency indicates the amount of
genetic diversity in a population
Eye color
• In humans is determined by 3 genes and
multiple alleles
• To simplify, let’s consider just 1 gene
– bey 2 gene
– B allele is dominant (brown eyes)
– b allele is recessive (blue eyes)
Allele frequency
BB
Bb
Bb
Bb
Bb
bb
• How many B alleles?
9
• How many b alleles?
11
• How many total?
20
bb
Bb
BB
bb
Allele frequency
•
•
•
•
9 B alleles
11 b alleles
20 total alleles (= B + b)
Let p = the frequency of the dominant allele
B
= 9/20
= 0.45
• Let q = the frequency of the recessive allele b
= 11/20
= 0.55
Allele frequency
•
•
•
•
The sum of allele frequencies always equals 1
p+q=1
0.45 + 0.55 = 1
This is possible because we know the
genotypes
• However, we don’t always know genotypes
• Can be difficult to distinguish between
homozygous dominant individuals and
heterozygous individuals
Hardy-Weinberg Equation
• Possible to estimate alleles frequencies using
the Hardy-Weinberg equation
• p2+2pq+q2 = 1.0
• This equation can only be used when there
are no selective pressures acting on a
population causing it to change
• i.e. this equation describes a population at
equilibrium
• Population that is not changing is at
equilibrium
Equilibrium
• Conditions for equilibrium (not changing):
– Population must be very large
– Mating must be random
– No mutations occurring nor evolutionary
pressures acting on the population
Back to eye color…
• p2 = Frequency of BB genotype (homozygous
individuals)
• q2 = Frequency of bb genotype (homozygous
individuals)
• 2pq = Frequency of Bb genotype
(heterozygous individuals)
Is this population at equilibrium?
•
•
•
•
•
BB
Bb
Bb
Bb
Bb
bb
bb
Bb
BB
bb
p2+2pq+q2 = 1.0 (at equilibrium)
p = 0.45
q = 0.55
(0.45)2 + 2(0.45)(.055) + (0.55)2 = ?
? = 1; therefore, the population is at equilibrium
Evolutionary change
• Populations are rarely at equilibrium
• Four pressures that can cause populations to
change
– Genetic drift
– Gene flow
– Non-random mating
– Natural Selection
Genetic drift
• Genetic drift: Changes in gene frequencies of
a small population due to chance
– In small populations, chance events can
permanently change the populations gene pool or
allele frequencies
Bottle Neck Effect
• The bottle neck effect can cause genetic drift
• Due to an event that leads to a significant
reduction in the population
• Only a few individuals survive to pass on their
genes – alters allele frequencies
Founder Effect
• The Founder Effect: Due to the migration of a
few individuals away from a population
• The new population is established in a new
location
• The allele frequency of the new population
may be very different from that of the old,
depending on the allele frequency of the
founding individuals
Gene Flow
• Gene flow: The change in a population’s allele
frequency resulting from migration
– Can be due to individuals entering or leaving a
population
– The frequencies of alleles change when individuals
enter or leave a population
Nonrandom mating
• Nonrandom mating: The
selection of mates on the
basis of a trait or group of
traits, not by chance
Nonrandom mating
• Assortative mating: Form of nonrandom
mating; partners resemble each other
– Although it does not change allele frequencies, it
does reduce the number of heterozygous
individuals
– Can lead to inbreeding: Mating between
genetically related individuals; increases the risk
that an individual will be born with a homozygous
damaging recessive trait
Nonrandom mating
• Sexual selection: another type of nonrandom
mating; selection based on the evolution of
traits among individuals of the same sex
– Results from either
• Selection by the opposite sex, or
• Competition among individuals of the same sex
Natural selection
• Natural selection
– Adapts organisms to their environments
– Facilitates evolution by increasing or decreasing
the odds that the organism successfully
reproduces
• Darwinian fitness: The contribution an
individual makes to the gene pool of the next
generation relative to others
– Difficult to measure
– Therefore, scientists look at “relative fitness”
Relative fitness
• Relative fitness: The contribution of a
genotype to the next generation relative to
other genotypes
• The reproductive success of an individual
ultimately depends on its genotype
• Differences in genotype arise from mutation
– Some mutations are harmful, some have no effect
and some are beneficial
– When a beneficial mutation occurs, the individual
has a reproductive advantage
Natural selection
• When an individual has a reproductive
advantage, the population is not at
equilibrium (Hardy-Weinberg equation does
not apply)
• The population evolves
• Favorable mutations accumulate and persist in
a population
Natural selection
• Natural selection changes allele frequency in
three ways
– Stabilizing selection: Minimizes extreme
phenotypes (example – average birth weights –
higher mortality rates in very small or very large
babies)
– Directional selection: Shifts phenotype to one
extreme (often caused by environmental shifts –
example – moth color)
– Diversifying selection: favors both extremes
(example – beak sizes of finches in different
habitats)
How is diversity maintained?
• What prevents natural selection from
eliminating unfavorable traits?
• Are several mechanisms at work:
– The diploid character of most eukaryotes hides
recessive alleles in heterozygous individuals
– Polymorphism: The co-existence of different
alleles in the same population
• Balanced polymorphism: The coexistence of different
alleles without any change in their frequency
Balanced polymorphism
• How does a balanced polymorphism occur?
• Heterozygote advantage: The reproductive
advantage of heterozygous individuals over
homozygous individuals
– Example: the allele for sickle cell disease is
maintained at relatively high levels in some
African countries
– Heterozygous individuals have the selective
advantage of surviving Malaria
Balanced polymorphism
• In plants, inbreeding often results in reduced
yields and increased sensitivity to disease
• Crossing of two inbred varieties produces
hybrid offspring that are more vigorous than
either parent
• Hybrid vigor: The superiority of a hybrid
offspring
– Unfavorable recessive alleles are “hidden”
– Promotes heterozygote advantage
Balanced Polymorphism
• The environment plays a role
• Different habitats
• Drive divergent evolution
• Act to preserve different phenotypes
Balanced polymorphism
• Frequency-dependent selection: A type of
selection in which the frequency of a certain
phenotype determines the reproductive success
of the organism
– Reproductive success declines when a phenotype
becomes too common
– Example: HIV evolves rapidly to avoid immune system
detection and destruction and drug intervention
– When a variant becomes too common it is more easily
recognized by the host’s immune system, or targeted
by drugs
Unit 5: Evolutionary Biology
Chapter 12: The Theory of Evolution
POPULATION GENETICS AND
EVOLUTION
Consider Blood Rh Factor
• When a person receives foreign blood, their
immune system may recognize the blood as
being foreign
• The blood may be recognized because of
proteins and sugars present on the outer
surface of the blood cells
• The molecules are classified into blood groups
• Example: ABO group or the Rh factor group
Rh factor
• The Rh blood group is a very complex blood
group polymorphism
• The molecules in this group are collectively
referred to as the Rh factor
• About 85% of the population posses the factor
and are Rh+
• About 15% lack these molecules and are Rh– Serious complications can arise when Rh- mothers
carry Rh+ babies as the mother’s immune system
targets the babies blood as being foreign
For our example…
• We will simplify the Rh factor down to just
two alleles
– One dominant (A) and one recessive (a)
– AA
Rh+
– Aa
– aa Rh-
Remember…
• If population size (N) = 100, the total number
of alleles = 200
• The number of A alleles in a population of 100
people = 120
• How many a alleles?
• 80
• Allele frequency = the number of allele copies
total alleles
Remember…
• The frequency of A = 120/200 = 0.60
• The frequency of a = 80/200 = 0.40
• In this example, we are given the allele
frequencies
• It can be difficult to determine allele
frequencies and genotype frequencies based
on phenotype alone
• We can use the Hardy Weinberg formula to
calculate genotype and allele frequencies
Hardy-Weinberg Conditions
• Random mating
• Large breeding population
• No differential migration (when individuals with
specific traits leave the population)
• No mutation of the alleles
• No natural selection
• When these conditions are met, populations do
not evolve and allele frequencies do not change
– The Hardy-Weinberg formula can be used to predict
frequencies of future generation