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Exceptions to Mendelian
• Some alleles are neither dominant nor
recessive, and some traits are controlled by
multiple alleles or multiple genes and may be
influenced by the environment
Incomplete dominance
• the heterozygous phenotype is somewhere
between (intermediate to) the two
homozygous phenotypes.
• We see this often in flowers.
incomplete dominance
• P generation parents are
homozygous red and
• All F1 offspring are pink
and heterozygous
• When F1 offspring are
allowed to self-pollinate,
there is a 1:2:1
phenotypic ratio for the
F2 offspring
• Both alleles are expressed to some degree in
the heterozygous phenotype
• Ex. “Erminette” chickens, roan cattle
• In chickens, the allele for
black feathers and the
allele for white feathers
are both being expressed.
• In roan cattle, the allele
for red coat color and the
allele for white coat color
are both being expressed.
Codominance in Cattle
• More than two alleles may exist for a given
trait. An example is coat color in rabbits.
• C - full (brown/gray) coat color - dominant to
all other alleles
• Cch - “Chinchilla” coat color - all gray dominant to Ch and c alleles
• Ch - “Himalayan” coat color - white with black
features - dominant to c allele
• c - Albino coat color - no pigmentation recessive to all alleles
Multiple Alleles
• Human blood types are the result of multiple
• Three alleles: IA , IB , and i
• IA and IB are both dominant over i, neither is
dominant over the other
• When the IA and IB alleles occur together, they
are codominant
Polygenic traits
• Complex traits that are controlled by multiple
• It can be difficult to accurately predict the
appearance of some polygenic traits in
• Ex. human eye color, coat color in labs
Coat color in Labs
Qualitative v. Quantitative
• Qualitative traits - have a clear quality, simple
inheritance, limited environmental influence
• Ex. attached earlobes
• Quantitative traits - measurable but has many
different qualities, complex, large
environmental influence
• Ex. milking ability
Charles Darwin
Introduction to Natural Selection
Darwin’s Journey
• In 1831, 22-year old Darwin leaves on a 5
year voyage as a naturalist on the HMS
• The Beagle travels around the world, with
numerous stops along the South American
• As a naturalist, Darwin must make
observations of the natural world, specifically
plants, animals, geology, and climate of the
various regions.
Darwin’s Journey
Darwin’s Journey
• While on his journey, Darwin makes a number
of observations:
• Animals living in similar environments
exhibit some of the same adaptive
• Fossil remains of extinct animals resemble
some current species.
Darwin’s Journey
• For example, fossils
of the Glyptodon very
closely resemble, but
are not the same as
the armadillo.
• This leads Darwin to
believe that species
must be changing
over time.
Darwin’s Journey
• This notion that species change over time
clashes with the beliefs of that time period.
• People are firm believers in divine creation the belief that a divine being was responsible
for the creation of all life forms on Earth.
• This view is static and does not allow for
Science in the 1800’s
• At the time, a comprehensive fossil record is
being established by geologists.
• The fossil record contradicts the theory of
divine creation.
• Several people in the science community
begin to discuss species change.
• In 1809, Lamarck
theorizes that:
• Physical features
change in size based
on how much or how
little the animal uses
them in it’s lifetime. Use
and disuse.
• These acquired
changes are heritable are passed from
parents to offspring.
• Lamarck theorized that these sorts of traits
were heritable. As we will soon see, evolution
is something that takes place over
generations, not in an individuals lifetime.
• Lamarck did lay the foundation for Darwin’s
theory by stating that species change with the
“physical conditions of life” - the species
• These changes are evolutionary
Darwin’s Journey
• Darwin returns from his journey in 1836.
• He has overwhelming evidence to show that
evolution exists.
• He doesn’t publish his theory until 1859.
• Why?
• Darwin finds the answer to his problem in an
unlikely place - an essay on economics by
Thomas Malthus.
• In the paper, Malthus is discussing the growth
of human populations and food supply.
• Due to different types of
growth, populations could
easily exceed resources.
• When resources become
scarce, war and famine
control the population.
• What does this have to
do with Darwin and
• To summarize the main point of Malthus’
essay, human populations are limited by their
• Humans are limited by their environment.
• When humans come to conflict over scarce
resources, the stronger, better-equipped will
• Sound familiar?
Natural Selection
• That’s because it’s the basis for Darwin’s
mechanism of evolution!
• Darwin believes that this applies not simply to
humans, but to all living species.
• Darwin theorizes that species evolve through
the process of natural selection.
Natural Selection
• Natural selection is defined as the differences
in survival and reproduction among individuals
in a population as a result of their interaction
with the environment.
• In other words, some individuals are better
able to survive and reproduce because they
have traits that make them better at coping
with their environment
Natural Selection
• Two major premises for Darwin’s theory:
• Species are not immutable.
• The driving force behind evolutionary
change is natural selection or “survival of
the fittest”.
Natural Selection
• Within a population, genetic diversity exists.
• In a particular environment, certain individuals
in a population are better suited to survive.
• Over time, genes that code for traits that
enable individuals to survive and reproduce
accumulate (greater allelic frequency).
• Evidence indicates modern species evolved
from organisms that are now extinct
QuickTime™ and a
are needed to see this picture.
Natural Selection
• Beneficial traits are evolutionary adaptations
to the environment, and increase the
individuals fitness - the relative ability to
survive and leave offspring.
• New alleles arise by chance out of random
• Some mutations better equip an organism to
survive and reproduce (increase fitness),
which will lead to those new traits being
passed on.
• We consider those traits “beneficial”
• If environmental conditions remain the same,
over time the beneficial trait will increase in
Natural Selection
• Darwin presented the following arguments to
support evolution:
• Populations have a huge reproductive potential
• Population sizes remain relatively stable.
• Resources are limited.
• Individuals compete for limited resources.
Natural Selection
• There is variation among individuals in a population.
• Most of this variation is heritable.
• Only the most fit individuals survive. Individuals with traits
best adapted for survival in the environment are able to
outcompete others.
• Over time, traits best adapted for survival in the
environment (and the genes behind them) accumulate in
the population.
Types of Selection
• Stabilizing
• Directional
• Disruptive
• Sexual Selection
• Artificial Selection
Directional Selection
• Case of the
Moth in
• PreIndustrializatio
n = light moth
is selected for.
Directional Selection
• After the Industrial
Revolution, thick soot
covers the trees that the
moths live on.
• Now, dark moths are
selected for.
African Swallowtail
Sexual Selection
Male Competition - Brooklyn Parrots
Male Competition
QuickTime™ and a
are needed to see this picture.
Sexual Dimorphism
Sexual Dimorphism
QuickTime™ and a
are needed to see this picture.
Sexual Selection
Female Choice - Blue-Footed Booby
Female Choice
QuickTime™ and a
are needed to see this picture.
Stabilizing Selection
• Eliminates
individuals that
have extreme or
unusual traits.
• Individuals with
the most common
trait are best
Height variation in
adheres to stabilizing
Directional Selection
• Favors traits that
are at one extreme
of a range of traits.
• Traits at the
opposite extreme Example: Industrial Melanism
are selected
Disruptive Selection
• Selection for
extreme or unusual
traits, while
selecting against the
common traits.
• Example: weeds
and disruptive
selection for height.
Sexual Selection
• Differential mating of males (sometimes
females) in a population.
• Male competition: awards mating opportunities
to the strongest males.
• Female choice: leads to traits or behaviors
that are attractive to females.
Artificial Selection
Directional selection carried out by humans
Artificial Selection
Artificial Selection
Artificial Selection