Advanced Topics in Evolutionary
Theory
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For natural selection
to operate, there must
be variation among
individuals in a
population. Variation
arises from:
Mutations
Sexual Reproduction
Balanced
Polymorphism
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Mutations are random
changes in the DNA
of an individual.
Mutations can
introduce new alleles
into a population.
Mutations provide the
raw material for
variation in a
population.
Mutations in fruit flies
include changes in eye
color and wing shape
This dog has more
muscle due to a
muatation. Good or
bad—what do you
think?
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Sexual reproduction
produces individuals
with new combinations
of alleles. 3 Events lead
to genetic variation:
1. Crossing over
2. Independent
Assortment of
homologs
3. Random joining of
gametes in fertilization
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Balanced
polymorphism is the
maintenance of
different phenotypes in
a population.
Often, a single
phenotype provides the
best adaptation and
becomes more common
in the population.
However, examples of
polymorphism exist in
many populations.
The Gouldian Finch possesses a
genetic color polymorphism in the
form of three genetically determined
head-colors (yellow, black and red)
that coexist in the same population.
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Some characters are
fixed in a
population—this
means that all
individuals in a
species have these
traits.
Other characters are
polymorphic,
meaning there are two
or more variants.
All tulips grow from bulbs—a trait that is fixed in their species. However,
there are numerous varieties of colors in their flowers.
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Sickle Cell
Heterozygote
is
Advantage: occurs when Anemia
caused by a
being heterozygous for a homozygous
recessive
trait has a greater
gene -- ss
selective advantage. Ex:
Sickle Cell Anemia—
individuals who are Ss
have an advantage
because they will not get
either Sickle Cell Disease
or Malaria
People who
are
homozygous
dominant for
this gene—SS
are suseptible
to malaria.
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Hybrid Vigor:
describes the superior
quality of some
hybrid offspring. This
comes from a reduced
number of deleterious
recessive conditions
and heterozygous
advantage Ex: Hybrid
Corn
The 2 corn plants on the left are the
original parents. The corn plant on
the right is a hybrid of the two.
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Frequency-dependent
selection (or minority
advantage) occurs when
the least common
phenotype have a
selective advantage.
Common phenotypes are
selected against!
However, since rare
phenotypes have a
selective advantage, they
soon increase in frequency
and become common.
Once they become
common—they are
selected against!
Some predators form a “search
image” or standard representation
of their prey. By standardizing on
the most common form of its prey,
the predator optimizes its search
effort. The prey that is rare,
however, escapes predation.
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Not all variation has
selective advantage.
Instead, much of the
variation observed,
especially at the
molecular level in DNA
and proteins, is neutral
variation.
In many cases, the
environment
determines whether a
variation is neutral or
not.
Every human has slightly different
fingerprints—even identical twins.
However, this is an example of
neutral variation.
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Natural selection is
not the only way that
evolution occurs.
Factors that can
change allele
frequencies are:
Natural selection
Mutations
Gene Flow
Genetic Drift
Nonrandom Mating
If there is no natural selection, gene flow,
mutation, genetic drift and there is
random mating, then Evolution Cannot
Occur!
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Mutations—introduce
new alleles that may
provide a selective
advantage (most,
however, are harmful)
Gene Flow—describes
the addition or
removal of alleles
from the population
by emigration or
immigration
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Genetic Drift is a random
increase or decrease of
alleles.
Some alleles may
increase or decrease for
no other reason than
just by chance.
When populations are
small (usually less than
100 individuals), the
effect of genetic drift
can be very strong and
can dramatically affect
evolution.
How will this random event
affect the evolution of this
beetle population?
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The Founder Effect—
occurs when allele
frequencies in a group
of migrating individuals
are, by chance, not the
same as that of their
population of origin.
Go to:
http://evolution.berkel
ey.edu/evolibrary/artic
le/_0/speciationmodes
_03
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A Bottleneck—occurs
when the population
undergoes a dramatic
decrease in size.
Regardless of the cause
of the bottleneck
(natural disaster, etc.),
the small population
that results becomes
severely vulnerable to
genetic drift.
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The Founder Effect:
One of the founding members of the small group of Germans that
began the Amish community in Pennsylvania possessed an allele for
polydactylism. After 200 years of reproductive isolation, the number of
cases of this trait among the 8,000 Amish exceeds the number of cases
occurring in the remaining world’s population
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Nonrandom mating
occurs when
individuals choose
mates based upon their
particular traits.
They may always
choose mates with traits
similar to their own (or
the opposite!)
Nonrandom mating
also occurs when mates
choose only nearby
individuals.
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When the allele
frequencies in a
population remain
constant from
generation to
generation, the
population is said to be
in genetic equilibrium,
or Hardy-Weinberg
Equilibrium.
There is No Evolution!
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1. All traits are selectively
neutral (no natural
selection)
2. Mutations do not occur
3. The population must be
isolated from other
populations (no gene
flow)
4. The population is large
(no genetic drift)
5. Mating is random
Can This Ever Occur in
Real Populations????
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Genetic equilibrium is
determined by
evaluating the
following values:
1. Allele frequencies
for each allele (p, q)
2. Frequency of
homozygotes (p2, q2)
3. Frequency of
heterozygotes
(pq + qp = 2pq)
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Also, the following
two equations:
1. p + q = 1
(all alleles sum to
100%)
2. p2 + 2pq + q2 = 1
(all individuals sum
to 100%)
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African Cichlids have evolved different
feeding mechanisms and anatomy to explore
different food niches. These are just a few,
including insect-eaters, snail eaters, algaeeaters, plankton feeders, fish-eaters, etc.
A species is a group of
interbreeding
organisms.
Speciation is the
process by which new
species evolve
Two forms of
speciation: Allopatric
and Sympatric
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Allopatric (“other
country”) speciation
occurs when some sort
of barrier separates a
single population into
two
The two populations
evolve independently,
and if they change
enough, then even if the
barrier is removed, they
cannot interbreed.
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An example of sympatric speciation
could occur if a new food source
became available. Only certain
organisms would exploit this new
resource , eventually leading to a
reduced gene flow between them
and the original.
Sympatric (“same
country”) speciation is
the formation of new
species without the
presence of a physical
barrier.
This may happen
when a new species
originates while still
living in the same
area as the parent
species.
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Balanced polymorphism—
When variations in a
population diverge so much
that the two variants can no
longer interbreed.
Polyploidy—A condition in
which an individual has
more than the normal
number of sets of
chromosomes (more
common in plants); these
will not be able to mate
with the original organisms.
The plant in the middle is a
polyploid individual while the
others are diploid. Because it is
polyploid, it cannot mate with its
diploid “cousins” and thus
becomes its own species.
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Adaptive radiation is a rapid series of speciation events
that occur when one or more ancestral species invades
a new environment.
If there are many ecological niches, several species will
evolve because each can fill a different niche.
An example of adaptive
radiation is the
evolution of finches on
the Galapagos Islands.
Each evolved to take
advantage of the special
food available on its
island.
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We define different species as organisms who
cannot (or will not) reproduce with each other.
Interbreeding between two different species
cannot produce viable, fertile offspring.
These two species are
very closely related, yet
one prefers to rest on
open, sunny areas and
the other rests on shady
branches of trees. They
have evolved different
behaviors and have
become different species.
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If species are not physically separated by a
geographic barrier, various mechanisms
commonly exist to maintain reproductive
isolation and prevent gene flow.
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Remember: a zygote is a fertilized egg—it is a
single cell made from the union of an egg and a
sperm
Prezygotic isolating mechanisms, then, will
prevent fertilization in some way:
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation
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Habitat isolation—species do not encounter each other
Temporal isolation—species mate or flower during
different seasons or at different times of the day
Behavioral isolation—when a species does not
recognize another species as a mating partner because
it doesn’t perform the correct courtship rituals, sing the
correct songs, or release the correct chemicals (scents).
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Mechanical isolation—when
male and female genitalia
are structurally incompatible
or when flower structure
select for different
pollinators
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Gametic isolation—when
male gametes do not survive
in the environment of the
female gamete (such as in
internal fertilization) or
when female gametes do not
recognize male gametes
Gametic isolation is particularly important in
aquatic environments because many aquatic
animals release their gametes into the water,
where fertilization takes place
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Postzygotic isolating
mechanisms prevent the
formation of fertile progeny.
So, the egg has been
fertilized but the offspring
doesn’t survive (or it
survives, but can’t
reproduce)
One example: Hybrid
inviability—when the zygote
fails to develop properly and
aborts (dies) before reaching
reproductive maturity.
Papilio glaucus (the Eastern Tiger
Swallowtail) and Papilio canadensis
(the Canadian Tiger Swallowtail)
are two species of North American
butterflies. Although similar in
appearance, they are adapted to
different climates, and they show
mild hybrid incompatibility
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Hybrid sterility occurs
when hybrids become
functional adults, but
are reproductively
sterile (eggs or sperm
are nonexistent or
dysfunctional).
Hybrid breakdown
occurs when hybrids
produce offspring that
have reduced viability
or fertility.
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Divergent
Evolution—describes
two or more species
that originate from a
common ancestor and
become increasingly
different over time.
This may happen as a
result of allopatric or
sympatric speciation
or by adaptive
radiation.
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Convergent Evolution—describes two
unrelated species that share similar traits. Their
similarities arise, not because they share a
common ancestor, but because each species has
independently adapted to similar ecological
conditions.
These species are not
closely related, nor
did they come from a
common ancestor,
yet all evolved wings
for flight
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Parallel Evolution—
describes two related
species or two related
lineages that have
made similar
evolutionary changes
after their divergence
from a common
ancestor.
This is one of several species of
rainforest moths that have
evolved coloration patterns
similar to those of butterflies.
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Coevolution is the titfor-tat evolution of one
species in response to
new adaptations that
appear in another
species.
Coevolution occurs
between predator and
prey, plants and planteating insects, and
flowering plants and
pollinators.
The Madgascar Orchid, shown
here, has a flower with a petal that
it 11” long with a tiny drop of
nectar at the very tip. The fly
shown with it has a proboscis that
is the exact length needed to get the
nectar and in the process, pollinate
the flower.