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•The Black Death…The
Dramatic Conclusion
•Population Genetics
Continued
•Origin of Species!
Microevolution:
• generation-to-generation
change in a population’s
frequencies of alleles
• Below the species level!
Four Primary Causes of Microevolution:
1. Genetic Drift
2. Natural Selection
3. Gene Flow
4. Mutation
2.Natural
2.Natural Selection:
Types of Selection: Examples??
Natural selection sorts through variations,
increasing the frequencies of certain genotypes
and “fitting” organisms to their environment.
It is the entire organism that is “sorted”, as
natural selection acts on phenotype.
Diversifying Selection: An Example
Looking at GENETIC VARIATION, the
Foundation of Natural Selection:
•Variation occurs both within and between
populations
•Not all variations are heritable! (Phenotype)
The female Papilio butterflies exist in several morphs, some of which
resemble two other species which are noxious. Intermediate butterflies
do not gain the advantage of mimicry and thus are more likely to be
preyed upon.
Seasonal variations of Arachnia levana, the map butterfly. Individuals that emerge in spring are orange and
brown; individuals that emerge in late summer are black and white.
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Variation may consist of QUANITATIVE or
DISCRETE Characters
Quantitative variation usually indicates polygenic
inheritance
If two + discrete characters are present in a
population, it is POLYMORPHIC.
Genetic Diversity may be measured most
directly as GENE DIVERSITY or
NUCLEOTIDE DIVERSITY
Gene Diversity: the percent of loci
that are heterozygous. Fruit flies
typically have ~14% gene diversity.
Nucleotide Diversity: percent difference of the
nucleotide sequence. Fruit flies typically have ~1%
sequence diversity. (humans = ~0.1%)
GEOGRAPHIC VARIATION may also be
observed BOTH within and between
populations.
Cline
Geographic
Variation may
also occur in
DISCRETE
CHARACTERS
Variation is generated by MUTATION
and SEXUAL RECOMBINATION
Variation is generated by MUTATION
and SEXUAL RECOMBINATION
•Many single point mutations have little or no effect
• However, organisms that reproduce rapidly, or
sloppily (i.e. HIV) can achieve massive variation
through mutation.
•Those that do have serious effects of the protein are
usually harmful
•Thus random change rarely improves the genome,
especially in a stable environment
•Sexual reproduction alters the combination of alleles
an individual receives
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Preserving Variation:
1. Diploidy- “hides” genetic variation from selection
Frequency-dependent selection-survival and
reproduction of any one morph declines if that
form becomes too common.
2. Balanced Polymorphism- natural selection favoring
2+ phenotypes
1. Heterozygote advantage- Sickle Cell Disease
2. Frequency-dependent selection- survival and
reproduction of any one morph declines if that
form becomes too common.
3. Neutral Variation- no known selective
advantage “shields” from natural selection.
Sexual selection
Darwin: "...the advantage which certain
individuals have over others of the same sex
and species solely in respect of reproduction“
or
Sexual Selection frequently involves a TRADEOFF between survival and attracting mates.
Sexual selection arises because one sex is a
limiting resource for another.
Selection that arises from
differential mating
success
Two Types of Sexual Selection:
Two Types of Sexual Selection:
1. INTRASEXUAL selection
2. INTERSEXUAL SELECTION (Mate Choice)
• Well documented
Females are usually
more selective in
mate choice than
males (why?)
• Arises (typically) from competition
among males for access to females or
mating sites
• Often results in larger body sizes or
weaponry that may be advantageous in
battle
More Problematic!
A male and female Bowerbird. Males build ornate nests to compete for mates.
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Female preference can be a complex trait!
Preference may serve to increase her
fitness (examples?)
Fitness may not be affected by her
preference (if not, why should she have a
preference?)
If females are generally the "choosy" ones,
how might you explain some of the
exceptions (like sea horses) where males
are the ones who are choosy?
Three Hypotheses to Explain Intersexual
Selection:
1.
2.
3.
Direct BenefitsBenefits- increased food, protection, etc
Indirect BenefitsBenefits- good genes?
Sensory BiasBias- artifact of other selective force
Reminders: Natural Selection is limited!
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Defining a Species:
the Biological Species Concept
The Next Step:
Macroevolution &
the Formation of
Species
Evolution limited by historical
constraints
Adaptations are often compromises
Not all evolution is adaptive (Chance!)
Selection can only sort existing variations
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Defines a species as…
a population (s) whose members
can potentially interbreed in
nature to produce viable, fertile
offspring, but who can’t produce
viable, fertile offspring with
members of other species
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Barriers leading to Biological Species:
Types of Prezygotic Barriers:
Habitat IsolationIsolation- two
species rarely encounter
each other, despite living in
same area
ExEx- Two species of Manzanita,
share geographic range in
the Sierra. One species is
better adapted to more
sheltered areas; the other to
exposed, rocky hillsides.
1.
Barriers can be
PREZYGOTIC
or
POSTZYGOTIC
Types of Prezygotic
Barriers:
Fireflies. Males use patterns of flashes to signal to females.
3. Temporal IsolationIsolation- two
species breed during different
times and thus do not
interbreed despite sharing the
same geographic area.
2. Behavioral Isolation:
specialized signals and
behaviors that prevent mating
between species.
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Blue-footed boobies performing a mating ritual.
Types of Prezygotic Barriers:
Ex. Many birds use complex
displays including tactile, visual,
and acoustical components to
attract and recognize mates.
Male fire flies use distinct
flashes of light to attract and
recognize mates.
The periodical cicada, Magicicada septendecim
Types of Prezygotic Barriers:
4. Mechanical IsolationIsolationanatomical differences prevent
mating. Can be direct or
indirect.
Ex.
Ex. The zebra orchid has evolved
a precise, anatomical fit with
the wasp species that pollinates
it, an indirect mechanical
isolation.
Ex.
Ex. This periodical cicada
emerges from the ground to
reproduce every 17 years. A
sibling species shares the same
habitat, emerging every 13 years
to reproduce
Types of Prezygotic
Barriers:
Balanophora fungosa pollen grain.
5. Gametic IsolationIsolationdifferences in the structure of
the gametes or reproductive
structures prevent gametes of
different species from fusing.
Ex.
Ex. Molecular recognition
enables many flowers to
discriminate between pollen
grains from different species.
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Types of POSTzygotic Barriers:
1. Reduced Hybrid ViabilityViabilityHybrid zygotes may have genetic
incompatibility that interferes
with development. Even
hybrids who do complete
development may be frail.
Ex.Ex.- Leopard frogs frequently
hybridize, but offspring that
survive are frail. Even Darwin’s
finches may occasionally form
viable hybrids.
Types of POSTzygotic Barriers:
Reduced Hybrid Fertility2.
FertilityTwo species mate, produce
vigorous offspring, but hybrids
are sterile
ExEx- Mules are robust but sterile
hybrids between a horse and
donkey
The leopard frog, Rana pipiens
Types of POSTzygotic Barriers:
Hybrid Breakdown3.
BreakdownFirst generation hybrids are
robust and fertile. Their
offspring are feeble or
sterile.
The BREAKDOWN of the
BIOLOGICAL SPECIES CONCEPT
Many limitations, including:
•Impractical for many existing,
and all extinct species!
•Doesn’t allow for classification
of asexual organisms!
ExEx- Many species of plants,
cotton, produce fertile
hybrids that “die off”.
A BETTER MODEL??
ALTERNATE MODELS:
The ECOLOGICAL species conceptconceptDefines a species in terms of its ecological niche
-Emphasizes the ROLE of an organism
-Accommodates asexual organisms
ALTERNATE MODELS:
The Pluralistic Species ConceptConceptDefines a species based on variable
characters on a case by case basis
Hookworm SEM
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ALTERNATE MODELS:
The Morphological Species ConceptConceptCharacterizes each species in terms of a unique set
of structural features
-Easy and most
common
-Doesn’t address
evolutionary
relationships
ALTERNATE MODELS:
The Genealogical Species Concept
Defines a species as a set of organisms with a
unique genetic history
Modes of Speciation
Biological Model is perhaps most useful in
understanding the causes of SPECIATION.
SPECIATION driven by reproductive barriers
to gene flow.
Two primary Modes: ALLOPATRIC and
SYMPATRIC Speciation
Modes of Speciation: Allopatric
Speciation
Allopatric (“other homeland”) Speciation occurs in
populations with geographically separate ranges
Gene flow is interrupted or reduced due to spatial
separation (geological remodeling).
Likelihood increased in SMALL, ISOLATED
populations–
populations– WHY?
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A few individuals of a
species on the mainland
reach isolated island 1.
Speciation follows genetic
divergence in a new habitat.
Later in time, a few
1
individuals of the new
species colonize nearby
island 2. In this new
habitat, speciation follows
genetic divergence.
Speciation may also
follow colonization of 1
islands 3 and 4. And it
may follow invasion of
island a by genetically
different descendants
of the ancestral species.
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2
4
ALLOPATRIC
SPECIATION
on an isolated
archipelago
ADAPTIVE
RADIATION
2
3
2
4
Ex- Hawaii, the
Galapagos, but not
the San Juans–
WHY??
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Hawaiian Honeycreepers:
KONA
FINCH
extinct
Key Question: Did speciation occur
during geographic isolation?
KAUAI AKIALAOA
LAYSAN
FINCH
AMAKIHI
AKIAPOLAAU
IIWI
MAUI
PARROTBILL
APAPANE
insect and nectar eaters
fruit and seed eaters
FOUNDER SPECIES
Allopatric Speciation in Process can
Produce RING SPECIES
Expected pattern
of genetic
relatedness?
Two Important Misunderstandings:
1.
2.
Geographic isolation prevents interbreeding,
but doesn’t necessarily equal reproductive
isolation!
Reproductive barriers usually evolve by
coincidence. (No selection pressure!)
In fact, when hybridization can
occur, it usually confers a
selective ADVANTAGE!
Used with permission; www.denniskunkel.com
Modes of Sympatric
Speciation
Red viscacha rat from
Argentina = 4n
One Possible ALLOPOLYPLOID Mechanism:
A new species can also arise in the
same geographic area as the parent
species.
Modes: Polyploid Speciation in Plants (and animals!)
Autopolyploids have 2+ chromosome sets from a
single species.
Allopolyploids have chromosome sets from two
different species.
Note: This causes a SPECIATION EVENT in a
single generation!
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Other Modes of Sympatric Speciation
•Populations in the same area can become isolated due to
resource use (habitat, food source, etc.)
•Rigid mate preference can also
make populations diverge.
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