Download IV. Natural Selection

IV. Natural Selection
A. Important points
(1) Natural selection does not cause genetic changes in individuals
(2) Change in allele frequency occurs in populations
(3) Fitness !" Reproductive Success
= survival to reproductive maturity, passing on genes, > # of offspring
(4) Evolutionary changes are not goal oriented
mutations are random
environments/selective forces constantly change>>
(5) Of the 5 agents of Evolutionary Change, ONLY selection produces
adaptive evolutionary change
C. Components of Fitness
(1) Competition:
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scarce resources favor the
best-adapted individuals
(2) Predator/Prey Interactions
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one organism (the predator) eats another (the prey)
– coevolution of predator and prey
(3) Symbiosis
•
leads to the most intricate
coevolutionary adaptations
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(4) Sexual selection (access to sexual partners)
" Favors traits that help an organism attract a mate
" Usually, all females get to breed, but only some males
" Result is sexual dimorphism
2 mechanisms:
Male-male combat (large structures for fighting; large
male size)
Female choice (males have attractive structures to lure
females)
***Under both scenarios – females get “good genes”
for offspring
C. Components of Fitness (cont.)
(5) Kin Selection
• Actions of an individual increase survival and reproductive success
of relatives
• Who have some of the same alleles!
Prairie Dogs
Spadefoot toad tadpoles
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Forms of Selection
Favors one
extreme
Favors average
Favors both
extremes
V. Forms of Selection: What type of Selection?
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70
50
30
20
15
10
10
5
7
5
3
2
Percent infant mortality
Percent of births in population
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2 3 4 5 6 7 8 9 10
Birth weight in pounds
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V. Forms of Selection: What type of Selection?
Peppered Moths and Industrialized Melanism
" Until the mid-nineteenth century, peppered moths, Biston betularia,
had predominately light-colored wings. Subsequently, dark
individuals became predominant
" Industrial smog helped turn lichens on tree trunks dark.
" Contrasting colors between trunk color and moth color led to
differential predation by birds.
VII. Constraints on Evolutionary Perfection
" Historical constraints
• Evolution tinkers with existing structures
" Adaptations are compromises
• Seal flippers on land and water
" Chance and selection interact
• No prediction of future conditions
" Selection can edit only existing variation
• Mutation is random
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I. What Is a Species?
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•
•
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groups of actually or potentially interbreeding natural populations
that are reproductively isolated from other species
maintain connectedness over geographic distances
gene flow maintains a species
production of fertile,
viable offspring
II. Isolating Mechanisms
Species maintain their genetic distinctiveness through
barriers to reproduction
A) Prezygotic Isolating Mechanisms
1. Geographical Isolation
2. Ecological Isolation
3. Temporal Isolation
Rana aurora - breeds January - March
Rana boylii - breeds late March - May
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A) Prezygotic Isolating Mechanisms
4. Behavioral Isolation
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different courtship rituals
5. Mechanical Incompatibility
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Structural differences prevent mating
6. Gametic Incompatibility
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Sperm from one species are unable to
fertilize eggs of another
B) Postzygotic Isolating Mechanisms: hybrid zygotes fail to
develop or develop abnormally, or hybrids cannot establish
1. Hybrid Inviability
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offspring fail to survive to maturity due
to developmental defects
2. Hybrid Infertility
•
hybrid offspring unable to produce
normal sperm or eggs
Animal hybrid examples:
mules (horse + donkey)
ligers (lion + tiger)
•
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often sterile
chromosomes don’t pair properly
during gamete formation (i.e.
meiosis)
3. Ecological inviability
•
hybrids develop normally, but cannot
find a suitable ecological niche
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III. Speciation
Similar populations of one species diverge until
reproductive isolation results ! Separate species
1. Populations adapt to their environment or random
changes may occur in separate populations.
2. Adaptation or random changes result in the accumulation
of many differences between the populations.
3. Isolating mechanisms prevent population hybridization.
4. The populations are no longer capable of interbreeding
successfully.
III. Speciation
A. Requirements of speciation
C
B
(1) restricted gene flow
" very low or no gene flow (migration) between
diverging populations
(2) genetic divergence
large genetic differences accrue between
populations
(3) reproductive isolation must evolve
to maintain the differences
" can no longer interbreed
" can no longer produce normal
offspring
A
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B. Mechanisms of Speciation
(1) Allopatric Speciation:
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populations geographically
separated
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no or little gene flow
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Differences accumulate
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Eventually, cannot
interbreed
B. Mechanisms of Speciation
(1) Allopatric Speciation:
Long distance colonization
Barriers to movement
Extinction of intermediate
populations
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B. Mechanisms of Speciation
(2) Sympatric Speciation:
• populations share the same
geographic locality
• but with no (or little) gene flow
• What reduces gene flow in these
cases?
(2) Sympatric Speciation: What reduces gene flow?
a. Ecological isolation / Disruptive selection
Rhagoletis pomonella
• Fruit fly: Parasite
• Lays eggs in fruit
HAWTHORN TREES (native)
APPLE TREES
(introduced)
• Flies have different emerging times
• Tree “loyalty”
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III. Speciation
C. Speciation produces Evolutionary Trees
= Canines
D. Adaptive Radiation
One species
new
new
new
new
species:
species:
species:
species:
A
B
C
D
Typically occurs when populations of a single species ...
• invade a variety of new habitats,
• evolve under different environmental pressures (selective
forces)
• Examples: Darwin’s Galapagos Finches, Hawaiian
Silverswords
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D. Adaptive Radiation
IV. Pace of Evolution
Gradualism
phenotypic change
Evolutionary change is slow and imperceptible from generation
to generation
Over the course of thousands and millions of years, major
changes could occur
Punctuated equilibrium
Evolution normally proceeds in spurts, with long periods of little
evolutionary change in between.
The proposed stasis would be expected in large populations
experiencing stabilizing selection over long periods of time.
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V. Extinction
Extinction = death of all the members of a species
How and why does it happen?
(1) Localized distributions & overspecialization
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Increase an organism’s likelihood of extinction
Devil’s Hole pupfish:
Lives in isolated water
pools in Mojave
Desert
What Causes Extinction?
(2) Interactions with other organisms
- competition, parasitism, predation
EX. the Panama land bridge: connected 2 continents, formed during the
Pleiocene (~3 mya)
• N. American species & South American species mixed
" For the first time in history
• Many S. American species were out-competed
" went extinct
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What Causes Extinction?
(3) Habitat change /
destruction
• leading causes of
extinction
• tropical forests
• ALSO: Habitat
Fragmentation
What Causes Extinction?
(3) habitat change / destruction
What can we do?
BOZEMAN PASS WILDLIFE LINKAGE
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VI. Speciation and Extinction Through Time
There have been 5 major mass extinctions interspersed within
relatively consistent extinction patterns.
most famous " end of Cretaceous period (65 mya); dinosaurs went
extinct
Mammals quickly experienced evolutionary radiation.
Biological diversity tends to rebound after mass extinctions
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