1. Section 15.2 Summary– pages 404-413
Population Genetics and Evolution
The principles of evolution are rooted in population genetics
Section 15.2 Summary– pages

2. Population Genetics and Evolution
A population consists of all the individuals of a species that live together in one place at one time and can successfully breed.
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3. Population characteristics
Population size~ number of individuals in a population
Density ~ number of individuals per unit of area
Dispersion~ pattern of spacing
•random ~ unpredictable, patternless spacing (a)
•clumped ~ patchy spacing (b)
•uniform ~ even spacing (c)
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4. Factors that affect Populations
Birthrate (natality)~ number of offspring produced
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5. Factors that affect Populations
Death rate (mortality)-how many people die
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6. Factors that affect Populations
Age structure~ relative number of individuals of each age
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7. Factors that affect Populations
Survivorship curve~ plot of numbers still alive at each age
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8. Carrying capacity is maximum number of individuals a habitat can support
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10. Population limiting factors
Density-dependent factors
Competition
Predation
stress/crowding
waste accumulation
Density-independent factors
•weather/climate
•periodic disturbances
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11. Population “strategies”
r-selected
opportunistic
Short lifespan
Many small offspring
reproduces once
no parental care
High death rate
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12. Population “strategies”
K-selected
Long lifespan
Few large offspring
several reproductions late in life
extensive parental care
Low death rate
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13. Population Growth Models
Exponential model (blue)
(J-curve)
r-selected
Logistic model (red)
(S-curve)
K-selected species
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14. Section 15.2 Summary– pages 404-413
Changes in genetic equilibrium
Another mechanism that disrupts a population’s genetic equilibrium is genetic drift— the alteration of allelic frequencies by chance events.
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15. Section 15.2 Summary– pages 404-413
Changes in genetic equilibrium
The movement of genes by migration is called gene flow.
When an individual leaves a population, its genes are lost from the gene pool.
When individuals enter a population, their genes are added to the pool.
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16. Section 15.2 Summary– pages 404-413
Natural selection acts on variations
Some variations increase or decrease an organism’s chance of survival in an environment.
There are three different types of natural selection that act on variation:
stabilizing selection
directional selection
disruptive selection
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17. Section 15.2 Summary– pages 404-413
Natural selection acts on variations
Stabilizing selection is a natural selection that favors average individuals in a population.
Selection for
average size
spiders
Normal variation
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18. Section 15.2 Summary– pages 404-413
Natural selection acts on variations
Directional selection occurs when natural selection favors one of the extreme variations of a trait.
Selection for longer beaks
Normal
variation
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19. Section 15.2 Summary– pages 404-413
Natural selection acts on variations
In disruptive selection, individuals with either extreme of a trait’s variation are selected for.
Selection for
light limpets
Normal variation
Selection for
dark limpets
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20. Section 15.2 Summary– pages 404-413
Natural selection acts on variations
Natural selection can alter the genetic equilibrium of a population’s gene pool over time.
Significant changes in the gene pool could lead to the evolution of a new species over time.
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21. Section 15.2 Summary– pages 404-413
In nature, physical barriers can break large populations into smaller ones.
Geographic isolation occurs whenever a physical barrier divides a population.
A new species can evolve when a population has been geographically isolated.
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22. Section 15.2 Summary– pages 404-413
The Evolution of Species
When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations.
Tree frogs are a single population.
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23. Section 15.2 Summary– pages 404-413
The Evolution of Species
The formation of a river may divide the frogs into two populations.
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24. Section 15.2 Summary– pages 404-413
The Evolution of Species
Over time, the divided populations may become two species that may no longer interbreed, even if reunited.
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25. Section 15.2 Summary– pages 404-413
As populations become increasingly distinct, reproductive isolation can arise.
Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring.
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26. Section 15.2 Summary– pages 404-413
Reproductive isolation can result in speciation
Reproductive Isolation can occur when there are changes in the species:
Genetic material (DNA)
Behavior
Section 15.2 Summary– pages

27. The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding
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