Populations

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Populations
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Definition

• Population - individuals of the same species that
  live in the same area at the same time
• Example - the human population of Grapevine, TX

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Characteristics of Populations

• Population size
• Number of individuals
• Population density
• Number of individuals in an area
• Dispersion
• How individuals are arranged in space

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Types of Dispersion
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Population Growth

• births - deaths
• If more individuals are born than die, the
  population grows
• If more individuals die than are born, the
  populations shrinks

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Growth Curves
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Carrying Capacity (K)

• of individuals the environment can sustain

Carrying capacity
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Density-Dependent Factors

• Limited Resources
• Food
• Water
• Shelter
• When limited resources become depleted,
  population growth slows

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Population cycles in the snowshoe hare and lynx
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Density-Independent Factors

• Environmental conditions that limit population
  growth
• Weather
• Climate

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r- and K-Strategists

• r-strategists
• Small
• Reproduce often and large numbers
• Unstable environment
• Exponential growth
• ex mouse
• K-strategists
• Large
• Reproduce less frequently, in numbers that they
  can manage parental care
• Stable environment
• Logistic growth
• ex elephant

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Populations evolve because allele frequencies
change

• Things that change allele frequencies
• Mutations
• Gene flow
• Non-random mating
• Genetic drift
• Natural selection

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Gene Flow

• Migration of fertile individuals or gametes
  between populations
• Reduces differences between populations

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Genetic Drift
Change in allele frequency due to chance.
Happens in small populations.
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Bottleneck Effect (Genetic Drift)
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Preserving Variation

• Why are unfavorable alleles not eliminated from
  population?
• Diploidy
• Heterozygote advantage
• Ex sickle-cell allele and malaria

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Natural Selection

• Darwinian fitness contribution an individual
  makes to the gene pool of the next generation
  relative to the contributions of others
• Modes of selection
• Directional
• Diversifying
• Stabilizing