Chapter 15 populations
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Chapter 15 - Populations. Section 15-1 - How Populations Grow. What is a population?. All individuals of a species that live together in one place at one time. What Is a Population?. Ex: humans, bacteria, animals Individuals usually have more than one offspring = population growth.

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Chapter 15 - Populations

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Chapter 15 populations

Chapter 15 - Populations

Section 15-1 - How Populations Grow


What is a population

What is a population?

  • All individuals of a species that live together in one place at one time.


What is a population1

What Is a Population?

  • Ex: humans, bacteria,

    animals

  • Individuals usually have more than one offspring = population growth


3 key features to populations

3 Key Features to Populations

  • 1. Population Size - # of individuals in a pop.

    • Important effect on ability of pop. to survive

    • Small pop. @ greater risk of extinction

    • Interbreeding = risk of genetic disorders


3 key features to populations1

3 Key Features to Populations

  • 2. Population Density - # of individuals in a given area

    • Large area may

      mean less

      reproduction


3 key features to populations2

3 Key Features to Populations

  • 3. Dispersion - how individuals of the population are arranged

    • Random

    • Even

    • Clumped

      • Most common

      • Microhabitats


Modeling population growth

Modeling Population Growth

  • Population Model – hypothetical population that attempts to exhibit the key characteristics of a real population.


Chapter 15 populations

  • What causes populations to grow?

    • More birth than death

  • What determines how fast they will grow?

    • Amount of resources

    • Rate of immigration

    • Success of birth rate

  • Is there anything that can slow their growth?

    • Death

    • Emigration

    • Lack of resources


Exponential growth curve

Exponential Growth Curve

  • Pop. size plotted against time.

    • (r) remains constant

    • (∆N) pop. growth –

      rises quickly as pop.

      increases

    • Ex: single bacteria produces

      over 1 million in 10 hrs.


Carrying capacity

Carrying Capacity

  • Pop. size the environment can support

    • (K)


Resources and population size

Resources and Population Size

  • Density-Dependent Factors – amount of resources available depends on population size.


Logistic model

Logistic Model

  • Accounts for declining resources as pop. grows.

    • Assumes birth and death rates are not constant

    • As pop. grows, births decline and death increases

    • As (N) approaches (K) growth decreases


Density independent factors

Density-Independent Factors

  • Environmental conditions


Two strategies of population growth

Two Strategies of Population Growth

  • r - strategists – mature rapidly

  • Ex: insects, bacteria, some plants

  • Exponential growth

  • Temporarily large

    pop. followed

    by crash in size

  • Produce lots of

    offspring with little

    investment or care


Two strategies of population growth1

Two Strategies of Population Growth

  • k - strategist – mature slowly

  • Ex: Redwood trees, whales, rhinos

  • Characterized by high degree of specialization

    • Environments stable and predictable

  • Competition

  • Produce few offspring

  • Danger of extinction


Rapidly growing human population

Rapidly Growing Human Population

  • What strategy?

    • r/K - strategist

  • Large investment in offspring

  • What led to human pop. growth?

    • Technology and agriculture increased carrying capacity, which led to rapid growth (∆N )

  • Population growing explosively

  • Will it continue to grow explosively?

    • Something has to give (human population will reach carrying capacity)


Current population 6 8 billion people

Current population = 6.8 billion people


Section 15 2

Section 15-2

How Populations Evolve


Allele frequencies

Allele Frequencies

  • Just because an allele is dominant doesn’t mean it will become more common

    • Dominant allele could be deadly


Hardy weinberg principle

Hardy-Weinberg Principle

  • States that the frequency of alleles in a population do not change unless evolutionary forces act on the population.

    • Pop. must be large

    • No inbreeding

    • Evolutionary forces change alter genetic makeup of a population


5 evolutionary forces

5 Evolutionary Forces

  • 1. Mutation

    • Rate in nature is very slow

    • Not all mutations result in phenotypic changes.

    • Source of variation,

      makes evolution

      possible


Chapter 15 populations

  • 2. Gene Flow

    • Result of migration

    • Immigrants = add alleles

    • Emigrants = take alleles away


Chapter 15 populations

  • 3. Nonrandom Mating

    • Individuals mating with others nearby or of the same phenotype.

      • inbreeding


Chapter 15 populations

  • 4. Genetic Drift

    • Random change in allele frequency

    • Small populations effected by catastrophic events

      • Less genetic drift = similar individuals = BAD


Chapter 15 populations

  • 5. Natural Selection

    • Directly changes allele frequency

    • Acts on the population, not individuals


How selection acts

How Selection Acts

  • Only on characteristics that are expressed.

  • Hemophilia – homozygotes may die, but heterozygotes may still produce offspring.

    • Only homozygous individuals will be eliminated from the population.


Why genes persist

Why Genes Persist

  • Individuals carry genes that are not expressed.

  • When two individuals that carry the gene reproduce, it may become expressed.


Chapter 15 populations

  • Polygenic trait – a trait that is influenced by several genes.

    • Produce a normal distribution

    • Height


Directional selection

Directional Selection

  • Frequency of a particular trait moves in one direction

    • Eliminates one extreme


Stabilizing selection

Stabilizing Selection

  • Reduces extremes from both ends

    • Individuals become more “average”


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