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Chapter 15 - Populations

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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  1. Chapter 15 - Populations Section 15-1 - How Populations Grow

  2. What is a population? • All individuals of a species that live together in one place at one time.

  3. What Is a Population? • Ex: humans, bacteria, animals • Individuals usually have more than one offspring = population growth

  4. 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

  5. 3 Key Features to Populations • 2. Population Density - # of individuals in a given area • Large area may mean less reproduction

  6. 3 Key Features to Populations • 3. Dispersion - how individuals of the population are arranged • Random • Even • Clumped • Most common • Microhabitats

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

  8. 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

  9. 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.

  10. Carrying Capacity • Pop. size the environment can support • (K)

  11. Resources and Population Size • Density-Dependent Factors – amount of resources available depends on population size.

  12. 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

  13. Density-Independent Factors • Environmental conditions

  14. 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

  15. 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

  16. 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)

  17. Current population = 6.8 billion people

  18. Section 15-2 How Populations Evolve

  19. Allele Frequencies • Just because an allele is dominant doesn’t mean it will become more common • Dominant allele could be deadly

  20. 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

  21. 5 Evolutionary Forces • 1. Mutation • Rate in nature is very slow • Not all mutations result in phenotypic changes. • Source of variation, makes evolution possible

  22. 2. Gene Flow • Result of migration • Immigrants = add alleles • Emigrants = take alleles away

  23. 3. Nonrandom Mating • Individuals mating with others nearby or of the same phenotype. • inbreeding

  24. 4. Genetic Drift • Random change in allele frequency • Small populations effected by catastrophic events • Less genetic drift = similar individuals = BAD

  25. 5. Natural Selection • Directly changes allele frequency • Acts on the population, not individuals

  26. 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.

  27. Why Genes Persist • Individuals carry genes that are not expressed. • When two individuals that carry the gene reproduce, it may become expressed.

  28. Polygenic trait – a trait that is influenced by several genes. • Produce a normal distribution • Height

  29. Directional Selection • Frequency of a particular trait moves in one direction • Eliminates one extreme

  30. Stabilizing Selection • Reduces extremes from both ends • Individuals become more “average”

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