Pack 21 - Population and Evolution

Population
1. A population is is a group of organisms of the same species occupying a particular space at a particular time that can interbreed
2. A gene pool is all the individuals in a population at a given time
3. Allele frequency = number of times an allele occurs within the gene pool
4. The Hardy-Weinberg Principle
  1. The proportion of dominant and recessive alleles of a gene will remain constant from one generation to the next
  2. 5 assumptions:
    1. No mutations occur
    2. Isolated population
    3. No selection
    4. Large population
    5. Random mating
  3. p + q = 1
    1. p = probability of dominant allele
    2. q = probability of recessive allele
  4. p^2 + 2pq + q^2 = 1
    1. p^2 = frequency of AA
    2. 2pq = frequency of Aa
    3. q^2 = frequency of aa
Evolution
1. Variation
  1. Individuals within a population show a wide range of variation in phenotype
    1. Genetic and environmental factors cause variation
  2. Primary source is mutations
  3. Also caused by random fertilisation
  4. Types of variation:
    1. Discontinuous:
      1. Organisms fit into few distinct forms with no intermediate types
    2. Continuous:
      1. Organisms with many intermediate types
2. Natural Selection
  1. Caused by competition, predation and disease
  2. A biotic factor is any living component that affects the population of another organism or the environment
  3. Competition:
    1. Intraspecific: Competition between organisms of the same species
    2. Interspecific: Competition between organisms of different species
  4. Types of Selection:
    1. Stabilising Selection:
      1. This occurs when the environment remains constant for a long period of time
      2. It selects against the extremes of the phenotypes
    2. Directional Selection:
      1. Occurs when the environment changes and so the selection pressure will be to select for individuals either to the right or the left of the mean
    3. Disruptive Selection:
      1. This occurs when the environment has two or more distinct forms and so the selection pressure will be to select for individuals away from the mean
        It selects for both extremes
3. A change in allele frequency of a population
4. Main stages:
  1. 1) Mutation
    1. 2) Variation
      1. 3) Struggle for existence (selection pressure)
        4) Survival of the fittest (advantageous allele survives)
        5) Reproduction of advantageous allele
        6) Natural selection (frequency of allele increases)
Isolation and Speciation
1. If a population becomes isolated, there will be no gene flow
  1. So there is an accumulation of genetic differences
    1. These genetic differences may eventually lead to different species
      1. The isolated populations cannot interbreed with each other to produce fertile offspring
2. Speciation is the evolution of a new species from existing ones
3. Types of Speciation
  1. Allopatric
    1. Speciation that occurs when biological populations of the same species become isolated from each other preventing interbreeding
      1. e.g. a river
  2. Sympatric
    1. Speciation that occurs without any geographical seperation
Genetic Drift
1. Genetic drift is a change in the allele frequency in a small population due to chance

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Pack 21 - Population and Evolution

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	Created by Jacob Shepherd about 7 years ago