The roles of genes and environment in evolution (part 1)

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  • THE ROLE OF GENES AND ENVIRONMENT IN SELECTION
    • Stabilising or evolutionary forces of natural selection
      • All organisms reproduce
        • Therefore all have potential to increase population size
          • Many reach carrying capacity and remain stable
            • Not all young reach adulthood
              • If they did the population would continue to grow
      • Environmental factors limit growth of a population
        • Not enough space
        • Availability of food
        • Light intensity/ availability
        • Availability of minerals or water
        • Predation
        • Infection by pathogens
        • All of these factors offer ENVIRON - MENTAL RESISTANCE
          • Some are abiotic (non living components)
          • Some are biotic (other living organisms)
      • Over time population size fluctuates around the mean
        • If a strong environmental resistance, population size reduces
          • This in turn reduces competition, and population size will grow
            • As it increases there is more intraspecific (within the population) competition such as for food, shelter or mates
              • Eventually population size falls again
    • Which individuals survive
      • Variation within a population will mean that some members will be better adapted and will be able to out compete other members
        • The organism that can run the fastest, camoflage itself the best, have sharper teeth and claws will have an advantage in the struggle for existence
          • It has a greater change of surviving to adulthood, producing offspring and passing on its advantageous alleles
    • Selection pressures are environmental factors that confer greater changes of survival to reproductive age on some members of the population
      • Mice with better camoflaging ability, such as a darker coat, will be more likely to escape predation and live to reproduce. The predation is the selective pressure
      • Stabilising selection, when natural selection selects the individuals that will survive and reproduce, to keep things the way they are
        • Mice with better camoflaging ability, such as a darker coat, will be more likely to escape predation and live to reproduce. The predation is the selective pressure
        • If a new phenotype arises in the same environmental conditions, it is unlikely to confer an advantage and will therefore not be selected
          • If the environment does change, then this is a new selection pressure, eg new weather, new predator, then animals with an advantageous phenotype will have a selective advantage
            • Frequency of these alleles would increase
              • Directional selection is shown here, which is an evolutionary force of natural selection

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