B17. Variation and natrual selection

  • Variation: differences between individuals of the same species

  • Phenotypic variation: the difference in features - most caused by genetic variation

    • continuous variation: many small degrees of difference for a particular characteristic, are arranged in order and can usually be measured on a scale. E.g: height, mass,…

      • graphs are a smooth curve bell

    • Discontinuous variation: limited number of phenotypes with no intermediates

      • graphs are a ‘step-like’ shape

      • e.g: ABO blood groups

  • Genetic variation:

    • blood group

    • eye color

    • gender

  • Environmental variation:

    • characteristics that can be affected by enviromental factors

    • an accident → scarring, eating too much,

  • Mutation: genetic changes → new alleles are formed

    • have small effects on the organism

    • new allele could give the individual a survival advantage

    • could lead to harmful changes

    • happens spontaneously and continuously

    • increased mutation could be cancerous

  • Adaptive features: inherited features that help an organism to survive and reproduce in its environment

  • Natural selection: individuals that have the best adaptive features are most likely to survive and reproduce to pass on those adaptive features

    • produce more offspring than the environment is able to support → competition for food and other resources which results in a ‘struggle for survival’

    • Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce→ alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival

    • This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics

  • Adaptation by natural selection: If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele

  • Selective breeding: select individuals with desirable characteristics and breed them together

    • doesn’t stop until offspring show the desired characteristics are selected and bred

    • repeated for many successive generations

    • Selective breeding in crop plants

      • Plants are selectively bred by humans for the development of many characteristics, including:

        • disease resistance in food crops

        • increased crop yield

        • hardiness to weather conditions (e.g. drought tolerance)

        • better tasting fruits

        • large or unusual flowers