18 Variation & Natural selection

  • Created by: lee8444
  • Created on: 24-02-20 12:41

Variation due to genetic factors

  • Within a population, all members have the same genes
  • Genetic differences is differnt alleles of the same gene
  • Change from generation to generation
  • Mutations - sudden changes that may or may not be passed on, main source of variation
  • Meiosis - special nuclear division, new combinations of alleles
  • Random fertilisation of gametes - Which gamete fuses is random adding the variety that two parents can produce
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Variation due to environmental influences

  • Genes set the limits but the environment determines where on that scale the organism is
  • Environmental conditions could be temperature, rainfall, sunlight, soil conditions, pH and food availability
  • e.g. height is mainly determined through multiple genes, however diet can influence how tall or short a person can become
  • Variation is a combination of genes possessed and environmental factors
  • It can be hard for scientists to distinguish between these two factors
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Natural selection

  • The process of selection is based on the suitability for survival under the existing conditions
  • Environmental factors are called selection pressures e.g. predation, disease and competition
  • Selection pressures determine the allele frequency in a gene pool
  • Evolution depends on many factors including
    • Organisms produce more offspring than can be supported by the resources e.g. light, food, space
    • There is genetic variety between the offspring
    • There are a variety of phenotypes that selection operates against
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Role of over-production of offspring in natural se

  • Darwin realised that in real life, populations didnt grow exponentially
  • High birth rates compensates for high death rates caused by predation, competition and extreme conditions
  • Where there are too many offspring for the available resources, there is intraspecific competition for the limited resources
  • More individuals will die in the struggle to survive
  • Those that are better suited to prevailing conditions e.g. better at hiding or running away from predators are morelikely to survive
  • They are more likely to breed and pass on their favourable alleles
  • This causes the next generation to have a different allele frequency
  • The process depends on organisms haveing genetic diversity
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The role of variation in natural selection

  • Variation may produce offspring that are unsuited to the environment that the parents live in
  • Conditions change over time and so having a wide range of phenotypes is advantageous to survive in multiple new circumstances
  • Populations with little genetic variation are susceptible to diseases and climate change
  • The larger a population, the more likely some of the individuals will be more suited to the new environment
  • This causes a population to evolve
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