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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Natural selection

Selection pressures

  • Environmental factors affect an individual’s chance of survival

    • They are said to act as selection pressures

  • These factors can be:

    • botic (living) factors, including predation, competition and disease

    • abiotic (non-living) factors, including light, temperature, water and soil pH

Example: environmental pressures on African lions

Environmental factor

Type

Effect on lions

Competition for food

Biotic

Limited prey leads to starvation if lions fail to hunt successfully

Competition for mates

Biotic

Males fight for access to females; losers cannot reproduce and may be injured

Water availability

Abiotic

Drought or dried water sources can cause dehydration and death

High temperature

Abiotic

Overheating limits hunting time, reducing food intake and survival chances

Survival and reproduction

  • In any species population, genetic variation means some individuals will have phenotypes that make them better adapted to survive

    • This means they have a selective advantage

  • Those with a selective advantage are likely to produce more offspring and pass on their favourable alleles to the next generation

    • Favourable alleles (that provide a selective advantage) will increase in frequency in a gene pool

    • Alleles that are not favourable (don’t provide a selective advantage) will decrease in frequency in a gene pool

  • This differential survival and reproduction is the basis of natural selection

    • Read more detailed notes on natural selection from Unit 4

Examiner Tips and Tricks

In exams, avoid saying well-adapted organisms are “guaranteed” to survive — say they are more likely to survive and reproduce.

Natural selection and evolution

  • Over generations, differential reproductive success and natural selection lead to an increase in advantageous allele (and phenotype) frequencies

  • When allele frequencies change significantly over time, the characteristics of the population change — if the changes are great enough, a new species may form

    • This process is known as evolution

  • Evolution can be defined as:

The formation of new species from pre-existing species over time, as a result of changes to gene pools and allele frequencies from generation to generation

  • The formation of new species via the process of evolution has resulted in a great diversity of species on planet Earth

Diagram showing theoretical origin of life with speciation, illustrating evolutionary time. Species increase from 1 to 7 as speciation occurs.
Speciation increases biodiversity over time by branching from one species into many

Examiner Tips and Tricks

There are some key differences between speciation and evolution:

  • Evolution is the gradual change in the genetic makeup of a population over time

  • Speciation is the formation of a new species from an existing one, usually when populations become reproductively isolated and genetically distinct

Note that speciation takes many generations, but it happens faster in organisms with short generation times (e.g. bacteria).