IB DP Biology: HL复习笔记5.1.3 Variation

• Differences exist between organisms of the same species
  • These differences are known as variation
  • Examples of variation include:
    • Coat colour in mammals
    • Body length in fish
    • Flower colour in flowering plants
• The process of natural selection can only take place when there is variation in a population
  • If every organism in a population is identical then no individual will be favoured over another
  • There will be no advantageous characteristics leading to increased survival and chances of reproduction, and so there would be no increased likelihood of passing on those advantageous alleles
  • In this situation, a population's characteristics would remain the same over time and it would be unable to adapt to any environmental changes

• Variation results from small differences in DNA base sequences between individual organisms within a population
• There are several sources of these differences in DNA base sequences:
  • Mutation
  • Meiosis
  • Random fertilisation during sexual reproduction

Mutation

• The original source of genetic variation is mutation
  • A mutation is a change in the DNA base sequence that results from a copying error during DNA replication
• Mutation results in the generation of new alleles
• Mutations that take place in the dividing cells of the sex organs lead to changes in the alleles of the gametes that are passed on to the next generation
  • A new allele may be advantageous, disadvantageous or have no apparent effect
  • An advantageous allele is more likely to be passed on to the next generation because it increases the chance that an organism will survive and reproduce
  • A disadvantageous mutation is more likely to die out because an organism with such a mutation is less likely to survive and reproduce
• Note that a mutation taking place in a body, or somatic, cell will not be passed on to successive generations, and so will have no impact on natural selection
• Mutation is the only source of variation in asexually reproducing species

Meiosis

• There are two main events during the process of meiosis that generate variation
  • Crossing over
  • Random orientation
• Crossing over is the process by which homologous chromosomes exchange alleles
  • During meiosis I homologous chromosomes pair up
  • The non-sister chromatids can cross over and get entangled
  • As a result of this, a section of chromatid from one homologous chromosome may break and rejoin with the chromatid from the other chromosome
• This swapping of alleles is significant as it can result in a new combination of alleles on the two homologous chromosomes

The process of crossing over can result in new combinations of alleles

• Random orientation occurs due to the independent arrangement of homologous pairs along the equator of the cell during metaphase I
  • Each pair can be arranged with either chromosome on either side of the cell; this is completely random
  • The orientation of one homologous pair is independent, or unaffected by the orientation of any other pair
  • This is sometimes described as independent assortment
• The homologous chromosomes on the equator of the cell are pulled apart to different poles, and will each end up in a separate daughter cell
• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• To work out the number of different possible chromosome combinations the formula 2ⁿ can be used, where n corresponds to the number of chromosomes in a haploid cell
  • E.g. for humans this is 2²³ which calculates as 8,324,608 different combinations

Random orientation of chromosomes

Random fertilisation during sexual reproduction

• Meiosis creates genetic variation between the gametes through crossing over and independent assortment
• This means each gamete carries substantially different alleles
• During fertilisation any male gamete can fuse with any female gamete to form a zygote
• This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles
• There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

The random fusion of gametes during fertilisation

Sources of Genetic Variation Table

• Adaptations can be defined as:
  • Characteristics that cause individuals to be well suited to their environment and methods of living
• When describing an adaptation it is always a good idea to relate the structure of a characteristic to its function, e.g.
  • Fish gills are an adaptation for survival underwater; they have a very large surface area, enabling them to maximise oxygen absorption from water
  • The thick fur of a polar bear is an adaptation for survival in a cold environment; it is thick, trapping a layer of warm air next to the polar bear's body and providing insulation
  • Different beak shapes in birds enable the consumption of different types of food; some species of finch have short, cone-shaped beaks that enable them to crack nuts and seeds
• Adaptations arise in species gradually by evolution through the process of natural selection
  • In a slowly changing environment, populations are able to adapt by natural selection and survival continues
  • If an environment changes quickly, the process of natural selection is too slow and adaptation cannot occur fast enough; in this situation, a population must migrate to a different environment or it will go extinct