• 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