6.3 Genetics and Evolution

• What did Charles Darwin propose the theory of?

  Evolution by natural selection
• What is evolution?


  The change in the frequency of a phenotype in a population over many generations
• Explain Darwin's theory


  1. Individual organisms within a particular species show a wide range of variation for a characteristic
  2. Individuals with characteristics most suited to the environment are more likely to survive to breed successfully
  3. The alleles that have enabled these individuals to survive are then passed on to the next generation
• Why was the acceptance of evolution gradual?

  • The theory challenged the idea that God made all the animals and plants that live on Earth
  • There was insufficient evidence at the time the theory was published to convince many scientists
  • The mechanism of inheritance and variation was not known until 50 years after the theory was published
• What was Lamarck's theory of evolution?

  • Theory was based mainly on the idea that changes that occur in an organism during its lifetime can be inherited, and involved two main ideas:
  • A characteristic used frequently by an organism becomes better and stronger, whereas a characteristic that isn't used gradually disappears
  • The beneficial characteristics that are used frequently are passed to offspring
• Explain Lamarck's theory on giraffes

  1. Giraffes had a short-necked ancestor which would frequently stretch its neck to reach the high branches
  2. This repeated stretching could very slowly elongate the neck and that would be passed on to the giraffe's offspring
  3. Over time and many generations, the giraffe would evolve to have the very long neck it has today
• Who was Wallace?

  Alfred Russel Wallace was a scientist who independently developed his own theory of evolution based on the process of natural selection

  • He published scientific papers on this theory with Darwin in 1858
• What was Wallace best known for?

  • His work on warning colouration in animals (evolved by natural selection)
  • His theory of speciation
• What is speciation?

  A process that results in the formation of a new species
• Explain the process of speciation

  1. Populations of the same species become isolated by a geographical barrier (e.g. sea, mountains)
  2. Each population has different alleles due to mutations
  3. Habitat variation on each side
  4. Natural selection occurs in each population
  5. Different advantageous genotypes on each side of barrier
  6. Advantageous genotypes passed on in each population
  7. Eventually populations cannot interbreed successfully
• What did Mendel do?

  In the mid-19th century Gregor Mendel carried out breeding experiments on plants

  • E.g. he conducted studies with pea plants and looked at how the height characteristic was inherited
  • One observation was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged
• Why was Mendel's discovery not recognised until after his death?

  • His studies were totally new to science in the 19th century
  • There was no knowledge of the mechanisms of his findings (DNA, genes, chromosomes hadn't been discovered yet)
• In the late 19th century, the behaviour of chromosomes during cell division was observed. Explain the effect this had on science.

  • Scientists realised that chromosomes behaved in a very similar way to Mendel's 'hereditary units'
  • Scientists believed Mendel's 'units' must be located on chromosomes
  • We now know this is true and they are called genes
• What led to the gene theory being developed?

  In the mid 20th century, the structure of the DNA was determined and the mechanism of gene function worked out
• Why has Darwin's theory of evolution by natural selection been widely accepted?

  • Evidence is now available as it has been shown that characteristics are passed on to offspring in genes
  • There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria
• What are fossils?

  Remains of organisms from millions of years ago, which are found in rocks
• What are the 3 ways fossils are formed?

  • From parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
  • When parts of the organism are replaced by minerals as they decay
  • As preserved traces of organisms, such as footprints, burrows and rootlet traces
• Why can scientists not be certain about how life began on Earth?

  • Many early forms of life were soft-bodied, meaning that they have left few traces behind
  • Soft tissues often decay fully, leaving no trace in the fossil record
  • What traces there were have been mainly destroyed by geological activity (e.g. tectonic plate movements crushing fossils)
• Name 5 factors which led to extinction

  • New diseases - species without resistance will not have resistant alleles within its population so will lead to very large numbers dying very quickly without enough time to develop immunity
  • New predators - no defence mechanisms -> heavily predated -> no time to adapt
  • New, better adapted and more successful competitors
  • Changes to the environment over geological time - e.g. climate change -> not well adapted
  • Single catastrophic events - e.g. asteroid collisions
• Explain the process of antibiotic resistance in bacteria

  1. Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed
  2. They survive and reproduce, so the population of the resistant strain rises
  3. The resistant strain will then spread because people are not immune to it and there is no effective treatment
• Give 3 examples of overuse of antibiotics

  • Treatment of non-serious infections
  • Routine treatment to animals in agriculture
  • Failure to finish prescribed course of antibiotics
• What is MRSA?


  A very dangerous bacterial strain that is resistant to most antibiotics

  • Cannot be treated easily
• What 3 things are done to reduce the rate of development of antibiotic resistant strains

  • Doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections
  • Patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
  • The agricultural use of antibiotics should be restricted
• What are the problems with antibiotic development?

  • Development of new antibiotics is costly and slow
  • It is unlikely to keep up with the emergency of new resistant strains
• The size of seeds eaten by each bird depends on the depth of the bird's beak. The range of beak depth of medium ground finches on Isabella Island is different from the range on Daphne Island. Explain why.

  • Colonisers of Isabela have a range of beak depths due to different alleles of one gene
  • There is a large range of seeds on Isabela, so there is more competition for seeds
  • Birds with larger beaks get enough food to survive and reproduce
  • These birds pass on the beneficial alleles to their offspring
• Explain the evolution of different lemur species on an island

  1. There is isolation of different populations
  2. There is a habitat variation between lemur populations.
  3. There is genetic variation / mutation in each population
  4. Some lemurs are better adapted to survive, reproduce (natural selection / survival of the fittest) and pass on favourable alleles to offspring
  5. Eventually they cannot reproduce successfully with other populations and cannot produce fertile offspring with other populations.
• What is the evidence for developing theories?

  • The fossil record
  • Chemical / DNA analysis of living species
  • Fossils