natural selection

• Alfred Russel Wallace


  Developed the theory of speciation, and therefore evolution by natural selection
• Alfred Russel Wallace


  • Had the idea that the individuals who did not have characteristics to help them survive a change in the environment would die out
  • Published joint studies with Darwin
  • Continued to work across the world to collect evidence – one of his most important works was on warning colouration in animals
• Charles Darwin


  Scientist and naturalist who put forward the theory of evolution
• Charles Darwin published 'On the Origin of Species' in 1859
• Theory of Evolution


  • Variation exists within species as a result of mutations in DNA
  • Organisms with characteristics most suited to the environment are more likely to survive to reproductive age and breed successfully – called survival of the fittest
  • The beneficial characteristics are then passed on to the next generation
  • Over many generations the frequency of alleles for this advantageous characteristic increase within the population
• There was lots of controversy surrounding Darwin's ideas for many reasons
• Antibiotic resistance


  Bacteria are labelled resistant when they are not killed by antibiotics which previously were used as cures against them
• Development of antibiotic resistance

  1. Bacteria reproduce at a fast rate

  2. Mutations during reproduction can result in new genes, such as the gene for antibiotic resistance

  3. Exposure to antibiotics creates a selection pressure, as those with antibiotic resistant genes survive and those without die

  4. Those with antibiotic resistance can reproduce and pass on the advantageous gene to their offspring

  5. The presence of these new, resistant bacteria supports Darwin's theory of natural selection
• MRSA


  • Called a 'superbug' as it is resistant to many different types of antibiotics
  • Common in hospitals: spreads when doctors and nurses move to different patients
• Fossil evidence


  Shows how developments in organisms arose slowly, which can be estimated using carbon dating and related techniques
• Ardi


  • Ardipithecus ramidus, the oldest known human ancestor - estimated to have lived 4.4 million years ago
  • Her fossilised skeleton contains many 'humanoid' features but also resembles an ape, suggesting natural selection and evolution occurred gradually
• Lucy


  • Fossilised skeleton dating from 3.2 million years ago
  • Her bone structure suggests she walked in an upright, human-like position but had a small, chimp-like skull and brain, representing an intermediate between apes and early humans
• Fossils discovered by Louis and Mary Leakey

  • Helped support the theory of natural selection, including an early fossil containing remnants of stone tools and Homo habilis, an important early human species
• Stone tools


  Became more complex as human brains evolved in complexity
• Dating stone tools


  1. Radiometric carbon dating - looking at the natural radioactive decay of Carbon-14 to estimate the age of once-living material found with the tools

  2. Stratifying rock layers - looking at the layer of sediment in which a rock was found to estimate when the tools were formed
• Pentadactyl limb


  • A limb with five digits, implying a common ancestor for organisms with this feature
• Five Kingdoms system


  Classifies all organisms into one of 5 groups: Animals, Plants, Fungi, Prokaryotes, Protists
• Three-domain system


  Classifies organisms into three large groups called domains: Archaea, Bacteria, Eukaryota
• Selective breeding


  When humans choose which organisms to breed in order to produce offspring with a certain desirable characteristic
• Selective breeding


  1. Parents with desired characteristics are chosen

  2. They are bred together

  3. From the offspring those with desired characteristics are bred together

  4. The process is repeated many times until all the offspring have the desired characteristic
• Inbreeding


  Breeding closely related individuals, which reduces the gene pool and can make a species vulnerable to changes in the environment or new diseases
• Inbreeding


  Breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals
• Gene pool


  The number of different alleles reduce (as they mostly have the same alleles)
• If the environment changes or there is a new disease

  The species could become extinct as they all have the same genetic make-up (so the chance of a few organisms having a survival advantage and not dying is reduced)
• This is particularly relevant in selective breeding of plants, as one disease could spread rapidly and destroy the entire population of crops
• Small gene pool


  Greater chance of genetic defects being present in offspring, as recessive characteristics are more likely to present
• This is particularly relevant in domesticated animals, which have a much higher frequency of genetic conditions than normal
• Tissue culture


  Culturing living tissue, i.e making it grow outside the organism, within a growth medium
• Tissue culture in plants


  1. Take the plant that you would like to clone

  2. Using tweezers, remove a piece of tissue from a fast-growing region of the plant, e.g the root or shoot tip

  3. Using aseptic technique (maintaining sterile conditions), place the tissue on a special growth medium (containing hormones and nutrients)

  4. Once the tissue has developed enough (e.g produced shoots and roots), it can be transferred to compost for further growth
• Genetic engineering


  Modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
• Genetic engineering applications


  • Plant cells have been engineered for disease resistance or to have larger fruits
  • Bacterial cells have been engineered to produce substances useful to humans, such as human insulin to treat diabetes
• Benefits of tissue culture


  • Produces lots of offspring with a specific desirable feature
  • Increasing the number of crops resistant to bad weather, for example, can increase crop yields
  • Can help extremely endangered species, or even bring back species that have become extinct
• Risks of tissue culture

  • The gene pool is reduced through producing clones, meaning it is less likely that the population will survive if a disease arises with low diversity in the population
  • Clones have a low survival rate, and tend to have some genetic problems
  • It may lead to human cloning
• Stages of genetic engineering

  1. Genes from chromosomes are 'cut out' using restriction enzymes

  2. The same restriction enzymes are used to cut the vector (such as a virus or bacterial plasmid) into which the genes will be placed

  3. Ligase enzyme is used to attach the sticky ends of the gene and the vector together, to produce a recombinant gene product

  4. The vector is placed in another organism at an early stage in development so the desired gene moves into its cells and cause the organism to grow with the desired characteristics

  5. In plants the vector is put into meristematic cells (unspecialised cells) which can then produce identical copies of the modified plant
• Perceived benefits of genetic engineering

  • It can be very useful in medicine to mass produce certain hormones in microorganisms (bacteria and fungi)
  • In agriculture it can be used to improve yields by: Improving growth rates, Introducing modifications that allow the crops to grow in different conditions, e.g. hotter or drier climates, Introducing modifications that allow plants to make their own pesticide or herbicide
  • Crops with extra vitamins can be produced in areas where they are difficult to obtain
  • Greater yields can help solve world hunger, which is becoming an increasingly bigger issue due to population growth
• Perceived risks of genetic engineering

  • GM crops might have an effect on wild flowers and therefore insects
  • GM crops are infertile and these genes could spread into wild plants, leading to infertility in other species, which affects the entire environment
  • Growing with herbicides and pesticides can kill insects and other plants, which would reduce biodiversity
  • People are worried that we do not completely understand the effects of GM crops on human health
  • Genetic engineering in agriculture could lead to genetic engineering in humans. This may result in people using the technology to have designer babies
  • They pose a selection pressure, which could lead to increased resistance in other species, creating super weeds and pests
• Benefits of selective breeding

  • It is possible to greatly increase the yield of a particular crop by selectively breeding only individuals that produce higher quality or a larger mass of food
  • Individual plants or animals can be bred to be resistant to a particular disease, which could increase crop yield
• Disadvantages of selective breeding

  • Selecting for advantageous characteristics can sometimes cause severe health problems in the offspring - e.g chickens that have been bred to have more meat (muscle) are sometimes too large to be able to walk
  • Lack of genetic variation - Despite the bred population being able to have resistance to a particular disease (or multiple diseases), if one of them has susceptibility to a different disease then they all do - and the entire population could be wiped out as a result
  • There are ethical issues associated with selective breeding -many people consider it unethical to selectively breed for characteristics wanted by humans if it means that the offspring will suffer, or have a reduced quality of life as a result