variation and evolution

• variation is differences in the characteristics of individuals in a population
• variation may be due to:

  • differences in the genes that have been inherited (genetic causes)
  • differences in the conditions in which individuals have developed (environmental causes)
  • a combination of genetic and environmental causes
• characteristics from genetic variation: free or fixed ear lobe, blood group, gender and natural eye colour
• characteristics from the environment: scars from injury, language spoken, tattoos and accent
• characteristics caused by both: height, weight
• although genes determine what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop
• there is usually extensive genetic variation within a population
• all variants arise from mutation. Most have no effect on the phenotype, some influence phenotype and very few determine phenotype
• • mutations occur continuously
  • very rarely a mutation will lead to a new phenotype
  • if the new phenotype (from mutation) is suited to an environmental change it can lead to a relatively rapid change in the species
• the number of mutations or chance of mutation can be increased by exposure to mutagens such as ionising radiation - these mutations can lead to cancer
• evolution is a change in the inherited characteristics of a population over time through natural selection which may result in the formation of a new species
• evolution:

  1. variation is present in a population from random mutations
  2. some organisms will have phenotypes that make them better adapted to their environment
  3. this gives them a survival advantage due to their advantageous alleles
  4. natural selection occurs where those with the advantageous alleles will have a higher chance of surviving and reproducing successfully to pass on those favourable alleles to their offspring
  5. these favourable alleles are passed down at a faster rate than the less suited alleles
  6. over generations this is repeated. Most of the population have the favourable allele
• if two populations of one species become isolated from each other, or become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species or speciation has occurred
• the theory of evolution is that all species of living things have evolved from simple life forms more than 3 billion years ago
• selective breeding (or artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics
• human have been doing selective breeding for thousands of years, since they first bred food crops from wild plants and domesticated animals
• selective breeding:

  1. parents with desired characteristics are chosen from a mixed population
  2. they are bred together
  3. this produces offspring with a range of characteristics
  4. From the offspring those with desired characteristics are bred together
  5. this cross breeding continues over many generations until all the offspring show the desired characteristics
• the desired characteristic in selective breeding can be chosen for usefulness or appearance:

  • disease resistance in food crops
  • animals which produce more milk or meat
  • domestic dogs with a gentle manner
  • large or unusual flowers
• selective breeding advantages --

  • economically beneficial as it can produce increase yield and quality of crops, and more and better animal products
• genetic engineering is a process which involved modifying the genome of an organism by introducing a gene from another organism to give it a desired characteristic
• genetic engineering:

  1. a useful gene is identified
  2. an enzyme is used to cut out the desired gene
  3. the gene is inserted into a vector, usually a bacterial plasmid or virus
  4. the plasmid containing the desired gene is inserted into the bacteria so it can multiply
  5. bacteria can be purified to obtain a desired chemical (eg insulin)
  6. or the vector is used to insert the gene into the required cells
  7. genes are transferred to the cells of organisms at an early stage in their development so they can develop desired characteristics
• bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes
• crops that have been genetically modified through cutting out genes using enzymes are called genetically modified crops
• genetic engineering : advantages --

  • modifying crops to be resistant to pests and diseases allows crops to grow better and increase yield
  • GM plants can produce more food, or more nutritious food in places where they can't grow lots of food
  • reduces the use of chemical pesticides and fertilisers
• genetic engineering disadvantages --

  • inserted genes could be transferred to wild populations of plants which could affect the biodiversity of plants (competition) and the organism that feed on them
  • affects flowers and insects
  • some people feel the effects of eating GM crops on human health have not been fully explored
• modern medical research is exploring the possibility of genetic modification to overcome inherited disordered
• GM for inherited diseases : advantages --

  • can produce large amounts of proteins (eg insulin) and other drugs rapidly
  • gene therapy can work on 'faulty genes' in people with inherited disorders
  • proteins made this way won't be rejected
• cloning : tissue cultures -> using small groups of cells from part of a plant to grow a new identical plant:

  1. cells are scraped from the parent plant
  2. cells are transferred to a petri-dish with agar and plant hormones
  3. they develop into a large mass of identical cells called callus
  4. growth hormones and conditions are used to stimulate the growth of plantlets (identical plants) from callus
• tissue cultures : advantages --

  • enables rare plant species to be preserved
  • thousands of plants are produced, each with the desired characteristics of the parent plant making it great for commerical use in nurseries
  • parent plant isn't damaged or destroyed
  • cheap, quick and can be done year long
• tissue cultures : disadvantages:

  • expensive as specialist equipment is required
  • contamination can kill all the plants
• cloning : cutting -> an older, simpler method used by gardeners to produce many identical new plants from a parent plant:

  1. a cutting is taken from a growing plant
  2. it's planted into soil where it will form roots and grow into a genetically identical plant
  3. this happens because all plant cells have the potential to differentiate into any type of cell
• cutting : advantages --

  • cheap
  • quick
  • easy
  • rotting powders increase the chance of success
• cutting : disadvantages --

  • original plant is damaged and could be destroyed when taking cuttings
  • the cutting doesn't always root
• cloning : embryo transplant -> splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers:

  1. a prize mother is given hormones to make it produce an egg cell which is extracted
  2. sperm from a prize male is used to artificially fertilise the egg cells
  3. the fertilised egg cell starts dividing to produce an embryo
  4. the embryo is split apart many times before the cells of the embryo become specialised
  5. the embryos are implanted into different surrogate mothers
  6. clone offspring are born
• embryo transplant : advantages --

  • top quality offspring can be produced many times in a prize female
  • embryos can be transported all around the world to improve local animal stock
  • GM animals can produce medically important compounds in large quantities
• embryo transplant : disadvantage --

  • genetic variation and diversity of organisms within a population is reduced, leaving the population susceptible to change and diseases
  • expensive as specialist equipment is required
• cloning : adult cell cloning -

  1. collect an unfertilised egg from the same species as the animal being cloned and remove its nucleus (enucleate it)
  2. get the nucleus from an adult body cell of the animal being cloned
  3. insert the nucleus into the enucleated egg cell
  4. give the egg cell a tiny electric shock to stimulate the egg cell to divide and form an embryo
  5. these embryo cells contain he same genetic information as the adult cell
  6. when the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue development
  7. surrogate will give birth to the clone
• adult cell cloning : advantages -

  • could be used to help preserve endangered species
  • cloning makes it possible to quickly produce commercial quantities of plants and animals year long
  • consistently good livestock
  • in humans, cloning can be used for transplants without rejection
• adult cell cloning : disadvantages -

  • reduces gene pool drastically, leaving the population susceptible to a change in the environment or disease
  • people worried the process could be used to clone humans, maybe replace children who've died
  • clones often have many health problems and may die at a much earlier age than other individuals in the species
• Charles Darwin as a result of observations on a round of the world expedition (backed by years of experimentation, discussion and linked to developing knowledge of geology and fossils) proposed the theory of evolution by natural selection