variation and natural selection

• what are the 2 main types of variation?

  continuous and discontinuous
• species

  a group of similar organisms capable of interbreeding to produce fertile offspring
• what kind of change is there in continuous variation?

  a gradual change in a characteristic across a population
• what is continuous variation plotted on?

  histogram and usually show 'normal distribution
• what is normal distribution?

  when most individuals are around the average value and relatively few are found at either extreme
• examples of continuous variation

  height, length, mass and hand span. these characteristics are usually polygenic and affected by genes and environment
• polygenic


  multiple genes affecting a given trait
• how is a histogram drawn?

  data is collected and placed into a tally chart with at least 5 groups. the bars are drawn with the same width and should touch to show that the divisions in categories are artificially chosen. on the y axis place frequencies and on the x axis place the lower axis at each interval
• what is a histogram?


  a bar graph showing a frequency distribution
• continuous data


  numerical data on a scale from one extreme to another
• in discontinuous variation what can the population be divided into?

  discrete groups or categories
• Examples of discontinuous variation

  tongue rolling, blood groups, hand dominance
• discontinuous data


  discrete categories (non-numerical)
• monogenetic


  single gene
• describe the characteristics of discontinuous data

  monogenetic and less effected by the environment
• what is discontinuous variation displayed on?

  bar graph
• causes of variation


  changes to chromosomes/genes (genetic variation) or due to the environment (environmental variation)
• what is genetic variation caused by?

  mutations - random changes in the number of chromosomes or sequence of gene.

  sexual reproduction - meiosis mixes up the chromosome arrangement during gamete formation. this results in all gametes being genetically unique. so when 2 gametes fuse the zygote produced is unique (this is why siblings show considerable variation in characteristics (phenotype))
• what can height of a person depend on?

  mainly our genes but also nutrition (particularly the nutrition they receive when they are growing)
• Who is Charles Darwin?

  (1809-1882) a naturalist who spent 5 years on the HMS beagle as it travelled around south america
• when were darwin's ideas published and what was the book called?

  'on the origin of species' in 1859
• describe the theory of natural selection

  1. there is variation among the phenotypes (characteristics) of individuals in a population

  2. competition for resources causes a struggle for existence

  3. the best adapted individuals survive and the less well-adapted individuals do not survive - this is differential survival

  4. the surviving phenotype (the fittest) is able to reproduce

  5. the surviving phenotype passes its beneficial gene to the next generation
• What is the theory of evolution? (definition of evolution)

  a continuing process of natural selection that leads to gradual changes in organisms over time, which may result in the formation of a new species
• Darwin's finches

  Darwin observed different shaped beaks on different islands (galapagos islands)

  - all originated from same ancestral finch population

  - variable ancestral population arriving on different islands - different beak shapes for specific foods on each island at an advantage
• what is natural selection?

  the process in which the better adapted individuals survive (at the expense of the less adapted individuals) and pass on their genes
• extinct definition


  no living organisms of the species remain
• describe natural selection in terms of antibiotic resistance

  1. variation - some strains of bacteria are resistant and some aren't

  2. competition - the non-resistant bacteria are killed by the penicillin

  3. survival of the fittest - the resistant bacteria survive

  4. passing on if genes - the resistant bacteria reproduce and pass on their adaptations to their offspring
• what is antibiotic resistance?

  if an antibiotic is used long enough, bacteria will emerge that cannot be killed by that antibiotic
• how does antibiotic resistance develop by natural selection?

  1. random changes/mutations occur in the genes of individual bacterial cells

  2. some mutations protect the bacterial cell from the effects of the antibiotic (make the resistant)

  3. bacteria without the mutation die or cannot reproduce with the antibiotic present

  4. the resistant bacteria are able to reproduce with less competition from

  non-resistant bacterial strains
• what are fossils?

  remains of living organisms that have been preserved in rock for millions of years
• why is it possible to accurately date a fossil?

  as it is possible to date rocks to when they were formed (and when the organism was fossilised)
• what is one of the most important fossils ever found and why?

  the fossil of Archaeopteryx. this and other fossils provide evidence to help scientists work out how birds evolved from reptiles
• what do fossils show?

  what a particular animal or plant looked like millions of years ago but also how the species changed over time
• if a species didn't change over a long period of time, what does this suggest?

  living in a very stable environment, to which it is well adapted
• what parts of animals do not decay (sometimes) and if they do how do we still see their shape?

  sometimes bones don't decay. in other cases the bodies of animals and plants were replaced by minerals. you can still see their shape in rocks
• why are species extinct?

  when there are no living examples left
• when does extinction happen?

  if a species fails to adapt to changes in its environment
• give 4 examples of extinct species

  dodo, dinosaurs, woolly mammoth, quagga
• why may species become extinct?

  1. climate change and natural disasters

  2. hunting by humans

  3. hunting by animals introduced by humans i.e. not natural predators

  4. the spread of disease

  5. loss of habitat - often caused by human activities e.g. deforestation and clearing land for roads and building
• how can we slow down the rate of extinctions?

  1. legislation preventing the hunting of endangered species

  2. international agreements, such as those which aim to reduce or slow climate change

  3. special programmes creating nature reserves to protect habitats

  4. education to encourage people to protect the environment and endangered species