Variation

• What is variation?

  The differences in phenotype of organisms of the same species
• Why is variation important?

  Adaptation.
• What are the heritable causes of variation?

  • Sexual reproduction
  • Mutation
  • Epigenetic modifications
• What are the non-heritable causes of variation?

  Environmental factors
• How does variation arise in asexually reproducing organisms?

  Only increased by mutation
• How does variation arise in sexually reproducing organisms?

  • crossing over between homologous pairs during prophase 1 of meiosis
  • independent assortment of chromosomes during metaphase 1 and chromatids during metaphase 2 of meiosis
  • random mating
  • random fusion of gametes
• What are the properties of discontinuous variation?

  • monogenic
  • distinct groups
  • limited number of phenotypes
  • environmental factors do not have an effect
  • e.g. blood groups
• What are the properties of continuous variation?


  • polygenic
  • a range of phenotypes
  • no distinct groups
  • environmental factors have an effect
  • e.g. human height
• When does competition occur?

  When there is no environmental resistance meaning organisms over-produce
• What is a selection pressure?

  An environmental factor that can alter the frequency of alleles in a population, when it is limiting.
• What is natural selection?

  The increased chance of survival and reproduction of organisms with phenotypes suited to their environment, enhancing the transfer of favourable alleles form one generation to the next.
• What are some examples of selection pressures?

  • overcrowding allows disease to spread
  • predation increases as a population grows
  • availability of nesting sites
  • temperature
  • human impact
• What do selection pressures do?

  Increase the chance of some alleles being passed on to the next generation and decreasing the chance of others
• What else determines if an allele is passed on to the next generation?

  The extent to which they have contributed to a phenotype that is advantageous in a given environment.
• What does the term 'fit' describe in biology?

  How likely an organism is to survive, reproduce and pass on alleles to the next generation. Selection pressures allow the fittest to survive which increases the overall fitness of a population
• What determines the phenotypic outcome?

  Many selection pressures acting at the same time in combination with mutation and genetic drift
• What is a gene pool?

  The total of all the alleles of all genes in a population at a given time
• What is the allele frequency?

  The proportion, fraction or percentage of all the alleles of that gene in a gene pool
• Why do allele frequencies change?

  An environment is unstable and some phenotypes will be advantageous and some will be selected against causing allele frequencies to change.
• What is genetic drift?

  Random chance variations in allele frequencies in a population.

  It is most significant in small or isolated populations.
• What does the Hardy -Weinberg principle state?

  In ideal conditions, allele and genotype frequencies in a population are constant
• What are the ideal conditions for the Hardy-Weinberg principle?

  Organisms- Diploid, equal allele frequencies in each sex, sexually reproduce, random mating, generations do not overlap

  Population- very large, no immigration or emigration, no selection, no mutation
• What is the Hardy-Weinberg equation?

  p^2 + 2pq + q^2 = 1

  where p+q=1

  p is the dominant allele

  q is the recessive allele
• What is the Hardy-Weinberg equation used for?

  To calculate the frequencies of alleles in a population and determine if a population is evolving.
• What is evolution?

  A change in the average phenotype of a population
• What is meant when population is said to be in Hardy-Weinberg equilibrium?

  The allele frequencies remain constant over generations
• What is speciation and how does it come about?

  The formation of a new species and comes about if the change in average phenotype is profound enough
• How do we know speciation has occurred?

  Organisms with the altered phenotype are unable to reproduce successfully with the initial population
• What is the founder effect?

  The founder effect is the loss of genetic variation that occurs when a small group of individuals establishes a new population from a larger population. The smaller the population, the more significant the effect is
• What are the three types of natural selection?

  • Stabilising selection-extremes are selected against
  • Directional selection-one extreme is selected for
  • Disruptive selection-both extremes are selected for
• How are changes in the average phenotype brought about?

  • Mutation
  • Gene flow
  • Genetic drift
  • Natural selection
• What is Darwins theory for evolution?

  • In any population there is variation due to over producing
  • New selection pressures arise due to environmental resistance
  • Individuals with an advantageous alleles are more likely to survive
  • These individuals reproduce more successfully
  • Their offspring are likely to inherit the beneficial allele
  • This allele frequency increases over generations in the gene pool
  • The average phenotype therefore changes
• What are the two types of evolutionary product?

  • Adaptive traits are features used in an original form
  • Exaptations are structures that appear to have had their original use modified
• What is a deme?

  A sub-group within a population
• What is a species?

  A group of phenotypically similar organisms that can interbreed to produce fertile offspring
• What is reproductive isolation?

  The prevention of reproduction and, therefore, gene flow between breeding groups within a species
• What happened if demes are isolated for many generations?

  They undergo changes in allele frequency and accumulate so many different mutations so that they are no longer able to interbreed successfully with the initial population. Speciation has occurred and the new species have their own gene pool
• What are the major mechanisms of speciation?

  The founder effect, coupled with genetic drift, and natural selection. They can be enhanced by reproductive isolation.
• What is pre-zygotic isolation?

  Gametes are prevented from fusing and so a zygote never forms
• What is post-zygotic isolation?

  Gametes fuse and a zygote forms. Even if the organism develops and grows it is sterile so genes of the parent species are kept separate and the species do not merge