populations

• population 


  defined as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed and produce fertile offspring
•  species 


  defined as a group of similar organisms that can reproduce to give fertile offspring
•  two organisms from a different species cannot produce fertile offspring is due to the fact that different species have a different diploid number of chromosomes in their cells
• Defining a species

  • The system used by biologists to organise living organisms into categories is based on dividing organisms into species
• several factors that need to be taken into consideration when defining a species or determining whether two organisms belong to the same species

  • Similarities/differences in observable features (morphology)
  • Similarities/differences in DNA
  • Similarities/differences in RNA
  • Similarities/differences in proteins
  • The ability to interbreed and produce fertile offspring
• • A gene pool is the collection of genes within an interbreeding population at a particular time
  • As these genes can have different alleles, a gene pool can also be thought of as the sum of all the alleles of the genes of a population (of a single species) at a particular time
• • How often different alleles occur in the gene pool of a population is known as the allele frequency
• • The gene pool (or allele frequencies) in a species population can change over time due to processes such as natural selection
• Conditions for the Hardy-Weinberg principle

  • Organisms are diploid
  • Organisms reproduce by sexual reproduction only
  • No overlap between generations, i.e. parents do not mate with offspring
  • Mating is random
  • The population is large
  • No migration, mutation, or selection
  • No individuals entering the population (immigration) or leaving (emigration)
  • Allele frequencies are equal in both sexes
• Selection

  Both natural and artificial selection
• The Hardy-Weinberg principle can be useful when building models and making predictions
• • As there are only two alleles at a single gene locus for a phenotypic trait in the population:

  p + q = 1

  •  As these are all the possible genotypes of individuals in the population the following equation can be constructed:

  p2 + q2 + 2pq = 1