Populations & evolution

• What is the Hardy-Weinberg principle?

  Calculates allele, phenotype & genotype frequency in a population. Assumes allele frequency in gene pool of population stays constant. Assumptions: no mutations within population, no evolution by natural selection, no changes to population (e.g, no migration), random mating & large population.
• In a gene:


  $p +$$q =$$1$
• In a population:


  $p2 +$$2pq +$$q2 =$$1$
• What is allopatric speciation?

  Populations geographically separated. Experience different conditions e.g, climate on each side of physical barrier. Different selection pressures & so different changes in allele frequencies could occur: genetic drift may affect allele frequencies, allele frequencies change as mutations occur independently in each population & different alleles more advantageous in different populations-natural selection occurs. Directional selection acts on alleles. Can lead to speciation due to changes in phenotype frequencies.
• What are the mechanisms of reproductive isolation?

  Occurs as changes in alleles, genotypes & phenotypes prevent individuals with these changes from successfully breeding with those without them. Changes include: seasonal changes-e.g, different mating seasons, mechanical changes-changes in size, shape or function of genitalia or behavioural changes-group of individuals may e.g, develop different courtship rituals or preference for breeding ground.
• What is genetic drift?

  Chance dictates which alleles passed on. In population, show genotype variation-by chance, allele for 1 genotype is passed on to more offspring than others-number with this allele increases. Can lead to evolution if allele becomes more common in population. Can lead to speciation & reproductive isolation. Evolution by genetic drift has greater effect in smaller populations where chance has greater influence.
• What is speciation?

  Development of a new species from an existing species. Occurs when populations of same species become reproductively isolated- changes in allele frequency cause changes in phenotype-means they can't interbreed to produce fertile offspring. (Gene pools vary due to random mutations & different selection pressures).
• What is sympatric speciation?


  Can occur when random mutations within a population prevent individuals that carry the mutation from breeding with other members of the population that don't carry the mutation. (No geographical isolation).
• What is a gene pool?


  Sum of all alleles of all genes of all individuals in population.
• What is a stabilising selection?

  Individuals with alleles for characteristics towards middle of the range more likely to survive & reproduce. Occurs when environment isn't changing & it reduces range of possible phenotypes.
• What is directional selection?

  Individuals with alleles for a single extreme phenotype are more likely to survive & reproduce-could be in response to an environmental change.
• What is disruptive selection?

  Individuals with alleles for extreme phenotypes more likely to survive & reproduce. Opposite of stabilising selection as characteristics towards middle of range are lost. Occurs when environment favours more than 1 phenotype.
• 3 ways genetic variation caused:

  Mutation, meiosis & random fertilisation.