Evolution may lead to speciation

• Explain why individuals within a population of a species may show a wide range of variation in Phenotype:

  1)Genetic Factors

  • Mutations = Primary source of genetic variation
  • Crossing over between homologous chromosomes during Meiosis
  • Independent Segregation of homologous chromosomes during meiosis
  • Random Fertilisation of gametes during sexual reproduction

  2)Environmental factors (depends on context e.g. food availability, light intensity)
• What is Evolution?

  • Change in allele frequency over time / many generations in a population
  • Occurring through the process of natural selection
• Describe factors that may drive natural selection:

  • Predation, disease, and competition for the means of survival
  • These result in differential survival and reproduction e.g. Natural Selection
• Explain the principles of Natural Selection:?

  1. Mutations = Random gene mutations can result in new alleles of a gene
  2. Advantage = Due to selection pressures the new allele might benefit its possessor which gives the organisms a selective advantage
  3. Reproductive success = Possessors are more likely to survive and have increased reproductive success
  4. Inheritance = Advantageous allele is inherited by members of the next generation
  5. Allele frequency = Over many generations alle frequency increases in the gene pool
• Explain the Effects of Stabilising Selection:

  • Organisms with alleles coding for average variations of a trait have a selective advantage (e.g. babies with an average weight)
  • So frequency of alleles coding for average variations of a trait increases and those coding for extreme variations of a trait decreases
  • So range / standard deviation is reduced
• Explain the effects of directional selection:

  • Organisms with alleles coding for one extreme variation of a trait have a selective advantage (e.g. bacteria with high resistance to an antibiotic)
  • So frequency of alleles coding for this extreme variation of the trait increases and those coding for the other extreme variation of the trait decrease
• Explain the effects of Disruptive Selection:

  • Organisms with alleles coding for either extreme variation of a trait have a selective advantage
  • So frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease
  • This can lead to speciation
• Describe Speciation:

  1. Reproductive separation of two populations (of the same species)
  2. This can result in accumulation of differences in their gene pools
  3. New species arise when these genetic differences lead to an inability of members of the population to interbreed and produce fertile offspring
• Describe Allopatric Speciation:

  1. Population is split due to geographical isolation
  2. This leads to reproductive isolation, separating gene pools by preventing interbreeding / gene flow between populations
  3. Random mutations cause genetic variations within each population
  4. Different selection pressures act on each population
  5. So different advantageous alleles are passed on in each population
  6. So allele frequencies within each gene pool change over generations
  7. Eventually different populations can't interbreed to produce fertile offspring
• Describe Sympatric speciation:

  1) Population is not geographically isolated

  2)Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding / gene flow within one population e.g.

  • Gamete incompatibility
  • Different breeding seasons
  • Different courtship behaviours prevent mating
  • Body shape prevents breeding

  3)Different selection pressures act on each population

  4)So different advantageous alleles are passed on

  5)So allele frequency within each gene pool changes

  6)Eventually different populations can't interbreed to produce fertile offspring
• Explain genetic drift and its importance in small populations:

  • Genetic drift = A mechanism of evolution in which allele frequencies in a population change over generations due to chance
  • As some alleles are passed onto offspring more/less often by chance

  -Regardless of selection pressures and whether alleles give selective advantage

  • So strongest effect in small populations as gene pool is small and chance has a greater influence
  • This can reduce genetic diversity - some alleles can become fixed or lost entirely