Natural selection

• 3 main types of natural selection

  • stabilising
  • directional
• stabilizing natural selection

  • selection pressure toward the centre increases the number of individuals at the modal value, intermediate phenotypes favoured. Environment usually unchanging
  • e.g. Human birth weight
• directional natural selection

  • selection pressure toward one extreme moves the mode in this direction. Caused by an environmental change
  • e.g. Antibiotic resistance in bacteria
• gene pool

  • all the genes and alleles of the breeding individuals in a population at a particular time
  • gene pool's composition changes from one generation to the next as the relative proportions of alleles vary
  • if there is a consistent change in allele frequency then a population is evolving
• genetic drift

  • genetic drift is where neither allele for a characteristic offers an advantage or disadvantage
  • random events such as floods, outbreaks of diseases, earthquakes or food shortages can lead to an allele disappearing from a population
  • there is no selection pressure, it is down to chance
  • has a greater effect in smaller populations
  • genetic drift can result from a genetic bottleneck or as the result of the founder effect
• population/genetic bottlenecks

  • population/genetic bottleneck occur when a large diverse population is reduced by a non-selective event and then increases again
  • total genetic diversity of the few survivors is likely to be lower than that of the original population, as the population re-establishes itself, this low level of diversity will be maintained. there could be a loss of advantageous alleles or an increase in harmful alleles
• population/genetic bottlenecks

  • the cheetah population has a low genetic diversity. This is thought to be due to a very narrow bottleneck, where only a single family group survived the last ice age
• the founder effect

  • occurs when a new population is established from a very small number of individuals who originate from a larger parent population
  • this is demonstrated in human migration and those that have established populations in new areas. The new population can become isolated (due to religious, cultural, geographical reasons) which will in turn reduce their gene pool