2.2 a - evolution, drift and selection

• evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits
• during evolution, changes in allele frequency occurs through the non-random processes of natural selection and the random process of genetic drift
• genetic drift

  occurs when chance events cause unpredictable fluctuations in allele frequencies from one generation to the next
• genetic drift is more important in small populations as alleles are more likely to be lost by chance in a small gene pool, and any change in allele frequency is likely to be more significant to the population as a whole
• a gene pool is altered by genetic drift because certain alleles may be under-represented and some may be over-represented
• the occurring examples of genetic drift are

  • population bottlenecks
  • founder effects
• population bottlenecks

  when a population size is randomly reduced for at least one generation, so lowering the range of alleles upon which any subsequent selection pressure may then act
• founder effects

  occur through the isolation of a few random members of a population from a larger population, so the gene pool of the new population is not representative of that in the original gene pool
• natural selection acts on genetic variation in population
• variation in traits arises as a result of mutation which is the source of new sequences of DNA which can be novel alleles
• most mutations are deleterious (harmful) or neutral, but in rare cases they can be beneficial to the fitness of an individual
• because populations tend to produce more offspring than the environment can support, theres competition for resources
• individuals with variations that are better suited to their environment tend to compete better, and so survive longer and produce more offspring than their less well suited competators
• survivors breed to pass on alleles, that conferred an advantage, to the next generation
• sexual selection is a non-random process involving the selection of alleles that increase an individuals chances of mating and producing offspring
• sexual selection doesnt always help with general surviving like obtaining food and avoiding predators, so its more important in habitats with low predation and a high abundance of food
• sexual selection may lead to the evolution of sexual dimorphism
• sexual dimorphism

  when the appearance and behaviour in males and females is different
• sexual selection can lead to male-male rivalry and female choice
• male-male rivalry

  when large size or weaponry increases an individuals access to females through successful conflict with other males
• female choice

  females assessing the fitness of males through honest signals related to display, colour, song, call and plumes
• selection pressures

  this is how strong a biotic or abiotic factor in the environment exerts its effect by influencing which individuals in a population survive and pass on their alleles to the next generation
• where selection pressures are strong, the rate of evolution can be rapid
• the hardy weinberg principle

  in the absence of evolutionary influences, allele and genotype frequencies in a population will remain constant over generations
• the HW equilibrium can only apply under a set of conditions

  • absence of natural selection
  • system of random mating
  • absence of mutations
  • gene flow by migration
  • large population size
• in the HW equilibrium the conditions would not always be expected in natural situations so changes to the HW frequencies can indicate if evolutionary influences are active
• changes in allele frequencies are revealed by HW suggests that evolution is occurring
• HW formula

  p2+2pq+q2=1