Evolution

• Evolution = the change in inherited characteristics of a group of organisms over time, occurs owing to changes in the frequency of different alleles within a population
• Factors affecting evolution:

  • mutation
  • sexual selection
  • gene flow
  • genetic drift
  • natural selection
• Gene flow = the movement of alleles between populations. Immigration and emigration result in changes of allele frequency within a population.
• Genetic drift = occurs in small populations. This is a change in allele frequency due to the random nature of mutation. The appearance of a new allele will have a greater impact in a smaller population than in a much larger population where there is a greater number of alleles present in the gene pool.
• Gene pool = sum total of all genes in a given population at a given time
• Allele frequency = how often different alleles occur in the gene pool of a population
• Allele = different versions of the same gene
• Allele frequencies can be calculated to track whether evolution is taking place or not.
• The Hardy-Weinberg Principle states that if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next.
• Conditions that must be met for the Hardy-Weinberg principle:

  • organisms are diploid
  • they reproduce by sexual reproduction only
  • parents do not mate with offspring
  • mating is random
  • population is large
  • no migration, mutation or selection
  • allele frequencies are equal in both sexes
• Calculating allele frequency:

  • the letter p represents the frequency of the dominant allele
  • the letter q represents the frequency of the recessive allele
• Calculating allele frequency:

  As 100% of the population will either have p or q or a combination of both:

  p+q=1
• There are only two alleles at a single gene locus for a phenotypic trait in the population:

  p+q=1
• Calculating genotype frequency:

  • p^2 is the proportion of homozygous dominant (BB)
  • q^2 is the proportion of homozygous recessive (bb)
  • 2pq is the proportion of heterozygous (Bb)
• Calculating genotype frequency:

  p^2 + q^2 + 2pq = 1
• The gene pool of a large population ensures lots of genetic diversity (presence of many different genes and alleles). Genetic diversity leads to variation within a population which is essential for natural selection. Selection pressures such as changes in environment, new diseases, prey, competitors or human influences lead to evolution. The population can adapt to change over time.
• Small populations with limited genetic diversity can’t adapt to change as easily. A new strain of pathogen could wipe out a whole population.
• The size of a population can be affected by many factors. Factors which limit or decrease the size of a population are called limiting factors.
• Limiting factors:

  • density-dependent factors = dependent on population size and include competition, predation, parasitism and communicable disease
  • density-independent factors = affect populations of all sizes in the same way- includes climate change, natural disaster, seasonal change and human activity
• Large reductions in population size which last for at least one generation are called population bottlenecks. The population is greatly reduced and the effect will be seen in future generations. It takes thousands of years for genetic diversity to develop in a population through the slow accumulation of mutations.
• A positive aspect of a generic bottleneck is that a beneficial mutation will have a much greater impact and lead to the quicker development of a new species.
• Founder effect= small populations can arise due to the establishment of new colonies by a few isolated individuals
• Founder effect:

  • these small populations have much smaller gene pools than the original population and display less genetic variation
• Founder effect:

  If carried to the new population, the frequency of any alleles that were rare in the original population will be much higher in the new, smaller population and so will have a bigger impact during natural selection.
• Evolutionary forces:

  • stabilising selection = allele frequencies are kept constant over generations
  • directional selection = gradual change in allele frequencies over generations
  • disruptive selection = extremes selected for and ‘normal’ selected against
• Evolutionary forces:

  • stabilising selection
  • directional selection
  • disruptive selection
• Stabilising selection = the norm/average is selected for and the extremes are selected against. Stabilising selection therefore results in a reduction in the frequency of alleles at the extreme and an increase in frequency of ‘average‘ alleles.
• Directional selection = when there is a change in the environment and the normal phenotype is no longer the most advantageous. Organisms which are less common and have extreme phenotypes are positively selected. The allele frequency then shifts towards the extreme phenotypes and evolution occurs.
• Disruptive selection = the extremes are selected for and the norm is selected against.