factors affecting allele frequencies


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factors affecting allele frequencies

  1. 1. 22/10/2012GENETIC VARIATION ANDCHANGEBiology 2.5 (external: 4 credits)
  2. 2. Factors affecting allele frequency in a gene pool 1. Evolution 2. Natural selection 3. Mutations 4. Migration 5. Genetic drift a. Founder effect b. Bottleneck effect.
  3. 3. Genetic Change: Gene pool is the total number of alleles present in a population. Genetic change is the change in frequency of alleles in the gene pool of a population. The processes of mutation, natural selection, migration and genetic drift all affect the gene pool and change the frequency of the alleles in that gene pool. Frequency of an allele = occurrence of allele _ total number of alleles
  4. 4. 1. Evolution: Is the process by which new species of organisms develop from earlier forms. Process normally occurs slowly. Most often in response to a change in a species’ environment. It is changes in the frequency of the alleles in a population (some alleles ‘do better’ than others). Evolution acts on populations (i.e. it is populations that evolve, not individuals).
  5. 5. 2. Natural Selection The theory of natural selection was proposed by Charles Darwin over 150 years ago. Populations typically produce more offspring than environmental resources can maintain – there is a competition for survival. Individuals with the best adaptations survive and reproduce (this is what is meant by fitness) and pass their successful alleles onto their offspring. The frequency of these successful alleles will then increase in the gene pool.
  6. 6.  Environmental factors (both biotic and abiotic) act as selecting agents of phenotypes. When environmental factors change, different phenotypes will be selected for. As phenotype is largely determined by genotype, successful genotype alleles will increase in frequency in the gene pool.
  7. 7.  Favourable alleles increase in frequency in a gene pool, while unfavourable alleles decrease. If the frequency of alleles changes, evolution is occurring. After a certain number of generations, the frequency of alleles and phenotypes might change so markedly that the population becomes reproductively isolated from others of that species. It is now a new species.
  8. 8. DD = warm tolerant Dd = warm tolerant Original dd = cold tolerant ancestral population Cold environment Genotypically isolated gene pools Environment changes Warm environmentCold region Further environmental changesMild region Selection for different genotypes as climateWarm region changes.
  9. 9. 3. Mutations: Are the source of new alleles in the gene pool – mutations are essential for evolution. If a mutation occurs in the gamete-producing cells during DNA replication for meiosis and that gamete is fertilised, then the mutation will enter the gene pool. Any mutation that enters a gene pool is acted upon by natural selection.
  10. 10.  Alleles resulting from unfavourable mutations are selected against, and only remain in the gene pool if they are recessive (remain ‘hidden’ in heterozygotes). Neutral or ‘silent’ mutations (neither favourable nor unfavourable) are not acted upon by selection. The frequency of these mutated alleles in the gene pool will be due to chance. Eg. a change in a base code (GGG to GGC) that codes for the same amino acid. The same protein is made - no change results from this mutation.
  11. 11. 4. Migration Migration is the movement of individuals from one population to another. Immigration = individuals migrate into a population. Emigration = individuals migrate out of a population.
  12. 12.  Both processes allow for gene flow between populations. Gene flow may change the frequency and/or the range of alleles in the populations. If populations are large, migration may have little or no effect on allele frequency. However, if populations are small, migration may have a big impact on allele frequency.
  13. 13.  Emigration may remove alleles from a population, reducing a population’s genetic diversity. Aa aa Time Population after emigrants have left – frequency of Emigrants leaving original allele A is greatly reduced population in the gene pool.
  14. 14.  Immigration may add new alleles to a population, increasing a population’s Aa aa genetic diversity. Original Population ImmigrantsFinal population afterimmigrants arrive – frequencyof allele a is greatly increasedin the gene pool.
  15. 15. 5. Genetic Drift Is the change in allele frequency due to change (not selection), and may include the loss of alleles from the gene pool. Most likely to have an effect in small populations.
  16. 16.  When populations are large, mating is random and the environment is stable (i.e. no natural selection) – the frequency of alleles tends to remain stable from generation to generation. However, when populations become small, allele frequencies can increase, decrease or even be completely loss by change alone. This has nothing to do with natural selection.
  17. 17. a. Founder effect: Occurs when a small group of individuals (founder population) colonises a geographically isolated area such as an island. The range and frequency of alleles in this small group are unlikely to be representative of those of the original population – alleles may not be present or may be more or less frequent.
  18. 18.  Likely to be more pronounced in a founder population. Evolution is likely to occur at a faster rate than in the original or other populations. In extreme cases, a founder population may be a single individual (e.g. a windblown seed).
  19. 19.  Many species arrived in NZ in small numbers by chance (e.g. birds such as silvereye, white-faced heron, spur winged plover). Or through introduction by humans (e.g. chamois, tahr). They therefore have the potential to become quite different from the original populations and their evolution is likely to progress faster as natural selection by the different environment occurs.
  20. 20. b. Bottleneck effect: Populations may be suddenly reduced in numbers. Usually from a catastrophic environmental event (e.g. fire, flood, landslide, or drought). Or by sudden, severe selection pressure (often human activities, e.g. rapid habitat destruction, introduction of predators/ competitors). After the event, the populations may recover to grow again to return to normal levels.
  21. 21.  As population numbers drop rapidly, it is likely that the range of alleles decreases and the frequency of alleles changes. When small, the population is more subject to genetic drift. When the population increases, it is likely that it will have reduced genetic diversity.