Genetic Diversity and Natural Selection

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  • Genetic Diversity and Natural Selection
    • Alleles
      • Genetic diversity is number of different alleles of genes in species or population
      • Genetic diversity within a population increased by mutation, different alleles being introduced when individuals from another population migrate and reproduce (gene flow)
      • Genetic diversity allows natural selection
    • Genetic Bottlenecks
      • Reduce genetic diversity
      • 1) Causes big reduction in population e.g. large number of organisms in population die before reproducing
      • 2) Reduces number of different alleles in gene pool and so reduces diversity
      • 3) Survivors reproduce, larger population produced from fewer individuals
      • Northern Elephant Seals- original population reduced to around 50 seals because of hunting. Since, 170 000 seals produced in new population. Very little genetic diversity compared to Southern Elephant Seals (never suffered such reduction in numbers)
    • Founder Effect
      • 1) Describes when just a few organisms from population start new colony, only small number of different alleles in initial gene pool
      • 2) Frequency of each allele in new colony may be very different to frequency in original population- allele that was rare in old colony may be common in new one. May lead to higher incidence of genetic disease
      • 3) Can occur as result of migration leading to geographical separation or if new colony separated from old one for another reason e.g. religion
      • Amish- descended from small number of Swiss who migrated to North America. Population shows little genetic diversity. Remained isolated due to religious beliefs. Few new alleles introduced. High incidence of certain genetic disorders
    • Natural Selection
      • Inceases advantageous alleles in population
      • Random mutations sometimes result in new allele being formed. Can be harmful which usually means mutated allele dies out. Some mutations cause beneficial mutations that help organism survive. These alleles increase in frequency in a population
      • 1) Not all individuals as likely to reproduce as one another. Differential reproductive success in a population- individuals that have beneficial allele more likely to survive, reproduce and pass on genes than other individuals
      • 2) Greater proportion of next generation inherits beneficial allele
      • 3) They in turn more likely to survive, reproduce and pass on genes
      • 4) Frequency of beneficial allele increases from generation to generation
      • 5) Over generations this leads to evolution as advantageous alleles become more common in the population
    • Adaptations
      • Help organism to survive in their environment
      • Behavioural: ways organism acts that increase chance of survival/ reproduction e.g. possums play dead when being threatened
      • Physiological: processes inside organisms body that increase chance of survival e.g. brown bears hibernate over winter. Lower rate of metabolism which conserves energy
      • Anatomical: Structural features of organisms body that increase chance of survival e.g. whales have thick layer of blubber that keeps them warm in cold sea
      • Adaptation and selection both key factors of evolution- gradual change in species over time. Evolution has led to huge diversity of organisms on Earth


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