Selection and Speciation

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  • Created by: Anon
  • Created on: 23-02-15 16:07
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  • Selection and Speciation
    • Selection
      • A condition of the Hardy-Weinburg principle is that 'there is no selection'
        • This is highly unlikely to be the case in the real world because some individuals have more reproductive success than others
      • Differences in reproductive success affects the allele frequency in the population
        • 1.All organisms produce more offspring than can be supported by the environment, causing competition
        • 2. In the gene pool there are already many different alleles
        • 3. Some alleles may cause an individual to be better suited to the environment, making them more likely to survive
        • 4. Individuals with the advantageous allele will then grow faster and be more likely to live longer and pass on that allele
        • 5. Over many generations the number of those with the advantageous allele will increase, whereas the frequency of the disadvantageous allele will decrease
      • Types of selection
        • Directional selection
          • Directional selection favours individuals that differ from the mean of the population
          • If conditions in the environment change some individuals will have a phenotype that is to the left or right of the mean, making them more likely to survive
          • The mean of the population will move in the direction of these individuals over time
        • Stabilising selection
          • Stabilising selection favours the average individual and so preserves the characteristics of the population
          • If environmental conditions remain stable the individuals with phenotypes closest to the mean will be more likely to survive and reproduce
          • Stabilising selection tends to eliminate phenotypes at the extremes
    • Speciation
      • Speciation is the evolution of one species from another species
      • Individuals in a single population breed with each other and form a single gene pool
      • However, the population may become separated and therefore the flow of genes will cease
        • Populations are usually separated by geography, in geographical isolation
        • Things such as mountains and oceans cause geographical isolation
      • The two populations may then experience different conditions in their separate environments
      • Different alleles will be favourable in each of the different conditions and so different phenotypes will be more likely to survive and breed
      • Over many millions of years the two populations may become so different that they are unable to interbreed to create fertile offspring. A new species has been formed


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