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Hardy-Weinberg calculations

Hardy-Weinberg calculations. Evolution & Homeostasis 2011. When we did computer modelling of beetle populations, for a large population it generally reached a stable state with more oranges than red & yellow Why?. Why don’t recessive phenotypes disappear from a population over time?.

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Hardy-Weinberg calculations

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  1. Hardy-Weinberg calculations Evolution & Homeostasis 2011

  2. When we did computer modelling of beetle populations, for a large population it generally reached a stable state with more oranges than red & yellow • Why?

  3. Why don’t recessive phenotypes disappear from a population over time?

  4. Hardy-Weinberg Equilibrium • Populations that show no phenotypic change over many generations are stable. • The frequency of phenotypes is stable. • This stability over time was described mathematicallyby: • Hardy: an English mathematician • Weinberg: a German physician Sharks and horseshoe crabs (Limulus) have remained phenotypically stable over many millions of years.

  5. Hardy-Weinberg equation p2 + 2pq + q2 = 1 • p2 is the frequency of RR genotype • 2pq is the frequency of Rr genotype • q2 is the frequency of rr genotype

  6. No. of dominant alleles aa AA AA Aa Aa Aa Aa Aa Aa X 100 Total no. of alleles DeterminingAllele Frequencies • Each individual has 2 alleles for a single gene A, so there are a total of 16 alleles in the population. • To determine the frequencies of alleles in the population, count up the numbers of dominant and recessive alleles.

  7. The Hardy-Weinberg principle is based on the stipulation that there will be no change in allele frequency of a population over time- genetic equilibrium. • For this to occur, the following conditions must be present: • Random mating • No mutations • Large population size • No migration • No natural selection • If these conditions aren’t met, the principle doesn’t apply and evolution occurs.

  8. NON-RANDOM MATING • The result of non-random mating is that some individuals have more opportunity to mate than others and thus produce more offspring (and more copies of their genes) than others.

  9. AA aa aa Aa Aa Aa • Non random mating includes: • Sexual selection • Mating with neighbors rather than with distant members of the population. • Choosing mates that are most like themselves. Aa Aa AA AA Random mating AA aa aa Aa aa Aa Aa Aa AA AA Assortative mating

  10. MUTATIONS • When a mutation occurs, the allele frequency is changed. • Mutations add to the genetic variability of populations over time and are thus the ultimate source of variation for evolution.

  11. SMALL POPULATION SIZE • In a large population, it is less likely that random fluctuations will change the allele frequencies (genetic drift).

  12. MIGRATION • Migration – the movement of breeding individuals into or out of isolated populations – results in evolutionary change because alleles move with the individuals.

  13. aa aa aa AA AA AA AA AA AA AA AA Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Immigration/Emigration • Later, one beetle (AA) joins the gene pool, while another (aa) leaves. This individual is entering the population and will add its alleles to the gene pool This individual is leaving the population, removing its alleles from the gene pool

  14. NATURAL SELECTION • Natural selection tends to reduce the genetic variability of populations by decreasing the frequency of some phenotypes and increasing the frequency of others. • Three kinds of selection cause changes in the normal distribution of phenotypes in a population. • Stabilizing selection • Directional selection • Disruptive selection

  15. aa aa aa aa aa AA AA AA AA AA AA AA Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Aa Two pale individuals died and therefore their alleles are removed from the gene pool Natural selection • Two pale individuals die due to the poor fitness of their phenotype.

  16. SUMMARY • As long as the conditions are met, the population remains stable. • If the Hardy-Weinberg equation doesn’t apply, then the population is changing and evolution occurs.

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