Gene Pools

GENE POOLS Gene Pools Allele Frequencies Evolution is the change in the allele frequency within a gene pool over several successive generations Genetic drift changes the composition of a gene pool due to random / chance events within the population • There is higher drift in smaller populations (faster change) • There is lower drift in larger populations (greater stability) A gene pool is the sum total of all the genes (and the alleles) that are present within an interbreeding population Natural selection changes the composition of a gene pool due to environmental selection pressures • Selection may be stabilising, directional or disruptive The allele frequency refers to the relative proportion of a particular allele within a population Genetic Drift Population Bottlenecks • Population bottlenecks occur when an event reduces the population size by an order of magnitude • Surviving population has less genetic variability (⬆ drift) Founder Effect • The founder effect describes the establishment of a new population by a fraction of a larger existing population • The new population has less genetic variability (⬆ drift) New Original New Original Types of Selection Frequency Stabilising Selection Culls extreme variations Narrows width of distribution ⇨ No antibiotic = ⬆ susceptibility Disruptive Selection • When both extremes are favored at the expense of the intermediate • Operates when conditions fluctuate • Example: Moth pigmentation ⇨ Pigmentation = camouflage ⇨ Benefit depends of conditions Disruptive Selection Directional Selection Directional Selection Favours one extreme Shifts distribution left / Favours one extreme Shifts distribution left / right Frequency ⇨ Too small = high infant mortality Directional Selection • When one phenotypic extreme is selected at the cost of the other • Operates when conditions change • Example: Antibiotic resistance ⇨ Antibiotic = ⬆ resistance Frequency Stabilising Selection • When an intermediate phenotype is favored at the expense of extremes • Operates when conditions are stable • Example: Human birth weights ⇨ Too large = birth complications Favours both extremes Creates bimodal distribution