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Evolutionary Processes

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Course Code
BIOL 1500
Alexander Mills

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BIOL1001 TEXTBOOK NOTES CHAPTER: Evolutionary Processes – Pages: 65-73 & Pages: 51-55 Evolutionary Processes  Evolution can be defined as a change in allele frequencies  Natural selection acts on individuals but evolutionary change occurs in populations  4 mechanisms that shift allele frequencies in populations: 1. Natural Selection: increases the frequency of certain alleles 2. Genetic Drift: causes allele frequencies to change randomly 3. Gene Flow: When individuals bring alleles from another population 4. Mutation: modifies allele frequencies by continually introducing new alleles  ^ Mutation and drift introduce a nonadaptive component into evolution The Hardy-Weinberg Principle  Hardy and Weinberg imagined that all of the gametes produced in each generation go into a single group called: GENE POOL and then combine at random to form offspring 2 2  A an1 A  A 2 + A1A 1 A A1= 2 + 22q 2 q = 1  Example: Frequency of A is 0.7 1nd frequency of A is 0.3. 2  ^ = p + 2pq + q 0.49 + 0.42 + 0.09 = 1  A in1this gene pool is = 0.49 + 0.21 = 0.7 (no change in allele frequency. Still 0.7!)  When no change in allele frequency, the result is called Hardy-Weinberg Principle  In order to follow the Hardy-Weinberg principle, 5 assumptions must be followed: - No Natural selection must occur - No genetic Drift in question - No gene flow - No mutation - Random mating must happen  This principle acts as a perfect null hypothesis because biologists often want to test whether natural selection is acting on a particular gene, non-random mating is occurring or one of the other evolutionary mechanisms is at work Types of Natural Selection  Occurs when individuals with certain phenotypes produce more offspring than individuals with other phenotypes do  If certain alleles are associated with the favoured phenotypes, they increase in frequency while other alleles decrease in frequency  ^ The result is evolution  Genetic Variation – the number and relative frequency of alleles that are present in a particular population  Selection can occur only if heritable variation exists in a population Directional Selection  The average phenotype of the population changed in one direction  It tends to reduce the genetic diversity of populations  If it continues over time, the favoured alleles will eventually approach a frequency of 1.0 while disadvantageous alleles will approach a frequency of 0  Alleles that reach a frequency of 0 are said to be lost  When disadvantageous alleles decline in frequency, purifying selection is said to occur Stabilizing Selection  Occurs when 2 factors that affect the population counteract each other  Example: Only small birds with small beaks can get small seeds that are available but big birds can survive better in cold weather. Food and weather counteract each other  When this is the case, individuals with intermediate body size should be favoured  This concept is known as fitness trade off  The optimal phenotype is intermediate  Reduces both extremes in a population  No change in the average value of a trait over time  Genetic variation in the population is reduced  Ex. Babies of average size survive the best  Alleles associated with high birth or low birth weight were subject to PURIFYING SELECTION Di
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