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Grade 11 University Biology – Unit 3 Evolution Mechanisms of Evolution – Types of Selection Overheads Section 9.1 Pages 356 Natural Selection Populations have many phenotypes and genotypes. If a single allele improves survivability, even slightly, it gives a selective advantage. As a result, the frequency of that allele will increase in the population AND in subsequent generations. Natural selection causes changes in the allele frequencies of a population...and thus, evolutionary change. There are three types of selection. 1. Stabilizing Selection Selection against individuals exhibiting variations in a trait that deviates from the current population average; rather, favours intermediate traits The most common phenotype is the favoured phenotype Most common form of selection Once a species adapts to its environment, selective pressures maintain the evolved feature Example is human birth weight. Low birth weight babies tend to be sicker, while high birth weight babies are difficult to deliver. Survivability in both cases is low. Thus, medium weight babies survive more, and thus, the intermediate trait is favoured. 2. Directional Selection Favours an increase or decrease in the value of a trait from the current population Occurs when the environment favours one EXTREME trait Causes average to move in one direction...in the illustration, the average has moved toward an extreme Example – fishing with nets captures certain sized fish. Smaller fish escape. This change moves the average size of fish in future generations to smaller size...and that size becomes the norm 3. Disruptive Selection Favours two or more variations of a trait that differ from the current population average Favours individuals at BOTH extremes Produces distinctive forms within a population (e.g., Darwin’s finches – small beaked ate small seeds and big beaked ate large seed; yet, medium beaked could not eat or compete for either seed size (maladaptive)) Sexual Selection Selects for any trait that influences mating success Leads to sexual dimorphism (i.e., differences between male and female) Most common, female selects male AND males compete Some traits, while sexually attractive, are detrimental (e.g., bright coloured plumage of the male peacock) For more information – See Figure 9.9 on Page 356