Summary – one summary at the end Unit 3 Evolution “Evolution as Genetic Change 3.6” Name Date Period EQ: Why does an advantageous trait increase in proportion within a species over time? Take notes here * Read Chapter 16 for additional support LT: Questions 25 Justify why an advantageous trait will increase in proportion from one generation to another when provided statistical evidence Summary – one summary at the end

Natural selection never acts on genes directly Natural selection never acts on genes directly. Natural selection acts on individuals who survive and reproduce If an individual dies without reproducing the individual does not contribute its alleles to the populations gene pool

Normal Distribution (of Finches) - starts as a bell-shaped curve Smaller Larger Beak Size Number of Birds in Population

Because less small seeds, more big seeds Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle, or at the other end. One extreme trait is favored EX: Bigger beaks are favored Because less small seeds, more big seeds Selection against an extreme Population after selection Original population

Key Food becomes scarce. Directional Selection Low mortality, high fitness Directional Selection High mortality, low fitness Food becomes scarce. Die Live out Die Live out

Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve. Selection against both extremes Original population Population after selection

Stabilizing Selection Average survives Selection against both extremes keep curve narrow and in same place. Selection against both extremes Original population Population after selection

Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Key Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness % of Population Die Live Die Out out Birth Weight

Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Selection against the middle Population after selection Original population

Disruptive Selection Opposite extreme traits are favored EX: Small & large beaks because only small & large seeds Average (middle) trait is eliminated EX: Less average size beaks because less average sized seeds

Disruptive Selection Key Disruptive Selection Low mortality, high fitness High mortality, low fitness Disruptive Selection Large and small seeds become more common. Live Die Live out Population splits into two subgroups specializing in different seeds. Birds in Population Live Die Live out Birds in Population Beak Size Beak Size

Organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait