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Natural Selection on Polygenic Traits. Effects of natural selection on polygenic traits are very complex Can affect the distributions of phenotypes in.

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Presentation on theme: "Natural Selection on Polygenic Traits. Effects of natural selection on polygenic traits are very complex Can affect the distributions of phenotypes in."— Presentation transcript:

1 Natural Selection on Polygenic Traits

2 Effects of natural selection on polygenic traits are very complex Can affect the distributions of phenotypes in any of three ways: –Directional selection –Stabilizing selection –Disruptive selection

3 Directional Selection Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end Entire curve moves as the trait changes –Ex. Finches and beak size – food became scarce; finches compete for food; bigger beaks are able to find food, survive and reproduce = beak size increases

4 Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

5 Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end of the curve Keeps the center of the curve at its current position, but narrows the graph –Ex. Weight of human infants – small babies less likely to survive and large babies have difficulty being born = average-sized babies are favored

6 Stabilizing Selection Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

7 Disruptive Selection Individuals at upper and lower ends of the curve have higher fitness than individuals near the middle Can cause graph to split into two, creating two distinct phenotypes –Ex. Bird – 2 different seed sizes (small and large); birds with large beaks and birds with small beaks survive = two distinct beak sizes

8 Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness

9 Genetic Drift Allele can become more or less common BY CHANCE Random change in allele frequency is genetic drift (caused BY CHANCE!) In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just BY CHANCE. Over time, a series of chance occurrences of this type can cause an allele to become common in a population

10 Genetic Drift (continued) May occur when a small group of individuals colonies a new habitat. These individuals may carry alleles in different relative frequencies than the larger population that they came from. If so, the population that they create will be genetically different from the parent population

11 Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Founding Population A Founding Population B Descendants

12 Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Founding Population A Founding Population B Descendants

13 Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population: Sample of Original Population Founding Population A Founding Population B Descendants

14 Genetic Equilibrium Hardy and Weinberg – 2 scientists that asked: Are there any conditions under which evolution will NOT occur? If allele frequencies stay the same – the population does NOT evolve

15 Genetic Equilibrium (continued) 5 conditions are required to maintain genetic equilibrium from generation to generation: –Random mating – equal chance of passing genes –Large population – genetic drift has less effect –No movement into or out of the population – must maintain gene pool –No mutation – cant have new alleles –No natural selection – no survival of the fittest

16 Genetic Equilibrium (continued) Meeting these five conditions is difficult to do and often cannot be met, so evolution will occur –Many organisms mate with chosen mates based on things like strength, color, etc –Not all populations are large –Migration occurs all the time –Mutations are bound to happen –Natural selection/survival of the fittest often cannot be avoided!


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