Modes of Natural Selection Modes of Natural Selection

Modes of Natural Selection

Modes of Natural Selection • Directional Selection - Favors individuals at one end of the phenotypic range Most common during times of environmental change or when moving to new habitats • Disruptive selection - Favors extreme over intermediate phenotypes - Occurs when environmental change favors an extreme phenotype 2

Directional Selection 3

Disruptive Selection 4

Modes of Natural Selection • Stabilizing Selection - Favors intermediate over extreme phenotypes Reduces variation and maintains the current average Example: Human birth weight 5

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Variations in Populations

Geographic Variations • Variation in a species due to climate or another geographical condition • Populations live in different locations • Example: Finches of Galapagos Islands & South America 8

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Heterozygote Advantage • Favors heterozygotes (Aa) • Maintains both alleles (A, a) instead of removing less successful alleles from a population • Sickle cell anemia • > Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age. • > Heterozygotes are less susceptible to malaria 10

Sickle Cell and Malaria 11

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Other Sources of Variation • Mutations - In stable environments, mutations often result in little or no benefit to an organism, or are often harmful - Mutations are more beneficial (rare) in changing environments (Example: HIV resistance to antiviral drugs) • Genetic Recombination - source of most genetic differences between individuals in a population • Co-evolution • -Often occurs between parasite & host and flowers & their pollinators 13

Coevolution 14

Hardy-Weinberg Principle

The Hardy-Weinberg Principle • Used to describe a non-evolving population. • Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool. • Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium. 16

The Hardy-Weinberg Principle • Deviation from Hardy-Weinberg equilibrium usually results in evolution • Understanding a non-evolving population, helps us to understand how evolution occurs 17

5 Assumptions of the H-W Principle 1. Large population size - small populations have fluctuations in allele frequencies (e. g. , fire, storm). 2. No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population. 3. No net mutations - if alleles change from one to another, this will change the frequency of those alleles 18

5 Assumptions of the H-W Principle 3. Random mating - if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. 4. No natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation. 19

Traits Selected for Random Mating 20

The Hardy-Weinberg Principle • The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change) • • The Hardy-Weinberg Equation: • 1. 0 = p 2 + 2 pq + q 2 • Where: • p 2 = frequency of AA genotype • 2 pq = frequency of Aa • q 2 = frequency of aa genotype 21

The Hardy-Weinberg Principle • Determining the Allele Frequency using Hardy. Weinberg: • • 1. 0 = p + q • Where: • p = frequency of A allele • q = frequency of a allele 22

Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 320 RR 160 Rr 20 white flowers 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0. 8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0. 2 (20%) r 23

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