Population Genetics

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Occurs when environmental change favors an extreme phenotype

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Directional Selection

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Disruptive Selection

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Modes of Natural Selection

Stabilizing Selection

Favors intermediate over extreme phenotypes

Reduces variation and maintains the cureent average

Example: Human birth weight

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Variations in Populations

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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

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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

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Sickle Cell and Malaria

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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

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Coevolution

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