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Chapter 21Genetic Drift and Gene Flow

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Three major factors alter allele frequencies and bring about most evolutionary change

Natural selection

Genetic drift

Gene flow

Concept 21.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

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CWCW

CRCR

CRCW

CRCR

CRCR

CRCR

CRCR

CRCW

CRCW

CRCW

p (frequency of CR) 0.7 q (frequency of CW) 0.3

Generation 1

CWCW CRCR

CRCW

CRCR

CRCR

CRCW

CRCW CRCW

CWCW

CWCW

5 plantsleave

offspring

p 0.5 q 0.5

Generation 2

CRCR CRCR

CRCR CRCR

CRCR

CRCR CRCR

CRCR CRCR

CRCR

2 plantsleave

offspring

p 1.0q 0.0

Generation 3

Genetic Drift

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The Founder Effect

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Originalpopulation

Bottleneckingevent

Survivingpopulation

(a) By chance, blue marbles are overrepresented in thesurviving population.

The Bottleneck Effect

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Gene Flow

Gene flow consists of the movement of alleles among populations.

Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen).

Gene flow tends to reduce genetic variation among populations over time.

© 2014 Pearson Education, Inc.

Centralpopulation

Vlieland,the Netherlands

Easternpopulation

NORTH SEA

2 km

Gene flow can decrease the fitness of a population.

Parus major

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Gene flow can increase the fitness of a population.