The Evolution of Populations

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

Natural selection - nonrandom

Genetic drift - random

Gene flow - random

Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Slide 16

Natural Selection and Genetic Drift

Natural Selection: Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions by the more fit individuals.

Genetic drift: describes how allele frequencies fluctuate randomly from one generation to the next.

The smaller a sample, the greater the chance of deviation from a predicted result.

Genetic drift tends to reduce genetic variation through losses of alleles.

Slide 17

Genetic Drift

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7

q (frequency of CW ) = 0.3

Generation 2

CR CW

CR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5

q = 0.5

Generation 3

p = 1.0

q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR

Slide 18

Genetic Drift: The Founder Effect

The founder effect occurs when a few individuals become isolated from a larger population.

Allele frequencies in the small founder population can be different from those in the larger parent population.

Slide 19

Genetic Drift: The Bottleneck Effect

The bottleneck effect is a sudden reduction in population size due to a change in the environment, such as a natural disaster.

The resulting gene pool may no longer be reflective of the original population’s gene pool.

If the population remains small, it may be further affected by genetic drift.

Slide 20

Genetic Drift: The BottleNeck Effect

Original

population

Bottlenecking

event

Surviving

population

Slide 21

Effects of Genetic Drift: A Summary

Genetic drift is significant in small populations.

Genetic drift causes allele frequencies to change at random.

Genetic drift can lead to a loss of genetic variation within populations.

Genetic drift can cause harmful alleles to become fixed.

Slide 22

Gene Flow: Immigration & Emmigration