Basic concepts of population genetics - Bioversity International

Consequences … (continued)
A bottleneck develops when the population size
drops sharply
The founder effect occurs when a few
individuals colonize and become established in
a new environment
p = 0.5 (frequency of A 1 )
q = 0.5 (frequency of A 2 )
N
A1A1 A1A1 A1A2 A1A1 A1A1 A1A2 A1A1 A2A2 A2A2 A2A2 A2A2 A1A2 A2A2 A1A1 A2A2 A2A2 A1A2 A1A1 A1A2 A1A2 A1A1 A1A2 A1A2 A2A2 A 2 A 2
Bottleneck
Copyright: IPGRI and Cornell University, 2003 Population genetics 41
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A 2 A 2
p = 0.0 (frequency of A 1 )
q = 1.0 (frequency of A 2 )
A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 A2A2 Small populations are highly vulnerable to extinction because the surviving sample
may not be representative of the pre-crash genepool.
Both effects depend on the number of survivors (or colonizers) and the rate of
population growth.
The graphic in the slide shows a bottleneck effect. To the left is a population in
Hardy-Weinberg equilibrium with allele frequencies of 0.5. If a sudden reduction
occurs, then the original size restored, the result may be a loss of alleles and the
fixation of others. In the example (right), only A 2 A 2 survived and allele A 1 was lost.
time