Evolutionary Analysis

Fourth Edition

Chapter 6

Mendelian Genetics in Populations I:

Selection and Mutation as Mechanisms of Evolution

Copyright © 2007 Pearson Prentice Hall, Inc.

Scott Freeman • Jon C. Herron

The flour beetles in this population vary in color: Some are red, others black

Population Genetics

• Changes in the relative abundance of traits in a population can be tied to changes in the relative abundance of the alleles that influence them.

Population Genetics

• Population genetics begins with a model of what happens to allele and genotype frequencies in an idealized population.

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• Once we know how Mendelian genes behave in the idealized population, we will be able to explore how they behave in real populations.

Population

• Population is a group of interbreeding individuals and their offspring.

6.1 The life cycle of an imaginary population of mice

Gene pool

6.2 A gene pool with allele frequencies of 0.6 for allele A and 0.4 for allele a

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An experiment

Blind Luck

• Genetic Drift

When blind luck plays no role

Figure 6.8 The general case for random mating in the gene pool of our model mouse population

Hardy-Weinberg Equilibrium

• Idealized population does not evolve.
• Two fundamental conclusions:
• 1. The allele frequency in a population will not change, generation after generation.
• 2. If the allele frequencies in a population are given by p and q, the genotype frequencies will be given by p2, 2pg, and q2

Assumptions of H-W Equilibrium

• 1. No selection
• 2. No mutation
• 3. No migration
• 4. No chance events (Genetic Drift)
• 5. Random mating
• A population in which conclusions 1 & 2 hold is said to be in Hardy-Weinberg Equilibrium

6.2 Selection

Figure 6.12 Persistent selection can produce substantial changes in allele frequencies over time

Figure 6.13 Frequencies of the Adh^F allele in four populations of fruit flies over 50 generations

Selection can change genotype frequencies so that they cannot be calculated by multiplying the allele frequencies

Figure 6.15 Predicted change in allele frequencies at the CCR5 locus due to the AIDS epidemic under three different scenarios

Initial frequency

Selection strength

Selection on Recessive and Dominant Alleles

Figure 6.16 Evolution in laboratory populations of flour beetles

The decline in frequency of a lethal recessive allele (red symbols)

The increase in frequency of the corresponding dominant allele.

Selection

• Natural selection is most potent as a mechanism of evolution when it is acting on common recessive alleles (and rare dominant alleles).
• When a recessive allele is rare, most copies are hidden in heterozygotes and protected from selection.

Selection on Heterozygotes & Homozygotes

• Heterozygote Superiority = Over-dominance

Heterozygotes enjoy superior fitness to either homozygote

VV VL LL

0.735 1.0 0

Selection Favoring Homozygotes

• Compound chromosomes

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• Heterozygotes have inferior fitness (Homozygotes have superior fitness)
• = under-dominance

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How populations evolve when heterozygotes have lower fitness than either homozygote

How populations evolve when heterozygotes have lower fitness than either homozygote

Underdominance

• When heterozygotes have inferior fitness, one allele tends to go to fixation while the other allele is lost.

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• However, different populations may lose different alleles.

Frequency-dependent selection

Frequency-dependent selection in Elderflower orchids

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Bumblebees

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The flowers attract bumblebees, but the flowers offer nothing.

Male function

Female function

Frequency-Dependent Selection

• Selection can also maintain two alleles in a population if each allele is advantageous when it is rare.

Compulsory Sterilization

• Eugenic sterilization program

Feeblemindedness (弱智) under a eugenic sterilization program that prevents homozygous recessive individuals from reproducing

6.4 Mutation

Mutation is a weak mechanism of evolution

Over very long periods of time, mutation can eventually produce appreciable changes in allele frequency

Mutation & Selection

Change over time in cell size of an experimental E. coli population

Mutation & Selection

• Research with bacteria illustrates that while mutation itself is only a weak mechanism of evolution, it nonetheless supplies the raw material on which natural selection acts.

Mutation-Selection Balance

• When the rate at which copies of a deleterious allele are being eliminated by selection is exactly equal to the rate at which new copies are being created by mutation, the frequency of the allele is at equilibrium. This situation is called mutation-selection balance.

Cystic Fibrosis 囊腫性纖維化

• Are the alleles that cause cystic fibrosis maintained by a balance between mutation and selection, since the allele have a relatively high frequency in the population.

A normal lung (left) versus a lung ravaged by the bacterial infections that accompany cystic fibrosis (right).

Heterozygotes for the ΔF508 allele are resistant to typhoid fever 傷寒

Data from 11 European countries suggest that S. typhi selects for carriers of the ΔF508 allele