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Population Genetics

Sven Delaney School of BABS

s.delaney@unsw.edu.au

Overview

Population Genetics: the Modern Synthesis

Evolutionary forces

Hardy-Weinberg Law

Questions:

Why are some phenotypes and genotypes common in a population, while others are rare?

Why do small populations have high levels of genetic disorders?

Why do couples (and their families) tend to look similar?

Darwin

Darwin proposed the Theory of Natural Selection in On the Origin of Species by Means of Natural Selection (1859)

Variation within a population

Struggle for survival

Fittest survive & reproduce

Adaptation

The Modern Synthesis Darwin could not explain how variation was inherited;

suggested blending or Lamarckian inheritance

Mendels work published in 1865, but ignored until early 20th century

Fusion of Mendels Laws and Darwins theory & developments in statistics and other fields resulted in the modern synthesis (neo-Darwinism)

Population and evolutionary genetics

Population Genetics

Definitions

Population: a group of interbreeding individuals of

the same species that inhabit a specific place at

the same time

Gene pool: all of the alleles in the population

Genetic description of populations?

Hardy-Weinberg Law

Species: a group of individuals that are capable of

breeding to produce fertile offspring

Hardy-Weinberg Law

A simple model for understanding phenotype, genotype and allele frequencies in an ideal, non-evolving population

Considers a single gene with two alleles: A, a

Phenotypes: Homozygous

dominant

Heterozygous Homozygous

recessive

Genotypes: AA Aa (or aA) aa

Genotype

frequencies:

p2 2pq q2

Allele frequencies are p (for A) and q (for a)

2

Hardy-Weinberg Law: Derivation

AA

Aa

Aa

aa

A

A a

a

Eggs

Sperm

Punnett square

Genotypic ratio = 1AA: 2Aa:1aa

Hardy-Weinberg Law: Derivation

AA

p2

Aa

pq

Aa

pq

aa

q2

p

p q

q

Eggs

Sperm

Terms with p and q are proportions, hence

(p + q) = 1

p = 1-q

q = 1-p

p2 + 2pq + q2 = 1 and

Punnett square

Hardy-Weinberg Equilibrium

Non-evolving populations will be in a state of Hardy-Weinberg equilibrium, in which allele frequencies will not change from one generation to the next

Equilibrium due to random segregation of alleles in heterozygotes

fixed allele frequencies constrain genotype frequencies since (p + q) = 1 and p2 + 2pq + q2 = 1

Genotype frequencies may change slightly, but remain close to an equilibrium value

Aa aa AA

Hardy-Weinberg Equilibrium

Allele and genotype frequencies at

equilibrium

Approach to equilibrium in an

ideal population

1 2 3 4 5

Generation 0

0

0.8

0.375

Fre

qu

en

cy

of A

a (

2p

q)

p = 0.75

q = 0.25

Hardy-Weinberg in Action

Mm mm MM

N = 50, p = q = 0.5, all heterozygotes (p(Mm) = 1.0)

Reproduction!

What are the allele and genotype frequencies at T1 and T2?

p(MM) = ? ; p(Mm) = ? ; p(mm) = ?

p = ? ; q = ?

One locus, two alleles: minty (M) and fruity (m)

Evolutionary Forces

H-W equilibrium will be maintained only in the absence of factors that change allele frequencies (i.e. evolution = change in allele proportions)

Deviation from H-W equilibrium is a useful indicator of evolution in action

Evolutionary forces:

1. Small population size

Small populations are more affected by random genetic drift (changes in allele frequencies due to chance events) than large populations

Why???

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Evolution in Action

Now what happens to allele proportions?

Evolutionary Forces

1. Small population size

Small population size may occur because of bottleneck or founder effects

Bottleneck effect

Examples: fire,

flood, famine,

disease

Evolutionary Forces

1. Small population size

Founder effect

Amish Norfolk Island

Evolutionary Forces

2. Non-random (assortative) mating

Individuals choose mates on the basis of phenotype (not randomly)

2 types: negative (like chooses unlike) and positive (like chooses like)

Positive

Narcissus

Negative

Eclectus parrot

Evolutionary Forces

3. Mutation

Only source of new alleles

Very slow: NOT responsible for rapid changes in allele frequency

Provides the raw material for selection

Evolutionary Forces

4. Migration

Refers to genetic exchange with other populations

Barriers between populations may be physical, sociocultural etc.

Isolated populations may develop reproductive incompatibility mechanisms that prevent interbreeding ( separate species)

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Human Migration Evolutionary Forces

5. Selection

May be natural or artificial, positive or negative

Modes of selection

Examples of Selection

Darwins finches Scale-eating fish

Heterozygote Advantage

Heterozygote has greater fitness than either homozygote

Example: sickle-cell anemia

-In malarial regions, two alleles of hemoglobin -chain (sickle-cell, HbS and normal, HbA) are often present

-HbS/HbA heterozygotes are less susceptible to malaria

than HbA/HbA homozygotes, and lack the severe symptoms

of sickle-cell anemia in HbS/HbS homozygotes

Sickle-cell Anemia Summary

Modern synthesis

Evolutionary forces

Examples of selection

Small population size

Non-random mating

Mutation

Migration

Selection

Hardy-Weinberg law & equilibrium

p2 + 2pq + q2 = 1

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References

Campbell, Reece & Meyers, Biology

6th, 7th, 8th or 9th Edition, Chapters 22, 23.

Griffiths et al., Introduction to Genetic Analysis 9th Edition (Ch. 17) or 10th Edition (Ch. 18)

Knox et al., Biology 3rd Edition, Chapter 32.