GENETICS & EVOLUTION:POPULATION GENETICS

Chapter 3.3

Overview

Microevolution and mutation Hardy-Weinberg Principle and

Equilibrium Natural Selection Genetic drift Gene flow

Overview: The Smallest Unit of Evolution One misconception is that organisms

evolve, in the Darwinian sense, during their lifetimes

Natural selection acts on individuals, but only populations evolve

Genetic variations in populations contribute to evolution

Microevolution is a change in allele frequencies in a population over generations

Fig. 23-1

Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

Mutation and sexual reproduction produce the genetic variation that makes evolution possible

Gene Pools and Allele Frequencies

• The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring

A gene pool consists of all the alleles for all loci in a population

A locus is fixed if all individuals in a population are homozygous for the same allele

Fig. 23-5Porcupine herd

Porcupineherd range

Beaufort Sea

NORTH

WES

T

TERRITO

RIES

MAPAREA

ALA

SK

A

CA

NA

DA

Fortymileherd range

Fortymile herd

ALA

SK

A

YU

KO

N

Fig. 23-5a

Porcupineherd range

Beaufort Sea

NORTH

WES

T

TERRITO

RIE

S

MAPAREA

ALA

SK

A

CA

NA

DA

Fortymileherd range

ALA

SK

A

YU

KO

N

The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles

at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2

alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies

The frequency of all alleles in a population will add up to 1 For example, p + q = 1

The Hardy-Weinberg Principle

The Hardy-Weinberg principle describes a population that is not evolving

If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation

In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

Mendelian inheritance preserves genetic variation in a population

Fig. 23-6

Frequencies of alleles

Alleles in the population

Gametes produced

Each egg:

Each sperm:

80%chance

80%chance

20%chance

20%chance

q = frequency of

p = frequency ofCR allele = 0.8

CW allele = 0.2

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of

the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Fig. 23-7-1

SpermCR

(80%)

CW

(20

%)

80% CR ( p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% ( pq) CRCW

4% (q2) CW CW

CR

(80

%)

64%

( p2) CRCR

16% (qp) CRCW

Eg

gs

Fig. 23-7-2

Gametes of this generation:

64% CRCR, 32% CRCW, and 4% CWCW

64% CR   +    16% CR    =   80% CR = 0.8 = p

4% CW     +    16% CW   =   20% CW = 0.2 = q

Fig. 23-7-3

Gametes of this generation:

64% CRCR, 32% CRCW, and 4% CWCW

64% CR   +    16% CR    =   80% CR = 0.8 = p

4% CW     +    16% CW   =   20% CW = 0.2 = q

64% CRCR, 32% CRCW, and 4% CWCW plants

Genotypes in the next generation:

Fig. 23-7-4

Gametes of this generation:

64% CR CR, 32% CR CW, and 4% CW CW

64% CR    +     16% CR    =   80% CR  = 0.8 = p

4% CW      +    16% CW    =  20% CW = 0.2 = q

64% CR CR, 32% CR CW, and 4% CW CW plants

Genotypes in the next generation:

SpermCR

(80%)

CW

(20

%)

80% CR ( p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% ( pq) CR CW

4% (q2) CW CW

CR

(80

%)

64% ( p2) CR CR

16% (qp)

CR CW

Eg

gs

Conditions for Hardy-Weinberg Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population

In real populations, allele and genotype frequencies do change over time

The five conditions for nonevolving populations are rarely met in nature: No mutations Random mating No natural selection Extremely large population size No gene flow

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

Applying the Hardy-Weinberg Principle

We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: The PKU gene mutation rate is low Mate selection is random with respect to whether

or not an individual is a carrier for the PKU allele

Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions

The population is large Migration has no effect as many other

populations have similar allele frequencies

The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01

The frequency of normal alleles is p = 1 – q = 1 – 0.01 = 0.99

The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 or approximately 2% of the U.S. population

Required conditions are rarely (if ever) met Changes in gene pool frequencies are likely When gene pool frequencies change,

microevolution has occurred

Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place

Hence, microevolution (via genetic mutations) The raw material for evolutionary change Provides new combinations of alleles Some might be more adaptive than others

Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow

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

You should now be able to:

1. Explain why the majority of point mutations are harmless

2. Explain how sexual recombination generates genetic variability

3. Define the terms population, species, gene pool, relative fitness, and neutral variation

4. List the five conditions of Hardy-Weinberg equilibrium

5. Apply the Hardy-Weinberg equation to a population genetics problem

6. Explain why natural selection is the only mechanism that consistently produces adaptive change

7. Explain the role of population size in genetic drift

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

Allele FrequenciesRed short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies?a. red = 0.36, white = 0.16b. red = 0.6, white = 0.4c. red = 0.5, white = 0.5d. Allele frequencies cannot be determined

unless the population is in equilibrium.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

Hardy-Weinberg Equilibrium DeterminationWhich of these populations are in Hardy-Weinberg equilibrium?

a. Ab. Bc. both A and Bd. neither A nor B

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

Cystic Fibrosis 1The frequency of cystic fibrosis, a recessive genetic disease, is 1 per 2,500 births among Northern Europeans. Assuming random mating, what is the frequency of carriers?

a. 1/2,500 b. 1/50c. 1/25d. The frequency cannot be calculated because

selection violates Hardy-Weinberg assumptions.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

Cystic Fibrosis 2Until the 1950s, infants born with cystic fibrosis did not survive longer than a few months. If the frequency of carriers was 4% in the year 1900, what proportion of CF alleles was eliminated in one generation?

a. 100%b. 50%c. 4%d. 2%e. <0.1%