AP BIOLOGYBig Idea #1 – Part A – Part 2

Natural Selection Acts on Phenotypes

Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving

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

Gene Pools and Allele Frequencies

• 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 NO

RTH

WEST

TERR

ITOR

IES

MAPAREA

AL

AS

KA

CA

NA

DA

Fortymileherd range

Fortymile herd

AL

AS

KA

YU

KO

N

Fig. 23-5a

Porcupineherd range

Beaufort Sea NO

RTH

WEST

TERR

ITOR

IES

MAPAREA

AL

AS

KA

CA

NA

DA

Fortymileherd range

AL

AS

KA

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 of

CR 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 non-evolving 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

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

Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

Genetic Variation

• Variation in individual genotype leads to variation in individual phenotype

• Not all phenotypic variation is heritable

• Natural selection can only act on variation with a genetic component

Fig. 23-2

(a) (b)

Fig. 23-2a

(a)

Fig. 23-2b

(b)

Variation Within a Population

• Both discrete and quantitative characters contribute to variation within a population

• Discrete characters can be classified on an either-or basis

• Quantitative characters vary along a continuum within a population

• Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels

• Average heterozygosity measures the average percent of loci that are heterozygous in a population

• Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

Variation Between Populations

• Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

Fig. 23-3

13.17 19 XX10.169.128.11

1 2.4 3.14 5.18 6 7.15

9.10

1 2.19

11.12 13.17 15.18

3.8 4.16 5.14 6.7

XX

• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

Fig. 23-4

1.0

0.8

0.6

0.4

0.2

046 44 42 40 38 36 34 32 30

GeorgiaWarm (21°C)

Latitude (°N)

MaineCold (6°C)

Ldh

-B b

alle

le f

req

uen

cy

Mutation

• Mutations are changes in the nucleotide sequence of DNA

• Mutations cause new genes and alleles to arise

• Only mutations in cells that produce gametes can be passed to offspring

Animation: Genetic Variation from Sexual Recombination

Point Mutations

• A point mutation is a change in one base in a gene

The effects of point mutations can vary:• Mutations in noncoding regions of

DNA are often harmless

• Mutations in a gene might not affect protein production because of redundancy in the genetic code

Mutations That Alter Gene Number or Sequence

• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

• Duplication of large chromosome segments is usually harmful

• Duplication of small pieces of DNA is sometimes less harmful and increases the genome size

• Duplicated genes can take on new functions by further mutation

Mutation Rates

• Mutation rates are low in animals and plants

• The average is about one mutation in every 100,000 genes per generation

• Mutations rates are often lower in prokaryotes and higher in viruses

Sexual Reproduction• Sexual reproduction can shuffle

existing alleles into new combinations

• In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible