© 2014 Pearson Education, Inc. Objective 5: TSWBAT recognize that genetic variation makes evolution...

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Objective 5:TSWBAT recognize that genetic variation makes

evolution possible.

© 2014 Pearson Education, Inc.

Overview: The Smallest Unit of Evolution

One common misconception is that organisms evolve during their lifetimes

Natural selection acts on individuals, but only populations evolve

Consider, for example, a population of medium ground finches on Daphne Major Island

During a drought, large-beaked birds were more likely to crack large seeds and survive

The finch population evolved by natural selection

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

Three mechanisms cause allele frequency change

Natural selection

Genetic drift

Gene flow

Only natural selection causes adaptive evolution

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Variation in heritable traits is a prerequisite for evolution

Mendel’s work on pea plants provided evidence of discrete heritable units (genes)

Concept 21.1: Genetic variation makes evolution possible

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Genetic Variation

Phenotypic variation often reflects genetic variation

Genetic variation among individuals is caused by differences in genes or other DNA sequences

Some phenotypic differences are due to differences in a single gene and can be classified on an “either-or” basis

Other phenotypic differences are due to the influence of many genes and vary in gradations along a continuum

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Figure 21.3

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Genetic variation can be measured at the whole gene level as gene variability

Gene variability can be quantified as the average percent of loci that are heterozygous

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Genetic variation can be measured at the molecular level of DNA as nucleotide variability

Nucleotide variation rarely results in phenotypic variation

Most differences occur in noncoding regions (introns)

Variations that occur in coding regions (exons) rarely change the amino acid sequence of the encoded protein

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Figure 21.4

1,000

Substitution resultingin translation ofdifferent amino acid

Base-pairsubstitutions Insertion sites

Deletion

Exon Intron

1 500

2,5002,0001,500

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Phenotype is the product of inherited genotype and environmental influences

Natural selection can only act on phenotypic variation that has a genetic component

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Figure 21.5a

(a) Caterpillars raised on a diet of oak flowers

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Figure 21.5b

(b) Caterpillars raised on a diet of oak leaves

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Sources of Genetic Variation

New genes and alleles can arise by mutation or gene duplication

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Formation of New Alleles

A mutation is a change in the nucleotide sequence of DNA

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

A “point mutation” is a change in one base in a gene

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The effects of point mutations can vary

Mutations in noncoding regions of DNA are often harmless

Mutations to genes can be neutral because of redundancy in the genetic code

Mutations that alter the phenotype are often harmful

Mutations that result in a change in protein production can sometimes be beneficial

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Altering Gene Number or Position

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

Duplication of small pieces of DNA increases genome size and is usually less harmful

Duplicated genes can take on new functions by further mutation

An ancestral odor-detecting gene has been duplicated many times: Humans have 350 functional copies of the gene; mice have 1,000

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Rapid Reproduction

Mutation rates are low in animals and plants

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

Mutation rates are often lower in prokaryotes and higher in viruses

Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses

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Sexual Reproduction

In organisms that reproduce sexually, most genetic variation results from recombination of alleles

Sexual reproduction can shuffle existing alleles into new combinations through three mechanisms: crossing over, independent assortment, and fertilization

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Objective 6

TSWBAT describe the basic principles of population

genetics including us of the Hardy-Weinberg theorem and

equation.

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Concept 21.2: 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

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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

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

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Figure 21.6Porcupine herd

Beaufort Sea

Fortymile herd

Porcupine herd range

Fortymile herd range

MAPAREA

AL

AS

KA

CA

NA

DA

NO

RT

HW

ES

T

TE

RR

ITO

RIE

S

YU

KO

NA

LA

SK

A

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Figure 21.6a

Porcupine herd

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Figure 21.6b

Fortymile herd

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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 times 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

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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

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Figure 21.UN01

CRCR

CRCW

CWCW

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For example, consider a population of wildflowers that is incompletely dominant for color

320 red flowers (CRCR)

160 pink flowers (CRCW)

20 white flowers (CWCW)

Calculate the number of copies of each allele

CR (320 2) 160 800

CW (20 2) 160 200

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To calculate the frequency of each allele

p freq CR 800 / (800 200) 0.8 (80%)

q 1 p 0.2 (20%)

The sum of alleles is always 1

0.8 0.2 1

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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

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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

Hardy-Weinberg Equilibrium

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

Consider, for example, the same population of 500 wildflowers and 1,000 alleles where

p freq CR 0.8

q freq CW 0.2

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Figure 21.7Frequencies of alleles

Gametes produced

p frequency of CR allele

q frequency of CW allele

Alleles in the population

Each egg: Each sperm:

0.8

0.2

80%chance

80%chance

20%chance

20%chance

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The frequency of genotypes can be calculated

CRCR p2 (0.8)2 0.64

CRCW 2pq 2(0.8)(0.2) 0.32

CWCW q2 (0.2)2 0.04

The frequency of genotypes can be confirmed using a Punnett square

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Figure 21.8

Sperm

Eggs

80% CR (p 0.8) 20% CW (q 0.2)

p 0.8 q 0.2 CR

CR

CW

CW

p 0.8

q 0.2

0.64 (p2)CRCR

0.16 (pq)CRCW

0.16 (qp)CRCW

0.04 (q2)CWCW

Gametes of this generation:

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

64% CR

(from CRCR plants)16% CR

(from CRCW plants)

4% CW

(from CWCW plants)16% CW

(from CRCW plants)

80% CR 0.8 p

20% CW 0.2 q

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

With random mating, these gametes will result in the samemix of genotypes in the next generation:

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Figure 21.8a

Sperm

Eggs

80% CR (p 0.8) 20% CW (q 0.2)

p 0.8 q 0.2 CR

CR

CW

CW

p 0.8

q 0.2

0.64 (p2)CRCR

0.16 (pq)CRCW

0.16 (qp)CRCW

0.04 (q2)CWCW

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Figure 21.8b

Gametes of this generation:

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

64% CR

(from CRCR plants)16% CR

(from CRCW plants)

4% CW

(from CWCW plants)16% CW

(from CRCW plants)

80% CR 0.8 p

20% CW 0.2 q

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

With random mating, these gametes will result in the samemix of genotypes in the next generation:

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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

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Figure 21.UN02

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Conditions for Hardy-Weinberg Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population that is not evolving

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

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The five conditions for nonevolving populations are rarely met in nature

1. No mutations

2. Random mating

3. No natural selection

4. Extremely large population size

5. No gene flow

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

Some populations evolve slowly enough that evolution cannot be detected

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Applying the Hardy-Weinberg Principle

We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that

1. The PKU gene mutation rate is low

2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

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

4. The population is large

5. Migration has no effect, as many other populations have similar allele frequencies

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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 0.99 0.01 0.0198

or approximately 2% of the U.S. population