POPULATION GENETICS

OVERVIEW: THE SMALLEST UNIT OF EVOLUTION

Natural selection acts on individuals On phenotypes

only populations evolve Genetic variations contribute to evolution

Microevolution change in the genetic makeup of a population

from generation to generation

THE MODERN SYNTHESIS

Population genetics study of how populations change

genetically over time integrates Mendelian genetics with the

Darwinian theory of evolution by natural selection

GENE POOLS AND ALLELE FREQUENCIES

• gene pool is the total aggregate of genes in a population at any one time gene pool consists of all gene loci in all

individuals of the population

LE 23-3MAP

AREA

CA

NA

DA

ALA

SKA

Beaufort Sea

Porcupineherd range NORTHW

EST

TERRITORIES

Fairbanks

Fortymileherd range Whitehorse

ALA

SKA

YUK

ON

THE HARDY-WEINBERG THEOREM

describes a population that is not evolving frequencies of alleles and genotypes in a

population’s gene pool remain constant from generation to generation

provided that only Mendelian segregation and recombination of alleles are at work

Mendelian inheritance preserves genetic variation in a population

LE 23-4

Generation3 25% CRCR

Generation4

50% CRCW 25% CWCW

50% CW

gametes50% CR

come together at random

25% CRCR 50% CRCW 25% CWCW

Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions

gametes

Generation2

Generation1

CRCR CWCW

genotypegenotypePlants mate

All CRCW

(all pink flowers)

50% CR 50% CW

gametes gametes

come together at random

X

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

p2 and q2 represent the frequencies of the homozygous genotypes

2pq represents the frequency of the heterozygous genotype

p + q = 1 p represents the frequency of one allele (dominant) q represents the frequency of the other allele (recessive)

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

POPULATION GENETICS AND HUMAN HEALTH

We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease

VARIATION THAT MAKES EVOLUTION POSSIBLE Mutation

changes in the nucleotide sequence of DNA cause new genes and alleles to arise point mutation is a change in one base in a gene

usually harmless but may have significant impact on phenotype

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

Gene duplication is nearly always harmful Mutation rates are low in animals and plants

one mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms

sexual recombination far more important than mutation in producing the

genetic differences that make adaptation possible

Deletion

Duplication

Inversion

Translocation

A POPULATION’S GENETIC COMPOSITION

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

LE 23-7

CRCR CRCR CWCW CRCR

CRCW

CRCR

CRCW

CWCW

CWCW

CRCW CRCW

CRCRCRCW

CRCWCRCR

CRCR

CRCW

CWCW

CRCW

CRCR

Only 5 of10 plantsleaveoffspring

Only 2 of10 plantsleaveoffspring

CRCR

CRCR

CRCR CRCR

CRCR

CRCR CRCR

CRCR

CRCR

CRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

LE 23-8

Originalpopulation

Bottleneckingevent

Survivingpopulation

GENETIC VARIATION

Genetic variation occurs in individuals in populations of all species

It is not always heritable

LE 23-9

Map butterflies thatemerge in spring:orange and brown

Map butterflies thatemerge in late summer:black and white

POLYMORPHISM

Phenotypic polymorphism a population in which two or more distinct

morphs for a character are represented in high enough frequencies to be readily noticeable

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

LE 23-11Heights of yarrow plants grown in common garden

Sierra NevadaRange

Great BasinPlateau

Seed collection sites

100

50

0

3,000

2,000

1,000

0

Mea

n he

ight

(cm

)A

ltit u

de (m

)

SELECTION

Selection favors certain genotypes by acting on the phenotypes of certain organisms

Three modes of selection: Directional Disruptive Stabilizing

Directional selection favors individuals at one end of the

phenotypic range Disruptive selection

favors individuals at both extremes of the phenotypic range

Stabilizing selection favors intermediate variants and acts

against extreme phenotypes

LE 23-12

Originalpopulation

Evolvedpopulation

Phenotypes (fur color)

Original population

Directional selection Disruptive selection Stabilizing selection

Freq

uenc

y of

indi

vidu

als

THE PRESERVATION OF GENETIC VARIATION

Diploidy maintains genetic variation in the form of hidden

recessive alleles Balancing selection

occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

leads to a state called balanced polymorphism Heterozygote Advantage

maintains two or more alleles at that locus Ex. sickle-cell allele

causes mutations in hemoglobin also confers malaria resistance

LE 23-13

Frequencies of thesickle-cell allele

0–2.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

10.0–12.5%

>12.5%

Distribution ofmalaria caused byPlasmodium falciparum(a protozoan)

In frequency-dependent selection fitness of any morph declines if it

becomes too common in the population

LE 23-14

Parental population sample

Experimental group sample

On pecking a mothimage the blue jayreceives a food reward.If the bird does notdetect a moth oneither screen, it pecksthe green circle tocontinue a new setof images (a newfeeding opportunity).

Plain background Patterned background

0.6

0.5

0.4

0.3

0.20 20 40 60 80 100

Generation number

Frequency-independent control

Phen

otyp

icva

riatio

n

SEXUAL SELECTION

Sexual selection natural selection for mating success can result in sexual dimorphism

marked differences between the sexes in secondary sexual characteristics

Intrasexual selection competition among individuals of one sex for mates of

the opposite sex Intersexual selection

occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex

Selection may depend on the showiness of the male’s appearance