Evolution of Populations

Chapter 23

Macroevolution

Evolution on a large scale Changes in plants & animals Where new forms replace old Major episodes of extinction

Microevolution

Changes within a population Changes in allele frequencies Leads to adaptation of an

organism

Variation

Gene variation Driving force behind evolution New genes & alleles can arise by

mutation or gene duplication Sexual reproduction

Genetic Variation from Sexual Recombination

Population genetics

Study of the properties of genes in populations

Population

Group of individuals Same species Interbreed Fertile offspring

Population

Contains a great deal of variation Variation-raw material for

evolution

Gene pool

All the alleles Of all individuals within a

population

Hardy-Weinberg Principle

Determines if population is evolving

Frequencies of alleles in population Used for baseline of genes in a

population

Hardy-Weinberg

Equilibrium When proportions of genotypes

remain the same Generation to generation

Hardy-Weinberg

Original proportions of genotypes in a population remain constant if

1. Large population 2. Random mating 3. No mutations 4. No gene flow 5. No natural selection

Hardy-Weinberg

P+q=1 alleles

p=dominant q=recessive

p2 + 2pq + q2 = 1 genotypes

Hardy-Weinberg

84 black 16 white (100 total)

Hardy-Weinberg

p2 + 2pq + q2 = 1 P + q=1

q2 = .16 q = .4 p = .6 p2 = .36 2pq = .48

Hardy-Weinberg

If the dominant allele is 30% of the gene pool

What is % dominant phenotype % recessive phenotype % hybrid

Hardy-Weinberg

Factors that affect evolutionary change

1. Mutations 2. Nonrandom mating 3. Gene flow 4. Genetic drift 5. Natural selection

Mutation

Occurs at a low rate Not a strong influence on

evolutionary change

Nonrandom mating

Individuals with one genotype mate with another at a greater rate

Not a strong influence on allele frequency

Gene flow

Movement of alleles from one population to another

Populations exchange genetic information

Example New animal comes into population Mates & survives

Gene flow

Bees and pollen Seeds Reduces genetic differences

between populations

Gene flow

Insecticide resistant alleles Mosquito West Nile & Malaria Spreading the allele

Gene flow

Advantage when a beneficial mutation enters a population

Select for the allele Disadvantage when an inferior

allele enters the population Select against the allele

Genetic drift

Change in allele frequency due to chance alone

Small populations

Genetic drift

Only a few possible alleles are present

Example: Red, blue, yellow seeds If blue & yellow are isolated from

red Eventually the population will only

have blue or yellow and no red

Genetic drift

May see a rise in harmful alleles Lose alleles

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Genetic drift

1. Founders effects Few individuals leave a population New isolated population Few alleles present Island populations Amish (polydactyly)

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Genetic drift

2. Bottleneck Occurs when a few surviving

individuals have only a few genes Loss of genetic variability Occurs when a natural event

happens– Flood, drought, disease etc.

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

Genetic drift

Northern elephant seal California Reduced to few seals in a

population due to hunting Has rebounded in numbers Organisms with limited genetic

variation

Fig. 23-10a

Rangeof greaterprairiechicken

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

(a)

Selection

Natural selection the process that causes evolutionary change

Adaptive evolution

Selection

Natural selection to happen & cause evolutionary change

1. Must have variation in individuals among population

Enables choice of traits that are better able to survive

Selection

2. Variation causes different number of offspring surviving

3. Variation must be genetically inherited

Selection

Individuals with a certain phenotype

Leave more surviving offspring than other phenotypes

Relative fitness

Reproductive success Number of surviving offspring left

for the next generation Green vs brown frogs Green leave 4 offspring Brown leave 2.5 offspring More green mating eventually lose

the brown phenotype

Relative fitness

1. Survival (how long) 2. Mating success 3. Number of offspring Examples: larger organisms mate

more Larger fish or frogs leave more

offspring

Forms of selection

1. Disruptive selection 2. Directional selection 3. Stabilizing selection

Forms of selection

1. Disruptive selection Eliminates intermediate type Favors extremes Example: African-bellied seed cracker finch Large beak Large seeds Small beak Small seeds

Original population

(b) Disruptive selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

Forms of selection

2. Directional selection Favors one extreme

Original population

(a) Directional selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Original population

Evolved population

Forms of selection

3. Stabilizing selection Eliminates both extremes Example: birth weight of newborns Small & large newborns can be

harmful Increased death rate Intermediate BW best survival

Original population

(c) Stabilizing selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

Selection

Environment imposes conditions Determines selection Cause evolutionary change.

Selection

1. Selection to avoid predators Adaptation that decreases the

chance of being captured

Selection

2. Selection to match climatic condition Enzyme alleles Vary depending on geographic location Fish enzyme for LDH Coverts pyruvate to lactate Works better in colder weather Fish swim faster

Selection

3. Selection for pesticide resistance

Housefly developed a resistant target receptor

Do not absorb the insecticide Rats have developed resistance to

Warfarin (blood thinner)

Sexual selection

Sexual dimorphism: Differences in secondary sexual

characteristics Intrasexual selection: Selection between same sex Competing for mates Male fighting

Sexual selection

Intersexual selection: Selection of mate Females choosing male mate “good genes”

Fig. 23-15

Fig. 23-19

Maintaining variation

1. Frequency-dependent selection 2. Oscillating selection 3. Heterozgote advantage

Frequency-dependent selection Fitness of a phenotype depends on

frequency within population Negative frequency-dependent

selection Rare phenotypes favored Predator preys on the more common

phenotype Allowing less common phenotype to

thrive

Frequency-dependent selection Positive frequency-dependent

selection Predator feeds on rare phenotype Favoring common phenotype

Oscillating selection

When one phenotype is favored at one time

Another phenotype is favored at a different time

Birds beak size and drought

Heterozygote advantage

Favored genotype has both alleles Example: sickle cell anemia Heterozygous for disease does

better against malaria

The Sickle-Cell Allele

Events at the Molecular Level

Sickle-cell alleleon chromosome

Template strand

Effects on IndividualOrganisms

Consequences for Cells

Fiber

An adeninereplaces a thymine.Wild-type

allele

Sickle-cellhemoglobin

Low-oxygenconditions

Sickled redblood cell

Normal redblood cell

Normal hemoglobin(does not aggregate

into fibers)

The Sickle-Cell Allele

Evolution in Populations

KeyFrequencies ofthe sickle-cell allele

Distribution of malariacaused by Plasmodium falciparum(a parasitic unicellular eukaryote)

3.0–6.0% 6.0–9.0% 9.0–12.0%12.0–15.0% 15.0%