Post on 18-Dec-2015
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%