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Genes and Evolution
What is a Population?• Populations Evolve.
• Populations are groups of interbreeding individuals that live in the same place at the same time
• Individuals in a population compete for resources with each other.
Genes and Evolution• Gene pool: the total collection of genes in a
population at any one time• Genetic variation: the differences genetically
between individuals in a population• Genotypic Frequencies: the frequency of
genotypes (BB, Bb, bb) in the population which usually determines genetic variation
• Allelic Frequencies: the frequency of alleles (B or b) in the population which usually determines genetic variation.
• Phenotypic Frequencies: the frequency of phenotypes or appearances of traits in the population.
Genotypic and Allelic Frequencies
Why is genetic variation important?
variation
no variation
north
south
north
south
Why is genetic variation important?
variation
no variation
divergence
NO DIVERGENCE!!
north
south
north
south
Why is genetic variation important?
EXTINCTION!!
variation
no variation
globalwarming survival
EVOLUTION= Change in Traits of the Population
= Change in the Gene Pool
= Change in Allelic Frequencies
5 Agents of evolutionary change
Mutation
Gene FlowGenetic Drift
Selection
Non-random mating
• mutation
• gene flow
• natural selection
• genetic drift
• non-random mating
spontaneous change in DNA•makes new alleles
How do allelic frequencies change?
• ultimate source of allgenetic variation
No Variation (Only short genes are present)
Tall Neck Genes(TT or Tt)
Short Neck Genes (tt)
MUTATION
Genetic Variation (both T and t genes are present)
Genetic Variation (both T and t genes are present)
Tall Neck Genes(TT or Tt)
Short Neck Genes (tt)
DIES
No Genetic Variation Less t genes are present – eventually none
Tall Neck Genes(TT or Tt)
Taller Neck Genes (T’T)
Tall Neck Genes(TT)
MUTATION
• migration
Introducing or removing genes from a population
How do allelic frequencies change?
• mutation
• gene flow
• natural selection
• genetic drift
• non-random mating
Founder Effect
• When small populations move to new areas, the new populations will contain genes similar to the “founders”
• differences in survival or reproduction
• leads to adaptation
differences in“fitness”
How does genetic structure change?
• mutation
• gene flow
• natural selection
• genetic drift
• non-random mating
Variation & natural selection • Variation is the raw material for natural
selection– there have to be differences within population
– some individuals must be more fit than others
Mean beak depth of parents (mm)
Medium ground finch8
8 9 10 11
9
10
11
1977 1980 1982 1984
Dry yearDry year
Dry year
Wet year
Bea
k de
pth
Bea
k de
pth
ofof
fspr
ing
(mm
)
Where does Variation come from?• Mutation
– random changes to DNA• errors in mitosis & meiosis
• environmental damage
• Sex – mixing of alleles
• recombination of alleles– new arrangements in every offspring
• new combinations = new phenotypes
– spreads variation• offspring inherit traits from parent
How Does Natural Selection Work?
• Populations produce more offspring than the environment can support
• Some offspring have genetic qualities that makes survival easier.
• The unequal ability of individuals to survive and reproduce leads to the gradual change in a population over many generations
Survival of the Fittest
• Biological fitness is measured by the ability to reproduce– Cockroach (40
offspring/month)– Manatee (1 baby/
two years)
Types of Adaptations• Protective Coloring
– Camouflage– Mimicry– Aposematic Coloration
• Structural Adaptations– Structures that attract mates– Structures that help meet
needs• Behavioral Adaptations
– Living in groups– Courtship Dance– Song Birds
Antibiotic Resistance: An Example of Natural
selectionResistance to antibacterial soap
Generation 1: 100% not resistant 0 resistant
Natural selection
Generation 1: 100% not resistant 0% resistant
Resistance to antibacterial soap
Natural selection
Resistance to antibacterial soap
mutation!
Generation 1: 100% not resistant 0% resistant
Generation 2: 96%not resistant 4% resistant
This mutation is not a miracle gene. It codes for a protein that allows the bacteria
to survive in the presence of the antibiotics.
Natural selection
Resistance to antibacterial soap
Generation 1: 100% not resistant 0% resistant
Generation 2: 96% not resistant 4% resistant
Generation 3: 76% not resistant 24% resistant
Natural selection
Resistance to antibacterial soap
Generation 1: 100% not resistant 0% resistant
Generation 2: 96% not resistant 4% resistant
Generation 3: 76% not resistant 24% resistant
Generation 4: 12% not resistant 88% resistant
Selection on sickle-cell allele
aa – abnormal ß hemoglobin sickle-cell anemia
very lowfitness
intermed.fitness
highfitness
Selection favors heterozygotes (Aa).Both alleles maintained in population
Aa – both ß hemoglobins resistant to malaria
AA – normal ß hemoglobin vulnerable to malaria
Types of SelectionDIRECTIONALSELECTION
STABILIZINGSELECTION
DISRUPTIVESELECTION
giraffe neckhorse size human birth weight rock pocket mice
Co-Evolution
• Coevolution occurs when one species adapts to anothers adaptations
• Evolutionary Arms Race– Prey adapts to not be
caught by predators– Predators adapt to catch
prey
genetic change by chance alone
How does genetic structure change?
• mutation
• gene flow
• natural selection
• genetic drift
• non-random mating
Genetic drift
8 RR8 rr
Before:
After:2 RR6 rr
50% R50% r
25% R75% r
• The random population that survives a genetic drift event repopulates with a different frequency of genes.
• mutation
• gene flow
• natural selection
• genetic drift
• non-random mating
• non-random mating
non-random allele combinations
mating combines alleles into genotypes
How does genetic structure change?
Non-Random Mating• In nature, no species
truly mates randomly.
• There is always a preference-usually with a mate that has similar genes.
• Selfish gene - a concept that individuals care more for others with similar genes. `
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Sexual selection
It’s FEMALE CHOICE, baby!
Artificial Selection• Human-guided
selection of traits– Dog Breeds– Crops
• Miniature horses were developed from multiple sources. Many different pony breeds were bred for small size, including the Shetland pony and the Dartmoor pony.
Darwin Awards
• In honor of Charles Darwin, the Darwin Awards commemorate those who improve our gene pool...by accidentally removing themselves from it. By necessity, the award is generally bestowed posthumously.
Darwin Award 2010
• (1 January 2010, South Africa) Pop quiz, class. Do you or don't you go swimming in the crocodile-infested Limpopo? Do, or don't, leave your friends on the banks of the great grey-green Olifants River (main tributary of the Limpopo) and swim in its limpid waters not once, not twice, but three times the day you are finally devoured by that old crocodile? Let's just say it was a short New Year for Mariska B., 27, a waitress and former swimmer. According to a long-time resident of Phalaborwa, locals know, "You don't even put a toe in the river. It's teeming with crocodiles and hippos." This local, on her third refreshing dip of the day, didn't have time to scream or struggle. Friends saw just a ripple on the water where seconds before she had been swimming.
• Did I mention that swimming was strictly prohibited? Police searched for Mariska's body with long poles, and with the chemical detectors known as sniffer dogs, but found nothing. The cycle of life continues.
HARDY - WEINBERG• If a population is evolving, the gene pool is
changing.• If a population that is not evolving, the gene
pool is not changing and the population is said to be at Hardy–Weinberg equilibrium.
• The equilibrium is a reference point to determine how much a population is evolving.
• Biologists determine the rate of change by comparing the population’s genotype frequencies with Hardy–Weinberg equilibrium frequencies.
HARDY - WEINBERG
• To be at Hardy–Weinberg equilibrium the following must be true:– population is large– mating is random– no migration– mutation can be ignored– natural selection is not acting on the
population.
HARDY – WEINBERG EQUATIONS
• Two Equations– Allelic Frequencies
• p + q = 1» p is the frequency of one allele in decimal form
(usually the dominant allele)
» q is the frequency of the other allele (usually recessive)
– Genotypic Frequencies• p2 + 2pq + q2 = 1
» p2 is the homozygous dominant frequency; it can be calculated at equilibrium using the allele frequency p
» 2pq is the heterozygous frequency
» q2 is the homozygous recessive frequency
HARDY-WEINBERG PROBLEM• Given: In a population of 100 individuals (200 alleles), sixteen
have attached earlobes (which are recessive).– Find the allele frequencies for A and a.– Find the genotypic frequencies of AA, Aa, and aa.
• Allele frequency– p + q = 1 or A + a = 1
– Homozygous Recessive (aa) = q2 = .16 q = .4 or 40%– p + q = 1 or 100% – p + .4 = 1 – p = .6 or 60%– or A = .6 and a = .4
HARDY - WEINBERG PROBLEM• Genotypic frequencies
– If: p = .6 and q = .4, then• p2 = (.6)(.6) = .36
• q2 = (.4)(.4) = .16
• 2pq = 2(.6)(.4) = .48
• Therefore, in the population:– Homozygous dominant = 36/100 or 36%– Heterozygous dominant = 48/100 or 48%– Recessive = 16/100 or 16%
ANOTHER PROBLEM
• Fraggles are mythical, mouselike creatures that live beneath flower gardens.
• Of the 100 fraggles in a population, 91 have green hair(F) and 9 have grey hair(f).
• Assuming genetic equilibrium:– What are the allelic frequencies of F and f?– What are the genotypic frequencies?
ANSWERS TO PROBLEM
• Gene frequencies:– F = 0.7 and f = 0.3
• Genotypic frequencies– FF = 49% or 0.49– Ff = 42% or 0.42– f f = 9% or .09
HARDY-WEINBERG & EVOLUTION• If a populations actual frequencies match
the HW frequencies, then the population is not changing—not evolving.
• Conversely, if the actual frequencies do not match the HW frequencies, then the frequencies have been changed—evolution has occurred.
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