Genes and Evolution What is a Population? Populations Evolve. Populations are groups of...

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


variation

no variation

divergence

NO DIVERGENCE!!

north

south

north

south


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)


Short Neck Genes (tt)

DIES

No Genetic Variation Less t genes are present – eventually none


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


• genetic drift


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


• genetic drift


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


mutation!


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





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


• mutation

• gene flow


• genetic drift


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


• genetic drift



non-random allele combinations

mating combines alleles into genotypes


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. `

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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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