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EVOLUTION OF POPULATIONS
Chapter 23
CLARIFYING EVOLUTION
Natural selection ACTS on individuals
 Evolutionary impact of affects populations over time
 Grants Finches

Drought = large,
deep beaks
 Seeds = large, hard
 Average beak size
increase



Population evolved


More large in population
Beak not change in life
Mutations ultimate
source of new alleles
GENETIC VARIATION

Mutation
Must be in gametes to be passed to offspring
 Point mutations

Phenotype & environment
 Wobble bases and introns


Altering gene number or sequence
Nondisjunction
 Olfactory receptors


Sexual reproduction

Allelic reshuffling


Crossing over, independent assortment, and fertilization
Makes evolution possible
POPULATIONS
Species interbreeding
to produce offspring in
an area
 Genetic makeup is the
gene pool

Alleles for all loci in
all individuals
 Each allele has a
frequency (proportion)

HARDY-WEINBERG PRINCIPLE

Frequency alleles & genotypes remain constant


Determines if evolution is occurring
Conditions necessary

No mutation


Usually not a huge effect anyway
Infinitely large, isolated population

No movement in or out of population to change allelic
frequency
Mating is random
 Neither allele gives reproductive (or early survival)
advantage over the other


Can apply to some, all or no genes
HARDY-WEINBERG EQUILIBRIUM

2 alleles


Allelic frequency


p+q=1
Genotypic frequency


p is more common, q is less common
p2 + 2pq + q2 = 1
Phenotypic frequency

Same or different then genotypic
HARDY-WEINBERG PRACTICE
Work with examples in section, end of chapter, and
study guide to become familiar with equation
 Wildflowers with 2 alleles (Cr Cw) demonstrating
incomplete dominance, what does this mean?


320 red, 160 pink, 20 white
500 individuals, 1000 copies of genes for flower color (2n)
 Frequencies of each allele, genotype, & phenotype?


Blood Type
M
 MN
 N

Genotype
LMLM
LMLN
LNLN
Number of Individuals
700
650
150
MICROEVOLUTION
Change in allelic frequency in a population over
generations
 3 mechanisms


Natural selection


Genetic drift


Improves match between individual and environment
Chance events that alter allele frequencies
Gene flow

Transfer of alleles between populations
GENETIC DRIFT
Chance events cause allele frequencies to fluctuate
unpredictably
 Doesn’t work to produce adaptations
 Founder effect and bottleneck effect are examples

FOUNDER EFFECT

Isolated individuals form a new
population


Gene pool differs from source
Few members blown to a new island
or an earthquake splits a population

Chance where some individuals and
their alleles, but not others are
separated
BOTTLENECK EFFECT
Sudden event
drastically reduces
population
 Chance allowed certain
alleles to survive
 Recovery may show low
variation levels
 Humans can impose


Cheetahs
Skin grafts and
immunity
 Low sperm count
 Variability too low to
flourish

GENE FLOW

Transfer of alleles into or out of a population


Fertile individuals and their alleles move
Reduces genetic differences between populations
Significant enough, 2 populations can = 1
 Human populations


Introduces new alleles to population

Natural selection can increase frequency
RELATIVE FITNESS

Contribution an individual makes to the gene pool
of the next generation relative to the contributions
of others
Natural selection is not survival of the fittest
 Individuals with most viable, fittest offspring pass on
the most genes
 Survival doesn’t guarantee reproductive success


Selection favors individuals with phenotypic
traits that provide higher reproductive
success than others
MODES OF SELECTION
Dark rocks
Beak size in
finches
Birth weight
SEXUAL SELECTION
Certain inherited characteristics enhance finding
mates
 Creates sexual dimorphism

Differences (2°) that don’t have direct effect on fitness
 Include size, color, ornamentation, and behavior


Intrasexual selection (within same sex)


Males defend status through force or psychologically
Intersexual selection (between sexes)

Female choice depends on showiness of male


Not always beneficial, pose risks by making more visible =
tradeoff
Females want mates with ‘good genes’
Midshipman Fish
 Male singers or
sneakers
 Singing induces egg
laying
 Male resumes singing


Attract more mates
Sneakers hangout
and sneak in to
fertilize eggs

Resemble females
NATURAL SELECTION ISN’T PERFECT
Selection can only act on existing variations
 Evolution limited by ancestry


Doesn’t scrap existing structure, adapts to new ones
Often compromises
 Interaction of chance, natural selection, and the
environment



Chance moves 1 organism to new
environmentColorado, but not necessarily to best fit
environment
New species are ‘better than’