A change in allele frequency
 A: They compare it to a non-changing population
 = Ideal population (like a “perfect” car… it only exists
in a showroom)
1. Population is very large
Large depends on the population,
but thousands or hundreds are
assumed
2. Mating is random
No phenotype has a greater chance
of mating
3. No mutation of alleles
4. No immigration or emigration
No movement into or out of population
5. No selection occurs
No survival advantage goes to one
phenotype over another
 There is no change in allele/genotypic frequencies…
 and no evolution
 When these rules are broken, microevolution is said
to be occurring.
 p stands for the frequency of the dominant allele Ex.
“A”
 q stands for the frequency of the recessive allele. Ex.
“a”
 In H-W,
p+q=1
or 100% of the alleles
p2 + 2pq + q2 = 1, or 100%
 p2 = frequency of homozygous dominant genotype Ex.
“AA”
 q2 = frequency of homozygous recessive genotype Ex.
“aa”
 2pq = frequency of heterozygous genotype Ex. “Aa”
 Approximately 9% of Americans of African descent
suffer from sickle cell anemia, which is inherited as a
recessive trait.
 What is the frequency of the sickle cell allele?
 Approximately what percentage of this subpopulation carries the sickle cell allele?
 Cystic fibrosis is known to occur as a recessive trait in
human populations. In a genetic study, the frequency of
the recessive allele for a population was found to be 2.0%.
 What percentage of the population would be expected to
exhibit Cystic Fibrosis?
 What percentage of the population would be normal, but
carry a CF allele?
 1. Genetic Drift
 Significant in small populations
 Can cause allele frequencies to change at random
 Can lead to a loss of genetic variability
 Can cause harmful alleles to become fixed
 Founder effect
 Small founding population does not reflect source gene
pool
2. Gene flow
Immigration/ emigrationmovement of alleles into/ out of
populations
May increase or decrease variation
in population
3. Natural Selection
Only Natural Selection is likely to adapt a
population to its environment
Do you remember the
basic tenets of natural
selection?
Fitness?
Not necessarily
Evolutionary fitness refers to beneficial
adaptations that allow an organism to
survive and reproduce
Not necessarily the biggest, fastest, or
strongest, rather, the best suited to a
particular place at a particular time.
 One extreme of the
variation spectrum is
favored
 Results in a shift in the
make-up of the
population
 Both extremes of the
variation spectrum is
favored
 Can result in speciation
 Both extremes are
selected against
 Reduces variationmaintains status quo
Sexual Selection
 Intrasexual Selection
(within the same sex)
 Usually male-to-male
competition
 Most encounters
don’t come down to
actual fighting (that
could be harmful to
both.)
 Sexual Selection
 Intersexual Selection (between sexes)
 a.k.a. mate choice – usually females choosing males with the most ornate
displays.
 There can be a trade-off- Attract a mate? More conspicuous to predators?
Which of the following pairs of
structures is least likely to
represent homology?
A) The wings of a bird and those of an insect
B) The brain of a cat and that of a dog
C) The mitochondria of a plant and those of an animal
D) The hemoglobin of a baboon and that of a gorilla
E) The wings of a bat and the arms of a human
Which of the following provides
evidence that modern species
have evolved from prior species?
A) molecular biology
B) comparative anatomy
C) biogeography
D) comparative embryology
E) All of the choices are correct.
Which of the following is NOT a
basic tenet of natural
selection?
A) Variation exists within a species
B) There are limited resources for which organisms much
compete
C) More offspring are born that can survive
D) Some genes are dominant, some are recessive
The fossil record shows all of
following except that
A) the earliest fossils of life are about 3.5 billion years old.
B) younger layers are on top of older layers.
C) within the vertebrates, fish were the first to evolve.
D) some fossils represent an evolutionary series of changes
that provide strong documentation of evolution.
E) the first life forms were eukaryotes.