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Evolution of Populations
Chapter 16
Genes and Variation
Relative (allelic) frequency - the percentage of
a particular allele in a gene pool.
Selection
Natural Selection- In nature, unequal
ability to survive and reproduce... Survival
of the fittest.
Natural Selection ACTS ON
PHENOTYPE but influences
genotype (thus, allelic frequency)
Artificial Selection- Mankind selects for
desired traits. Also known as “selective
breeding”
Over time, the zebra herd becomes faster as
the slower zebra (and their genes) are
removed from the population… survival of
the fittest
Darwin’s Theory = Evolution by means of
natural selection
Artificial Selection
ADAPT OR DIE!
Camouflage- organisms blendin with surrounding environment
Mimicry- species copy another
to insure their own survival
NatGeo
Genes and Variation
Gene pool - all the genes that
exist within a population
Gene flow – movement of alleles into
or out of a population
Immigration – new alleles move IN
Emigration – alleles move OUT
Genetic drift - change
in allelic frequencies
by chance
Ex: sudden extinction of
a dominant species;
small populations
most affected
Genetic equilibrium - when alleles stay the same
from generation to generation
1.
2.
3.
4.
5.
The Hardy Weinberg Principle: Allele
frequencies will remain constant under five
conditions
Random Mating
Large Population
No movement (immigration or emigration)
No Mutations
No Natural Selection: equal change of
survival
Hardy-Weinberg Equation
(p + q)2 = 1, which is the same as
p2 + 2pq + q2 = 1
p = frequency of “A” allele and
q = frequency of “a” allele
p2 = expected freq. of homozygotes for
one allele
2pq = expected freq. of heterozygotes
q2 = expected freq. of homozygotes for
the other allele
Natural Selection effects Genetic
Change in Populations
Natural Selection has three
affects on phenotype
distribution
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
Directional Selection
1. Directional SelectionIndividuals on one end
of a curve are “better
fitted” than the middle
or other end
Peccaries naturally choose to consume those
cactus plants with the fewest spines As a result,
at flowering time there are more cacti with
higher spine numbers; thus, there are more of
their alleles going into pollen, eggs, and seeds
for the next generation.
Stabilizing Selection
2. Stabilizing SelectionIndividuals near center
of a curve are “better
fitted” (have highest
fitness) than both ends
Peccaries are consuming the low-spine
number plants, and the insects are killing
the high-spine-number plants. As these
gene combinations are removed from the
cactus gene pool, there is less and less
variety possible in subsequent
generations.
Disruptive Selection
3. Disruptive SelectionIndividuals at upper and
lower ends are “better
fitted” than the ones in the
middle
Years of collecting have left their toll on
the roadside cacti. In this environment, it is
maladaptive to be good looking and have
a reasonable number of spines. Low
spine-number plants are not picked
because they don't "look right", and high
spine-number varieties are left alone
because they are too hard to pick.
Gradually, the gene pool changes in favor
of the two extreme spine number types.
Patterns of Evolution
1.
2.
3.
4.
Extinction
Divergent Evolution (adaptive radiation)
Convergent Evolution
Coevolution
Extinction
Why do species go extinct?
Extinction
Natural selection, climate
changes, and
catastrophic events have
caused 99 percent of all
species that have ever
lived to become extinct.
Mass extinctions –
caused by continents
moving, sea level
changing, volcano
eruptions, large meteors
Predict what each ecosystem will look like after the event.
Catastrophic
Event
Catastrophic
Event
Question
When a mass extinction happens, what do
you think will happen next?
Divergent Evolution
(adaptive radiation)
Divergent evolution – natural selection
causes 1 species to evolve into many
species with many different adaptations
(homologous structures)



After mass extinctions, many environments
will be open for inhabitation
Species will migrate to that area and new
environmental pressures will cause the
population to change over time
This is also known as Adaptive Radiation
Adaptive Radiation in
honeycreepers
Convergent Evolution
Convergent Evolution – when unrelated
organisms come to resemble one another
(analagous structures)
Coevolution
When 2 species
evolve in response to
one another
Evidence of Evolution
Homologous structures - similar
structures found in related organisms that
are adapted for different purposes.
Ex: human arm and bat wing or whale
flipper
---DIVERGENT EVOLUTION--the process of two or more related species
becoming more and more dissimilar.
Homologous structures 
Divergent evolution
Analogous structures - structures found
in unrelated organisms that have a similar
function but may be structurally different
Ex: bird wing and insect wing
---CONVERGENT EVOLUTION--independent evolution of similar
features in species of different
lineages
Process of Speciation
speciation - evolution of a new species
4 Main Isolating Mechanisms
A. Reproductive
B. Behavioral
C. Geographic
D. Temporal
A. Reproductive Isolation: Two populations
cannot interbred and produce fertile offspring
B. Behavioral Isolation: Two populations
capable of breeding but cannot because of
courtship rituals
C. Geographic Isolation: Two populations are
separated by geographic barriers
Ex: Rivers, Oceans, Mountains
D. Temporal Isolation: Two or more
populations reproduce at different times
Genes and Variation
Sources of genetic variation:
1.
2.
Mutations- change in DNA sequence
Gene Shuffling- random assortment of genes
during gamete production (SEXUAL
reproduction)
Gene Expression Variation

Single-gene trait- controlled by one gene


Ex: Widow’s Peak
Polygenic trait- controlled by many genes

Skin color, eye color
Original Source of Genetic
Variation?
MUTATION!
the evolution of sexual reproduction
increased genetic diversity within a
population and therefore accelerated the
rate of evolution