I. Populations and
Variations
• Population - a group
of individuals
belonging to the
same species that
occupy a given area.
• In most natural
populations, the
manifestations of
traits are not the
same from one
individual to the
next. (variations)
• Variations in traits are inherited.
• Information about traits resides in
hundreds of genes.
• The genetic variation results in
different phenotypes within a
population.
• more than 10600 combinations of
genes are possible in human gametes.
• Far more genetic variation is possible
than can ever be expressed in the
individuals of any population.
II. Allele Frequencies
and Gene Pools
• Gene pool - all the
genotypes that
exist in a
population
• Remember that the different forms
of a gene are called alleles.
• Allele Frequencies - The relative
abundance of each kind of allele in a
population.
• The number and type of alleles in a
population is constantly changing.
• Evolution occurs when there is a
change in the allele frequency of a
population
• Changes in allele frequencies can
occur in a number of different ways:
1) Mutation
2) Genetic Drift
3) Gene Flow
4) Natural Selection
III. Mutation
• Mutation - a heritable change in the
genetic code (DNA)
• Mutations are random events, they
may be helpful or harmful
Harmful Mutations:
• Most mutations are harmful. Why?
• Each individual inherits a combination of
many genes that are already fine tuned by
selection.
• Therefore, the product of a mutant gene is
likely to be less functional not more so.
Helpful and Neutral Mutations
• (mutations that survive in the
population)
• Every once in a great while a helpful
mutation may arise and be beneficial
to the organisms.
• Some mutations have neutral effects
or sometimes the effect of the
mutant gene is masked by a dominant
allele.
• These genes are passed on by chance.
IV. Genetic Drift
• Allele frequencies can change
randomly through generations
because of chance. This process is
called genetic drift
GENETIC DRIFT
Start with five different alleles
Due to random events, only two
different alleles are left after
several generations.
• Genetic drift is most rapid when
population size is small.
• During extreme cases of genetic
drift, a population originates or is
rebuilt from very few individuals
• Founder Effect - a few individuals
leave a population and establish a new
one. By chance the allele frequencies
for many traits may not be the same
as in the original population.
• Bottlenecks - disease, starvation, or
some other disaster can nearly wipe
out large populations. Even though the
population recovers, the relative
abundance of alleles has been altered
at random
V. Gene Flow
• Allele frequencies change when
individuals leave a population
(migration) or enter a population.
• This movement of individuals is called
gene flow.
VI. Natural Selection
• Natural Selection - the differential
survival and reproduction of
individuals in a population based on
the traits they posses
• Natural selection may have a
directional, stabilizing, or disruptive
effects on the range of traits
(phenotypes) in a population
a)Directional
selection - shifts
allele frequencies
in a constant
direction in
response to the
environment
b) Stabilizing selection - favors the
most common phenotype in a
population
• Peccaries are eating those
plants with low-spine-number
causing their alleles to
vanish from the gene pool
• STABILIZING
SELECTION
• Preferring densely
spined cacti, egglaying parasites
more often destroy
varieties of plants
with larger numbers of
spines. An infested
cactus rarely survives.
c)Disruptive Selection- favors
characteristics at both ends of the
range of traits in the population and
operates against intermediate forms.
• DISRUPTIVE
SELECTION
• Low-spine-number
plants are not picked
because they don't
"look right", and highspine-number varieties
are left alone because
they are too hard to
pick.
VII. Speciation
• Speciation - the
process by which
species originate
• How does
speciation occur?
• Sometimes barriers arise between
the parts of the population and
create local breeding units. Then,
two or more gene pools exist when
there was only one.
• If over time there is no gene flow
between the two populations then
selection, mutation, and genetic drift
can operate differently in each
population.
• This can cause different traits to
arise in each population. This is
called divergence
• When the divergence is great enough
that the two populations will no longer
interbreed, the two population have
become different species.
• Geographic Isolation -Populations can
be separated by geographic barriers
ex. mountains, rivers
• Ex. The Blue-headed wrasse was
separated from the Rainbow wrasse
when the Isthmus of Panama was
created.
• Reproductive Isolation - any aspect
of structure or behavior that
prevents interbreeding
• ex. the development of different
breeding seasons or different mating
behaviors
• Over time, two species
of frog have
developed different
mating calls through
isolation.
• Females respond only
to the calls of their
species.