Ch. 16 Evolution
of Populations
Variations and Gene Pools
pool – consists of all genes,
including all the different alleles, that are
present in a population.
 Gene
frequency – the number of times
that the allele occurs in a gene pool,
compared with the number of times other
alleles for the same gene occur.
 Relative
Variations and Gene Pools
Sources of Genetic Variation
 The
2 main sources of genetic variation
are mutations & the genetic shuffling
that results from sexual reproduction.
Single-Gene & Polygenic
Traits
 The
# of phenotypes produced for a given
trait depend on how many genes control
the trait.
 Single-gene trait - controlled by a single
gene, that
has 2 alleles

Ex.) Widow’s Peak
Single-Gene & Polygenic
Traits
 Polygenic
traits - trait controlled by 2 or
more genes

Ex.) Height in humans
Evolution as Genetic Change

Evolutionary fitness can be viewed as an
organism’s success in passing genes to the next
generation.
 Evolutionary adaptation can be viewed as any
genetically controlled trait that increases an
individual’s ability to pass along its genes.
Evolution as Genetic Change
 NS
never acts directly on genes.
 NS can only affect which individuals
survive & reproduce, & which do not.
 Also, remember that it is populations, not
individual organisms, that can evolve over
time.
NS on Single-Gene Traits
 NS
on single-gene traits can lead to
changes in allele frequencies & thus to
evolution.
NS on Polygenic Traits
 NS
can affect the distributions of
phenotypes in any of 3 ways:
-directional selection
-stabilizing selection
-disruptive selection
NS on Polygenic Traits
 Directional
Selection - occurs when
individuals at 1 end of the curve have
higher fitness than individuals in the
middle or at the other end.
NS on Polygenic Traits
 Stabilizing
selection - occurs when
individuals near
the center of a curve
have higher fitness
than individuals at
either end.
NS on Polygenic Traits
 Disruptive
selection - when individuals at
the upper & lower ends of the curve have
higher fitness than individuals near the
middle.
Genetic Drift
drift – random change in allele
frequencies that occurs in small
populations.
 Founder effect - situation in which allele
frequencies change as a result of the
migration of a small subgroup of a
population.
 Genetic
Genetic Drift
Evolution vs. Genetic
Equilibrium
 Hardy-Weinberg
Principle-allele
frequencies in a population will remain
constant unless one or more factors cause
those frequencies to change.
 Genetic equilibrium - when allele
frequencies remain constant, the
population will not evolve.
Evolution vs. Genetic
Equilibrium
5
conditions are required to maintain genetic
equilibrium:
-random mating
-large population
-no movement into/out of pop.
-no mutations
-no NS
Overuse of Antibiotics
 People
may be overusing antibiotics.
 Doctors may prescribe them for diseases
which they are not effective.
 This wide use of antibiotics has caused
many bacteria to evolve resistance to
antibiotics.
The Process of Speciation
 Speciation
- formation of a new species
 The gene pools of 2 populations must
become separated for them to become
new species.
Isolating Mechanisms
 As
new species evolve, populations
become reproductively isolated from each
other.
 Reproductive isolation - when members
of 2 populations can’t interbreed &
produce fertile offspring
Isolating Mechanisms
 Behavioral
isolation - when 2
populations are capable of interbreeding
but have differences in courtship rituals or
other reproductive strategies that involve
behavior

Ex.) Birds with
mating songs
different
Isolating Mechanisms
 Geographic
isolation - 2 populations are
separated by geographic barriers like
rivers, mountains, or bodies of water

Ex.) Squirrels split by Colorado River
Isolating Mechanisms
 Temporal
isolation - when 2 or more
species reproduce at different times

Ex.) Orchids releasing pollen on
different days
Testing NS
in
Nature
Speciation in
Darwin’s Finches
 Speciation
in the Galapagos
finches occurred by:





founding a new population
geographic isolation
changes in the new population’s gene
pool
reproductive isolation
ecological competition
Patterns of Evolution

Macroevolution - large-scale evolutionary
patterns & processes that occur over long
periods of time
 6 important topics in macroevolution are:
•
•
•
•
•
•
extinction
adaptive radiation
convergent evolution
coevolution,
punctuated equilibrium,
changes in developmental genes
Patterns of Evolution
 Extinction:



More than 99% of all species are now extinct
It usually happens for a reason; species
compete for resources, & environments
change
Some species adapt &
survive, others become
extinct
Patterns of Evolution
 Adaptive
radiation - when a single species
has evolved, through NS, into diverse
forms that live in different ways

Ex.) Darwin’s Finches
Patterns of Evolution
 Convergent
evolution - when unrelated
organisms begin to resemble one another


It has occurred in both plants
animals
Ex.) Swimming animals
&
Patterns of Evolution
 Coevolution
- when 2 species evolve in
response to changes in each other over
time

Ex.) Orchid has long spur with nectar in its tip,
a Hawk moth has equally long feeding tube
that allows it to feed on the nectar
Patterns of Evolution
 Punctuated
equilibrium - a pattern of long,
stable periods interrupted by brief periods
of more rapid change