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Chapter 16
Evolution of
Populations
REMEMBER !
•
A _______________
is a group of
POPULATION
individuals of the same species that
can interbreed and produce fertile
offspring.
Chapter 16-1
1.______
____- all genes
Gene Pool
(dominant and recessive
alleles) that are present
in a population.
• Ex. What genes are present in
Sweden
• ________
Japan
• ________
• ________
Mexico
• Because members of a population interbreed, they
share a common group of genes called a
___________
GENE POOL
ALL the genes present in a
• A gene pool consists of ____________
alleles
population, including all the different _________.
Remember: alleles code for a gene which determines
the phenotype. Genes will have 2 alleles, one given
by each parent. The combination of the two alleles
on the gene will determine the expressed physical
characteristic or phenotype.
Mutations changes
1. ___________=
or random errors in DNA
(by chance)
Gene ______
flow
2. _____
• Migration
Drift random
3. Genetic
______ ____=
change in allele frequency
• affects small,
isolated populations
most
• Ex: Amish
Polydactilysm
_____________
in the Amish
Population
Natural selection is NOT the only
source of evolutionary change.
REMEMBER !
Genetics is controlled by
_________________
PROBABILITY
The smaller the population . . .
the farther the ________results
may be
actual
from the ___________
predicted outcomes.
In a small population random
changes in allele frequency based on
chance is called _________________
GENETIC DRIFT
Genetic drift can occur when
a _______
small group
of individuals
colonizes a
new
_____habitat.
Genetic Drift
Nonrandom __________
mating
4. __________
• mates selected based on
how “fit” they are
•more fit = better chance of
surviving offspring
5. Natural selection!
Types of Natural
Selection
Chapter 16-2
Natural selection can affect the distribution of
phenotypes in 3 ways:
DIRECTIONAL selection
______________________
STABILIZING selection
______________________
______________________
DISRUPTIVE selection
Stabilizing selection
1. _________
________= favors
average individuals in a population
Ex: lizards –
large
captured
easily & small
cannot run
fast enough
STABILIZING SELECTION
CENTER
Individuals in _____________
of the curve
have higher fitness than
individuals at
either end
Graph stays in same place but narrows as
more organisms
in middle are produced.
EXAMPLE OF STABILIZING SELECTION
Human babies born with low birth
weight are less likely to survive.
Babies born too large have difficulty
being born.
Average size babies are selected for.
STABILIZING SELECTION
Section 16-2
Male birds use
their plumage to
attract mates.
Male birds in the
population with
less brilliant and
showy plumage
are less likely to
attract a mate,
while male birds
with showy
plumage are more
likely to attract a
mate.
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Brightness of
Feather Color
Selection
against both
extremes keep
curve narrow
and in same
place.
Male birds with
showier, brightlycolored plumage
also attract
predators, and are
less likely to live
long enough to
find a mate.
The most
fit, then, is the male
bird in the middle-showy, but not too
showy.
Directional________=
selection favors one of
2. ________
the extreme variations of a trait
Ex:
Anteaters –
long
tongues are
best to get
food
• DIRECTIONAL SELECTION
KEY
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Individuals at _____________
ONE END of the curve
have higher fitness than individuals in
middle or at other end.
Graph shifts as some individuals fail to survive at one end
and succeed and reproduce at other
EXAMPLE OF DIRECTIONAL SELECTION
Beak size varies in a population
Birds with bigger beaks can feed
more easily on harder, thicker
shelled seeds.
Suppose a food shortage causes
small and medium size seeds to
run low.
Birds with bigger beaks would be
selected for and increase in numbers
in population.
3. ________
_______– favors
Disruptive selection
both extreme variations of a trait
• Ex: Limpets – white
colored mimic white
barnacles & dark
colored camouflage
on dark rocks
•no intermediate
forms survive
• DISRUPTIVE SELECTION
EXTREMES of the curve
Individuals at _____________
have higher fitness than individuals in middle.
Can cause graph to split into two.
TWO DISTINCT
Selection creates __________________PHENOTYPES
EXAMPLE OF DISRUPTIVE SELECTION
Suppose bird population lives in area
where climate change causes medium
size seeds become scarce while large
and small seeds are still plentiful.
Birds with bigger or smaller beaks
would have greater fitness and the
population may split into TWO
GROUPS. One that eats small
seeds and one that eats large seeds.
SPECIATION
Chapter 16-3
I. Formation of Species
a. SPECIES = organisms
look alike ______
breed
that ________&
to produce ________
fertile
offspring
_________.
• Similar gene pool
Speciation = forming a
b. _________
new species
• must have isolation
•Ex: Galapagos finches
isolated from mainland
What events lead to the formation of
new species?
Isolation
(1) Reproductive
________ _________:
•When two members
cannot interbreed and
produce fertile offspring
•Each species is now their
own separate group
Isolation Mechanisms
Behavioral isolation
(2) _________
________= differ
in courtship rituals or don’t
respond to mating signals
Ex: Birds using different
songs to attract mates
Geographic _________=
isolation
(3) _________
population separated by
physical barriers
Ex: island formation,
river/canyon
Temporal isolation
(4) ________
_______=
species reproduce at
different times of the year
Ex: orchids pollinate on
different days
•
•
EVOLUTION OF POPULATION
Evolution as Genetic Change
• 16–2
• GAPS IN DARWIN’S THINKING:
•How do heritable traits pass from
one generation to the next?
• How does variation in the population
appear?
• When Darwin developed his
• theory of evolution, he didn’t
• know how ____________
HEREDITY
• worked.
inheritance in peas was
• Mendel’s work on ______________
published during Darwin’s lifetime, but
NOT recognized as important until
________________
__________________.
decades later
• TODAY we can understand how evolution
works better than Darwin ever could
because of our knowledge about
genes
DNA
• ____________ and ________
IN GENETIC TERMS
__________________
EVOLUTION
is any change in the relative
frequency of alleles in a population
If the relative frequency
of the B allele in this
mouse population
changed over time to
30%, the population is
evolving.
NATURAL SELECTION on
____________________
single-gene frequencies can lead to
ALLELE FREQUENCIES
changes in ____________________
and thus to EVOLUTON
EX: A population of normally brown
lizards. Mutations produce new color
choices.
If red lizards are more
visible to predators, they
might be less likely to survive.
In real populations, phenotypic
ratios are determined not only by
which allele is _____________,
but
DOMINANT
by _______________
of the allele in
FREQUENCY
the population
Presence of widow’s
peak in population
can be less common even if it is
DOMINANT!
The ____________________
relative frequency is the
number of times that an allele occurs in
the gene pool compared to the
occurrence of other alleles in the gene
pool.
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
RELATIVE FREQUENCY is often
expressed as a __________________.
percentage
EX: In this population
Dominant B allele (black) = 40%
Recessive b allele (brown) = 60%
NOTHING
RELATIVE FREQUENCY has _________
to do with whether an allele is
______________
DOMINANT or _____________
RECESSIVE
In this population, the recessive allele is
more frequent.
Black lizards absorb more heat to warm up
faster on cold days so they can move faster
to get food and avoid predators. The allele
for black may increase in frequency.
_______________________IS
THE
SEXUAL REPRODUCTION
MAJOR SOURCE OF VARIATION IN
POPULATIONS, but it does _______
NOT
change the __________________
of
relative frequency
alleles in population!
Shuffling a deck of cards
can shuffle to produce
many different hands,
but doesn’t change the
number of kings or
queens in the deck.
REMEMBER !
MEIOSIS
GENE SHUFFLING during _________
Crossing over
_____________
Segregation
_______________
Independent
Assortment
___________________
RESULT IN GENETIC RECOMBINATION
____________________
Independent
assortment during
__________
Anaphase I of _______
meiosis is a lot like
shuffling a deck of cards. There are
probability
always 52 cards, and the _________
of you being dealt any one card is
always the same.
independent assortment
During ____________________,
there are 23 ___________
chromosome “cards” that
can be “shuffled” and “dealt” in
8.4
million combinations! Also,
_________
sometimes the “cards” you are dealt
are brand-new and unique due to
___________.
crossing
over
The number of _______________
PHENOTYPES
produced for a given trait depends
GENES
on the number of ______
that control that trait.
EX: Widow’s peak
No widow’s peak
Single gene traits result in
only _______phenotypes.
TWO
POLYGENIC
_______________
traits are
controlled by two or
more genes.
A bell shaped curve is
typical of polygenic traits
When traits are controlled by
more than one gene, the effects
_______
are more complex.
polygenic traits
Remember ______________
show a bell-curve distribution
BIOLOGISTS can now explain how the
variation
_____________
that DARWIN saw is produced!
mutation is any change in a
A __________
DNA
a sequence of _______
SOURCES OF MUTATIONS:
Mistakes in replication
__________________________
Chemicals
_____________________
__________________
Radiation
Mutations that change phenotype can
affect an organism’s ____________
fitness
(ability to survive and reproduce)
Each time an organism reproduces,
it passes its genes on to the next
generation.
FITNESS
So __________
= success in passing on genes
The ___________
FITNESS of individuals near
each other will not be very different, but
fitness may vary from one end of curve
to the other.
Where fitness varies,
NATURAL SELECTION
________________
can act!
Many mutations are ___________
LETHAL
and result in
death.
silent
Some mutations are ____________
and
PHENOTYPE
DON’T affect and organism’s ______________
at all.
EX: Changing the code
from GGA to GGU
doesn’t change the
amino acid used
Individuals may carry alleles in
different
______________
relative frequencies
than in the larger population.
The population they “found” will be
different from the parent population
. . . not through
natural selection
but by _________
chance
A situation in which allele
frequencies change as a result of the
migration of a small subgroup of the
FOUNDER EFFECT
population = _________________