Transcript Chapter 16 Evolution and Populations

CHAPTER 16
EVOLUTION AND POPULATIONS
GENES AND VARIATION
Fitness, adaptation, species, and evolutionary
change are now defined in genetic terms.
 Gene pool: all the genes (and its alleles) present
in a population.
 Relative Frequency: the number of times an
allele appears in a gene pool compared with the
number of times other alleles for that same gene
occur.
 In genetic terms, evolution is any change in
the relative frequency of alleles in a
population.
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SOURCES OF GENETIC VARIATION
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Mutations
- are random
- occur in the DNA
- most are harmless but some can be lethal
Gene Shuffling
- occurs during sexual reproduction
- is responsible for most differences
- each chromosome has a pair of homologous
chromosomes that may sort independently
during meiosis; 23  8.4 million different gene
combos
- crossing over
Sexual reproduction produces different phenotypes,
but it does not change the relative frequency. The
probability remains the same.
SINGLE GENE TRAITS
The number of phenotypes produced for a given
trait depends on how many genes control the
trait.
 A single-gene trait is a trait that is controlled by
a single gene with two alleles.
ex: widow’s peak vs. straight hairline
 Just because a trait is dominant does not
mean that more people have it.
 A recessive allele may contribute to an
organism’s fitness, therefore leading to a
population with more organisms having the
recessive allele.
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POLYGENIC TRAITS
Polygenic traits are controlled by two or more
genes.
 Each gene often has two or more alleles.
 As a result, one polygenic trait may have many
possible genotypes and phenotypes.
ex: height in humans
 Refer to pg. 396 in your textbook showing a bellshaped curve (or normal distribution).
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EVOLUTION AS GENETIC CHANGE
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Natural selection never directly acts on genes.
It acts on the entire organism- not a single gene.
It is the organism that survives and reproduces or
dies without reproducing.
Natural selection can only affect which individuals
survive and reproduce and which do not.
If an organism dies without reproducing, then it does
not contribute its genetics to the populations gene
pool; if it reproduces before it dies, then it contributes
its genetics to the population’s gene pool.
If an individual produces many offspring, then its
alleles stay in the gene pool and contribute to the
relative frequency of the allele.
NATURAL SELECTION ON
SINGLE-GENE TRAITS
Natural selection on
single-gene traits can
lead to changes in
allele frequencies and
thus to evolution.
 An example is the
story of the peppered
moth…
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Break for activity….
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HHMI DVD on mice…
NATURAL SELECTION ON
POLYGENIC TRAITS
When traits are controlled by more than one
gene, the effects of natural selection are more
complex.
 Remember, the distribution of a polygenic trait
can be seen on a typical bell curve.
 There won’t be much difference in fitness
between individuals who are next to each other
on the curve, but the individuals at either end of
the curve will be very different.
 Natural selection can affect the distribution of
phenotypes in any of three ways: 1) directional
selection, 2) stabilizing selection, or 3)
disruptive selection.
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DIRECTIONAL SELECTION
Individuals at one end of the
curve have more fitness than
individuals in the middle or at
the other end of the curve. The
range of phenotypes shifts as
some individuals fail to survive
and reproduce while others
succeed.
Directional Selection
Population
after
selection
Original
population
See page 398 for the
example.
STABILIZING SELECTION
Selection Against Both Extremes
Individuals near the
center of the curve
have more fitness than
the individuals at
either end of the cure.
Population after
selection
Original population
DISRUPTIVE SELECTION
Selection against the mean
Individuals at the upper
and lower ends of the
curve have higher fitness
than the individuals near
the middle.
Population
after selection
Original
population
GENETIC DRIFT
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Natural selection is not the only source of evolutionary
change.
In small populations, an allele can become more or less
common simply by chance.
Probability laws are applied to genetics to make genetic
predictions.
But the smaller the population, the more unlikely it
becomes that the laws of probability can be applied.
Genetic Drift: In small populations, individuals can
carry a particular allele that may leave more
descendants than other individuals, just by chance.
Over time, a series of occurrences of this type can
cause an allele to become common in a population.
GENETIC DRIFT
Sample of original
population
Descendants
Founding Population A
Founding Population B
FOUNDER AFFECT
It is a situation in which allele frequencies
change as a result of the migration of a small
subgroup of a population.
 Therefore, two small groups from a large, diverse
population could produce new populations that
differ from the original group.
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Founder Effect: a few individuals from a population
start a new population with a different allele than the
original population.
Founder Effect also referred
to as the Bottle Neck Effect.
island
EVOLUTION VERSUS GENETIC
EQUILIBRIUM
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Hardy-Weinberg Principle: allele frequencies
in a population will remain constant unless one
or more factors cause those frequencies to
change.
Genetic equilibrium is when allele frequencies remain
constant and do not change; populations do not
evolve.
5 conditions required to maintain genetic equilibrium
from generation to generation are:
1. random mating
2. population must be large
3. there can be no movement in or out of a
population
4. no mutations
5. no natural selection
THE PROCESS OF SPECIATION
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Natural selection and chance
occurrences can change allele
frequencies, but how do entirely
new species form?
Species: a group of organisms
that breed with one another
and produce fertile offspring.
In order for a species to evolve
into a new species, the gene pools
of two populations must become
separated.
As new species evolve,
populations become
reproductively isolated from each
other.
When the members of two
populations cannot interbreed
and produce fertile offspring,
reproductive isolation has
occurred.
ISOLATING MECHANISMS
(TYPES OF REPRODUCTIVE ISOLATION)
Behavioral isolation
 Geographic isolation
 Temporal isolation
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BEHAVIORAL ISOLATION
Occurs when two populations are
capable of interbreeding, but
have differences in courtship
rituals or other reproductive
strategies that involve behavior.
 Ex: Eastern Meadowlark vs.
Western Meadowlark
 They have overlapping habitats.
 Won’t mate with each other
because they use different songs
to attract mates.
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Notice these birds are
singing different mating
songs even though they are
the same species. This
prevents them from
hooking up.
GEOGRAPHIC ISOLATION
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Two populations are separated
by geographic barriers such as
rivers, mountains, or bodies of
water.
Ex: the Albert squirrel in the
Southwest.
Its population got separated
when the Colorado River split
the area.
The Albert and Kaibab squirrels
have many similarities, but fur
color differs.
1. Original beetle population
2. River arises, effectively splitting
the population.
3. After many generations, each
population evolves genetic differences.
4. After the river dries up, genetic
differences prevent interbreeding.
TEMPORAL ISOLATION
Two or more species
reproduce at different
times.
 Ex: three species of
orchid are found in the
same rain forest.
 They release pollen only
on a single day.
 Since they each release
pollen on different days,
they cannot pollinate each
other.
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SPECIATION IN DARWIN’S FINCHES
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Read pgs. 408-409