Download NB Honors_Pop & Speciation

Chap 16: Evolution of Populations
Darwin never knew how heredity actually
worked
Biologists connected Mendel’s work to
Darwin’s in the 1930’s
Advances in genetics since then has
redefined many of Darwin’s ideas in
genetic terms
Neo-Darwinism
Chap 16: Evolution of Populations
Single-gene & Polygenic Traits
# of phenotypes produced for given trait
depends on how many genes control trait
Single-gene traits controlled by a single
gene with two alleles
Ex.- widow’s peak
100
80
60
Frequency of Phenotype
(%)
40
20
0
Widow’s peak
No widow’s peak
Phenotype
Chap 16: Evolution of Populations
Many traits controlled by two or more genes
Called polygenic traits
Each gene often has two or more alleles
Result: one polygenic trait can have many
possible genotypes & phenotypes
Ex.- height
Distribution of phenotypes usually takes on
bell shape on graph (normal distribution)
Chap 16: Evolution of Populations
How Common is Genetic Variation?
All genes have at least two forms (alleles)
Plants & animals often have many alleles for
a single trait
Some variation is “invisible” since it involves
biochemical processes
Organisms may also be heterozygous for
many genes
Chap 16: Evolution of Populations
Sources of Genetic Variation
Two main sources of genetic variation:
- mutations
- genetic shuffling resulting from sexual
reproduction
Chap 16: Evolution of Populations
Mutations
Any change in a sequence of DNA
Can occur due to mistakes in replication or
due to environmental factors (radiation)
Do not always affect organism’s phenotype
Those that do, may affect an organism’s
fitness
Other mutations have no effect on fitness
Chap 16: Evolution of Populations
Gene Shuffling
Even though parents provided you your
genes, you don’t look exactly like them
Most heritable differences due to gene
shuffling that occurs during meiosis
Each member of homologous pair moves
independently:
23 chromosomes can produce 8.4 million
combinations
Chap 16: Evolution of Populations
Crossing-over also increases the # of
different genotypes that can appear in
offspring
Sexual reproduction major source of
variation within populations
Does not change relative allele frequency on
its own
Chap 16: Evolution of Populations
Variation and Gene Pools
Genetic variation studied in populations
Population- group of individuals of the
same species that interbreed
Since they interbreed, share a common
group of genes called a gene pool
Gene pool- consists of all genes, including
all different alleles, present in a population
Chap 16: Evolution of Populations
Relative frequency of an allele is the # of
times that allele occurs in the gene pool,
compared with # of times other alleles for
same gene occur
Often expressed as a percentage
In genetic terms, evolution is any change in
the relative frequency of alleles in a
population
Sample Population
allele for
brown fur
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
black fur
Chap 16: Evolution of Populations
Evolution versus Genetic Equilibrium
Are there any conditions under which no
evolution will occur?
Is there any way to recognize when this is
occurring?
Answer is provided by the Hardy-Weinberg
principle
Chap 16: Evolution of Populations
Hardy-Weinberg principle:
allele frequencies in a population will remain
constant unless one or more factors cause
those frequencies to change
Situation called genetic equilibrium
Chap 16: Evolution of Populations
Five conditions required to maintain genetic
equilibrium from generation to generation:
- Must be random mating
- Population must be very large
- No movement into/out of population
- No mutations
- No natural selection
Chap 16: Evolution of Populations
Some populations, these criteria met for
long time periods
If conditions are not met, genetic equilibrium
disrupted, & population evolves
Chap 16: Evolution of Populations
Random Mating
All members of population have equal
chance of producing offspring
In natural populations, mating is rarely
completely random
Nonrandom mating means genes for certain
traits are under strong selection pressure
(sexual selection)
Chap 16: Evolution of Populations
Large Population
Genetic drift has less effect on large
populations
No Movement Into/Out of Population
New individuals could bring new alleles into
population
Gene pool of population must be kept
separate from others
Chap 16: Evolution of Populations
No Mutations
If genes mutate, then new alleles might be
introduced into gene pool
Gene frequencies will change
Chap 16: Evolution of Populations
No Natural Selection
All genotypes in the population must have
equal chance of survival & reproduction
No phenotype can have selective advantage
over another
Chap 16: Evolution of Populations
Evolution as Genetic Change
Each time an organism reproduces, passes
along a copy of its genes to offspring
Can then view evolutionary fitness as an
organism’s success in passing along genes
Can view evolutionary adaptation as any
genetically controlled functional, structural,
or behavioral trait that affects ability to pass
along genes to next generation
Chap 16: Evolution of Populations
Natural selection never acts directly on genes
Entire organism (not single gene) that either
reproduces or does not (pass along genes)
Can only affect which organisms survive &
reproduce or do not
Individual does not contribute its alleles to
gene pool unless it survives & reproduces
Chap 16: Evolution of Populations
If organism produces many offspring, its
alleles stay in gene pool & may increase in
frequency
Evolution is change over time in relative
frequencies of alleles in a population
So, populations can evolve, not individual
organisms, over time
Chap 16: Evolution of Populations
Natural Selection on Single-gene Traits
Natural selection on single-gene traits can
lead to changes in allele frequency
Ex.- population of lizards
Normally brown, experiences mutation
causing red and black phenotypes
If red more visible to predators, those
individuals less likely to survive &
reproduce
Chap 16: Evolution of Populations
Black lizards might warm up faster on cold
days
If high body temp. allows faster movement,
then black moves faster
Avoids predators & feeds easier: might
produce more offspring than brown form
If color change has no effect on fitness, then
no selection pressure would occur
Chap 16: Evolution of Populations
Natural Selection on Polygenic Traits
Fitness of individuals close to one another on
bell curve will not be very different
Fitness can vary great deal from one end of a
curve to the other
Where fitness varies, natural selection can act
Chap 16: Evolution of Populations
Genetic Drift
N.S. not only source of evolutionary change
In small populations, an allele can become
more or less common by chance
Probability can be used to determine results
of crosses in large populations
Smaller the population, farther the results
might be from predicted values
Chap 16: Evolution of Populations
This random change in allele frequency is
known as genetic drift
In small populations, individuals that carry a
particular allele may leave more
descendants than others, just by chance
Over time, a series of chance events of this
type can cause an allele to become
common in a population
Chap 16: Evolution of Populations
Genetic drift may occur when a small group
of individuals colonizes a new habitat
These individuals may carry alleles in a
different frequency than the parent
population
Population founded will be genetically
different from parent population
Cause is chance (that particular alleles were
in founding individuals), not N.S.
Chap 16: Evolution of Populations
Natural selection can affect the distributions
of phenotypes in any of three ways:
- Directional selection
- Stabilizing selection
- Disruptive selection
Chap 16: Evolution of Populations
Directional Selection
Directional selection- when individuals at
one end of curve have higher fitness than
individuals in middle or at other end
Range of phenotypes shifts towards one end
due to some individuals failing to S&R
Chap 16: Evolution of Populations
Ex.- Darwin’s finches (each with diff. beak
size)
Food shortage causes supply of small &
medium-sized seeds to run low
Birds with beaks that enable them to open
large seeds will have advantage
Big-beaked birds will have higher fitness
Average beak size in population would
probably increase
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Chap 16: Evolution of Populations
Stabilizing Selection
Stabilizing selection- when individuals
near center of curve have higher fitness
than individuals at either end
Keeps center of curve at current position,
but narrows the overall graph
Chap 16: Evolution of Populations
Ex.- human birth weight
Babies born much smaller than average less
likely to be healthy, less likely to survive
Babies much larger than average are likely to
have difficulty being born
Fitness of larger or smaller individuals is
lower than that of average-sized individuals
Stabilizing Selection
Key
Low mortality, high fitness
High mortality, low fitness
Birth Weight
Selection against both
extremes keep curve
narrow and in same
place.
Chap 16: Evolution of Populations
Disruptive Selection
Disruptive selection- when individuals at
upper & lower ends of curve have higher
fitness than individuals near middle
Selection acts against intermediate phenotype
If selection pressure is strong and lasts long
enough, two distinct phenotypes produced
Chap 16: Evolution of Populations
Ex.- If population of birds lives in area where
medium-sized seeds become less
common
Large and small-sized seeds become more
common
Birds with unusually small or large beaks
would have higher fitness
Population might split into large seed eaters
and small seed eaters
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Chap 16: Evolution of Populations
Process of Speciation
How do changes caused by natural selection
and genetic drift lead to formation of new
species?
Gene pools of two populations must become
separated for them to become new species
Biological Species Concept
Chap 16: Evolution of Populations
Isolating Mechanisms
As new species evolve, populations become
reproductively isolated from each other
When members of two populations can no
longer interbreed and produce fertile
offspring- reproductive isolation
Populations have separate gene pools
Respond to N.S. or genetic drift differently
Chap 16: Evolution of Populations
Reproductive isolation can develop in variety
of ways, including:
- Behavioral isolation
- Geographic isolation
- Temporal isolation
Chap 16: Evolution of Populations
Behavioral isolation
Occurs when two populations are capable of
interbreeding, but have differences in
courtship rituals or other reproductive
behaviors
Ex.- bird species using different songs to
attract mates
Chap 16: Evolution of Populations
Geographic isolation
Two populations are separated by
geographic barriers such as rivers,
mountains, or bodies of water
Ex.- Two species of squirrel (Albert &
Kaibab) formed after Colorado river cut the
Grand Canyon
Chap 16: Evolution of Populations
Does not guarantee formation of new
species:
Barrier might be breached/broken down
shortly
If two populations still can interbreed, no
new species has formed
Also, geographic barriers can exist for some
organisms, but not for others (birds can fly
over a large river or mountain range)
Chap 16: Evolution of Populations
Temporal isolation
Two or more species reproduce at different
times
Ex.- plants
Three plants all live in same forest, only
release pollen on one day
Each releases pollen on different day
Impossible to pollinate one another
Chap 17-4: Patterns of Evolution
Punctuated Equilibrium
Darwin felt that biological change needed to
be slow and steady – idea known as
gradualism
Fossil record confirms that some
populations of organisms change
gradually over time
Also evidence that this pattern does not
always hold true
Chap 17-4: Patterns of Evolution
Some species have changed little since they
appeared in the fossil record
Much of the time these species are in a
state of equilibrium
Every now and then, something upsets the
equilibrium
Changes in populations then occur relatively
rapidly
Chap 17-4: Patterns of Evolution
Rapid evolution after long periods of stability
can occur for several reasons:
- Small pop. isolated from larger pop.
- Small group migrates to new environment
- Mass extinctions open up niches
Term punctuated equilibrium used to
describe pattern of long, stable periods
interrupted by brief periods of rapid change