Transcript Nerve activates contraction - Jackson County School District

Evolution of
Population
s
Topics:
1. Types of Selection
2.Sources of Variation in a Population
• Causes of Microevolution
3.Sources of Stability in Populations
• Hardy-Weinberg Equilibrium
Introduction
• One obstacle to understanding evolution is the
common misconception that organisms
evolve, in a Darwinian sense, in their lifetimes.
• Natural selection does act on individuals by impacting
their chances of survival and their reproductive
success.
• However, the evolutionary impact of natural
selection is only apparent in tracking how a
population of organisms changes over time.
It is the population, not its
individuals, that evolves.
• Evolution on the scale of populations, called
microevolution, is defined as a change in the
allele frequencies in a population.
• Macroevolution leads to Speciation.
• For example, the bent grass (Argrostis tenuis) in this
photo is growing
on the tailings of
an abandoned mine,
rich in toxic
heavy metals.
• Individual plants
do not evolve to
become more
metal-tolerant
during their
lifetimes.
A population’s gene pool is defined
by its allele frequencies
• A population is a localized group of individuals
that belong to the same species.
• One definition of a species (among others) is a group
of populations whose individuals have the potential to
interbreed and produce fertile offspring in a nature.
• The total aggregate of genes in a population
at any one time is called the population’s
gene pool.
• It consists of all alleles at all gene loci in all
individuals of a population.
Review of natural selection
Types of selection
Directional Selection
Diversifying Selection
(Disruptive)
Stabilizing Selection
Sexual Selection
Animation
Directional Selection
 Changing environmental
conditions give rise to
directional selection,
where one phenotype
replaces another in the
gene pool.
 Can produce rapid shift in allelic frequencies.
 Ex: Peppered moth
– peppered moths, pesticide
resistance, antibiotic resistance
Occur in response to/when:
* in response to directional change in the environment
* in response to one or more new environmental conditions
* when mutation appears and proves to be adaptive
Diversifying
(Disruptive)
Selection
 Increases the extreme types in a population
at the expense of the intermediate forms.
 One population divided into two.
(bill size in seedcrackers)
• Diversifying selection can result in balanced
polymorphism.
• For example, two distinct bill types are present in
black-bellied seedcrackers in which larger-billed
birds are more efficient when feeding on hard seeds
and smaller-billed birds are more efficient when
feeding on soft seeds.
Stabilizing Selection
 Intermediate forms of a trait are favored
and alleles that specify extreme forms are
eliminated from a pop.
 Ex: Human birth weight stay between 6-8
lbs. Lower or higher has higher mortality.
Class Quiz
Sexual Selection
 Sexual dimorphism – when sexually
reproducing species have a distinctly male or
female phenotype (Ex. Peacocks)
 Sexual selection comes into play when
certain traits are advantageous simply
because males or females prefer them.
 Coloration, strength- other “mate attractants”
 Females are the main agents of selection here
The two main causes of microevolution
are genetic drift and natural selection
•
Four factors can alter the allele frequencies in
a population:
1. genetic drift
2. natural selection
3. gene flow
Overview Clip
4. mutation
•
All represent departures from the conditions
required for the Hardy-Weinberg equilibrium.
Causes of microevolution
Genetic Drift
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EVOLUTION BY CHANCE
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loss of genetic diversity
Genetic Drift
Short Clip
A random change in allele frequencies over generations,
brought about by chance alone.
Impact minor in large pops, but significant in small pops.
In the absence of other forces, random change in allele
frequencies leads to the homozygous condition and a loss
of genetic diversity over the generations.(This happens in all
pops; it just happens faster in small ones.)
Genetic drift is pronounced when very few individuals
rebuild a pop or found a new one – get a BOTTLENECK:
(a severe reduction in pop size, due to intense selection
pressure or natural calamity) see page 257 – founder
effect is one type of bottleneck.
Inbreeding – form of genetic drift in a small population. (pg
257)
ANIMATION
• The bottleneck effect occurs when the
numbers of individuals in a larger population are
drastically reduced by a disaster.
Genetic Drift
• By chance, some alleles may be overrepresented and
others underrepresented among the survivors.
• Some alleles may be eliminated altogether.
• Genetic drift will
continue to impact
the gene pool until
the population is
large enough to
minimize the impact .
Causes of microevolution
Causes of microevolution
Bottlenecking is an important concept in
conservation biology of endangered species.
• Populations that have suffered bottleneck incidents
have lost at least some alleles from the gene pool.
• This reduces individual variation and adaptability.
• For example, the genetic variation
in the three small surviving wild
populations of cheetahs is very low
when compared to other mammals.
• Their genetic variation is
similar to highly inbred
lab mice.
Causes of microevolution
Genetic Drift
An example of a bottleneck:
Northern elephant seals have
reduced genetic variation
probably because of a population
bottleneck humans inflicted on
them in the 1890s. Hunting
reduced their population size to
as few as 20 individuals at the
end of the 19th century. Their
population has since rebounded
to over 30,000—but their genes
still carry the marks of this
bottleneck: they have much less
genetic variation than a
population of southern elephant
seals that was not so intensely
hunted.
Causes of microevolution
Genetic Drift
Causes of microevolution
Genetic Drift
• The founder effect occurs when a new population is
started by only a few individuals that do not
represent the gene pool of the larger source population.
• At an extreme, a population could be started by single
pregnant female or single seed with only a tiny fraction of the
genetic variation of the source population.
• Genetic drift would continue from generation to
generation until the population grew large enough for
sampling errors to be minimal.
• Founder effects have been demonstrated in human populations
that started from a small group of colonists.
Causes of microevolution
Gene Flow
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Gene Flow
Animation
Gene flow is the physical flow of alleles into
or out of a population.
Immigration – alleles coming in (added)
Emigration – alleles moving out (lost)
Gene flow counteracts differences that
arise through mutation, natural selection,
and genetic drift.
Gene flow helps keep separated populations
genetically similar.
Causes of microevolution
Gene Flow
Gene flow tends to reduce differences
between populations.
If extensive enough, gene flow can
amalgamate neighboring populations into a
single population with a common genetic
structure.
 The migration of people throughout the
world is transferring alleles between
populations that were once isolated,
increasing gene flow.
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Causes of microevolution
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Mutation
Mutation
A mutation is a change in an organism’s DNA.
A new mutation that is transmitted in
gametes can immediately change the gene
pool of a population by substituting the
mutated allele for the older allele.
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For any single locus, mutation alone does not have
much quantitative effect on a large population in
a single generation.
An individual mutant allele may have greater
impacts later through increases in its relative
frequencies as a result of natural selection or
genetic drift.
Causes of microevolution
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While mutations at an individual
locus are a rare event, the
cumulative impact of mutations at
all loci can be significant.
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Each individual has thousands of
genes, any one of which could
experience a mutation.
Populations are composed of
thousands or millions of individuals
that may have experienced mutations.
Over the long term, mutation is a
very important to evolution
because it is the original source of
genetic variation that serves as
the raw material for natural
selection.
Clip: Review of factors that
contribute to gene pool
Mutation
Frequency Dependent Selection
 Preserves Variety
 Also called “minority advantage”
 Acts to decrease the frequency of the more
common phenotype and increase frequency of the
less common one.
 EX: Sometimes predators tend to concentrate on
common varieties of prey and overlook rare ones.
Within prey species, this could result in the
fitness of each variety being inversely related to
its frequency in the population.
 could maintain polymorphism
Allows us to calculate
the frequencies of
alleles in a population
The Hardy-Weinberg Theorem describes a nonevolving
population
• The Hardy-Weinberg theorem describes the
gene pool of a nonevolving population.
the frequencies
of alleles and genotypes in a
population’s gene pool will remain
constant over generations unless acted upon
• This theorem states that
by agents other than Mendelian segregation and
recombination of alleles.
Allele Frequencies
 Allele frequencies: the abundance of
each kind of allele in a population.
 To compare, use Hardy-Weinberg
equilibrium:
p2 + 2pq + q2 = 1
p2= frequency of homo dom
2pq = frequency of hetero
q2 = frequency of homo recess
p+q=1
p2 + 2pq + q2 = 1
•Consider a population whose gene pool
contains the alleles A and a.
•Hardy and Weinberg assigned the letter
p to the frequency of the dominant allele
A and the letter q to the frequency of
the recessive allele a.
•Since the sum of all the alleles must
equal 100%, then p + q = 1.
•They then reasoned that all the
random possible combinations of the
members of a population would equal
•(p+q)2
•or
p+q=1
p2 + 2pq + q2
p2 + 2pq + q2 = 1
The frequencies of A & a will remain unchanged generation
after generation if the following conditions are met:
1. Large population.
-The population must be large to minimize random sampling errors.
2. Random mating.
-There is no mating preference. For example an AA male does not prefer an aa
female. (NO SEXUAL SELECTION)
3. No mutation.
-The alleles must not change.
4. No migration.
-Exchange of genes between the population and another population must not
occur.
5. No natural selection.
-Natural selection must not favor any particular individual.
Example problem:
1 in 1700 US Caucasian newborns have cystic fibrous.
(C for normal is dominant over c for cystic fibrous).
1-What percent of the population have cystic fibrous.
2-What percent of the population are carriers?
•HWE= p + q = 1 or p2 + 2pq + q2 = 1
•Since we know cc = q2, calculate q2
•q2 =cc= 1/1700= 0.00059= 0.059% (Answer to #1)
•c =√.00059 =0.024= 2.4%
•So, the c allele frequency is 2.4%
•Now, Calculate C allele
•HWE= p + q= 1
•So, p= 1-q
•p= 1- .024
We still haven’t answered question #2 
•p= 0.976 or 97.6 %
Example problem:
1 in 1700 US Caucasian newborns have cystic fibrous.
(C for normal is dominant over c for cystic fibrous).
1-What percent of the population have cystic fibrous.
2-What percent of the population are carriers?
(0.059%)
(4.68%)
• p = 0.976 or 97.6 %
• q = 0.024 or 2.4%
• HWE= p2 + 2pq + q2 = 1
• 2pq= Heterozygous condition (carriers)
• 2pq= 2 (0.976 x 0.024)
• 2pq= 0.0468 or 4.68% of US Caucasians are
carriers. (Answer to #2)
Practice Problem:
• If 9 percent of the population has blue
eyes, what percent is hybrid for brown
eyes? Homozygous for brown eyes?
1. q2 = 9% = 0.09 (convert to decimal)
2. Solve for q: q = 0.3
3. Solve for p:
• p + q = 1 ; p = 1-0.3 ; p= 0.7
4. Solve for 2pq; 2(0.7 x 0.3)= 42% are hybrid
5. Solve for p2:
• p2 = (.7)2 =49% are homozygous dominant.
Practice Problem:
• This is a classic data set on wing coloration in the scarlet
tiger moth (Panaxia dominula). Coloration in this species had
been previously shown to behave as a single-locus, two-allele
system with incomplete dominance. Data for 1612 individuals
are given below:
• White-spotted (AA) =1469 Intermediate (Aa) = 138 Little
spotting (aa) =5
• What are the frequencies of the A & a alleles?
A = (2 * (1469) + (138))/(2 * (1469 + 138 + 5)) = .954
or 95.4%
a = 1 - .954 = .046 or 4.6%
AA = (.954) 2 = .910 or 91%
Aa = 2 (.954)(.046) = .087 or 8.7%
aa = (.046) 2 = .002 or .2%
It is the changes in gene
frequencies over time that
result in evolution.