Transcript Here - Angelfire

MICROEVOLUTION
THE SUBTLE CHANGES IN A
GENE POOL
Genetics of Populations
• Continuous variation can be quantified
– Eye color and height
– Bell curve
– Refer to pages 186 - 187
• Polygenic inheritance
– Additive effect of 2 or more genes on a single
phenotype character
Environmental conditions can
modify the expression of a gene
• Hydrangea colors: blue or pink
– Only true for that individual
– Not inherited
Inherited Alleles depend on
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Gene mutation
Crossover
Independent assortment
Fertilization
Change in chromosome number or structure
Tracking the Rate of
Genetic Change
• Calculate the allele frequency
• Compare with the ideal population as
outlined in H-W Rule
– Involves a population in genetic equilibrium
– Frequencies are stable generation after
generation
5 Conditions for an
Ideal (life-population)
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Random mating
No mutations
Very large
Isolation
Equal successful reproduction
– No natural selection
– If reproductive success were different, it would
alter the frequencies in the gene pool
Calculation “particulars”
• Use p + q = 1
– when finding the frequency of the allele
• Use p2 + 2pq + q2
– When finding the frequency of the genotype
500 flowered plants
Pink (A) white (aa)
• 20 white
• 480 pink
– 320 AA
– 160 Aa
• Find the frequency of the A allele
– 640 + 160 = 800
– 800 / 1000 = .8
• Allele “a” has frequency of 0.2
– Now let’s apply this information to specific
genotypes
• Each allele will occur in the same frequency
as the original population
– Gamete that will have “A” is 0.8
• Calculate the frequencies of the 3 possible
genotypes in the next generation
• Frequency of AA?
– 0.8 x 0.8 = .64 or 64%
• Frequency of aa?
– 0.2 x 0.2 = .04 or 4%
• Frequency of Aa?
– 0.2 x 0.8 = 0.16
• BUT THERE IS ALSO “aA” possible
– That’s why the formula is 2pq
– Heterozygote frequency is 0.32 or 32 %
• If you only knew the genotype information,
you can still find the allele frequency
– Use square root to solve for p and/or q
• Imagine 1 in 10,000 births has PKU
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Q2 = 0.0001
Q = 0.01
P= 1-q
P= 0.99
Carriers?
2pq = 2 (0.99)(.01)
= 0.0198 which is nearly 2% of the population
• In a population with 2 alleles B and b, the
frequency of the allele B is 0.7. What
would be the frequency of heterozygotes?
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A. 0.7
B. 0.49
C. 0.21
D. 0.42
E. 0.09
• Answer D
• If 16% of the individuals in a population
show the recessive trait, what is the
frequency of the dominant allele?
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A. 0.84
B. 0.36
C. 0.6
D. 0.4
E. 0.48
• Answer C
Microevolution
• Most common driving forces away from
equilibrium:
– Natural selection
– Gene flow
– Genetic drift
• Others
– Gene mutations
• Only source of new alleles in a population
• Rare enough not to have an immediate effect on the
allele frequency
• What would decrease the frequency of an
allele?
– Lethal mutation
• A mutation that turns out to be an advantage
can be maintained through natural selection
• Mutations create new alleles BUT
• Natural selection, gene flow, and genetic
drift change the frequencies of alleles in the
gene pool
Genetic Drift
Change in allele frequencies over the
generations
– Gene pool will change
– Especially true if population is 100 or less
– Negligible in a very large population
• Due to chance alone
– Just like rolling dice or flipping coins
Bottleneck Effect
Genetic Drift Example
• Segments of a population are destroyed by
disasters or hunting
• Usually reduces genetic variability
• Serious threat to the survival of a species
– Cheetahs –ice age victims 10.000 years ago then hunted
near extinction 1900’s
– Northern elephant seal had been hunted down to 20;
now 30,000 but electrophoresis shows no variability in
genes
Founder Effect
• Small sample of a population colonizes a
new habitat
– Darwin’s finches strayed from S. A.
– Inherited disorders among humans
• Retinitis pigmentosa frequency higher among an
isolated population due to colonists carrying the
gene
Gene Flow
• Alleles enter and leave a population as an
outcome of immigration and emigration
– Most populations are certainly not closed
systems
• Wind carries pollen
– Result is that over time gene flow reduce
differences between populations
• Neighboring populations may have been
affected by natural selection
• Gene flow will eventually amalgamate the
neighbors
• Human migration reduces the variability
Mutations
• Usual rate is 1 in a million
• Not significant source of variation in a gene
pool
• But it is the original source of variation
• Serves as raw material for natural selection
Nonrandom mating
• In reality, we do NOT mate randomly
• Assortative mating—select partners like
themselves in certain characteristics
• Inbreeding
• Extreme “selfing” is self-fertilization of
plants!
• Frequencies of genotypes shows a decrease
in heterozygotes
• Wildflower population
– Self-fert will increase the frequency of
homozygotes at the expense of heterozygotes
– AA begets AA
– aa begets aa
– “Aa” selfs; only half of their offspring will be
heterozygous
– Each generation decreases that number
• Result?
• More hom recessive
– Greater than H-W prediction would be
• However the p and q frequency of alleles
remains the same
– p+q=1
Natural Selection
• H-W says everyone has to be equal in their
ability to produce viable, fertile offspring
• Reality is differential success
– Some have more offspring
– Maybe red flowers produce more offspring that
white flowers (white visible to predators)
– Freq of “A” would increase
• This is the only agent of microevolution that
can be adaptive
– Accumulates and maintains favorable
genotypes in a population
– If the environment changes, selection responds
by favoring genotypes adapted to the new
conditions
Causes of Microevolution
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Genetic Drift
Gene Flow
Mutation
Nonrandom mating
Natural selection
• As a mechanism of microevolution, natural
selection can be most closely equated with
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A. Assortative mating
B. Genetic drift
C. Differential reproductive success
D. Bottlenecking of a population
E. Gene flow
• Answer next page
• C Differential reproductive success
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Selection acts directly on
A. Phenotype
B. Genotype
C. The entire genome
D. Each allele
E. The entire gene pool
• A
Phenotype
• Most of the variation we see in coat coloration and
pattern in a population of wild mustangs in any
generation is probably due to
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A. New mutations in the preceding generation
B. Sexual recombination
C. Genetic drift
D. Geographic variation within the population
E. Environmental effects
• B Sexual recombination
• The most likely effect of assortative mating on the
frequencies of alleles and genotypes would be
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A. Decrease in p2 compared to q2
B. Trend toward zero for q2
C. Convergence of p2 and q2 toward equal values
D. A change in p and q
E. A decrease in 2pq below the value expected by H-W
• E decrease in 2pq
– Reduce heterozygotes
– Does not affect p and q
• A founder event favors microevolution in the
founding population mainly because
– A. Mutations are more common in a new environment
– B. Small population is subject to sampling error in the
composition of its gene pool
– C. The new environment is likely to be patchy, favoring
diversifying selection
– D. Gene flow increases
– E. Members of a small population tend to migrate
• B small population leads to sampling error
Adaptive Evolution
Modes of Natural Selection
• Response to the environment
• Adaptive mutation
– Allele frequency shifts
– Phenotype frequency shifts
Directional Selection
• Most common during periods of
environmental change
• Or when members migrate to a new habitat
with different environmental conditions
• Frequency shifts in one direction
– Favors rare individuals that deviate from the
average
Peppered Moth
• Case of industrial melanism
• Bird predation contributed to the selection
Protective Coloration
• Rabbits
• Mice
• Tigers
Antibiotic Resistance
• Antibiotics “killed” susceptible cells
• Also allowed (favored) CELLS THAT ARE
RESISTANT
• All this in the last 60 years
Pesticide Resistance
• Kills insects, worms, etc.
• Has allowed resistant forms to increase
• Genetically engineered plants
– Pesticide resistant
– Will still trigger pests to evolve
– Coevolution
• Biological controls
– Natural enemies of the pests are raised and
released on a particular area
– Allows pests (prey) and “predator” to coevolve
Stabilizing Selection
• Culls the extreme variants from the
population
• Reduces phenotypic variation
• Maintains the status quo
– Favors the “average” for a trait
– Human birth weight around 7 lbs
– Mortality increases for higher or lower values
Gall Stories
• Page 262 was about a crown gall caused by
bacterium Agrobacterium tumefasciens
• Page 288 is a gall caused by a fly larvae
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Fly lays eggs on stem
Egg develops into pupa then larva
Larva bores into stem
Eats plant juice and tissues (yum)
• Plant cells respond by rapid growth of tissue
which forms a tumor (gall)
• These galls can vary in range depending on
the phenotypes of the fly
• Small galls—not favored because a wasp
will puncture it and lay its eggs
• Those eggs develop into larvae and eat. . .
– Eat the fly larvae!
– Reduces that phenotype of fly larvae
– Fly larvae that make small galls are selected
against
• Large galls are not favored because birds
will chip into the gall and eat the larvae
• Flies natural enemies (wasp and birds) act
as predators
• Cause stabilizing selection in favor of
intermediate sized galls
Disruptive Selection
• Phenotypes of two extremes are favored
• Intermediates are selected against
• This category also includes sexual selection.
– Females are main agents of selection.
– It’s their reproductive success that matters
most!
• Balanced polymorphism includes sickle cell
story.
• Heterozygotes favored
• Malaria is the selective force in tropical and
subtropical habitats.
• African Finch—balanced polymorphism
– Only found small or large billed species
– Why?
• Available food (sedge—grasslike plant)
• Wet season: Two species of sedge grow
– One has hard seeds, other soft
• Birds mate
• Both species of sedge available
• Birds with both sizes of beaks
• Dry season?
– Sedge with soft seeds decrease in population
– Birds with large bills are favored
– Small-billed may not survive
Beak of the Finch
• Darwin’s finches follow the same natural
selection path that the story of African
finches do.
• Darwin’s finches were the first “natural” (in
the wild) evidence for natural selection.
• Artificial selection was already in place.
– Breeding of domesticated animals
• The volcanic Galapagos give rise to a rich
diversity of environmental habitats.
• Darwin maintains that the beaks are
adaptations to different food sources.
– Question of seed-beak compatibility if you will.
– Harder the seed, the larger and stronger the bill.
• Wet years, the ground finch prefers small
seeds
– Easier no matter what the bill size is
– Selection favors those finches with smaller bills
and the population evolves
– Directional stabilization
– Just don’t bother with the large, hard seeds
• Dry years, the small and large seeds are less
plentiful.
• Finches with larger bills are favored
because they simply have an advantage of
being able to eat more food.
• Current research includes work done by
Princeton researchers Peter Grant and his
wife.
• I have one of their books if you want to see
it!
• Island with most species. . .Daphne major
– Trivia that someone may want to know
1858
• Alfred Wallace had been researching in the
East Indies and was ready to submit his
findings.
– He asked Darwin (whom he respected) to read
it and forward it to Lyell for publication.
– Lyell included excerpts on Darwin’s 1844
unpublished essay
• In 1859, On the Origin of Species, was finally
published
• Darwin developed and supported the theory much
more extensively than Wallace.
• Darwin’s notebook also collaborated that he had
developed the theory 15 years before Wallace.
• Even Wallace felt that Darwin deserved most of
the credit.
• Darwin only collected data from a few
different species. Later learned of the 13
species that inhabit the island chain.
• He also thought that the changes took place
s-l-o-w-l-y through gradual adaptations.
• That’s because of the geologic gradualism
that he was reading from Lyell.
• “Evolution” or “change through time” was
not the debated issue as many think today.
• It has taken many years of genetic evidence
(hence post Mendelian time) for the theory
to be accepted.
The Darwinian View of Life
• One facet is that evolution is the basis of
unity and diversity of life
• Darwin never used the word evolution in
The Origin of Species
• He DID use the terms “descent with
modification”
• He perceived that all organisms are related
through descent from some unknown
prototype that lived in the remote past.
• Descendants spilled into various habitats
over millions of years and accumulated
diverse modifications (adaptations).
• History of life is like a tree.
• At each fork is an ancestor common to all
lines of evolution branching from that fork.
• This was called “common descent”.
• Reality is that most branches of evolution
are dead ends.
• 99% of all species that have ever lived are
extinct.
• Darwin actually devoted very little space to
the origin of species.
• He really concentrated on how populations
became better adapted to their local
environments through natural selection.
• This is the theory that spurred controversy.
• Natural selection is a mechanism Darwin
proposed to explain the facts of evolution
documented by fossils, biogeography, and
other types of historical evidence.