The Evolution of Populations

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Transcript The Evolution of Populations

The Evolution of Populations
AP Chapter 23
What is microevolution?
• Changes in the
frequencies of a
population from
generation to
Sources of genetic variation
• Mutations – more common in
• Sexual reproduction – more
common in eukaryotes
• Can be discrete characters
determined by a single gene
• Can be a quantitative character
– varying along a continuum by
more than one gene * most
common in populations
Measures of genetic variability
• Average heterozygosity of a population
• Nucleotide variability
• Geographic variations
(cline – graded variation in a character
across a geographic area, may parallel
an environmental gradient)
A cline
About mutations…
• Most occur in somatic cells. Have to be
in gametes to be inherited.
• Point mutations that do not change the
amino acids or phenotype are harmless
Otherwise mostly harmful
• Chromosomal mutations often
• Rates in animals and plants – 1/100,000
genes per generation – so rare
• More rapid in prokaryotes and viruses
About variation in sexual
• Due to reshuffling of alleles in
recombination, crossing-over,
independent assortment of
chromosomes during meiosis, random
combination of gametes in fertilization
How can we tell if gene
frequencies are changing?
How to determine gene frequencies
• HardyWeinberg
• Gene pool – all
alleles at all the
loci present in a
• If all individuals are homozygous for the
same allele, the allele is fixed.
• Otherwise:
p = frequency of dominant alleles
q = frequency of recessive alleles
Therefore, p + q = 1
Hardy-Weinberg equation
• The gene pool of a population will
remain constant (no evolving) from
generation to generation IF
• No mutations
• Random mating
• Large population
• No natural selection
• No migration
• The probability that two gametes containing
the same allele will come together is equal
to (p + q)2.
• p2 + 2pq + q2 = 1
• P2 = homozygous dominants
• q2 = homozygous recessives
• 2pq = heterozygous ones
When doing problems:
• That will equal “q”
• Subtract that from 1 to find “p”
• Plug in equation
• Example of a Hardy-Weinberg calculation
• Species: Sciurus carolinensis (Grey Squirrel)
Alleles: A (dominant; wild type agouti fur) and a
(recessive; melanistic black fur)
• We are studying a population of 1000 squirrels. Of
these, 60 (60/1000, or 0.06) are melanistic.
If each of these melanistic squirrels carries two
recessive alleles, we can use this to calculate the
expected frequency of q, since q2 is the frequency
of the alleles in the homozygous recessive
• The square root of q2 is equal to q. (duh)
In our example, the square root of 0.06 (no. of recessives)
= .25.
• Since p + q = 1.0, you can now solve for p
• 1 - 0.25 = 0.75
• Our predicted frequencies, based on the assumption that
the squirrel population is in HW equilibrium, are p = 0.75
and q = 0.25
• plug these values into the HW equation to calculate
expected relative genotype frequencies:
• (0.75)2 + 2(0.75)(0.25) + (0.25)2
• This means that if our population of 1000 squirrels is in
HW equilibrium, then
• p2 = 0.56 - (0.56 x 1000, or 560 squirrels should be AA)
• 2pq = 0.38 - (0.38 x 1000, or 380 squirrels should be Aa)
• q2 = 0.06 - (0.06 x 1000, or 60 squirrels should be aa)
• Notice that these three frequencies add up to 1.0, 100% of
the 1000 squirrels in the population.
• A Toxic Salamander
• Western Newt is the vernacular name for the genus Taricha of
which there are three species: torosa, granulosa, and
rivularus. These are toxic salamanders found exclusively in
particular regions of California, the western halves of Oregon
and Washington, and western costal Canada up through parts
of Alaska (3). Being newts, they are salamanders that spend
the majority of their time on land
A Chi Square test can be done to
validate your predictions.
Probability of exceeding the critical value
0.05 0.025
0.01 0.001
2.706 3.841 5.024 6.635 10.828
4.605 5.991 7.378 9.210 13.816
6.251 7.815 9.348 11.345 16.266
7.779 9.488 11.143 13.277 18.467
9.236 11.070 12.833 15.086 20.515
How can allele frequencies be
• Natural selection
• Genetic drift
• Gene flow - migration
Gene Flow
Gene flow increases the variability of the gene pool by
adding new alleles.
Genetic Drift
• occurs in a small population when
some members “drift” off and form a
new colony – may not be representative
of the original population
• Can lead to loss of alleles in a
population or fixation of harmful alleles
Types of genetic drift:
• Bottleneck Effect – some disaster
affects the population
• The resulting population may not have
the same characteristics as the original.
Genetic Drift
Bottleneck Effect
• Founder Effect – some members drift off
voluntarily – may not be representative of
the original population
Breast cancer in the Jewish women
A disproportionate number of Jewish women have the
BRCA1 and BRCA2 mutation: Where the odds in the
general population are 1 in 450, for Jewish women, the
likelihood that they have a mutation is 1 in 40.
• Geneticists attribute this to the founder effect, a theory
suggesting that genes in certain isolated communities can
be traced back to a small number of "founders" who marry
only within the group. Intermarriage normally gets rid of
unhealthy genetic mutations, since only the children who
inherit the healthy genes survive. When the founders only
marry each other, though, those unhealthy genes stick
• For Ashkenazi Jews, the founders were a few thousand
people who lived in Eastern Europe 500 years ago.
A result of genetic drift
The Founder Effect in Action: Among the Amish, babies
with Ellis-Van Creveld Syndrome are born with six
This figure demonstrates 1) the chance of an allele becoming lost from
the population is equal to its initial frequency, and 2) the larger the
population size, the slower (weaker) drift is.
Natural Selection
• Relative fitness – measure of an
individual’s contribution to the gene
pool by reproduction
• Acts on the phenotype
Jim's genes allow him to live to be 100, but make him, er,
unappealing to prospective mates. Joe's genes make
him attractive to the ladies, but at a cost: he's not likely
to live past 65.
Who has the higher evolutionary fitness?
Types of natural selection
• Directional selection – individuals on
one end of a phenotypic range are
• Disruptive Selection – when
environment selects individuals on both
• Stabilizing Selection – favors more
intermediate forms, tending to reduce
phenotypic variation
Sexual selection
• Selection for a trait that enhances
• Can lead to sexual dimorphism
(distinction of males and females by
secondary sexual characteristics)
Types of sexual selection
• Intrasexual selection – same
sex competing for mates
• Intersexual selection – mate
choice, females choose
“sexier” male
So, what is the significance of
the “tusk” of the narwhal?
You got it!
They also use them for fighting! Probably for females!
Type of sexual selection?
Type of sexual selection?
Type of sexual selection?
Preservation of Genetic Variety
• Diploidy (1n + 1n = 2n)
• Balancing selection – maintaining two
or more phenotypes in the population
• Heterozygote advantage
• Frequency-dependent selection –
phenotype’s reproductive success
declines if too many in the population
Balancing selection
A case of frequency-dependent
There’s too many
of us already!
Heterozygote Advantage in Sickle Cell Anemia and Malaria
• People with normal
hemoglobin are susceptible to
death from malaria.
• People with sickle cell disease
are susceptible to death from
the complications of sickle cell
• People with sickle cell trait,
who have one gene for
hemoglobin A and one gene
for hemoglobin S, have a
greater chance of surviving
malaria and do not suffer
adverse consequences from
the hemoglobin S gene.
Neutral variations
• Do not confer a selective advantage or
• Ex in noncoding regions of DNA
Natural selection does not result in
perfect organisms!
• Many structures co-opted for new situations
• Often many compromises
ex: human knee is amazing in function, but often weak in
• Also chance events affects population’s
evolutionary history.
ex: pregnant female turtle washing ashore a remote island
• The environment changes what is “perfect”
Remember, natural selection…
• can only work with the alleles
present in the gene pool
• New alleles cannot be made in
response to new environments!
the “perfect” organism
• There are 100 students in a class. 91 did well
in the course whereas 9 blew it totally and
received a grade of F. Sorry. In the highly
unlikely event that these traits are genetic
rather than environmental, if these traits
involve dominant and recessive alleles, and if
the nine (9%) represent the frequency of the
homozygous recessive condition, please
calculate the following:
– The frequency of heterozygous individuals.
• You sample 1,000 individuals from a large
population for the MN blood group, which
can easily be measured since co-dominance
is involved (i.e., you can detect the
heterozygotes). They are typed accordingly:
• M (MM) 480
• MN (MN) 420
• N (NN)
• Calculate the gene frequencies of M and N by
COUNTING alleles in the population.