Population Genetics

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Population Genetics The Modern Synthesis of
evolution is Darwinism enlightened by the
understanding of molecular genetics which has
been gained since Darwin. The key to
understanding how evolution occurs is a move from
viewing genetics in terms of individuals and
their alleles to -- the frequencies of those
alleles among the genes of all individuals
comprising a population.
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We know about genes and particulate
inheritance. Darwin did not. He was neither the
first not the last to accept blended inheritance.
He wrote before Mendel had described recessive
traits. To explain evolution, he fell back into a
second error the inheritance of acquired
traits. Most phenotypes, resulting from the
influence of many genes, do seem to be inherited
as if blended. Without a mechanism for
particulate inheritance, it was hard to establish
the concept.
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Mendels genetics disappeared into the literature
until the beginning of the 20th century. The
rediscovery of Mendelian genetics led to a number
of leading biologists claiming that
evolution resulted from inheritance of mutations.
Evolution, in this view, moved rapidly and by
jumps, rather than gradually, as Darwin had
believed. Failures to accept the modern
synthesis of Mendelian genetics and Darwinian
evolution persisted into and after WWII - e.g.
Lysenko.
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To understand the modern synthesis, we need
to consider the genetics of populations, rather
than individuals. Consider a Punnett square for a
single trait cross ? (male)
½A ½a ½A AA Aa ? (female) ½a
Aa aa in describing this cross, we have shown
the effects of meiosis 1/2 the sperm carry A,
1/2 a, and similarly for the eggs.
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Now recognize that the fractions of these two
alleles in the population may not be equal, and
there may be more than two alleles. The sum of
all alleles for a trait in the population is the
gene pool for that trait. We measure fraction p
of the genes in this gene pool are of type A, and
fraction q are type a (assuming males and females
are genetically similar). Now the Punnett square
looks like this
male pA qa pA p2AA pqAa
female qa pqAa q2aa
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A mating like this does not change gene
frequencies. Evolution is a change in the
composition of the gene pool. Gradual change is
called microevolution. In a population that had
those allele frequencies, they would remain
unchanged indefinitely if the conditions for
Hardy-Weinberg equilibrium held.

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What are the conditions? 1. Large population
size 2. No migration (gene flow occurring
through immigration or emigration) 3. No
mutation 4. Random mating (no assortative
mating) 5. No natural selection Do the
conditions often apply (or apply for long)?
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The Hardy-Weinberg Law is a null hypothesis. It
holds (as what is called the Hardy-Weinberg equili
brium) when things dont change, i.e. 1. In large
populations there is no genetic drift. In
small populations random events (mortality of a
single individual) may materially affect gene
frequency. This happens in small island
popula- tions or populations of endo
(internal) parasites. 2. There is no movement
between populations, that would be gene flow.
The genes moved would change the frequencies
in both source and recipient populations.
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3. There is no mutation. If one A mutated to a
per 100 alleles, then what was 50 A in the
starting population would become 49A after
mutation. Actual mutation rates are about
1/106 per gene, but that translates to about
1 mutation per gamete for us. We are, thus,
each unique. 4. Mating (fertilization) occurs
randomly. If blondes would only marry blondes
(real ones) (blond hair being recessive),
there would be a much higher frequency of the
blond phenotype. Lets look at an example of
this
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We can figure out gene frequencies in a
population if we know the frequency of the
recessive phenotype. For these individuals,
knowing the phenotype frequency we also know the
genotype and gene frequencies. The frequency of
the recessive phenotype is q2. That is also the
frequency of the homozygous recessive genotype.
Then the frequency of the recessive gene is the
square root of q2 ? q. Now for the example
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We start with 100 people (50 male). 1 out of
10 is a natural blond. That means q2 .1, and
q.316. p 1 q .684. Those would be
the values indefinitely if mating were random,
but If blondes only mate with blondes, then the
5 blond males mate with the 5 blond females, and
produce 10 blond children in the next generation.
As to the other 90 (or 180 genes) p2
(.684)2 (100) ? 46 are homozygous for dark hair
(or 92 dark-haired genes), and 2pq
2(.684)(.316) (?100) ? 44 are heterozygous
(another 44 genes for dark hair)
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The overall frequency for the dark hair gene
among the mating population of dark-haired
individuals is 136/180
.755 Assuming that the dark haired individuals
mate randomly male gametes
.75B .25b .75B .5625 BB .1875
Bb female gametes .25b .1875 Bb .0625
bb BB and Bb have the dark hair phenotype. Take
these fractions and use them to correct to total
90 individuals to keep the population constant in
size ? 51BB 34Bb are dark haired, 5bb are blonds
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Add these to the 10 blonds from assortative
mating, and now there are 15 blonds instead of
10 out of 100, and 85 instead of 90 with dark
hair. The phenotypic and genotypic frequencies
have changed microevolution has occurred. But,
how often does assortative mating of the sort
presented in this example occur in nature?
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5. No natural selection occurs. When natural
selection occurs the survival and reproduction
of different phenotypes differs. Some have
higher survival and/or reproduction they
leave behind a larger fraction of the
offspring that form the next generation
(differential reproductive success). Their
genes represent a greater fraction of the
gene pool in the next generation. A
numerical example selection against the sickle
cell gene. We will conveniently forget the
advantageous effects of being heterozygous.
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An example of selection Sickle cell anemia Begin
with 50 of the genes S and 50 s. The initial,
randomly mated cross is .5S .5s .5S .25
SS .25 Ss .5s .25 Ss .25 ss We will assume the
.25ss die without reproducing. Now calculate new
gene frequencies. The 75 of the population of
offspring surviving to reproduce are the whole
population. Now 66 of the genes are S and .33
are s
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The cross in the 2nd generation
is .66S .33s .66S .44SS .22Ss .33s
.22Ss .11ss Natural selection against the
homozygous recessives has reduced the fraction
from 25 to 11 in one generation. It would
further reduce the fraction each generation, but
since there are fewer of them, fewer would be
selected against, as well. N.B. natural selection
- acts on phenotypes - selects only among
variants present
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Natural selection acts on phenotypic
variation. Where does the variation come
from? Ultimately, all genetic variation in living
organisms originates as mutations. The variation
we observe in a population is also determined
by 1) recombination (sexual reproduction)
2) the spread of variants in a population due to
drift, and 3) the effects of environmental
variation on the relative success of
different phenotypes.
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One view of the amount of genetic variation in a
species is the fraction of its genes that are
hetero- zygous. That fraction in part indicates
the amount of outcrossing (breeding with
unrelated members of the species) and in part
reflects the history of the species. Cheetahs
went through a severe bottleneck within the last
10,000 years only 0.07 of their genes are
heterozygous. Humans have not gone through a
bottleneck like that 7 of our genes are
heterozygous.
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Why are so many genes not heterozygous? -
because altered alleles are not as good as
the ones that persist. Others have been
removed by selection.
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While examples indicate how gene frequencies can
change, the most common cause of genetic change
(microevolution) in natural populations is
natural selection Natural selection can occur
in different ways. We categorize the basic types
of natural selection into three forms
stabilizing selection, directional selection, and
diversifying (or disruptive) selection.
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Modes of Natural Selection
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1) Stabilizing selection - - acts against
extreme forms, favors intermediates - one
example human birth weights
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2) diversifying (or disruptive) selection -
acts against intermediates, favors extremes
- example - selection of different coloration
patterns in Papilio to resemble noxious
but unrelated butterflies
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3) directional selection - favors one extreme,
selects against the opposite extreme -
shifts the phenotype distribution curve in one
direction. Numerous examples industrial
melanism pesticide or drug resistance
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There is another form of selection 4) sexual
selection - leads to evolution of secondary
sexual characters - results in sexual
dimorphism - usually males evolve showy
characters, e.g. a) tails of peacocks
peahens are drably colored b) antlers of deer
or caribou - females lack antlers c) colors
of male mallards at breeding time,... -
Why? usually females choose mates, showiest or
most dominant male gets a large harem, others
remain generally unmated
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So, to take a human view, imagine John Travolta
in Saturday Night Fever, or...
(Sorry, I couldnt find a good copy of the
classic pose in a white polyester suit, strutting
his stuff)
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These are Wodaabe men from Niger in a pose
off, where the women select the most beautiful
men. They are wearing lipstick and other
makeup, where the males of many animal species
are naturally decorated (e.g. cardinals,
peacocks, birds of paradise).
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Questions about selection 1) Are most genes
subject to the intense natural of (most) of
these examples? No! These extreme examples make
evolution more apparent, and occurring more
rapidly. 2) Are some genes strongly conserved
through the varieties of living
things? Yes! For example, there have been only a
handful of changes in the base sequence of
cytochrome C from bacteria to man. 3) Is all
genetic variation adaptive? No! Much of the
variation is neutral. None of the variants
confers a selective advantage.
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Does natural selection perfect organisms?
No! Why? 1. Organisms are locked into
historical constraints. 2. Adaptations are
compromises. 3. Not all evolution is
adaptive. Chance frequently plays a large role.
4. Selection can only act on (and edit)
variations (phenotypes) that exist.