Download Post- Modern Synthesis: Genomic Conflict as a Driving Force in

Post- Modern Synthesis:
Genomic Conflict as a Driving Force
in Evolution
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Meiotic-drive alleles (segregation distorters)
Transposable elements
Genomically-imprinted genes
Organelle genes vs. nuclear genes
(in mitochondria and chloroplasts)
• Cellular endosymbionts (e.g., Wolbachia)
• Selection on selfish genetic elements
counterselection for
suppression
perpetual antagonistic coevolution
EVOLUTIONARY ARMS RACE
• Tug of war metaphor borrowed from David Haig’s work on genomic
imprinting (parent-of-origin gene expression)
• Loss of tension
breakdown in normal function
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Most of the
eukaryotic genome
consists of
transposable elementderived sequences
which do not code for
proteins useful to
their vehicles
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The Evolution of Populations I
• Genetic variation is the raw material of
evolutionary change; how do we measure
it?
• What are the forces that cause genetic
changes within populations, that is, what
are the forces of evolution?
Evolution in Mendelian
Populations
• Evolution can be defined as a change in
gene frequency through time. Population
genetics tracks the fate, across generations,
of Mendelian genes in populations.
Population genetics is concerned with
whether a particular allele or genotype will
become more or less common over time,
and why.
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What is a Population?
• A group of individuals in a particular place that
mate with each other and produce fertile
offspring, i.e., they interbreed; e.g., population of
coyotes in Washoe Valley
• A geographic population extends over so great an
area that its members in some places are unlikely
to mate with those in other places (e.g., coyotes in
Maine & Nevada)
• A locally interbreeding group within a geographic
population may be called a subpopulation, deme
or Mendelian population
Basic Definitions
• Gene pool: the sum total of genetic information
present in a population at any given point in time
• Gene (Allele) frequency: the relative proportion of a
particular allele at some gene locus (a number
between 0 and 1, inclusive)
• Genotype frequency: the relative proportion of a
particular genotype (a number between 0 and 1,
inclusive)
Calculating allele frequencies allows us to quantify and
better understand precisely what genetic variation is,
and how evolution works.
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How do We Calculate Allele Frequencies?
• Assume 2 alleles, A, a
• Yields 3 genotypes, AA, Aa, aa
• NUMBERS of individuals, genotypes and alleles in
upper case; frequencies in lower case
• NAA = number of individuals of genotype AA
• NAa = number of individuals of genotype Aa
• Naa = number of individuals of genotype aa
• NAA + NAa + Naa = N = the total number of individuals
in the population sampled
Genotype Frequencies: with Incomplete
Dominance, They Can be Observed Directly
• Frequency of AA = NAA/N = nAA
• Frequency of Aa = NAa/N = nAa
• Frequency of aa = Naa/N = naa
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Allele Frequencies: Must be Calculated
from Genotypic Frequencies
• Let the frequency of A = p
• and the frequency of a = q
…. then
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p = (2NAA + NAa)/2N = (NAA + 0.5NAa)/N
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q = (2Naa + NAa)/2N = (Naa + 0.5NAa)/N
Sample Calculations
Assume N = 200 indiv in each of two popns, 1 & 2
• Popn 1: 90 AA, 40 Aa, 70 aa
• Popn 2: 45 AA, 130 Aa, 25 aa
In Popn 1
• p = 90 + 0.5(40)/200 = 110/200 = 0.55
• q = 70 + 0.5(40)/200 = 90/200 = 0.45
In Popn 2
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p = 45 + 0.5(130)/200 = 110/200 = 0.55
q = 25 + 0.5(130)/200 = 90/200 = 0.45
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Calculations Reveal Two Significant
Points:
• p + q = 1 (more generally, sum of allelic
frequencies equals one)
• two populations with markedly different
genotype frequencies can have the
same allele frequency
Why the ps and qs?
• This mathematical formalism can be used
to model the conditions under which
evolution will and will not occur. When
genotype and gene frequencies do not
change from generation to generation, the
population is said to be at equilibrium,
i.e., there is no evolutionary change.
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Hardy-Weinberg Theorem (Rule):
• Developed independently by Hardy in
England and Weinburg in Germany in 1908
• H-W Rule: Specifies the conditions
which must be met for a population to
remain at equilibrium, i.e., for no
evolutionary change to occur. It
provides a “null model” for evolution
A Population Will Remain at
Equilibrium if:
• there is no mutation
• popn is very large, essentially infinite
• mating must combine gametes at random to form
genotypes (random mating)
• there is no migration of individuals into or out of
the population
• all genotypes survive and reproduce equally well
• (there is no meiotic drive [transmission distortion)
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Sabotage at t-complex in Mus
• Meiotic drive: violates Mendel’s first
law, the principle of segregation
(each member of pair of alleles or
homologous chromosomes has equal
probability of being transmitted to
offspring)
• Extreme segregation distortion
occurs most commonly in
spermatogenesis: meiotic drive
alleles increase their transmission to
the next generation by sabotaging
gametes carrying alternative alleles
• Causes defective, “curlicue” flagella,
poor motility and impairs
capacitation in wild-type sperm
A Population Will Remain at
Equilibrium if:
• there is no mutation
• popn is very large, essentially infinite
• mating must combine gametes at random to form
genotypes (random mating)
• there is no migration of individuals into or out of
the population
• all genotypes survive and reproduce equally well
• (there is no meiotic drive [transmission distortion)
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If these conditions hold, it can be shown that:
Allele frequencies remain constant, and, after one generation
of random mating, the genotype frequencies become:
nAA = p2
--> Genotypic frequency of AA is p-squared
nAa = 2pq = 2p(1-p)
--> Genotypic frequency of Aa is two times p times q
naa = q2 = (1-p)2
--> Genotypic frequency of aa is q-squared
• Total frequency = 1 = p2 + 2pq + q2
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This can be shown with Punnett Square
Forces of Evolution
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Mutation
Genetic drift
Non-random mating
Migration
Selection
—Natural selection
—Sexual Selection
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