Chapter 23: The Evolution of Populations

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Chapter 23The Evolution ofPopulations
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Important Point
If you are having trouble understanding lecture
material Try reading your text before
attending lectures. And take the time to read it
well!
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Gene Pools
A gene pool is the sum of alleles at all loci
within a population
One species, but members are more likely to mate
within their herd than the other
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Polymorphism
A polymorphism is more than one allele present at
a given locus within a single population of
organisms
Population genetics is essentially the study of
allele and genotype frequencies within
populations of organisms
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Mendel meets H.W.
Recall Mendelian genetics
Hardy-Weinberg Equilibrium means genotype
frequencies stay the same
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Hardy-Weinberg Theorem
x2
diploid
320
20
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Hardy-Weinberg Theorem
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Hardy-Weinberg Theorem
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Hardy-Weinberg Theorem
Note same
The triumph of Darwinism occurred with the
Modern synthesis, the integration of the
mechanics of Darwinian evolution with those of
Mendelian genetics (1930s)
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H.W. Equilibrium
Hardy-Weinberg means that both genotype and
allele frequencies stay the same over time
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H.-W. Frequencies (2 alleles)
Calculated H.W. frequencies,1 locus, 2 alleles
Fixed allele

• Note how genotype frequencies are 100 a function
  of previous-generation allele frequencies.
• This is precisely what the H.W. equation tells
  us.
• It is the default evolutionary assumption
• (i.e., no evolution is occurring)

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H.-W. Assumptions

• To assume Hardy-Weinberg equilibrium all of the
  following must be true
• The population must be very large (no sampling
  error/genetic drift)
• There must be no net mutation
• There must be no natural selection (though as we
  will see that this assumption can be temporarily
  suspended in the course of using the
  Hardy-Weinberg theorem)
• No migration between populations
• Random mating (equivalent to mixing all sperm and
  eggs in population into a common bucket to foster
  fertilization)
• In other words, no mechanisms that can affect
  genetic structurei.e., allele or genotype
  frequenciesmay be operating

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Eggs Milt (Sperm) in Bucket
http//wdfw.wa.gov/wildwatch/salmoncam/hatchery.ht
ml
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Non-Random Mating
Anything that interferes with the random mating
between individuals is nonrandom mating
Nonrandom mating results in deviations from a
Hardy-Weinberg generation of genotypes from a
given frequency of alleles
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H.-W. Equilibrium

• If no mechanisms that can affect genetic
  structure are operating, then
• Hardy-Weinberg genotype frequencies will be
  established in a single generation
• And these frequencies will persist indefinitely
• (I.e., so long as there are no mechanisms
  operating that can affect genetic structure)
• Remember that an organism can be homozygous for a
  given allele even if within the population is
  polymorphic (meaning that more than one allele
  exists)
• Indeed, three alleles can exist within a
  population, even if only at best two can exist
  within a single individual

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Chalk discussion of H.W. theorem, including,
especially, p2 2pq q2 1
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Solving H.-W. Problems

• Work with Decimals, not percentages, not
  fractions, not absolute numbers
• Convert Phenotypes to Genotypes, whenever you are
  given phenotype information you should be
  pondering (i) how can I convert phenotypes to
  genotypes? and (ii) how can I convert known
  phenotype frequencies to genotype frequencies?
• Convert Genotypes to Alleles, once you know
  genotype frequencies it should be trivial to
  convert to allele frequencies dont let this
  step trip you up
• Convert Alleles to Genotypes, if you know allele
  frequencies, but not genotype frequencies, then
  chances are you will need to figure out the
  latter
• Incorporating Selection, usually selection only
  operates at the diploid stage ? make sure
  frequencies always add up to one
• Practice, Practice, Practice, Practice, Practice!

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Working with Decimals

• Convert percentages to decimals (I.e., by
  dividing by 100) 25 ? 0.25
• Convert fractions to decimals (I.e., by dividing
  by the denominator) ¼ ? 0.25
• Convert absolute numbers to decimals (I.e., by
  dividing number by total) 60/240 ? 0.25
• Many a Hardy-Weinberg solution has been tripped
  up by not employing decimals, i.e., by not
  employing frequencies
• E.g., 25 x 25 625! (which is incorrect)
• E.g., 0.25 x 0.25 0.0625! (which is correct)
• Yes, 25/100 x 25/100 625/100/100 0.0625
• But isnt that absurdly complicating???

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Phenotype ? Genotype

• Phenotype to Genotype conversions are going to
  depend on the genetics of your locus
• Always in these problems genotypes will be
  diploid
• If alleles have a dominance-recessive
  relationship, then the heterozygote will have the
  same phenotype as the dominant homozygote
• Therefore, if the relationship is
  dominant-recessive you will know with certainty
  only the genotypes of recessive homozygotes
• If the relationship is codominant or incomplete
  dominant, however, then there will be a
  one-to-one mapping of genotype to phenotype
• That is, for the latter ( only for the latter)
  genotype frequencies will be the same as
  phenotype frequencies

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Dominant Genotypes

• If a population is in Hardy-Weinberg equilibrium
  then the frequency of all genotypes, even
  dominant genotypes, may be estimated
• Start with the frequency of the recessive
  homozygote ? this equals q2
• q therefore is equal to the square root of the
  frequency of the recessive homozygote
• p, the frequency of the dominant allele,
  therefore (if 2 alleles) can be assumed to be
  equal to 1 q
• The dominant homozygote therefore can be assumed
  to have a frequency of (1 q)2
• The heterozygote therefore can be assumed to have
  a frequency simply of 2pq
• Always assume Hardy-Weinberg equilibrium unless
  you have a compelling reason not to

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Genotype ? Allele

• Once you know genotype frequencies, going from
  genotype frequencies to allele frequencies is
  easy
• Dont let it trip you up!
• There are two formulas one can use and which one
  you use depends on whether you are working with
  absolute numbers versus genotype frequencies
• f(A) 2f(AA) 1f(Aa) 0f(aa) / 2
• note that 2 2f(AA) 2f(Aa) 2f(aa) since
  all frequencies should add up to 1
• Note that this is just a ratio of number of
  alleles of a one type to total number of alleles
  present in a population
• Alternatively, with X AA, Y Aa, Z aa
• f(A) (2X 1Y 0Z) / 2(X Y Z)
• Note also that f(A) 1 f(a) (for 2 allele
  system)
• for ABO (3-allele) system, f(IA) 1 - f(IB) -
  f(i)

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Allele ? Genotype

• Genotype frequencies can be estimated from allele
  frequencies
• First, you must assume Hardy-Weinberg equilibrium
• Then simply calculate genotype frequencies from
  allelic frequencies using the Hardy-Weinberg
  theorem
• (recall that p and q are allele frequencies)
• If you had 70 A alleles and 120 a alleles, then
  what are the expected frequencies of AA, Aa, and
  aa?
• f(A) 70 / (70 120) 0.37 ? f(a) 0.63
• f(AA) 0.372 0.14 f(aa) 0.632 0.40 f(Aa)
  2 0.37 0.63 0.47
• Check your answer ? 0.14 0.40 0.47 1.01,
  which is pretty close to 1.0 (rounding error?)

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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Non-Random Mating

• Random mating violates statistical independence,
  which would complicate our math
• Recall the Rule of Multiplication from Chapter
  14
• How do we determine the chance that two or more
  independent events will occur together in some
  specific combination? The solution is in
  computing the probability for each independent
  event, then multiplying these individual
  probabilities to obtain the overall probability
  of the two events occurring together. (p. 254 C
  R, 2002)
• It is because matings are random that the odds,
  e.g., of one A allele (from mom) being paired
  with another A allele (from dad) is p p or p2
• If matings were not random then the probability
  of the above pairing could be gtp2 or ltp2,
  depending on whether opposites repel or
  opposites attract (respectively)

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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Sampling Error Genetic Drift
Errors get bigger (as fraction of sample) as
samples get smaller!
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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift Bottleneck
• Mutation
• Migration
• Non-Random mating

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Sampling Error Bottleneck
When a population is reduced in size randomly,
sampling error results in the allele frequencies
of the new population not likely matching what
were the allele frequencies in the old population
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Cheetah, Product of Bottleneck
The longer a population remains at a reduced
size, the greater the effect of genetic drift on
allele frequency
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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift Founder effect
• Mutation
• Migration
• Non-Random mating

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Sampling Error Founder Effect
Note that the alleles lost are not necessarily
the same alleles as may have been lost due to
natural selection
New population
Genetic drift is sampling error
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Products of Genetic Drift
A locus for which only a single allele exists for
an entire gene pool is considered to be fixed,
i.e., a fixed locus
Isolated populations by chance fixed different
karyotypes
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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Mutation Neutral Variation
Note change in allele frequencies
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Mutation (1/2)

• Mutation (or, at least, net mutation) also
  automatically changes allele frequency
• For example, a mutation involves the conversion
  of one allele into another allele
• Typically mutation does not play a big, direct
  role in changing allele frequency because
  mutation rates per locus tend to be low
• However, indirectly mutation is absolutely
  essential to microevolutionary processes because
  all allelic variation ultimately has a mutational
  origin
• Mutations represent random changes in highly
  evolved (i.e., information laden) nucleotide
  sequences, so often give rise to losses in gene
  function (thus most mutations are recessive)

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Mutation (2/2)

• "Organisms are the refined products of thousands
  of generations of past selection, and a random
  change is not likely to improve the genome any
  more than firing a gunshot blindly through the
  hood of a car is likely to improve engine
  performance.
• Every now and then, though, a mutational change
  is adaptive (and even less often, both adaptive
  and dominant or codominant), i.e., novel
  functions or novel expression of old functions
• "On rare occasions, however, a mutant allele may
  actually fit its bearer to the environment better
  and enhance the reproductive success of the
  individual. This is not especially likely in a
  stable environment, but becomes more probable
  when the environment is changing and mutations
  that were once selected against are now favorable
  under the new conditions." your text

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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Migration (Gene Flow)
Migration (movement of individuals) makes allele
frequencies become more similar
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Non-Darwinian Evolution

• Generally natural selection is the evolutionary
  force most closely associated with Darwinism
  (i.e., Darwinian evolution)
• Keep in mind, though, that selection cannot
  operate without genetic variation
• Genetic variation, in turn, ultimately is a
  consequence of mutation
• Non-Darwinian mechanisms generally are not
  adaptive and include
• Genetic drift
• Mutation
• Migration
• Non-Random mating

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Natural Selection (1/2)

• Make sure that you understand that
• Natural selection acts on phenotypes
• Genotypes underlie phenotypes
• Alleles underlie genotypes
• Therefore, natural selection ultimately acts on
  allele frequencies, though selection occurs
  through the filter of both phenotype and genotype
• "An organism exposes its phenotypeits physical
  traits, metabolism, physiology, and behaviornot
  its genotype, to the environment. Acting on
  phenotypes, selection indirectly adapts a
  population to its environment by increasing or
  maintaining favorable genotypes in the gene
  pool." your text

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Natural Selection (2/2)

• Natural selection can act during the haploid or
  diploid stage
• The effect of natural selection is to reduce (not
  to increase) the absolute number of genotypes or
  alleles
• That is, mutation places alleles into a gene
  pool, other microevolutionary forces can serve to
  increase the frequency of the allele, but
  selection acts to selectively remove maladaptive
  alleles (mutation in, selection out)
• In the absence of natural selection an organism
  contributes x gametes to the next generation in
  the presence of natural selection an organism
  contributes ltx gametes to the next generation
• Natural selection is differential reproductive
  success
• Natural selection serves to increase the
  information content found within genomes

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Incorporating Selection
Recall, for example, that we are diploid, and
assume that natural selection is acting only at
the diploid stage
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Chalk discussion of effect of natural selection
on H.W. frequencies
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Selection for Toxin Resistance
"The modern synthesis emphasizes the importance
of populations as the units of evolution, the
central role of natural selection as the most
important mechanism of evolution, and the idea of
gradualism to explain how large changes can
evolve as an accumulation of small changes
occurring over long periods of time." your text
Seeds that drift onto mine tailings die unless
they are genetically predisposed toward
heavy-metal resistant
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Darwinian Fitness

• Darwinian fitness is the contribution an
  individual makes to the gene pool of the next
  generation relative to the contributions of other
  individuals. p. 457, Campbell Reece, 2002
• Darwinian fitness is the allelic contribution an
  individual makes to the next generation
• Darwinian fitness is a quantity equal to the
  average reproductive output associated with a
  given genotype
• The more likely an individual is to survive and
  reproduce (i.e., to contributes its alleles to
  the next generation), the higher that
  individual's Darwinian fitness
• Darwinian fitness is often simply called fitness
• People typically consider Darwinian fitness on a
  locus-by-locus basis

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Relative Fitness

• In a more quantitative approach to natural
  selection, population geneticists define relative
  fitness as the contribution of a genotype to the
  next generation compared to the contributions of
  alternative genotypes for the same locus The
  relative fitness of the most reproductively
  successful variants is set at 1 as a basis for
  comparison. pp. 458-459, Campbell Reece, 2002
• Restatement Typically the genotype with the
  highest Darwinian fitness is given a relative
  fitness of 1.0
• All other genotypes, i.e., those with lower than
  the highest Darwinian fitness, then have relative
  fitness values of less than 1.0
• If one genotype produces on average 4 progeny per
  generation and another produces on average 1
  progeny per generation, then what is the relative
  fitness of the latter genotype? The former?

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Modes of Selection
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Stabilizing Selection
Stabilizing selection eliminates phenotypic
extremes within a population, thus increasing the
frequency of genotypes underlying intermediate
phenotypes
Stabilized populations tend to be reasonably well
adapted to their environments
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Directional Selection
Directional selection is natural selection
against only one phenotypic extreme
Directional selection is what people typically
think of when they think of natural selection
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Disruptive Selection
In disruptive selection the intermediate is
selected against
Disruptive selection can result in balanced
polymorphisms
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Sickle-Cell Prevalence
Selection by malaria exposure
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Directional Selection (in macroevolution)
"Of all the causes of microevolution, only
natural selection generally adapts a population
to its environment. The other agents of
microevolution are sometimes called non-Darwinian
because of their usually non-adaptive nature."
your text
Note This example is Macroevolutionary, not
Micro
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Sexual Selection
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Sexual Selection

• Sexual selection are forces that impact on mate
  procurement
• If you dont mate, you dont make babies
• Mate procurement involves competing with same
  gender individuals (e.g., other males) and
  attracting other-gender individuals
• Intrasexual selection is a consequence of direct
  competition (e.g., fighting) with ones own
  gender
• Intersexual selection (mate choice) is
  competition for the other genders eye
• How these mechanisms operate can differ greatly
  from gender to gender
• Basically, for some species (e.g., us), procuring
  a mate can be a very complicated experience

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Sexual Selection
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Cost of Sex (Why bother?)
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Sexual Dimorphism
Nyala sexual dimorphism
Ammonite sexual dimorphism
In sexual dimorphism, males and females differ
phenotypically in addition to their possessing
different sexual organs
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Sexual Dimorphism (elephant seals)
Hey, I was bottlenecked, too!
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Genetic Polymorphism

• Genetic polymorphism is the presence of multiple
  alleles at a given locus within a gene pool
• In general, there is a lot more genetic
  polymorphism in populations than meets the eye
• This in part is because of hidden recessive
  alleles, and also because different alleles do
  not necessarily give rise to different phenotypes
• Heritable variation within a population is
  synonymous with polymorphism
• Therefore, the raw material of natural selection
  are polymorphisms

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Genetic Polymorphism
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Balanced Polymorphism

• Balanced polymorphisms are stably maintained
  multiple alleles at a given locus
• Heterozygous advantage, a.k.a., balancing
  selection
• E.g., Sickle cell anemia but otherwise probably
  not too important
• Hybrid Vigor ? a product of heterozygous
  advantage and the masking of deleterious alleles
• E.g., Hybrid corn, but can this maintain
  polymorphisms in the wild?
• Frequency-dependent selection ? selection for
  alleles because they are rare, e.g., Major
  Histocompatibility Complex
• Neutral variation ? selection not strong enough
  to remove alleles (unless environment changes)
• There is more neutral variation in larger
  populations due reduced strength of genetic drift

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Environmental Variation A Cline
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Temporal Phenotypic Variation
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Why no Perfect Organisms?

• "An organism's phenotype is constrained by its
  evolutionary history
• "Adaptations are often compromises
• "Not all evolution is adaptive
• It takes too much energy to optimize everything
  so much of most organisms is simply good enough
  to get the job done (a.k.a., the principle of
  allocation)
• "Selection can only edit variations that exist
• Even if a perfect organism existed, it would only
  remain perfect so long as its environment
  remained unchanged
• To make matters worse, environments even change
  over single individual's life spans

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The End