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Selection and Fitness
What is Natural Selection?
• Natural selection can be defined simply as the differential survival and
reproduction of alternate genetic variants in a population
• It’s a process that promotes adaptation and keeps the disorganizing
effects of the other evolutionary processes in check
• Natural selection is the most critical evolutionary process, because
only selection accounts for the adaptive and highly organized nature
of living things; also explains the diversity of organisms because it
promotes their adaptation to different ways of life
• Natural selection is a process that occurs whenever two conditions are
(1) variation in a trait between individuals
(2) a relationship between the trait and reproductive success
• Natural selection, then, is not caused by differential reproduction,
it is differential reproduction.
Four Postulates of Natural Selection
1. Individuals within a species are variable
2. Some of these variations are passed along to offspring
3. In every generation, more offspring are produced than
can survive
4. Survival and reproduction are not random; those with
favorable variations survive and go on to reproduce
1. Individuals within a species are variable
2. Some of these variations are passed along to offspring
3. In every
generation, more
offspring are
produced than can
4. Survival and reproduction are not random; those with
favorable variations survive and go on to reproduce
4. Survival and reproduction are not random; those with favorable variations survive and go
on to reproduce
Additional Comments regarding Selection
• Heritability is an essential component for natural selection.
• However, it's important to realize that it's phenotypic differences
alone that affect reproductive success.
• And, if traits are not heritable, there will be what is called
• Because natural selection occurs through the differential
reproduction of phenotypes, it's possible to have natural selection
without evolution - differential reproduction but no evolutionary
response to selection.
• So, while selection and evolution are related, selection and
evolution are not synonymous.
• Selection can occur in the absence of genetic variation, but an
evolutionary response to selection requires genetic variation.
• Under a given set of environmental conditions, some phenotypes are
more successful reproductively than others.
• This difference in reproductive success is referred to as RELATIVE
• Fitness is a relative measure in two ways:
(1) Relative to the local environment at the time
(2) Relative to the quality of other phenotypes present
• Relative fitness is the parameter that we can use to measure natural
Computation of Fitness
• For mathematical
convenience, fitness is
defined to range from 0-1.
• A phenotype with the
greatest reproductive
success has a fitness of 1;
the success of all other
phenotypes is measured
relative to the success of
this most successful
Additional Comments about Fitness
• If a phenotype has a fitness of 1, it doesn't mean that this is the best
possible phenotype; it's just the best available phenotype.
• Fitness measured locally: a phenotype with a fitness of 1 may be the
best available phenotype in its population, but that doesn't mean
there aren't phenotypes in other populations that would do better.
• If environmental conditions change, the phenotype that is the most fit
may also change.
• In summary: The fitness of a phenotype is measured relative to other
phenotypes in the same population under the current set of
environmental conditions.
Fitness Components
• There are many aspects in the life of an individual that may
contribute to fitness
• These various aspects are collectively called fitness components
• The main fitness components are: survival and fertility; other
components can be treated separately or incorporated into these
Selection against Recessive Defects
• Multi-legged frog trait assumed to be due to a defective recessive
allele: A – dominant, normal legs; a – recessive, multi-legged
• Possible bullfrog genotypes: AA (normal), Aa (normal but a
carrier), or aa (multi-legged)
• Given genotypes of the initial generation: 24 AA, 48 Aa, and 24
• Assume that aa individuals are unable to mate; thus, parents of the
next generation will have either AA or Aa genotypes
• Q. Will the incidence of the multi-legged trait decline and
eventually disappear from the population?
Selection against Recessive Defects cont.
Heterozygotes are twice as numerous as homozygous dominants:
• 2/3 (48/72) of the total breeding members are Aa and 1/3
(24/72) are AA
• The different types of matings and their genotypes are as follows:
Selection against Recessive Defects cont.
• Given at total of 36 matings, we can express the frequencies as whole numbers:
AA male x AA female – 1/9 x 36 = 4
AA male x Aa female – 2/9 x 36 = 8
AA female x Aa male – 2/9 x 36 = 8
Aa male x Aa female – 4/9 x 36 = 16
Q. What is the outcome of each
type of cross?
Assume that each mated pair
contributes 4 progeny to the next
Assume that the actual number of
offspring is a reflection of the
different types of mating
Example: A single AA female x Aa male mating yield 4 offspring in the Mendelian
ratio of 2AA:2Aa
However, there are 8 matings of this kind so that the number of offspring would be
16AA:16 Aa
Selection against Recessive Defects cont.
• To calculate the frequency of a recessive allele after n generations
of selection:
qn = q0
1 + nq0
where q0 is the original frequency of the recessive
allele, qn is the frequency after n generations
• Thus, with the initial value q0 = 0.5, the frequency of the recessive
allele after two generations (n=2) will be:
q2 =
1 + 2(0.5)
= 0.25
• If the frequency of the recessive a allele is q, then the frequency of
the recessive individual (aa) is q2; the frequency of the recessive
heterozygote is (0.25)2 or 0.0625 (6.25%)
• Therefore, in the second generation, the incidence of the multilegged condition (aa) decreases to 6.25%
Outcome of Selection against a Recessive
Trait over Numerous Generations
The Interplay of Mutation and Selection
• In theory, if the process of selection against a homozygous
recessive trait were to continue over 100s of generations, the
detrimental recessive allele would be present at a very low frequency
in the population
• But, deleterious recessive alleles are continually be replenished by
mutation (e.g., from A -----> a)
• If a certain portion of A alleles are converted to a alleles, then a
population will carry a certain proportion of a recessive mutant allele
(a) regardless of selection against it
• An equilibrium state will be reached when the rate at which
recessive alleles are lost by selection equals the rate at which variant
recessive alleles are produced by mutation
Selection Against Dominant Defects
• Imagine a situation in which AA or Aa individuals do not leave any
• You might expect the A allele to be quickly eliminated, and for the
population to be comprised only of aa individuals
• However, it is important to realize that detrimental dominant alleles
are likely to be maintained in a population due other factors,
including mutation and the effects of partial selection
• Begin with a population of 500,000 individuals, all of which are homozygous
recessive (aa); 1,000,000 recessive a alleles
• 10 dominant mutant alleles occur in the first generation; but, assume the dominant
mutant gene is semi-lethal - only 5 of the newly arisen dominant alleles are
transmitted to the second generation
• The second generation would contain at total of 15 dominant alleles – 5 carried over
from the first generation and 10 new ones added by mutation
Selection Against Dominant Defects cont.
• In the third generation, there would be 17.5 dominant alleles, etc.
• Overall, the total number of dominant alleles would increase
slightly with each subsequent generation
• Equilibrium is achieved (about 12 generations) when the rate of
elimination of abnormal dominant alleles balances the mutation rate
Balanced Polymorphisms
• When a genes(s) and their alleles are balanced or approximate an
equilibrium state, e.g., due to interplay of mutation and selection
The Role of Heterozygote Advantage
• Selection in favor of heterozygotes over homozygotes
Sickle-cell Anemia
• Due to homozygosis for a recessive HbS
allele that produces abnormal hemoglobin
instead of the normal hemoglobin due to a
HbA allele
• Genetics of sickle cell anemia are as
HbS HbS - sickle cell anemia
HbA HbS - normal RBCs (codominance)
HbA HbA - normal RBCs
Distribution of the Sickle-cell Allele
Why is there are relatively high frequency of the HbS allele?
• Heterozygotes are more fit than either of
the homozygotes
• Heterozygotes are resistant to malarial
infections, whereas the HbA HbA are not
• HbA HbS red blood cells do not normally
• However, when malaria attacks an HbA HbS red blood cell, oxygen
concentration of the cell drops, inducing red blood cells to sickle
• This destroys the red blood cell, but it also destroys the parasite.
• So, in areas where malaria is rife, the heterozygotes have a selective
advantage over both the homozygotes, which have a fairly high
probability of dying from either anemia (HbS HbS) or malaria (HbA
Heterozygote Advantage and
Relative Fitness
Balanced Polymorphism and
Frequency-Dependent Selection
• Imagine a scenario in which allele frequencies in a
population remain near an equilibrium, but the
reason is because the direction of selection
• First selection favors one allele, then it favors
• This scenario is called frequency dependent
selection - the fitness of an allele depends on its
• The scale eating cichlid fish Perissodus microlepis from Lake
Tanganyika in Africa
• This fish bites the scales off of other fish – attacks from behind,
grabbing the scale off the victim’s flank
• Within the species are right-handed (mouth twisted to the right)
and left-handed (mouth twisted to the left) fish
• Also, there is evidence to suggest that handedness is determined
by a single locus with 2 alleles, with right handedness is dominant
over left handedness
• Behavior observations and an examination of scales recovered
from the fish’s stomach indicate that right—handed fish always
attack their victim’s left flank; left-handed fish attack the right
• The prey species are usually wary and alert, with the scale-eating
predators only successful in 20% of their attacks
• If right-handed scale eaters were more abundant than left-handed
individuals, then the prey species would be more vigilant against
attacks from the left
• This would then give left-handed scale eaters and advantage in
catching prey
• Left-handed scale eaters would secure more food, leave more
offspring, and pass on more of their left-handed genes
• This would ultimately increase the frequency of left-handed scale
eaters in the population
• After left-handed scale eaters had become more abundant, the prey
fish would start to be more vigilant for attacks from the right
• This would, in turn, give right-handed scale eaters and advantage
• As a result of this, left-handed and right-handed fish should be just
about equally abundant in the population at any given time
• Hori (1993) sampled fish from Lake Tanganyika every year for 11
years and found that the frequency of the 2 phenotypes oscillated
around 0.5 for each
• In any given year, one of the two phenotypes may have a slightly
higher frequency, but invariable the pendulum would swing back to
the other direction
Multiple Niche Polymorphism and Environmental Heterogeneity
• Polymorphisms can occur as a result of heterogeneous
environments, an idea some times referred to as multiple niche
• Jones and Probert (1980)
experiments with normal
and white-eyed (mutant)
Drosophila simulans
• Set up cages with a
mixture of the two
genotypes, in either red or
white light; mutants were
at a disadvantage and
selection reduced their
Multiple Niche Polymorphism cont.
• They also placed
the two groups of
flies together in a
cage that was
illuminated with red
light in one half and
white light in the
other half of the
cage; both
genotypes were
• The white flies
concentrated in the
red light half of the
cage and the normal
red-eyed flies in the
half with white light
– the flies showed
“habitat selection”
Types of Selection
Selection type
mean trait value
no change
no change
trait variance
usually no change
Stabilizing Selection
• This form of selection
occurs when
phenotypes have a
higher fitness that
extreme phenotypes.
• In fact, uncommon
phenotypes may be
• Under stabilizing
selection, the mean
value of a trait is
unaffected but trait
variance is reduced
Stabilizing Selection and New Born Birth Weights
Directional Selection
• Directional selection
occurs when an extreme
phenotype has higher
fitness than other
• It is usually in response
to new environmental
• Under directional
selection, the mean value
of the trait is affected and
trait variance is often not
Directional Selection and Cheetahs
Disruptive Selection
• Disruptive
selection occurs
when intermediate
phenotypes are of
lower fitness than
• Under disruptive
selection, the
mean value of the
trait remains the
same, but trait
variance increases.
Disruptive Selection and Cacti