Download Natural Selection

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Natural Selection
Background
Processes we've talked about so far (e.g., , genetic sampling error, inbreeding, and gene flow)
affect allele frequencies and/or genotype frequencies within a population, causing deviations
from the predicted H-W frequencies; they all cause evolutionary changes to occur.
In general, these processes make organisms less suited to survive and reproduce in their
environment -- they cause changes in allele/genotype frequencies that are independent of
whether such changes increase or decrease the adaptation of organisms to their environments
Because these processes are random with respect to adaptation, they would, by themselves,
destroy the organization and adaptation characteristic of living beings
NATURAL SELECTION can
be defined simply as the differential survival and reproduction. of
alternate genetic variants in a population
Its is the process accounts for the adaptive and highly organized nature of living things
It also explains the diversity of organisms because it promotes their adaptation to different ways
of life
In short, natural selection makes individuals better suited to survive and reproduce in their
environment.
Natural selection should not be viewed as a force or a pressure that acts on individuals, causing
some to reproduce and others to fail.
Instead, natural selection is a process that occurs whenever two conditions are met:
(1) variation in a trait between individuals
(2) a relationship between the trait and reproductive success
Whenever these conditions are met natural selection will automatically occur: individuals differ
in their traits, and they differ in their reproductive success because of differences in these traits.
Natural selection, then, is not caused by differential reproduction, it is differential
reproduction.
e.g. Think of white and brown hares living in the arctic.
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Natural Selections 4 Postulates (from Freeman and Herron)
Natural selection can be viewed as the outcome of 4 postulates:
1.
2.
3.
4.
Individuals within a species are variable
Some of these variations are passed along to offspring
In every generation, more offspring a re produced than can survive
Survival and reproduction are not random; those with favorable variations survive and go on
to reproduce
This unequal ability of individuals to survive and reproduce will lead to gradual change in the
population, with favorable characteristics accumulating over time and may eventually result in
the formation of a new species
Darwin’s Hypothesis is Testable
The beauty of Darwin’ theory of natural selection is that it is testable (e.g. the 4 postulates that
were outlined previously can be tested)
An excellent test of NS, as outlined in Freeman and Herron was performed by Rosemary and
Peter Grant and Grant who conducted research are the Galapagos finches
Both within and across finch species, Grant and Grant were able to show that beak size is
correlated with the size of the seeds harvested
In general, birds with bigger beaks eat larger seeds, and birds with smaller beaks eat smaller
seeds
Testing Postulate 1: Are Populations Variable?
The Grants measured a number of morphological variables from populations of Geospiza fortis,
including beak width depth and length
When these data were plotted (e.g., beak length) the data took the form of a bell-shaped curve
(see Fig 3.3 - Freeman and Herron)
Testing Postulate 2: Is Some of the Variation among Individuals Heritable?
To determine the degree to which the variability was due to genetic effects, the Grants estimated
heritability, that proportion of the total variation in a population that is due to the effects of
genes.
In this particular case, they estimated the similarity in the bill length between pairs of relatives
There was a strong correlation between relatives in that parents with deep beaks tended to have
offspring with deep beaks (see Fig 3.4 - Freeman and Herron)
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Testing Postulate 3: Is There an Excess of Offspring, so that only some Individuals Live to
Reproduce?
In 1977, the area the Grants were working in suffered a severe drought
Over a period of about 20 months, 84% of the medium ground finch population disappeared (Fig
3.5a Freeman and Herron)
It was inferred that the finches probably died of starvation since there was a strong
correspondence with seed availability (Fig 3.5 Freeman and Herron)
Thus, it was clear that only a fraction of the population survived to reproduce
Testing Postulate 4: Is Survival and Reproduction Nonrandom?
Did a selected or nonrandom portion of the ground finch population survived the 1977 drought?
It was determined that only those finches with the deepest beaks had survived (Fig 3.6 Freeman
and Herron)
Because the average survivor had a deeper bill than the average non-survivor, the average bill
size of the population changed (Fig 3.7 Freeman and Herron)
Finches with deep, narrow beaks were better able to crack and eat the most prevalent source of
fruit in the area
Additional Notes on Selection
Although heritability (that fraction of phenotypic variation that is due to genetic variation) is an
essential component for natural selection, it's important to realize that phenotypic differences
alone (regardless if they are the result of the environment or genetically determined) affect
reproductive success.
If traits are not heritable, there will be what is called PHENOTYPIC SELECTION.
Because natural selection occurs through the differential reproduction of phenotypes, it's possible
to have natural selection without evolution.
That is, there will be differential reproduction but no evolutionary response to selection.
Only if genetic variation for a trait occurs will there be an evolutionary response.
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.
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Fitness
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.
Relative fitness is the parameter that is used to measure NS
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
For mathematical convenience, fitness is defined to range from 0-1.
We say that the 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
phenotype.
Example
Phenotype
Black
Gray
White
Total
______________________________________________________________________________
Number of zygotes in one generation
40
50
10
100
Number of zygotes produced by each
80
90
10
180
80/40=2
90/50-1.8
10/10=1
2/2=1
1.8/2=0.9
1/2=0.5
phenotype in next generation
Computation
Average number of progeny
per individual in next generation
Fitness
_____________________________________________________________________________________________
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Fitness can be calculated when the number of progeny produced by each phenotype is known
First, calculate the average number of progeny produced per individual by each phenotype in the
next generation
Second, divide the average number of progeny of each phenotype by that of the best phenotype
(in terms of reproductive success)
One potentially confusing thing about fitness is that it's a relative measure.
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.
Also, 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.
So the fitness of a phenotype is measured relative to other phenotypes in the same population
under the current set of environmental conditions.
Many things determine the fitness of an individual.
These include such things as color, territory size, foraging ability, resistance to disease, sperm
count, etc.
These features are termed FITNESS COMPONENTS; each contributes to the overall fitness of an
individual.
The overall fitness of an individual tends to be a product of these different fitness components.
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Adding Selection to the HW Analysis: Changes in Allele Frequencies
Recall: Selection happens when individuals with certain genotypes survive and produce more
offspring than individuals with other genotypes
Selection can lead to evolution when the phenotypes that exhibit differences in reproductive
success are heritable – when certain phenotypes are associated with certain genotypes
Selection and HW Analysis
Imagine in a population of mice there is a locus (e.g., B locus with 2 possible alleles) that affects
the probability of survival
1
2
The frequency of the alleles is as follows: B = 0.6; B = 0.4
After random mating we observe the following genotype frequencies among 100 zygotes:
Genotype
Genotype frequency # of zygotes
1 1
0.36
36
1 2
0.48
48
2 2
0.16
16
BB
BB
BB
(See also Figure 5.9, Freeman and Herron)
Those individuals that survive to maturity and mate will yield the gene pool for the next
generation
Note: Selection is incorporated, by stipulating that the various genotypes have different rates of
1 1
1 2
2 2
survival: B B 100% survival, B B 75% survival, and B B 50% survival
Thus, the frequency of adult genotypes differs from that of the initial zygotes above:
Genotype
# of zygotes
# of adults
1 1
36
36
1 2
48
36
2 2
16
8
BB
BB
BB
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Now let’s assume that each surviving adult contributes 10 gametes to the next generation:
Genotype
1
2
# of adults
# gametes
B allele
1 1
36
360
360
0
1 2
36
360
180
180
2 2
8
80
0
80
BB
BB
BB
B allele
1
2
Thus for a total of 800 gametes, we have 540 carrying B and 260 carrying B
1
The frequency of B in the new gene pool is 540/800 = 0.675
2
The frequency of B in the new gene pool is 260/800 = 0.325
The population has evolved in response to selection
Persistent selection can lead to substantial changes in allele frequencies over time
Changes in the frequency of an allele will obviously be determined by the strength of selection
(See Figure 5.12, freeman and Herron)
Empirical Research on Allele Frequency Change by Selection
Cavener and Clegg (1981) documented cumulative changes in allele frequencies in a lab –based
selection experiment involving fruit flies
Fruit flies like most animals possess the enzyme ADH, which is responsible for breaking down
ethanol in rotting fruit, beer, wine, etc.
F
These researchers worked with a population of flies that had 2 alleles at this locus: Adh and
S
Adh
Cavener and Clegg maintained 2 populations of flies on food not spiked with ethanol and 2
experimental populations of flies on food that was spiked with ethanol
Every few generations, flies were randomly sampled from each population and the ADH
genotypes were scored and allele frequencies calculated (See Figure 5.13, p. 158 freeman and
Herron)
F
Control population showed no large or consistent change in the frequency of Adh over time
F
Experimental population exhibited large and consistent increase in the frequency of the Adh
allele
F
In the experimental population, flies with the Adh allele appear to have a higher fitness than
S
flies with the Adh allele
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Selection against Recessive Defects (Outcome or response to selection with a recessive allele)
The multilegged frog example from Volpe and Rosenbaum
Background
In 1958, in southern Mississippi a large number of multi-legged frogs were discovered in an 85
acre lake
Thought to be due to a genetic abnormality that followed simple Mendelian genetics (alteration
of a single gene)
One gene, two allele system: A – dominant, normal legs; a – recessive, multi-legged
Bullfrogs genotypes were AA (normal), Aa (normal but a carrier), or aa (multilegged)
Assume that genotypes of the initial generation were: 24 AA, 48 Aa, and 24 aa; assume equal
number of males and females
Selection: Assume that aa individuals are unable to mate; thus, parents of the next generation
have either AA or Aa genotypes
Heterozygotes are twice as numerous as homozygous dominants: N=72: 48 (2/3) Aa; 24 (1/3)
AA
Thus, 2/3 of the total-breeding members are Aa and 1/3 are AA
The different types of mating and their genotypes are as follows:
Given at total of 36 matings:
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
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Q. What is the outcome of each type of cross?
Assume that each mated pair contributes the same number of progeny (N=4) to the next
generation
Also 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
There are 8 matings of this kind so that the number of offspring would be 16AA:16 Aa
The detrimental recessive gene is still transmitted to the next generation even though multilegged frogs fail to mate due to mating between to heterozygous individuals
But the frequency of aa individuals decreases from 25% (24/96) to 11.11% in one generation
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We can continue to calculate the frequency against multi-legged (aa) frogs using this approach or
we can apply a formula that will indicate the frequency of these individuals after any number of
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 =
0.5
=
1 + 2(0.5)
0.5
2.0
= 0.25
If the frequency of the recessive a allele is q, then the frequency of the recessive individual (aa)
2
is q
2
Thus, the frequency of the recessive heterozygote is (0.25) or 0.0625 (6.25%)
Therefore, in the second generation, the incidence of the multi-legged condition (aa) decreases to
6.25%
Table 6.3 and Figure 6.1 indicates the outcome of this process over numerous generations
Notice that the elimination of the trait is rapid at the beginning, but becomes slower as selection
proceeds over many generations
As a recessive trait becomes rare, selection against it becomes less effective
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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 eventually be eliminated (more likely
remain at a very, very low frequency) from the population and members of the population would
express the normal homozygous condition
But, in reality, deleterious recessive alleles are continually be replenished by mutation
e.g., from a normal A allele to an abnormal a allele
All genes undergo mutations at some definable rate; so, if a certain portion of A alleles are
converted to a alleles, 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 offspring
Under these circumstances, you would expect the dominant A allele to be quickly eliminated,
and for the population to be comprised only of aa individuals (lethal dominant alleles are not
masked in heterozygotes)
It is, however, important to realize that detrimental dominant alleles are likely to be maintained
in a population due to mutation and the effects of partial selection (e.g., causing death late in life)
Example
Imagine a population of 500,000, all of which are homozygous recessive (aa); the population
contains 1,000,000 recessive a alleles
Assume that in the first generation 10 dominant mutant alleles arise (mutant rate is 1 in 100,000
alleles)
Also, assume that the dominant mutant gene is semi-lethal; only 5 of the newly arisen dominant
alleles are transmitted to the next (second) generation
As a consequence, 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
In the third generation, there would be 17.5 dominant alleles – 5 dominant alleles carried over
from the first generation would be reduced to 2.5, the 10 dominant alleles of the second
generation would be reduced to 5, and 10 new alleles would arise by mutation
Overall, the total number of dominant alleles would increase slightly would increase slightly
with each subsequent generation
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This would continue until an equilibrium is achieved where the rate of elimination of abnormal
dominant alleles balances the mutation rate
Selection on Heterozygotes: Maintenance of Balanced Polymorphisms in Populations
When you examine the genetic structure of populations and look at polymorphisms, under some
circumstances they appear to be transitional - you observe a population while one allele is
replacing another
But in most populations after the phase of transitional polymorphism comes balanced
polymorphism
Alleles arrive at a stable intermediate frequency or stable equilibrium
There are a number of conditions or processes that are known to lead to balanced polymorphisms
These include: Heterozygote Advantage, Frequency-Dependent Selection, and Heterogeneous
Environments.
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Heterozygote Advantage
HETEROZYGOTE ADVANTAGE is
selection in favor of heterozygotes over homozygotes
Example
A well-known example of heterozygote advantage in human populations is sickle-cell anemia.
Sickle cell anemia is due to homozygosis for a recessive allele HbS that produces an abnormal
hemoglobin instead of the normal hemoglobin produce by the allele HbA
The genetics of sickle cell anemia are as follows:
HbS HbS - sickle cell anemia
HbA HbS - normal RBCs (codominance - anemia when oxygen levels are low)
HbA HbA - normal RBCs
People with the HbS HbS genotype have sickle cell anemia, and about 80% die before
reproducing.
We would expect strong selection against the HbS allele, yet it persists at a fairly high frequency
(10% or higher in some populations).
Precisely in those areas of the world where a certain form of malaria, caused by the parasite
Plasmodium is common
HbS allele persists because the heterozygote is fitter than either homozygote
The heterozygotes are resistant to malarial infections, whereas the HbA HbA are not
In areas here 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 HbA)
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The fitnesses of the different genotypes can be calculated by comparing their frequency in a
population with their expected frequency based on H-W
Expected allele frequencies:
HbA = 0.756 + 1/2 (0.2416) = 0.877
HbS = 0.0023 +1/2 (0.2416) = 0.123
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 fluctuates
First selection favors one allele, then it favors another
This scenario is called frequency dependent selection
Example
Experiments involving scale eating cichlid fish Perissodus microlepis from Lake Tanganyika in
Africa
There is evidence to suggest that handedness is determined by a single locus with 2 alleles
And, right-handedness is dominant over left handedness
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Hypothesis:
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
Of course, 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
The dominant (0.3) and recessive alleles (0.7) should be held at a constant frequency
Under HW equilibrium, the frequency of the 2 alleles should be about 0.5
The Experiment
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
Figure 5.20 Herron
Multiple Niche Polymorphism and Environmental Heterogeneity
(Balanced) polymorphisms can occur as a result of heterogeneous environments, an idea some
times referred to as multiple niche polymorphisms
Example: Jones and Probert (1980) experiments involving normal and white-eyed Drosophila
simulans
Normal Drosophila with red eyes are typically attracted to light; white-eyed mutants are more
sensitive and will normally select low light intensity
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Jones and Probert used these 2 groups of flies to perform an experiment
They first set up cages with a mixture of the two genotypes, in either red or white light
In both cases, the mutants were at a disadvantage and selection reduced their frequency
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
In this set up, the two genotypes were maintained
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”
When both red and white habitats are available, each fly genotype chooses to live in the type of
habitat where it has the highest fitness
Two conditions are needed to create multiple niche polymorphisms: 1) members of a species
occupy a variety of niches that vary in their physical (dark-light, damp-dry, etc.) and biological
(presence of predators, parasites, food availability, etc.) conditions and 2) different genotypes
should have different fitness in the various habitats
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Types (Modes) of Selection
There are three primary modes of selection: STABILIZING SELECTION, DIRECTIONAL SELECTION,
and DISRUPTIVE SELECTION.
The mode of selection depends on the relationship between phenotype and fitness
The type of selection can be described by its effect on average trait values and trait variance.
selection
stabilizing
directional
disruptive
mean trait value
no change
change
no change
trait variance
decreases
usually no change
increases
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STABILIZING SELECTION
This form of selection occurs when intermediate phenotypes have a higher fitness that extreme
phenotypes.
Under stabilizing selection, the mean value of a trait is unaffected but trait variance is reduced
Trait variance is reduced because the extreme phenotypes are eliminated due to their low fitness.
DIRECTIONAL SELECTION
Directional selection occurs when an extreme phenotype has higher fitness than other
phenotypes.
It is usually in response to new environmental conditions
Under directional selection, the mean value of the trait is affected and trait variance is often not
affected.
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DISRUPTIVE SELECTION
Disruptive selection occurs when intermediate phenotypes are of lower fitness than extreme
phenotypes.
Under disruptive selection, the mean value of the trait remains the same, but trait variance
increases.
Disruptive selection can thus increase the genetic diversity of a poulation.
It can also potentially promote speciation, a process you'll hear more about later in the course
Example
Years of collecting have left their toll on the roadside cacti.
In this environment, it is maladaptive to be good looking and have a reasonable number of
spines.
Low-spine-number plants are not picked because they don't "look right", and high-spine-number
varieties are left alone because they are too hard to pick.
Gradually, the gene pool changes in favor of the two extreme spine number types.
Disruptive selection works against those phenotypes that fall into the midrange of values.