вход по аккаунту


SELECTION ??? - Bio 200 Evolutionary Biology

код для вставкиСкачать
Selection Lecture
Wed. June 2 .
Recap from last time:
There are three essential mechanisms
underlying evolution.
1. Variation ( mutation, gene migration,
genetic recombination)
2. Heritability or those traits that are
3. Natural Selection or the differential
capacity for survival.
Different things produce selection
Internal environment
The external environment
How do measure such a concept
Based on non-random breeding and on “fitness”
Fitness = W = ability of some genotypes to
leave more offspring to the next generation
When W = 1 indicates best fit genotype (all
offspring )
When W = 0 lethal genotype ( 0 offspring)
When W = 0.5 genotype leaves ВЅ offspring for
next generation
What is Fitness?
• Fitness =
species' fitness lies at the heart of Darwin's original
The genetic contribution of an individual to
succeeding generations. The ability to produce
healthy offspring.
Source: Webster's Revised Unabridged Dictionary, В© 1996, 1998 MICRA, Inc.
What is relative Fitness?
• The fitness of an individual relative to other
individuals in a population.
Calculation of fitness
W = Reproductive rate of unfavored alleles
reproductive rate of favored alleles
Consider the condition that for “normals”
W = 1
Measure of the intensity of selection
S = 1-W
S = 0 is most fit
S = 1 is least fit
Think of the lab example
• Fish A and Fish D
• Fish A had W= 1 so most fit
Fish D had W= 0.6
• For Fish D’s Fitness
W= Reproductive rate of unfavored types (D)
Reproductive rate of favored (A)
The opposite could hold true
• It will depend on the internal and external
environments of the fish. Thus, Fish D may
have W= 1 in a different pond so now
W= Reproductive rate of unfavored types (A)
Reproductive rate of favored (D)
• S will be 1- W or equal to 0
A. Selection against dominant allele
Given the genotypes
•Dominant genes can be good (favorable), bad
(unfavorable) or neutral
•Most disadvantageous: dominant lethal
time of lethal events effects W
•Environmental event may change fitness of
dominant genes—could disappear in single
•Partial selection against dominants
Ex. Dwarfism- Achondroplasia
• Achondroplasia is a genetic disorder of bone
growth that is evident at birth. A major type of
• It affects nearly one in every 25,000 births
• Non specific to races, creeds or sexes.
• Achondroplastic dwarfism is characterized by an
average-size trunk, short arms and legs, and a
slightly enlarged head and prominent forehead.
A case of Dwarfism
Achondroplastic dwarfs produce 19.6% offspring
as normal population (no differences in survival)
Dwarfism is a dominant allele
W = 19.6/100 = 0.196 fitness value
Since S = 1-W S = 0.804
selection coefficient against dwarfs
i.e. 80.4% of the expected offspring are
removed solely by selection
In this dominant case
• Achondroplasia is not favored for even
though has a dominant A allele
• Non dominant alleles are now favored for
normal births to occur
• How does this work in terms of
Note that A is gradually
lost over time
W = 0.2
W = 0.1
This level of selection leads to
elimination of dominant and fixation of
the recessive. Ignores mutation rate.
B. Selection against recessive allele
Aa not affected in complete dominance
Therefore, elimination of recessive is very slow.
If there is co-dominance or incomplete
dominance, elimination of recessive can be
faster I.e. Aa is disadvantageous
Ex. Is Cystic Fibrosis
• CF is a genetic disorder that affects the
respiratory, digestive and reproductive
systems. There are approximately 30,000
people in the United States with CF
• There is an inflammation of the mucous
membranes causing excess mucous to form
and clump in the lungs.
The presence of two
mutant genes (g) is
needed for CF to appear.
Each parent carries one
defective gene (g) and
one normal gene (G). The
single normal gene is
sufficient for normal
function of the mucus
glands, and the parents
are therefore CF-free.
Each child has a 25
percent risk of inheriting
two defective genes and
getting CF, a 25 percent
chance of inheriting two
normal genes, and a 50
percent chance of being
an unaffected carrier like
the parents.
C. Selection favoring heterozygotes
Co-dominance or incomplete dominance must be
involved if Aa is favored
Reminder: co-dominance= Situation in which two different alleles for a
genetic trait are both expressed autosomal dominant, recessive gene
incomplete dominance=A condition where a heterozygous
off- spring has a phenotype that is distinctly different from, and intermediate to, the
parental phenotypes
Specific Terms
• Sickel Cell anemia- abnormal blood
cells with irregular sickle cell shape.
• When these hard and pointed red cells go
through the small blood tube, they clog the
flow and break apart. This can cause pain,
damage and a low blood count, or anemia.
What makes the red cell sickle?
• Hemoglobin . This protein carries oxygen inside the cell. Any
changes in this protein causes the hemoglobin to form long rods
in the red cell when it gives away oxygen.
• The hemoglobin allele S, is what is responsible for the disease.
Why has this disease not been depleted if it is caustic?
• The heterozygotes for the S allele are resistant to malaria.
The Statistics
phenotype normal
Mild anemia
genotypes HbN,
In Africa
HbN, HbS
HbS, HbS
What sort of Selection will favor
heterozygotes like the sickle cell
anemia case?
The choices
• Disruptive
• Stabilizing
• Directional
The Answer:
Stabilizing Selection
• Selection eliminates the extremes.
• It prevents the changes of the middle range
• Doesn’t change the more common
phenotypic traits shown in the population
D. Selection for polygenic traits
1. Stabilizing selection = elimination of extremes
from the population
Before selection
After selection
In nature, natural selection is most commonly
stabilizing. The average members of the
population, with intermediate body sizes,
have higher fitness than the extremes.
Natural selection now acts against change in
form, and keeps the population constant
through time.
• The phenotypic distribution before
selection is a relatively broad bellcurve.
• The Variance is also reduced
Text material В© 2002 by Steven M. Carr
Stabilizing Selection ex.
• Human infants with an average/intermediate
birth weight will have the higher survival
Disruptive Selection
• The middle of the range of phenotypes are
selected against
• A bimodal distribution results
2. Disruptive selection
= elimination of intermediate individuals from
population i.e. increases population variability
The implication of the distribution
As a result, the population will be
monomorphic for one of the homozygous
genotypes spanning one of the two peaks in
the population distribution. Which
homozygous genotype comes to
predominate, however, depends on the
initial allele frequencies in the population.
Most common Disruptive Selection
• Diverse beak sizes in finch populations.
Has high amounts of large and small beaks
but few middle sized beaks.
• This suggests considerable variation in
body and beak size (large beaks are better
for large seeds but can also eat small seeds
favored by finches with small beaks).
Directional Selection
• Tries to eliminate one of the two extremes
• Distributions are either right or left skewed
3. Directional selection
=elimination of individuals from the population
causing progressive shift in an average trait
through time
The implications
• Often, shifts in environmental conditions, such as
climate change or the presence of a new disease or
predator, can push a population toward one
extreme for a trait.
• In periods of prolonged cold temperatures, natural
selection may favor larger animals because they
are better able to withstand extreme temperatures.
Directional Examples
• Components of fitness, such as survival
after food depletion in Cliff Swallows
• Insecticide resistance in insects
• heavy metal tolerance in plants
• selection coefficient (S)
• Relative Fitness coefficient (W)
• There are three kinds of natural selection
1. Stabilizing- peak will get narrower
2. Disruptive- 2 peaks form
3. Directional- peak shifts to one side
• One would think most populations are in a
normal distribution.
• However, we can now suggest that “The selection
on the traits affected by many genes ( or
environmental pressures) can favor both
extremes, the intermediate values, or only one
• Stabilizing Selection and Directional Selection
are fairly common in various populations.
• Disruptive Selection-this is uncommon, but of
theoretical interest because it suggests a
mechanism for species formation without
geographic isolation
Final Thought
• Selection is the agent of evolution that
solely produces adaptive evolutionary
Размер файла
318 Кб
Пожаловаться на содержимое документа