Download Directional Selection • For a population of giraffes, suppose we

Trends in Natural Selection | MCAT 2015
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Directional Selection
• For a population of giraffes,
suppose we graphed neck length
vs. the number of individuals
who possessed necks of that
length
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There is still genetic variability
among the giraffes
o Neck lengths are still
quite varied
However, there has been a shift
towards longer necks
Such a shift, where one extreme
of the spectrum is favored, is
called directional selection
Disruptive Selection
• Consider the peppered moth
living outside London
• At some point in time, the moths
spanned a range of shades
o Some light, some dark
Neck Length among a Population of Giraffes
Within the population, some giraffes have very
short necks, some have very long necks, but
most fall somewhere in the middle.
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Peppered Moths
At some point in time, peppered moths spanned
a broad array of shades; some were light and
some were dark.
The giraffes with long necks will
be naturally selected
o They will survive and
reproduce more than their
brethren
Over successive generations,
there will be a shift towards
longer necks within the
population
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And the shade distribution
among the population of moths
looked something like this:
Shade Distribution of Peppered Moths
Some moths were very light, some were very
dark, but most fell somewhere in the middle.
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Changes in Neck Length after Evolution
After many generations of evolution (dotted
line), there is still a parabolic distribution of neck
sizes among the giraffes. However, there has
been an overall shift towards longer necks.
In urban environments:
o Soot accumulated on
trees
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Trends in Natural Selection | MCAT 2015
•
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o Dark moths blended in
well with the darkened
trees
Thereby
camouflaging
themselves
In rural environments:
o Lichen covered the trees
o Light moths blended in
well with the light trees
Thereby
camouflaging
themselves
Camouflage allowed the moths to
be protected against predators
So, in urban environments, there
was as evolution towards darker
color, but in rural environments
there was as evolution towards
lighter color
Over time, the shade distribution
evolved to this:
Stabilizing Selection
• Consider the nest of a robin
Robin’s Nest
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Number of Eggs Laid by Robins in a
Population
Some robins are genetically programmed to lay
few eggs, some are programmed to lay many
eggs, but most lay a few eggs.
Evolution of the Peppered Moth
After many generations of evolution, there were
two observable shifts (dotted line) in moth color.
In urban environments, moths became darker to
camouflage with soot-covered trees. In rural
environments, moths became lighter to
camouflage with lichen-covered trees.
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The robin may be genetically
programmed to lay:
o A single egg
o A few eggs
o Many eggs
The distribution of the number of
robins that are genetically
programmed to lay a given
number of eggs might look like
this:
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This shift, in which both
extremes of the spectrum are
favored, is called disruptive
selection
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When a robin lays just one egg,
there’s a high risk that she will
have no surviving progeny
o If that one chick does not
survive infancy, there are
no more offspring
When a robin lays many eggs,
she might be unable to
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Trends in Natural Selection | MCAT 2015
•
adequately nurture them through
infancy
So, the most fit robins would lay
a few eggs
o There will be an
evolutionary trend
towards laying a few eggs
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Evolution of Robins
Over time, there will be an evolutionary trend
(dotted line), such that more robins lay an
intermediate number of eggs at a time.
•
This shift, in which the middle of
the spectrum is favored, is called
stabilizing selection
Heterozygote Advantage
• Sickle-cell anemia converts
otherwise-healthy red blood cells
into a sickle-shape
Allelic Combinations of Sickle-Cell Allele
An individual who is homozygous for the
dominant allele (AA) will not suffer from sicklecell anemia, but will have vulnerability to
malaria. An individual who is homozygous for
the recessive allele (SS) will have resistance to
malaria, but will suffer from sickle-cell anemia.
The heterozygous individual (AS) has normal
red blood cells and has amplified resistance to
malaria.
(a)
(b)
Sickle-Cell Anemia
Sickle-cell anemia converts (a) healthy red blood
cells into (b) a sickle-shaped cell.
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o Is homozygous for the
dominant, non-sickle-cell
allele (AA)
o Is heterozygous (AS)
Although it’s associated with
disease, the recessive sickle-cell
allele also confers resistance to
malaria
It is therefore most advantageous
for an individual to be
heterozygous for the sickle-cell
allele
o He/she will not have
sickle-cell anemia, as it is
a homozygous receissive
disorder
o He/she will have
amplified resistance to
malaria
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An individual will have sicklecell anemia if he/she:
o Is homozygous for the
(recessive) sickle-cell
allele (SS)
An individual will not have
sickle-cell anemia if he/she:
This phenomenon, in which the
heterozygous allelic pattern is
most favorable, is called the
heterozygote advantage
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