1: directional selection: a single extreme phenotype favoured. 2, stabilizing selection: intermediate favoured over extremes. 3: disruptive selection: extremes favoured over intermediate. X-axis: phenotypic trait Y-axis: number of organisms Group A: original population Group B: after selection
Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait
value. This is thought to be the most common mechanism of action for
natural selection because most traits do not appear to change
drastically over time.
Stabilizing selection commonly uses negative selection (a.k.a.
purifying selection) to select against extreme values of the character.
Stabilizing selection is the opposite of disruptive selection.
Instead of favoring individuals with extreme phenotypes, it favors the
intermediate variants. Stabilizing selection tends to remove the more
severe phenotypes, resulting in the reproductive success of the norm or average phenotypes. This means that most common phenotype in the population is selected for and continues to dominate in future generations.
Depending
on the environmental conditions, a wolf may have an advantage over
wolves with other variations of fur color. Wolves with fur colors that
do not camouflage appropriately with the environmental conditions will
be spotted more easily by the deer, resulting in them not being able to
sneak up on the deer (leading to natural selection).
History
The Russian evolutionary biologist Ivan Schmalhausen
founded the theory of stabilizing selection, publishing a paper in
Russian titled "Stabilizing selection and its place among factors of
evolution" in 1941 and a monograph "Factors of Evolution: The Theory of
Stabilizing Selection" in 1945.
Influence on population structure
Stabilizing
selection causes the narrowing of the phenotypes seen in a population.
This is because the extreme phenotypes are selected against, causing
reduced survival in organisms with those traits. This results in a
population consisting of fewer phenotypes, with most traits representing
the mean value of the population. This narrowing of phenotypes causes a
reduction in genetic diversity in a population.
Maintaining genetic variation is essential for the survival of a
population because it is what allows them to evolve over time. In order
for a population to adapt to changing environmental conditions they must
have enough genetic diversity to select for new traits as they become
favorable.
Analyzing stabilizing selection
There
are four primary types of data used to quantify stabilizing selection
in a population. The first type of data is an estimation of fitness of
different phenotypes within a single generation. Quantifying fitness in a
single generation creates predictions for the expected fate of
selection. The second type of data is changes in allelic frequencies or
phenotypes across different generations. This allows quantification of
change in prevalence of a certain phenotype, indicating the type of
selection. The third type of data is differences in allelic frequencies
across space. This compares selection occurring in different populations
and environmental conditions. The fourth type of data is DNA sequences
from the genes contributing to observes phenotypic differences. The
combination of these four types of data allow population studies that
can identify the type of selection occurring and quantify the extent of
selection.
However, a meta-analysis of studies that measured selection in
the wild failed to find an overall trend for stabilizing selection.
The reason can be that methods for detecting stabilizing selection are
complex. They can involve studying the changes that causes natural
selection in the mean and variance of the trait, or measuring fitness
for a range of different phenotypes
under natural conditions and examining the relationship between these
fitness measurements and the trait value, but analysis and
interpretation of the results is not straightforward.
Examples
The
most common form of stabilizing selection is based on phenotypes of a
population. In phenotype based stabilizing selection, the mean value of a
phenotype is selected for, resulting a decrease in the phenotypic
variation found in a population.
Humans
Stabilizing selection is the most common form of nonlinear selection (non-directional) in humans.
There are few examples of genes with direct evidence of stabilizing
selection in humans. However, most quantitative traits (height,
birthweight, schizophrenia) are thought to be under stabilizing
selection, due to their polygenicity and the distribution of the
phenotypes throughout human populations.
Birth Weight − A classic example of this is human birth weight.
Babies of low weight lose heat more quickly and get ill from infectious
diseases more easily, whereas babies of large body weight are more
difficult to deliver through the pelvis. Infants of a more medium weight
survive much more often. For the larger or smaller babies, the baby
mortality rate is much higher. The bell curve of the human population peaks at a birth weight that the newly born babies exhibit the minimum death rate.
Plants
Height
− Another example of a trait, that might be acted on by stabilizing
selection, is plant height. A plant that is too short may not be able to
compete with other plants for sunlight. However, extremely tall plants
may be more susceptible to wind damage. Combined, these two selection
pressures select to maintain plants of medium height. The number of
plants of medium height will increase while the numbers of short and
tall plants will decrease.
Cacti Spine Number − Desert populations of spiny cacti experience predation by peccaries,
which consume the fleshy part of the cactus. This can be prevented by
increasing the number of spines on the cactus. However, there is also a
selection pressure in the opposite direction because there is a
parasitic insect that will lay its eggs in spines if they are densely
populated. This means that in order to manage both of these selection
pressures the cacti experiences stabilizing selection to balance the
appropriate number of spines to survive these different threats.
Insects
Bicyclus anynana with wing eyespot, which experiences stabilizing selection to avoid predation.Butterfly's Winged Eyespots − The African butterfly Bicyclus anynana exhibits stabilizing selection with its wing eyespots.
It has been suggested that the circular eyespots positioned on the
wings are favoured functionally compared to other shapes and sizes.
Gall Size − The Eurosta solidaginis fly lays its eggs on the tip of plants, which then encase the larvae in a protective gall.
The size of this gall is under stabilizing selection, as determined by
predation. These larvae are under threat from parasitic wasps, which lay
a single egg in galls containing the flies. The single wasp offspring
then consumes the fly larvae to survive. Therefore, a larger gall is
favored to allow more places for larvae to hide from the wasp. However,
larger galls attract a different type of predation from birds, as they
can penetrate large galls with their beak. Therefore, the optimal gall
is moderately sized in order to avoid predation from both birds and
wasps.
Birds
Clutch
Size − The number of eggs laid by a female bird (clutch size) is
typically under stabilizing selection. This is because the female must
lay as many eggs as possible to maximize the number of offspring.
However, they can only lay as many eggs as they can support with their
own resources. Laying too many eggs could expend all of the energy of
the mother bird causing her to die and the death of the chicks.
Additionally, once the eggs hatch the mother must be able to obtain
enough resources to keep all of the chicks alive. Therefore, the mother
typically lays a moderate amount of eggs in order to increase offspring
survival and maximize the number of offspring.
Mammals
The Siberian husky experiences stabilizing selection in terms of their leg muscles, allowing them to be strong but light.The
Siberian husky experiences stabilizing selection in terms of their leg
muscles. These dogs have to have enough muscle in order to pull sleds
and move quickly. However, they also must be light enough to stay on top
of the snow. This means that the leg muscles of the husky are most fit
when they are moderately sized, to balance their strength and their
weight.
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