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What's the difference between stabilizing selection and balancing selection?

What's the difference between stabilizing selection and balancing selection?


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I came across these terms in Darwin's "Origin of Species" and I wasn't sure what the difference is.


Usual meaning

Usually, Stabilizing selection is a concept that applies to a phenotypic trait while balancing selection is a concept that applies to a given locus.

Balancing selection can either be due to negative-frequency dependence selection or due to overdominance (=heterozygous advantage at a single locus).

What Darwin may have meant

Because Darwin didn't know about genes, he was necessarily not using the term balancing selection as I am using it. I can think of several more or less related pattern of selection that might fall withing the definitions for "stabilizing selection" and/or "balancing selection". Maybe he meant "frequency-dependent selection", "positive selection for an intermediate trait", "varying selection through time (temporally heterogenous environment)" and "varying selection through space (spatially heterogenous environment)". Eventually again, that might mean "positively correlated traits that undergo opposite selection pressure (or the opposite)" but that would definitely be surprising.


How does balancing selection preserve genetic variation?

In respect to this, does balancing selection maintain genetic variation?

Balancing selection, on the other hand, brings the favored allele to an intermediate equilibrium, where it is maintained as a genetic polymorphism that potentially increases the variability in the genomic region nearby the selected locus.

Secondly, how does Diploidy preserve genetic variation? It allows recessive alleles that may not be favored in the current environment to be preserved in the gene pool by propagation in heterozygotes.

Similarly, what natural selection maintains genetic variation?

Finally, several forms of natural selection act to maintain genetic variation rather than to eliminate it. These include balancing selection, frequency-dependent selection, and changing patterns of natural selection over time and space.

What is the difference between balancing and stabilizing selection?

1 Answer. Usually, Stabilizing selection is a concept that applies to a phenotypic trait while balancing selection is a concept that applies to a given locus. Balancing selection can either be due to negative-frequency dependence selection or due to overdominance (=heterozygous advantage at a single locus).


What is the difference between directional selection and stabilizing selection?

Each type of selection contains the same principles, but is slightly different. Disruptive selection favors both extreme phenotypes, different from one extreme in directional selection. Stabilizing selection favors the middle phenotype, causing the decline in variation in a population over time.

One may also ask, what does directional selection mean? Directional selection is a type of natural selection in which the phenotype (the observable characteristics) of the species tends toward one extreme rather the mean phenotype or the opposite extreme phenotype.

Also Know, what is an example of stabilizing selection?

Stabilizing selection in evolution is a type of natural selection that favors the average individuals in a population. Classic examples of traits that resulted from stabilizing selection include human birth weight, number of offspring, camouflage coat color, and cactus spine density.


What are examples of stabilizing selection?

Click to read further detail. Also know, what is an example of disruptive selection?

Disruptive Selection Examples: Color If an environment has extremes, those who don't blend into either will be eaten the most quickly, whether they're moths, oysters, toads, birds or another animal. Peppered moths: One of the most studied examples of disruptive selection is the case of ?London's peppered moths.

One may also ask, what is a common cause of stabilizing selection? Common Causes of Stabilizing Selection Some of the most common forms of selection are from predation, resource allocation, coloration of the environment, food type, and a wide variety of other forces.

Additionally, what is meant by stabilizing 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 means that most common phenotype in the population is selected for and continues to dominate in future generations.


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An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females blue males are medium-sized and form strong pair bonds with their mates and yellow males are the smallest and look a bit like female, allowing them to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. The big, strong orange males can fight off the blue males to mate with the blue&rsquos pair-bonded females the blue males are successful at guarding their mates against yellow sneaker males and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males.

Figure (PageIndex<1>): Frequency-dependent selection in side-blotched lizards: A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.

In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes. In one generation, orange might be predominant and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for.Finally, when blue males become common, orange males will once again be favored.

An example of negative frequency-dependent selection can also be seen in the interaction between the human immune system and various infectious microbes such as pathogenic bacteria or viruses. As a particular human population is infected by a common strain of microbe, the majority of individuals in the population become immune to it. This then selects for rarer strains of the microbe which can still infect the population because of genome mutations these strains have greater evolutionary fitness because they are less common.


Frequency-dependent selection

The second important mechanism by which natural selection can preserve two or more phenotypic forms is known as frequency-dependent selection. Frequency-dependent selection is a form of selection in which the relative fitness of a specific phenotype declines if the frequency of that phenotype becomes too high. An example of this type of selection is between parasites and their hosts. An example follows: suppose that a certain parasite can recognize one of two receptors in its host, receptor Alpha or receptor Beta , if many parasites with receptor Alpha exist then hosts with receptor Beta will be selected for, and this will subsequently increase the selective pressure on parasites which use receptor Beta and this relationship will continue rocking back and forth.

Frequency-dependent selection has been observed in the banding and colour polymorphism in the European land snails, Cepaea nemoralis, where thrushes preferentially predate the most common morph. Frequency-dependent selection also appears in the form of mate preference, a type of sexual selection.


What do stabilizing selection and directional selection have in common?

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.

Subsequently, question is, what causes directional selection? Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

In respect to this, what is the difference between stabilizing directional and disruptive selection?

Disruptive selection favors both extreme phenotypes, different from one extreme in directional selection. Stabilizing selection favors the middle phenotype, causing the decline in variation in a population over time.

What is stabilizing selection example?

Stabilizing selection in evolution is a type of natural selection that favors the average individuals in a population. Classic examples of traits that resulted from stabilizing selection include human birth weight, number of offspring, camouflage coat color, and cactus spine density.


Notes on the Types of Natural Selection of Evolution (with Examples)

Selection is the process by which those organisms which appear physically, physiologi­cally and behaviourally better adapted to the environment survive and reproduce those organisms not so well adapted either fail to reproduce or die.

Image Courtesy : upload.wikimedia.org/wikipedia/commons/c/cc/Natural_Tunnel_State_Park.jpg

The former organisms pass on their successful characters to the next generation, whereas the latter do not. Selection depends upon the existence of phenotypic variation within the population and is part of the mechanism by which a species adapts to its environment.

A population has three types of individuals on the basis of their size average-sized, large-sized and small-sized. There are three types of selection process occurring in natural and artificial populations and they are described as stabilising, directional and disruptive.

1. Stabilising Selection (Balancing Selection):

This type of selection favours average sized individuals while eliminates small sized individuals. It reduces variation and hence does not promote evolutionary change. However, it maintains the mean value from genera­tion to generation. If we draw a graphical curve of population, it is bell-shaped.

Example:

It occurs in all populations and tends to eliminate extremes from the popula­tion, e.g., there is an optimum wing length for a hawk of a particular size with a certain mode of life in a given environment. Stabilising selection, operating through differences in breeding potential, will eliminate those hawks with wing spans larger or smaller than this optimum length.

2. Directional Selection (Progressive Selection):

In this selection, the population changes towards one particular direction. It means this type of selection favours small or large-sized individuals and more individuals of that type will be present in next generation. The mean size of the population changes.

Examples:

Evolution of DDT resistant mosquitoes, industrial melanism in peppered moth and evolution of giraffe.

3. Disruptive Selection (Diversifying Selection):

This type of selection favours both small-sized and large-sized individuals. It eliminates most of members with mean expression, so produces two peaks in the distribution of the trait that may lead to development of two different populations. This kind of selection is opposite of stabilizing selection and is rare in nature but is very important in bringing about evolutionary change.

Example:

Stebbins and his co-workers studied an example of disruptive selection in a population of sunflowers in the Sacramento Valley of California over a period of 12 years. In the beginning the genetically variable population of these sunflowers was a hybrid between two species. After five years this population had split into two subpopulations separated by a grassy area.

One of these subpopulations occupied a relative dry site and other occupied comparatively wet site. During the next seven years the size of the population fluctuated greatly in response to differences in rainfall, but the differences between the two subpopu­lations were maintained.

Examples of Natural Selection:

1. Industrial Melanism:

It is an adaptation where the moths living in the industrial areas developed melanin pigments to match their body to the tree trunks. The problem of industrial melanism in moths has been originally studied by R.A. Fischer and E.B. Ford and in recent times, by H.B.D. Kettlewell.

The occurrence of industrial melanism is closely associated with the progress of the industrial revolution in Great Britain, during the nineteenth century. It has occurred in several species of mothes. Of these, peppered moth (Biston betularia) is the most intensely studied.

Industrial melanism can be written briefly as follows.

(i) The peppered moth existed in two strains (forms): light coloured (white) and melanic (black).

(ii) In the past, bark of trees was covered by whitish lichens, so white moths escaped unnoticed from predatory birds.

(iii) After industrialisation barks got covered by smoke, so the white moths were selectively picked up by birds.

(iv) But black moths escaped unnoticed so they managed to survive resulting in more population of black moths and less population of white moths.

Thus industrial melanism supports evolution by natural selection.

2. Resistance of insects to Pesticides:

The DDT, which came to use in later 1945, was thought to be an effective insecticide against household pests, such as mosquitoes, houseflies, body lice, etc. But, within two to three years of the introduction of this insec­ticide, new DDT resistant mosquitoes appeared in the population. These mutant strains, which are resistant to DDT, soon became well established in the population, and to a great extent, replaced the original DDT-sensitive mosquitoes.

3. Antibiotic Resistance in Bacteria:

This is also true for disease causing bacteria against which we use antibiotics or drugs to kill these bacteria. When a bacterial population encounters a particular antibiotic, those sensitive to it die. But some bacteria having muta­tions become resistant to the antibiotic. Such resistant bacteria survive and multiply quickly as the competing bacteria have died.

Soon the resistance providing genes become widespread and entire bacterial population becomes resistant.

Genetic Basis of Adaptation the Lederberg Replica Plat­ing Experiment to illustrate Role of Natural Selection (Fig. 7.54):

By an Experiment Joshua Lederberg and Esther Lederberg were able to show that there are mutations which are actually pre-adaptive. Generally bacteria are cultivated by plating dilute suspensions bacterial cells on semi-solid agar plates containing complete medium with antibiotic like Penicillin. After some period colonies appear on the agar plates. Each of these colonies develops from a single bacterial cell by mitotic cell divisions. Lederberg inoculated bacteria on an agar plate and obtained a plate with several bacterial colonies. This plate is called as ‘master plate’.

They then formed several replicas from this master plate. For this, they took a sterile velvet disc, mounted on a wooden block, which was gently pressed on the master plate. Some of the bacteria cells from each colony sticked to the velvet cloth. By pressing this velvet on new agar plates of minimal medium, they were able to obtain exact replicas of master plate.

This was due to the fact that the bacterial cells were transferred from one plate to the other by the velvet. After that they tried to make replicas on the agar plates of minimal medium containing an antibiotic penicillin, the replica colonies were not formed. The new colonies that did grow were naturally resistant to streptomycin/penicillin.

The new colonies that did not grow were sensitive colonies. Therefore, there was an adaptation in some bacterial cells to grow on a medium containing the antibiotic (penicillin). This proved that mutations had occurred before bacteria were exposed to penicillin.

4. Sickle Cell Anaemia:

One of the best examples has been discovered in the human populations, inhabiting in tropical and subtropical Africa. The sickle cell gene produces a variant form of the protein haemoglobin, which differs from the normal haemoglobin by a single amino acid. In people, homozygous for this abnormal haemoglobin, the red blood cells (RBCs) become sickle-shaped, and this condition is described as sickle cell anaemia.

The people affected by this disease usually die before reproductive age, due to a severe haemolytic anaemia. In-spite of its disadvantageous nature, the gene has a high frequency in some parts of Africa, where malaria is also in high frequency. Subsequently, it has been discovered that the heterozygotes for the sickle cell trait are exceptionally resistant to malaria.

Thus in some parts of Africa, people homozygous for the normal gene tend to die of malaria, and those homozygous for sickle cell anaemia tend to die of severe anaemia while the heterozygous individuals survive and have the selective advantage over either of homozygotes. Sickle cell anaemia is caused by the substitution of glutamic acid by valine at sixth position of beta chain of haemoglobin.

5. Glucose 6-Phosphate Dehydrogenase Deficiency (G-6-PD):

It occurs as inborn error of metabolism in some persons. It is also called favism because beans cause haemolysis in the patients. Antimalarial drugs like primaquin causes haemolysis in such persons. The haemolysis is due to production of H202 which is not removed because of Glucose 6-PD deficiency and the result is lack of NADPH2. Malaria parasite cannot complete schizogony in Glucose 6-PD deficient patients due to premature death of RBCs.

6. Genetic Polymorphism:

Polymorphism plays a significant role in the process of natural selection. It is defined as the existence of two or more forms of the same species within the same population and can apply to biochemical, morphological and behavioural characteristics. There are two forms of polymorphism Balanced polymorphism and Tran­sient polymorphism.

Balanced Polymorphism:

This occurs when different forms coexist in the same population in a stable environment. It is illustrated most clearly by the existence of the two sexes in animals and plants. The genotypic frequencies of the various forms exhibit equi­librium since each form has a selective advantage of equal intensity. In humans, the existence of the А, В, AB and О blood groups are examples of balanced polymorphism.

Whilst the genotypic frequencies within different populations may vary, they remain constant from generation to generation within that population. This is because none of them has a selective advantage over the other.

Statistics reveal that white men of blood group О have a greater life expectancy than those of other blood groups, but, interestingly, they also have an increased risk of developing a duodenal ulcer which may perforate and lead to death. Red- green colour blindness in humans is another example of polymorphism, as is the existence of workers, drones and queens in social insects and pin-eyed and thrum-eyed forms in primroses.

Transient Polymorphism:

This arises when different forms or morphs, exist in a population undergoing a strong selection pressure. The frequency of the phenotypic appear­ance of each form is determined by the intensity of the selection pressure, such as the melanic and non-melanic forms of the peppered moth. Transient polymorphism usually applies in situations where one form is gradually being replaced by another.


Additional file 1: Table S1.

List of C. rubella accessions included in this study. Table S2. Summary of SNPs called from populations of each species. Table S4. Primers for PCR amplification and sequencing. Table S5. Statistics of the SNPs at CpG and non-CpG sites in the genic regions of 16,014 orthologous genes. Table S6. Simulation results for different demographic models. Table S7. Number of homologous genes for each of the confirmed genes with trans-species polymorphism signals in green plants. Table S10. Correlation between the structure and allelic type in 80 A. thaliana samples for each of the five genes under balancing selection. (DOCX 63 kb)

Additional file 2: Table S3.

Demographic inference results from fastsimcoal2. (XLSX 15 kb)

Additional file 3: Figure S1.

Allelic trees across the two species based on the 100-bp window around the TSP sites for each of the five genes under balancing selection. All A. thaliana accessions are colored in red and numbered according to the accessions listed on the 1001 Genomes site (http://1001genomes.org/projects/MPICao2010/index.html) see Additional file 5: Table S8C for details. All C. rubella accessions are shown in black and numbered according to Additional file 1: Table S1. Figure S2. Distribution of the (A) nucleotide diversity (π) and (B) MAF values of the simulated neutral sequences of 100 bp under the estimated model in each species. Triangles in different colors in (A) indicate the average values for all qualified windows in the five genes. See Table 1 for the details of each site (labeled TSP-1 to TSP-10). Figure S3. Cross-validation errors for various numbers of clusters (K) in an ADMIXTURE analysis. Figure S4. Geographic distributions of samples of different allelic types for the four genes under long-term balancing selection excluding AT5G38460. (DOCX 5905 kb)

Additional file 4:

Text S1. Coalescence-based simulation to support the candidate TSPs. (DOCX 45 kb)

Additional file 5: Table S8.

Ecological analysis: source data and results. A. The 48 ecological factors used in the analysis and the divergence of the two types of samples for each gene with regard to these factors. In each column of the four genes, the FDR-corrected P values are listed, and the significant ones are highlighted in red. B. Sample size distribution based on the haplotype phasing for each gene before and after thinning of the 1135 genomes. C. Sample information and classification for each gene. “-” indicates that the corresponding sample is not phased properly in the corresponding gene. D. The ecological significance of the four genes. (XLSX 52 kb)

Additional file 6: Table S9.

Structure of the 80 A. thaliana samples. The values for Group 1 and Group 2 indicate the corresponding ancestral fraction, respectively. (XLSX 12 kb)


Watch the video: Balancing Selection (September 2022).


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