Diversifying selection example

Diversifying selection example DEFAULT

Types of Natural Selection: Disruptive Selection

Disruptive selectionis a type of natural selection that selects against the average individual in a population. The makeup of this type of population would show phenotypes (individuals with groups of traits) of both extremes but have very few individuals in the middle. Disruptive selection is the rarest of the three types of natural selection and can lead to the deviation in a species line.

Basically, it comes down to the individuals in the group who get to mate—who survive best. They are the ones who have traits on the extreme ends of the spectrum. The individual with just middle-of-the-road characteristics is not as successful at survival and/or breeding to further pass on "average" genes. In contrast, population functions in stabilizing selection mode when the intermediate individuals are the most populous. Disruptive selection occurs in times of change, such as habitat change or change in resources availability.

Disruptive Selection and Speciation

The bell curve is not typical in shape when exhibiting disruptive selection. In fact, it looks almost like two separate bell curves. There are peaks at both extremes and a very deep valley in the middle, where the average individuals are represented. Disruptive selection can lead to speciation, with two or more different species forming and the middle-of-the-road individuals being wiped out. Because of this, it's also called "diversifying selection," and it drives evolution.

Disruptive selection happens in large populations with lots of pressure for the individuals to find advantages or niches as they compete with each other for food to survive and/or partners to pass on their lineage.

Like directional selection, disruptive selection can be influenced by human interaction. Environmental pollution can drive disruptive selection to choose different colorings in animals for survival.

Disruptive Selection Examples: Color

Color, in regards to camouflage, serves as a useful example in many different kinds of species, because those individuals that can hide from predators the most effectively will live the longest. 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. In rural areas, the peppered moths were almost all a very light color. However, these same moths were very dark in color in industrial areas. Very few medium-colored moths were seen in either location. The darker-colored moths survived predators in the industrial areas by blending in with the polluted surroundings. The lighter moths were seen easily by predators in industrial areas and were eaten. The opposite happened in rural areas. The medium-colored moths were easily seen in both locations and were therefore very few of them left after disruptive selection.​​

Oysters: Light- and dark-colored oysters could also have a camouflage advantage as opposed to their medium-colored relatives. Light-colored oysters would blend into the rocks in the shallows, and the darkest would blend better into the shadows. The ones in the intermediate range would show up against either backdrop, offering those oysters no advantage and make them easier prey. Thus, with fewer of the medium individuals surviving to reproduce, the population eventually has more oysters colored to either extreme of the spectrum.

Disruptive Selection Examples: Feeding Ability

Evolution and speciation isn't all a straight line. Often there are multiple pressures on a group of individuals, or a drought pressure, for example, that is just temporary, so the intermediate individuals don't completely disappear or don't disappear right away. Timeframes in evolution are long. All types of diverging species can coexist if there are enough resources for them all. Specialization in food sources among a population might occur in fits and starts, only when there is some pressure on supply.

Mexican spadefoot toad tadpoles: Spadefoot tadpoles have higher populations in the extremes of their shape, with each type having a more dominant eating pattern. The more omnivorous individuals are round-bodied, and the more carnivorous are narrow-bodied. The intermediate types are smaller (less well-fed) than those at either extreme of body shape and eating habit. A study found that those at the extremes had additional, alternate food resources that the intermediates didn't. The more omnivorous ones fed more effectively on pond detritus, and the more carnivorous ones were better at feeding on shrimps. Intermediate types competed with each other for food, resulting in individuals with ability on the extremes to eat more and grow faster and better.

Darwin's finches on the Galapagos: Fifteen different species developed from a common ancestor, which existed 2 million years ago. They differ in beak style, body size, feeding behavior, and song. Multiple types of beaks have adapted to different food resources, over time. In the case of three species on Santa Cruz Island, ground finches eat more seeds and some arthropods, tree finches eat more fruits and arthropods, vegetarian finches feed on leaves and fruit, and warblers typically eat more arthropods. When food is abundant, what they eat overlaps. When it's not, this specialization, the ability to eat a certain type of food better than other species, helps them survive.

Sours: https://www.thoughtco.com/what-is-disruptive-selection-1224582

Natural Selection and Adaptive Evolution

Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population.

Learning Objectives

Explain how natural selection leads to adaptive evolution

Key Takeaways

Key Points

  • Natural selection increases or decreases biological traits within a population, thereby selecting for individuals with greater evolutionary fitness.
  • An individual with a high evolutionary fitness will provide more beneficial contributions to the gene pool of the next generation.
  • Relative fitness, which compares an organism’s fitness to the others in the population, allows researchers to establish how a population may evolve by determining which individuals are contributing additional offspring to the next generation.
  • Stabilizing selection, directional selection, diversifying selection, frequency -dependent selection, and sexual selection all contribute to the way natural selection can affect variation within a population.

Key Terms

  • natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce
  • fecundity: number, rate, or capacity of offspring production
  • Darwinian fitness: the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype

An Introduction to Adaptive Evolution

Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and, thus, increasing their frequency in the population, while selecting against deleterious alleles and, thereby, decreasing their frequency. This process is known as adaptive evolution. Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce ( fecundity ), but if that same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary fitness (or Darwinian fitness).

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Adaptive evolution in finches: Through natural selection, a population of finches evolved into three separate species by adapting to several difference selection pressures. Each of the three modern finches has a beak adapted to its life history and diet.

Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing additional offspring to the next generation and, thus, how the population might evolve.

There are several ways selection can affect population variation:

  • stabilizing selection
  • directional selection
  • diversifying selection
  • frequency-dependent selection
  • sexual selection

As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate. In the end, natural selection cannot produce perfect organisms from scratch, it can only generate populations that are better adapted to survive and successfully reproduce in their environments through the aforementioned selections.

Galápagos with David Attenborough: Two hundred years after Charles Darwin set foot on the shores of the Galápagos Islands, David Attenborough travels to this wild and mysterious archipelago. Amongst the flora and fauna of these enchanted volcanic islands, Darwin formulated his groundbreaking theories on evolution. Journey with Attenborough to explore how life on the islands has continued to evolve in biological isolation, and how the ever-changing volcanic landscape has given birth to species and sub-species that exist nowhere else in the world.

Stabilizing, Directional, and Diversifying Selection

Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.

Learning Objectives

Contrast stabilizing selection, directional selection, and diversifying selection.

Key Takeaways

Key Points

  • Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations.
  • In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.
  • Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
  • In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.

Key Terms

  • directional selection: a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction
  • disruptive selection: (or diversifying selection) a mode of natural selection in which extreme values for a trait are favored over intermediate values
  • stabilizing selection: a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population’s genetic variance will decrease.

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Stabilizing selection: Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

image

Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

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The Evolution of the Peppered Moth: Typica and carbonaria morphs resting on the same tree.The light-colored typica (below the bark’s scar) is nearly invisible on this pollution-free tree, camouflaging it from predators.

Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

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Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection

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Types of natural selection: Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

Frequency-Dependent Selection

In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.

Learning Objectives

Describe frequency-dependent selection

Key Takeaways

Key Points

  • Negative frequency -dependent selection selects for rare phenotypes in a population and increases a population’s genetic variance.
  • Positive frequency-dependent selection selects for common phenotypes in a population and decreases genetic variance.
  • In the example of male side-blotched lizards, populations of each color pattern increase or decrease at various stages depending on their frequency; this ensures that both common and rare phenotypes continue to be cyclically present.
  • Infectious agents such as microbes can exhibit negative frequency-dependent selection; as a host population becomes immune to a common strain of the microbe, less common strains of the microbe are automatically favored.
  • Variation in color pattern mimicry by the scarlet kingsnake is dependent on the prevalence of the eastern coral snake, the model for this mimicry, in a particular geographical region. The more prevalent the coral snake is in a region, the more common and variable the scarlet kingsnake’s color pattern will be, making this an example of positive frequency-dependent selection.

Key Terms

  • frequency-dependent selection: the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population
  • polygynous: having more than one female as mate

Frequency-dependent Selection

Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).

Negative Frequency-dependent Selection

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’s 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.

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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.

Positive Frequency-dependent Selection

An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. The scarlet kingsnake, a harmless species, mimics the coloration of the eastern coral snake, a venomous species typically found in the same geographical region. Predators learn to avoid both species of snake due to the similar coloration, and as a result the scarlet kingsnake becomes more common, and its coloration phenotype becomes more variable due to relaxed selection. This phenotype is therefore more “fit” as the population of species that possess it (both dangerous and harmless) becomes more numerous. In geographic areas where the coral snake is less common, the pattern becomes less advantageous to the kingsnake, and much less variable in its expression, presumably because predators in these regions are not “educated” to avoid the pattern.

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Lampropeltis elapsoides, the scarlet kingsnake: The scarlet kingsnake mimics the coloration of the poisonous eastern coral snake. Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.

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Micrurus fulvius, the eastern coral snake: The eastern coral snake is poisonous.

Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.

Sexual Selection

Sexual selection, the selection pressure on males and females to obtain matings, can result in traits designed to maximize sexual success.

Learning Objectives

Discuss the effects of sexual dimorphism on the reproductive potential of an organism

Key Takeaways

Key Points

  • Sexual selection often results in the development of secondary sexual characteristics, which help to maximize a species ‘ reproductive success, but do not provide any survival benefits.
  • The handicap principle states that only the best males survive the risks from traits that may actually be detrimental to a species; therefore, they are more fit as mating partners.
  • In the good genes hypothesis, females will choose males that show off impressive traits to ensure they pass on genetic superiority to their offspring.
  • Sexual dimorphisms, obvious morphological differences between the sexes of a species, arise when there is more variance in the reproductive success of either males or females.

Key Terms

  • sexual dimorphism: a physical difference between male and female individuals of the same species
  • sexual selection: a type of natural selection, where members of the sexes acquire distinct forms because members choose mates with particular features or because competition for mates with certain traits succeed
  • handicap principle: a theory that suggests that animals of greater biological fitness signal this status through a behavior or morphology that effectively lowers their chances of survival

Sexual Selection

The selection pressures on males and females to obtain matings is known as sexual selection. Sexual selection takes two major forms: intersexual selection (also known as ‘mate choice’ or ‘female choice’) in which males compete with each other to be chosen by females; and intrasexual selection (also known as ‘male–male competition’) in which members of the less limited sex (typically males) compete aggressively among themselves for access to the limiting sex. The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision.

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Sexual selection in elk: This male elk has large antlers to compete with rival males for available females (intrasexual competition).Tn addition, the many points on his antlers represent health and longevity, and therefore he may be more desirable to females (intersexual selection).

Sexual Dimorphism

Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. Males are often larger, for example, and display many elaborate colors and adornments, such as the peacock’s tail, while females tend to be smaller and duller in decoration. These differences are called sexual dimorphisms and arise from the variation in male reproductive success.

Females almost always mate, while mating is not guaranteed for males. The bigger, stronger, or more decorated males usually obtain the vast majority of the total matings, while other males receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to obtain those matings, resulting in the evolution of bigger body size and elaborate ornaments in order to increase their chances of mating. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.

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Sexual dimorphism: Morphological differences between males and females of the same species is known as sexual dimorphism.These differences can be observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and (c) wood ducks.

Sexual dimorphism varies widely among species; some species are even sex-role reversed. In such cases, females tend to have a greater variation in their reproductive success than males and are, correspondingly, selected for the bigger body size and elaborate traits usually characteristic of males.

The Handicap Principle

Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual’s survival, even though they maximize its reproductive success. For example, while the male peacock’s tail is beautiful and the male with the largest, most colorful tail will more probably win the female, it is not a practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place. Because large tails carry risk, only the best males survive that risk and therefore the bigger the tail, the more fit the male. This idea is known as the handicap principle.

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A male bird of paradise: This male bird of paradise carries an extremely long tail as the result of sexual selection.The tail is flamboyant and detrimental to the bird’s own survival, but it increases his reproductive success.This may be an example of the handicap principle.

The Good Genes Hypothesis

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be so selective because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.

BBC Planet Earth – Birds of Paradise mating dance: Extraordinary Courtship displays from these weird and wonderful creatures. From episode 1 “Pole to Pole”. This is an example of the extreme behaviors that arise from intense sexual selection pressure.

No Perfect Organism

Natural selection cannot create novel, perfect species because it only selects on existing variations in a population.

Learning Objectives

Explain the limitations encountered in natural selection

Key Takeaways

Key Points

  • Natural selection is limited by a population ‘s existing genetic variation.
  • Natural selection is limited through linkage disequilibrium, where alleles that are physically proximate on the chromosome are passed on together at greater frequencies.
  • In a polymorphic population, two phenotypes may be maintained in the population despite the higher fitness of one morph if the intermediate phenotype is detrimental.
  • Evolution is not purposefully adaptive; it is the result of various selection forces working together to influence genetic and phenotypical variances within a population.

Key Terms

  • linkage disequilibrium: a non-random association of two or more alleles at two or more loci; normally caused by an interaction between genes
  • genetic hitchhiking: changes in the frequency of an allele because of linkage with a positively or negatively selected allele at another locus
  • polymorphism: the regular existence of two or more different genotypes within a given species or population

No Perfect Organism

Natural selection is a driving force in evolution and can generate populations that are adapted to survive and successfully reproduce in their environments. However, natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it cannot create anything from scratch. Therefore, the process of evolution is limited by a population’s existing genetic variance, the physical proximity of alleles, non-beneficial intermediate morphs in a polymorphic population, and non-adaptive evolutionary forces.

Natural Selection Acts on Individuals, not Alleles

Natural selection is also limited because it acts on the phenotypes of individuals, not alleles. Some alleles may be more likely to be passed on with alleles that confer a beneficial phenotype because of their physical proximity on the chromosomes. Alleles that are carried together are in linkage disequilibrium. When a neutral allele is linked to beneficial allele, consequently meaning that it has a selective advantage, the allele frequency can increase in the population through genetic hitchhiking (also called genetic draft).

Any given individual may carry some beneficial alleles and some unfavorable alleles. Natural selection acts on the net effect of these alleles and corresponding fitness of the phenotype. As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; similarly, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.

Polymorphism

Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency because the intermediate morph is detrimental.

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Polymorphism in the grove snail: Color and pattern morphs of the grove snail, Cepaea nemoralis.The polymorphism, when two or more different genotypes exist within a given species, in grove snails seems to have several causes, including predation by thrushes.

For example, consider a hypothetical population of mice that live in the desert. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of black rock. The dark-colored mice may be more fit than the light-colored mice, and according to the principles of natural selection the frequency of light-colored mice is expected to decrease over time. However, the intermediate phenotype of a medium-colored coat is very bad for the mice: these cannot blend in with either the sand or the rock and will more vulnerable to predators. As a result, the frequency of a dark-colored mice would not increase because the intermediate morphs are less fit than either light-colored or dark-colored mice. This a common example of disruptive selection.

Not all Evolution is Adaptive

Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and gene flow, often do the opposite by introducing deleterious alleles to the population’s gene pool. Evolution has no purpose. It is not changing a population into a preconceived ideal. It is simply the sum of various forces and their influence on the genetic and phenotypic variance of a population.

Sours: https://courses.lumenlearning.com/boundless-biology/chapter/adaptive-evolution/
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Stabilizing, Directional, and Diversifying Selection

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population's genetic variance will decrease.

Stabilizing selection

Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

Directional selection

Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population's genetic variance toward the new, fit phenotype.

The Evolution of the Peppered Moth

Typica and carbonaria morphs resting on the same tree.The light-colored typica (below the bark's scar) is nearly invisible on this pollution-free tree, camouflaging it from predators.

Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male's territory. In this case, both the alpha males and the "sneaking" males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

Diversifying (or disruptive) selection

Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection

Types of natural selection

Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

Sours: http://kolibri.teacherinabox.org.au/modules/en-boundless/www.boundless.com/biology/textbooks/boundless-biology-textbook/the-evolution-of-populations-19/adaptive-evolution-132/stabilizing-directional-and-diversifying-selection-535-11742/index.html

Disruptive selection

These charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the amount of organisms. Group A is the original population and Group B is the population after selection. Graph 1 shows directional selection, in which a single extreme phenotype is favored. Graph 2 depicts stabilizing selection, where the intermediate phenotype is favored over the extreme traits. Graph 3 shows disruptive selection, in which the extreme phenotypes are favored over the intermediate.
A chart showing three types of selection

Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.[1][2]

Overview[edit]

Natural selection is known to be one of the most important biological processes behind evolution. There are many variations of traits, and some cause greater or lesser reproductive success of the individual. The effect of selection is to promote certain alleles, traits, and individuals that have a higher chance to survive and reproduce in their specific environment. Since the environment has a carrying capacity, nature acts on this mode of selection on individuals to let only the most fit offspring survive and reproduce to their full potential. The more advantageous the trait is the more common it will become in the population. Disruptive selection is a specific type of natural selection that actively selects against the intermediate in a population, favoring both extremes of the spectrum.

Disruptive selection is inferred to oftentimes lead to sympatric speciation through a phyletic gradualism mode of evolution. Disruptive selection can be caused or influenced by multiple factors and also have multiple outcomes, in addition to speciation. Individuals within the same environment can develop a preference for extremes of a trait, against the intermediate. Selection can act on having divergent body morphologies in accessing food, such as beak and dental structure. It is seen that often this is more prevalent in environments where there is not a wide clinal range of resources, causing heterozygote disadvantage or selection against the average.

Niche partitioning allows for selection of differential patterns of resource usage, which can drive speciation. To the contrast, niche conservation pulls individuals toward ancestral ecological traits in an evolutionary tug-of-war. Also, nature tends to have a 'jump on the band wagon' perspective when something beneficial is found. This can lead to the opposite occurring with disruptive selection eventually selecting against the average; when everyone starts taking advantage of that resource it will become depleted and the extremes will be favored. Furthermore, gradualism is a more realistic view when looking at speciation as compared to punctuated equilibrium.

Disruptive selection can initially rapidly intensify divergence; this is because it is only manipulating alleles that already exist. Often it is not creating new ones by mutation which takes a long time. Usually complete reproductive isolation does not occur until many generations, but behavioral or morphological differences separate the species from reproducing generally. Furthermore, generally hybrids have reduced fitness which promotes reproductive isolation.[3][4][5][6][7][8][9][10][11]

Example[edit]

Suppose there is a population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur, represented by "B", and white fur, represented by "b". A rabbit in this population with a genotype of "BB" would have a phenotype of black fur, a genotype of "Bb" would have grey fur (a display of both black and white), and a genotype of "bb" would have white fur.

If this population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, the rabbits with black fur would be able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation.

As a consequence of this type of selective pressure, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not grey. This is an example of underdominance (heterozygote disadvantage) leading to disruptive selection.

Sympatric speciation[edit]

It is believed that disruptive selection is one of the main forces that drive sympatric speciation in natural populations.[12] The pathways that lead from disruptive selection to sympatric speciation seldom are prone to deviation; such speciation is a domino effect that depends on the consistency of each distinct variable. These pathways are the result of disruptive selection in intraspecific competition; it may cause reproductive isolation, and finally culminate in sympatric speciation.

It is important to keep in mind that disruptive selection does not always have to be based on intraspecific competition. It is also important to know that this type of natural selection is similar to the other ones. Where it is not the major factor, intraspecific competition can be discounted in assessing the operative aspects of the course of adaptation. For example, what may drive disruptive selection instead of intraspecific competition might be polymorphisms that lead to reproductive isolation, and thence to speciation.[13][14][15][16][12][17][18][19]

When disruptive selection is based on intraspecific competition, the resulting selection in turn promotes ecological niche diversification and polymorphisms. If multiple morphs (phenotypic forms) occupy different niches, such separation could be expected to promote reduced competition for resources. Disruptive selection is seen more often in high density populations rather than in low density populations because intraspecific competition tends to be more intense within higher density populations. This is because higher density populations often imply more competition for resources. The resulting competition drives polymorphisms to exploit different niches or changes in niches in order to avoid competition. If one morph has no need for resources used by another morph, then it is likely that neither would experience pressure to compete or interact, thereby supporting the persistence and possibly the intensification of the distinctness of the two morphs within the population.[20][21][22][23][24][25] This theory does not necessarily have a lot of supporting evidence in natural populations, but it has been seen many times in experimental situations using existing populations. These experiments further support that, under the right situations (as described above), this theory could prove to be true in nature.[16][19]

When intraspecific competition is not at work disruptive selection can still lead to sympatric speciation and it does this through maintaining polymorphisms. Once the polymorphisms are maintained in the population, if assortative mating is taking place, then this is one way that disruptive selection can lead in the direction of sympatric speciation.[14][16][17] If different morphs have different mating preferences then assortative mating can occur, especially if the polymorphic trait is a "magic trait", meaning a trait that is under ecological selection and in turn has a side effect on reproductive behavior. In a situation where the polymorphic trait is not a magic trait then there has to be some kind of fitness penalty for those individuals who do not mate assortatively and a mechanism that causes assortative mating has to evolve in the population. For example, if a species of butterflies develops two kinds of wing patterns, crucial to mimicry purposes in their preferred habitat, then mating between two butterflies of different wing patterns leads to an unfavorable heterozygote. Therefore, butterflies will tend to mate with others of the same wing pattern promoting increased fitness, eventually eliminating the heterozygote altogether. This unfavorable heterozygote generates pressure for a mechanism that cause assortative mating which will then lead to reproductive isolation due to the production of post-mating barriers.[26][27][28] It is actually fairly common to see sympatric speciation when disruptive selection is supporting two morphs, specifically when the phenotypic trait affects fitness rather than mate choice.[29]

In both situations, one where intraspecific competition is at work and the other where it is not, if all these factors are in place, they will lead to reproductive isolation, which can lead to sympatric speciation.[18][25][30]

Other outcomes[edit]

Significance[edit]

Disruptive selection is of particular significance in the history of evolutionary study, as it is involved in one of evolution's "cardinal cases", namely the finch populations observed by Darwin in the Galápagos. He observed that the species of finches were similar enough to ostensibly have been descended from a single species. However, they exhibited disruptive variation in beak size. This variation appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive.

While it is true that disruptive selection can lead to speciation, this is not as quick or straightforward of a process as other types of speciation or evolutionary change. This introduces the topic of gradualism, which is a slow but continuous accumulation of changes over long periods of time.[34] This is largely because the results of disruptive selection are less stable than the results of directional selection (directional selection favors individuals at only one end of the spectrum).

For example, let us take the mathematically straightforward yet biologically improbable case of the rabbits: Suppose directional selection were taking place. The field only has dark rocks in it, so the darker the rabbit, the more effectively it can hide from predators. Eventually there will be a lot of black rabbits in the population (hence many "B" alleles) and a lesser amount of grey rabbits (who contribute 50% chromosomes with "B" allele and 50% chromosomes with "b" allele to the population). There will be few white rabbits (not very many contributors of chromosomes with "b" allele to the population). This could eventually lead to a situation in which chromosomes with "b" allele die out, making black the only possible color for all subsequent rabbits. The reason for this is that there is nothing "boosting" the level of "b" chromosomes in the population. They can only go down, and eventually die out.

Consider now the case of disruptive selection. The result is equal numbers of black and white rabbits, and hence equal numbers of chromosomes with "B" or "b" allele, still floating around in that population. Every time a white rabbit mates with a black one, only gray rabbits results. So, in order for the results to "click", there needs to be a force causing white rabbits to choose other white rabbits, and black rabbits to choose other black ones. In the case of the finches, this "force" was geographic/niche isolation. This leads one to think that disruptive selection can't happen and is normally because of species being geographically isolated, directional selection or by stabilising selection.

See also[edit]

References[edit]

  1. ^Sinervo, Barry. 1997. Disruptive Selection [1]Archived 2010-06-24 at the Wayback Machine in Adaptation and Selection. 13 April 2010.
  2. ^Lemmon, Alan R. 2000. EvoTutor. Natural Selection: Modes of Selection [2]. 13 April 2010.
  3. ^Abrams, P.A., Leimar, O., Rueffler, C., Van Dooren, J.M. 2006. Disruptive selection and then what? Trends in Ecology & Evolution Vol. 21 Issue 5:238-245.
  4. ^Boam, T.B.; Thoday, J.M. (1959). "Effects of disruptive selection: Polymorphism and divergence without isolation". Heredity. 13 (2): 205–218. doi:10.1038/hdy.1959.23.
  5. ^Bolnick, D.I. (2004). "Can Intraspecific competition drive disruptive Selection? An experimental test in natural population of sticklebacks". Evolution. 58 (3): 608–618. doi:10.1111/j.0014-3820.2004.tb01683.x. PMID 15119444.
  6. ^Cook, L.M.; Grant, B.S.; Mallet, J.; Saccheri, I.J. (2012). "Selective bird predation on the peppered moth: the last experiment of Michael Majerus". Biology Letters. 8 (4): 609–612. doi:10.1098/rsbl.2011.1136. PMC 3391436. PMID 22319093.
  7. ^DeLeon, L.F.; Harrel, A.; Hendry, A.P.; Huber, S.K.; Podos, J. (2009). "Disruptive Selection in a Bimodal Population of Darwin's Finches". Proceedings: Biological Sciences. 276 (1657): 753–759. doi:10.1098/rspb.2008.1321. PMC 2660944. PMID 18986971.
  8. ^Kingsolver, J.G.; Pfenning, David W. (2007). "Patterns and Power of Phenotypic Selection in Nature". BioScience. 57 (7): 561–572. doi:10.1641/b570706.
  9. ^Rice, W.R.; Salt, G.W. (1988). "Speciation Via Disruptive Selection on Habitat Preference: Experimental Evidence". The American Naturalist. 131 (6): 911–917. doi:10.1086/284831.
  10. ^Seehausen, M. E.; Van Alphen, J.J.M. (1999). "Can sympatric speciation by disruptive sexual selection explain rapid evolution of cichlid diversity in Lake Victoria?". Ecology Letters. 2 (4): 262–271. doi:10.1046/j.1461-0248.1999.00082.x.
  11. ^Smith, T.B. (1993). "Disruptive selection and the genetic basis of bill size polymorphism in the African finch Pyrenestes". Letters to Nature. 363 (6430): 618–620. Bibcode:1993Natur.363..618S. doi:10.1038/363618a0. S2CID 4284118.
  12. ^ abSmith, J.M. (1966). "Sympatric speciation". The American Naturalist. 100 (916): 637–950. doi:10.1086/282457. JSTOR 2459301.
  13. ^ abMather, K. (March 1955). "Polymorphism as an outcome of disruptive selection". Evolution. 9 (1): 51–61. doi:10.2307/2405357. JSTOR 2405357.
  14. ^ abSmith, J.M. (July 1962). "Disruptive selection, polymorphism and sympatric speciation". Nature. 195 (4836): 60–62. Bibcode:1962Natur.195...60M. doi:10.1038/195060a0. S2CID 5802520.
  15. ^Thoday, J.M.; Gibson, J.B. (1970). "The probability of isolation by disruptive selection". The American Naturalist. 104 (937): 219–230. doi:10.1086/282656. JSTOR 2459154.
  16. ^ abcKondrashov, A.S.; Mina, M.V. (March 1986). "Sympatric speciation: when is it possible?". Biological Journal of the Linnean Society. 27 (3): 201–223. doi:10.1111/j.1095-8312.1986.tb01734.x.
  17. ^ abSharloo, W (1969). "Stable and disruptive selection on a mutant character in drosophila III polymorphism caused by a developmental switch mechanism". Genetics. 65 (4): 693–705. PMC 1212475. PMID 5518512.
  18. ^ abBolnick, D.I.; Fitzpatrick, B.M. (2007). "Sympatric speciation: models and empirical evidence". Annual Review of Ecology, Evolution, and Systematics. 38: 459–487. doi:10.1146/annurev.ecolsys.38.091206.095804.
  19. ^ abSvanback, R.; Bolnick, D.I. (2007). "Intraspecific competition drives increased resource use diversity within a natural population". Proc. R. Soc. B. 274 (1611): 839–844. doi:10.1098/rspb.2006.0198. PMC 2093969. PMID 17251094.
  20. ^Merrill, R.M.; et al. (1968). "Disruptive ecological selection on a mating cue". Proceedings of the Royal Society. 10 (1749): 1–8. doi:10.1098/rspb.2012.1968. PMC 3497240. PMID 23075843.
  21. ^Bolnick, D.I. (2007). "Can intraspecific competition drive disruptive selection? An experimental test in natural populations of stickleback". Evolution. 58 (3): 608–618. doi:10.1554/03-326. PMID 15119444. S2CID 16739680.
  22. ^Martin, R.A.; Pfennig, D.W. (2009). "Disruptive selection in natural populations: the roles of ecological specialization and resource competition". The American Naturalist. 174 (2): 268–281. doi:10.1086/600090. PMID 19527118.
  23. ^Alvarez, E.R. (2006). "Sympatric speciation as a byproduct of ecological adaptation in the Galician Littorina saxatilis hybrid zone". Journal of Molluscan Studies. 73: 1–10. doi:10.1093/mollus/eyl023.
  24. ^Martin, A. R.; Pfenning, D.W. (2012). "Widespread disruptive selection in the wild is associated with intense resource competition". BMC Evolutionary Biology. 12: 1–13. doi:10.1186/1471-2148-12-136. PMC 3432600. PMID 22857143.
  25. ^ abRice, W.R. (1984). "Disruptive selection on habitat preference and evolution of reproductive isolation: a simulation study". Evolution. 38 (6): 1251–1260. doi:10.2307/2408632. JSTOR 2408632. PMID 28563785.
  26. ^Naisbit, R.E.; et al. (2001). "Disruptive sexual selection against hybrids contributes to speciation between Heliconius cyndo and Heliconius melpomene". Proceedings of the Royal Society of London. Series B: Biological Sciences. 268 (1478): 1849–1854. doi:10.1098/rspb.2001.1753. PMC 1088818. PMID 11522205.
  27. ^Dieckmann, U.; Doebeli, M. (1999). "On the origin of species by sympatric speciation"(PDF). Letters to Nature. 400 (6742): 353–357. Bibcode:1999Natur.400..354D. doi:10.1038/22521. PMID 10432112. S2CID 4301325.
  28. ^Jiggins, C.D.; et al. (2001). "Reproductive isolation caused by colour pattern mimicry"(PDF). Letters to Nature. 411 (6835): 302–305. Bibcode:2001Natur.411..302J. doi:10.1038/35077075. PMID 11357131. S2CID 2346396.
  29. ^Kondrashov, A.S.; Kondrashov, F.A. (1999). "Interactions among quantitative traits in the course of sympatric speciation". Nature. 400 (6742): 351–354. Bibcode:1999Natur.400..351K. doi:10.1038/22514. PMID 10432111. S2CID 4425252.
  30. ^Via, S (1999). "Reproductive Isolation between sympatric races of Pea Aphids I. gene flow restriction and habitat choice". Evolution. 53 (5): 1446–1457. doi:10.2307/2640891. JSTOR 2640891. PMID 28565574.
  31. ^ abcRueffler, C.; et al. (2006). "Disruptive selection and then what?". Trends in Ecology & Evolution. 21 (5): 238–245. doi:10.1016/j.tree.2006.03.003. PMID 16697909.
  32. ^Lande, R (1980). "Sexual Dimorphism, sexual selection, and adaptation in polygenic characters". Evolution. 34 (2): 292–305. doi:10.2307/2407393. JSTOR 2407393. PMID 28563426.
  33. ^Nussey, D.H.; et al. (2005). "Selection on heritable phenotypic plasticity in a wild bird population". Science. 310 (5746): 304–306. Bibcode:2005Sci...310..304N. doi:10.1126/science.1117004. PMID 16224020. S2CID 44774279.
  34. ^McComas, W.F.; Alters, B.J. (September 1994). "Modeling modes of evolution: comparing phyletic gradualism & punctuated equilibrium". The American Biology Teacher. 56 (6): 354–360. doi:10.2307/4449851. JSTOR 4449851.
Sours: https://en.wikipedia.org/wiki/Disruptive_selection

Example diversifying selection

19.3B: Stabilizing, Directional, and Diversifying Selection

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Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.

Learning Objectives

  • Contrast stabilizing selection, directional selection, and diversifying selection.

Key Points

  • Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations.
  • In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.
  • Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
  • In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.

Key Terms

  • directional selection: a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction
  • disruptive selection: (or diversifying selection) a mode of natural selection in which extreme values for a trait are favored over intermediate values
  • stabilizing selection: a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population’s genetic variance will decrease.

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Stabilizing selection: Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

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Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

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Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

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Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection

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Stabilizing, directional and disruptive selection

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