Disruptive selection in which selection favours individuals with the smallest and largest values of the trait. These individuals have the highest fitness and individuals with intermediate values are at a fitness disadvantage. One example of this type of selection involves feather colour in male lazuli buntings Passerina amoena , a bird species native to North America Figure 15a.
The feather brightness of males varies, ranging from brown to bright blue. In a habitat with limited nesting sites, both the dullest and the brightest yearling males are more successful in obtaining high quality territories and therefore attract females. This is because adult males tolerate non-threatening dull yearlings and also leave brightly coloured yearlings alone.
Because of their dominance, both these groups are able to establish territories and attract females but yearling males with intermediate plumage are attacked by adults and therefore fail to obtain territories and mate Figure 15b.
In the long term, directional selection and disruptive selection can have the most dramatic evolutionary impact and can lead to the formation of a new type from an existing type, the process of speciation. This contrasts with the action of stabilising selection, which maintains the existing type without change in mean phenotype over long periods of time. Stabilising selection eliminates the extremes in a distribution of phenotypes, and as such it leads to a refinement of the existing type.
Because the trait under selection has a genetic basis, the differential reproduction of individuals carrying the genetic variants - the alleles - that underlie the trait e. Genetic variants that underlie phenotypes that are reproductively successful will increase in frequency, while those that underlie phenotypes that are not successful in reproducing will decrease. Thus, genetic changes across generations result from differences in reproductive success of genetically determined phenotypes.
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Not ready for formal University study? 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.
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. 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 either decrease, shift, or increase the genetic variance of a population. 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.
Stabilizing selection : Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases. 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 Evolution of the Peppered Moth : Typica and carbonaria morphs resting on the same tree. 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. 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. 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. In frequency-dependent selection, phenotypes that are either common or rare are favored through natural 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.
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. 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. 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.
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. Actively scan device characteristics for identification.
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