The theory of natural selection was first founded by Charles Darwin. The process of natural selection is important and is a driving force for evolution. For organisms to evolve, there needs to be differences in traits between organisms that provide certain advantages or disadvantages, and it is these traits that natural selection acts upon. When it comes to natural selection, there are three different types of selection that can occur.
These types include the following:. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:. Example: For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over. The plants that are very short fail to get enough sunlight to prosper.
Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind. This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:. Example: Giraffes with the longest necks are able to reach more leaves to each.
Selective pressures will work in the advantage of the longer neck giraffes and therefore the distribution of the trait within the population will shift towards the longer neck trait. This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait.
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. Learning Objectives Contrast stabilizing selection, directional selection, and diversifying selection. Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
This may be an example of the handicap principle. 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. This is an example of the extreme behaviors that arise from intense sexual selection pressure. Natural selection cannot create novel, perfect species because it only selects on existing variations in a population. 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. 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.
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. 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. Finally, it is important to understand that not all evolution is adaptive. 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. Privacy Policy. Skip to main content. The Evolution of Populations. Search for:. Adaptive Evolution. 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. 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.
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. 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.
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. Positive frequency-dependent selection selects for common phenotypes in a population and decreases genetic variance.
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