For more information or to contact an Oxford Sales Representative click here. Not a member? Sign up for My OBO. Already a member? Publications Pages Publications Pages. Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username. Password Please enter your Password. Forgot password? Don't have an account?
Sign in via your Institution. You could not be signed in, please check and try again. Sign in with your library card Please enter your library card number. Related Articles Expand or collapse the "related articles" section about About Related Articles close popup. Your current browser may not support copying via this button. Introduction Directional selection occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other.
General Overviews Most undergraduate textbooks on evolution, such as Futuyma and Barton, et al. How to Subscribe Oxford Bibliographies Online is available by subscription and perpetual access to institutions. Jump to Other Articles:. 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.
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. Example: An area that has black, white and grey bunnies contains both black and white rocks.
Both the traits for white and black will be favored by natural selection since they both prove useful for camouflage. The intermediate trait of grey does not prove as useful and therefore selective pressures act against the trait. Sch oolTutoring Academy is the premier educational services company for K and college students. We offer tutoring programs for students in K, AP classes, and college.
To learn more about how we help parents and students in Hastings, Nebraska visit: Tutoring in Hastings, Nebraska. Login Get started. Biology Extra Credit Science Uncategorized. Teaching Staff April 17, No Comments. 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.
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.
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. A male bird of paradise : This male bird of paradise carries an extremely long tail as the result of sexual selection.
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.
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