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. Directional Selection 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. Disruptive Selection This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait.
When looking at a trait distribution, there are two higher peaks on both ends with a minimum in the middle as such: Example: An area that has black, white and grey bunnies contains both black and white rocks. You might also like. How to Multiply Fractions. Heats of Dissociation. Projectile Motion. Tips for Factoring Polynomials. 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. 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.
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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.
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. 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.
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