Wednesday, May 27, 2009

Types of Selection

Many factors affect how selection can operate on individual phenotypes in a population. The population can live in a stable environment for long periods of time or live in an environment that is constantly changing. Variable or diverse environments may favour multiple phenotypes. Competition for mates adds further selection pressures that influence the evolutionary pathway of populations. A number of general selection types are a result of different sets of conditions. The table below is an example changes in population, in terms of traits. It will be used as a "control" graph that will be referred to in other articles.
Types of Selection covered in this blog:
Stabilization Selection
Directional Selection
Disruptive Selection
Sexual Selection
Runaway Selection
Artificial Selection
Cumulative Selection

Natural selection is a natural process resulting in the evolution of organisms best adapted to the environment and conditions. Chance and selection play an important role in evolution. Anatomical, morphological and behaviour traits of organisms evolve chiefly through selection mechanisms like sexual dimorphism. The behavioural traits directly influence the daily survival and reproductive success of individuals. At the molecular level, roles of random chance and selection are less clear.
Some biologists think most differences in genome are because of neutral mutations. Others think there are differences because of selective pressures. The basis of evolution is that the ultimate fate of all genetic combinations rests in an organism’s ability to produce individuals that are best suited to survive and reproduce in their habitat. Natural selection is the mechanism that drives the evolution of species.

Artificial Selection

Artificial selection is the intervention of an animal or plant species by humans. Humans cultivate certain desirable traits in future generations. Artificial selection is used to alter the appearance and behaviour of domesticated plants and animals. Darwin postulated that natural processes could act as agents of natural selection in much the same way as artificial selection.

Due to canine domestication, canines have been mated to make them smaller and to change their behaviour to less vicious and aggressive. Human intervention has changed the mate selection of canines and other domesticated organisms.

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Sexual Selection

Sexual Selection is categorized by individuals that mate and reproduce frequently make a substantial contribution to the gene pool of later generations. This type of selection favours the selection of any trait that influences mating success of the offspring. Sexual dimorphism as well as mating and courtship behaviours result from the differential reproductive success caused by the variation in the ability to obtain mates.
Traits favoured in sexual selection include sexual dimorphism and behavioural difference between the sexes. Sexual dimorphism is defined as the noticeable differences in physical appearances of males and females.
In terms of behavioural differences between the sexes, the most common forms of sexual selection result from female mate choice and male competitions. In most species, females choose males based on physical traits or behavioural traits. In other species, males are equipped with physical features that assist them in gaining control of area and defending that territory against other males. This territory provides an area to attract, and sometime detain females to mate with. Some traits are not produced by selective pressures from environmental conditions. Other traits produced by selective pressures, like sexual dimorphism, would be expected in both sexes. Some species produce traits that are beneficial for mating but detrimental to the survival of the species. This sexual diversity in not limited to animals.


The fiddler crab has an enlarged limb which gives him a physical advantage due to sexual dimorphism.

Penguins do not have noticeable sexual dimorphism. The male and female penguins look similar. A male has to drop a stone at the feet of a would be mate. If the stone is rejected, the penguin is male.

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Disruptive Selection

Disruptive selection favours multiple variation or forms of a trait that differs from the current population average. It also favours individuals with variations at opposite extremes of a trait over those with intermediate variations. Environment conditions may favour more than one phenotype. Disruptive selection is a significant evolutionary mechanism used to create distinctive forms within a population. These groups of distinctive forms within the population may eventually become isolated breeding population and have separate gene pools. The following graph illustrates the effects of disruptive selection on trait distribution in a population.
Two species of flowers may be available as a food source. Only the hummingbirds that possess a certain length of bill can use the flowers as a food source.

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Runaway Selection

Runaway selection was first proposed by R. A. Fisher, an English statistician in the 1930s. He believed the runaway selection accounted for the rapid evolution of specific physical traits in male animals of certain species. Some traits are so strongly preferred by certain females that they may mate only with males that possess the traits they want. In subsequent generations, the males usually possess the same traits and the females have the same affinity for the trait. Over time, the species may be characterized by extreme sexual dimorphism.

Organisms develop extreme traits that are used for mate selection that pose a threat to themselves as it makes them more noticeable to predators. Such as the stalk-eyed fly and the tungara frog.

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Directional Selection

Directional Selection favours an increase or decrease in the value of a trait from the current population average. This type of selection occurs when the environment favours individuals with more extreme variations of a trait. An organism will encounter new forces of selection when it enters a new environment or when aspects of its habitat change which may result in an observable change in a population. In species with large populations and short generation times many offspring are produces. In these populations, the amount of genetic variation from both recombination and mutation increase. Human activity also results in directional selection. The following graph illustrates the effect of direction selection within a population in terms of a specific trait.
Hummingbirds move to a new habitat that has long-length flowers, their mid-length bills are no longer suitable. The long-billed hummingbirds are better suited for the new environment and have access to more food and therefore contribute more offspring to the next generation.

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Cumulative Selection

Cumulative selection is the accumulation of many small evolutionary changes over long periods of time and also through many generations. This relatively new type of selection is a significant and important adaptation compared to ancestral species. Kin Selection is the natural selection of an organism based on a particular behaviour or trait that enhances the overall quality of life for any closely related individuals. This increases the fitness of the particular individual indirectly.

Over a period of time, an organism slowly develops adaptations/features through silent mutations or many small evolutionary changes. These changes may assist them in their current environment. From figure a to figure d, an area of light sensitive cells are formed into a lens. This is a result of prolonged exposure to light.

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Stabilization Selection

Stabilization selection is the most common form of selection.Although the appearance of any species changes slowly over very long periods of time, they do occur. Selection pressures will tend to prevent change in a species once the species has become well-adapted to their environment. Selection works against individuals exhibiting variations in a trait that deviates from the current, well-adapted, average population.
The following graph illustrates stabilization selection will do to a population, in terms of a specific trait.

Stabilization selection occurs when the most common phenotypes within the population are most favourable for the environment in which it lives. By doing so, the extreme variation of a particular trait resulting in a population where the traits are ideal for the environment and selective pressures maintain these ideal traits and features. Mutations that differ from the well-adapted traits and features will be selected against.

Human babies are a prime example of stabilization selection. Most babies are shown being born just over 3 kg than any other weight. Babies with significantly lower weight are more likely to have mental defects and have a lower survival rate while babies at a heavier weight pose risks to themselves and the mother. Natural selection reduces the extreme variations of a particular trait.

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