what is positive selection

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14-10-2024

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Positive Selection: The Evolutionary Force Driving Adaptation

Positive selection is a powerful force in evolution, responsible for the emergence of new traits and adaptations that help organisms survive and reproduce in their environment. It's the driving force behind the diversity of life on Earth, from the intricate camouflage patterns of a chameleon to the complex human brain.

But what exactly is positive selection, and how does it work?

Understanding Positive Selection

Imagine a population of bacteria. Some bacteria possess a gene that makes them resistant to a specific antibiotic. This resistance gene is a beneficial mutation, giving those bacteria a survival advantage. When the antibiotic is introduced, the resistant bacteria are more likely to survive and reproduce, passing on the resistance gene to their offspring. Over time, the frequency of the resistance gene in the population increases. This process is known as positive selection.

In essence, positive selection favors beneficial mutations, increasing their frequency in a population. This leads to the evolution of adaptations, which are traits that improve an organism's chances of survival and reproduction.

Examples of Positive Selection in Action:

• Antibiotic Resistance: As we've seen, the emergence of antibiotic resistance in bacteria is a classic example of positive selection.
• Sickle Cell Anemia and Malaria: Individuals carrying a single copy of the sickle cell gene have a higher resistance to malaria. This selective advantage has kept the sickle cell allele at relatively high frequencies in regions where malaria is prevalent (Allison, 1954). This is an example of heterozygote advantage, a special case of positive selection.
• Lactose Tolerance: The ability to digest lactose, a sugar found in milk, is a recent adaptation in humans. The gene for lactase persistence evolved independently in different populations where dairy farming was practiced (Tishkoff et al., 2007). This example demonstrates how environmental factors can influence the direction of positive selection.

How to Identify Positive Selection

Scientists use various methods to detect positive selection in populations. One approach involves comparing the frequency of genetic variation between different populations. If a specific gene has a higher frequency in one population than another, it might indicate that this gene has been subject to positive selection in that population.

Beyond the Basics: The Role of Selection in Complex Traits

While positive selection is often associated with single gene changes, it also plays a role in the evolution of complex traits. Think of human intelligence, for instance. This trait is likely influenced by multiple genes, and positive selection may have acted on variations in these genes over time, leading to the development of our sophisticated cognitive abilities.

Positive Selection: A Vital Force in Evolution

Positive selection is a fundamental evolutionary force, shaping the diversity and complexity of life on Earth. By favoring beneficial mutations, it drives the adaptation of organisms to their environments, allowing them to thrive and evolve over time. Understanding positive selection is crucial for comprehending the evolution of species, including our own.

References:

• Allison, A. C. (1954). Protection afforded by sickle-cell trait against subtertian malarial infection. British Medical Journal, 1, 290-294.
• Tishkoff, S. A., et al. (2007). Convergent adaptation of human lactase persistence in Africa and Europe. Nature Genetics, 39, 31-40.