Selección Natural, Selección Disruptiva y Selección Estabilizadora

Natural Selection, Disruptive Selection, and Stabilizing Selection

Introduction

Natural selection is a fundamental concept in biology that describes the process through which organisms adapt to their environments. It involves genetic changes that occur over generations due to differential reproductive success, resulting in the survival and reproduction of individuals with advantageous traits. Natural selection can take various forms, including disruptive selection and stabilizing selection.

Disruptive Selection (Diversifying Selection)

Disruptive selection is a type of natural selection where extreme phenotypes have higher fitness than intermediate ones. This results in two groups of organisms evolving along different paths to exploit distinct ecological niches. In this process, both extreme phenotypes are favored over moderate individuals, leading to an increase in genetic variance within the population. An example of disruptive selection is the peppered moth's adaptation to industrial pollution, where dark-colored specimens had a higher survival rate due to their camouflage against sooty environments.

Speciation through Disruptive Selection

Disruptive selection can potentially lead to allopatric speciation when two populations adapt to different environments that favor opposite extremes of a given trait. If these populations remain isolated from each other, they may eventually develop into separate species. For instance, if a lake were divided by a dam, fish on one side might evolve larger fins for swimming upstream in the faster current, while those on the other side would need smaller fins for navigating the slower waters. Over time, these differences could result in two separate fish species adapted to their respective habitats.

Stabilizing Selection

Stabilizing selection is a form of natural selection that occurs when individuals with average or intermediate traits have the highest fitness compared to those with extreme traits. It tends to reduce genetic variation within a population as it favors individuals whose traits fall close to the mean value. For example, in a population of mice living in a forest, natural selection will tend to favor individuals that best blend in with the brown forest floor, reducing the genetic variance of fur color among the population.

Stabilizing Selection and Fitness

In a population undergoing stabilizing selection, the frequency of the dominant alleles for favored traits increases while the frequency of recessive alleles decreases. This is because intermediate phenotypes, which often represent a balance of opposing forces, generally have lower fitness compared to extreme phenotypes. Over time, natural selection acts to shift the population towards one of the extremes.

Evidence of Stabilizing and Disruptive Selection in Humans

Recent studies suggest that stabilizing selection is prevalent in contemporary human populations, while disruptive selection is less common. However, evidence of directional selection favoring certain traits, such as increased height, has also been observed.

Conclusion

Stabilizing and disruptive selection play crucial roles in shaping biodiversity and driving evolutionary changes. Understanding these mechanisms helps us appreciate the complexity of natural selection and the interplay between genetic variation and environmental pressures.