Mechanisms of Evolution: Natural Selection, Genetic Drift, Gene Flow

Introduction

Key Concepts

Natural Selection

Natural selection is a primary mechanism of evolution, first articulated by Charles Darwin. It refers to the process where organisms better adapted to their environment tend to survive and produce more offspring. This concept is foundational to understanding how traits that enhance survival and reproduction become more common in successive generations.

The process of natural selection can be broken down into several key components:

• Variation: Within a population, individuals exhibit variations in their traits. These variations can be morphological, physiological, or behavioral.
• Inheritance: Some of these traits are heritable and can be passed down from parents to offspring through genes.
• Differential Survival and Reproduction: Individuals with advantageous traits are more likely to survive and reproduce, passing these traits to the next generation.
• Adaptation: Over time, the frequency of advantageous traits increases within the population, leading to adaptation to the environment.

An example of natural selection is the peppered moth in England. Before the industrial revolution, the majority of these moths had light coloring, which camouflaged them against lichen-covered trees. However, pollution darkened the tree bark, making the light-colored moths more visible to predators. Dark-colored moths had a selective advantage, leading to an increase in their population. As pollution levels decreased, the light-colored moths became more prevalent again.

Genetic Drift

Genetic drift is a mechanism of evolution that refers to random changes in allele frequencies within a population. Unlike natural selection, genetic drift does not necessarily favor traits that enhance survival or reproduction. Instead, it is driven by chance events that can lead to significant genetic changes, especially in small populations.

There are two primary forms of genetic drift:

• Bottleneck Effect: This occurs when a population is drastically reduced in size due to an event such as a natural disaster. The surviving population may have a different allele frequency than the original population, leading to reduced genetic diversity.
• Founder Effect: When a new population is established by a small number of individuals from a larger population, the new population may have different allele frequencies compared to the original population. This can lead to the prevalence of certain traits within the new population.

An example of the bottleneck effect is seen in the northern elephant seal population. Historically, their numbers were reduced to as few as 20 individuals due to hunting. This drastic reduction has led to low genetic diversity, making the population more susceptible to diseases and reducing their ability to adapt to environmental changes.

Gene Flow

Gene flow, also known as gene migration, involves the transfer of genetic material between separate populations. It occurs when individuals from one population migrate and breed with individuals from another population, introducing new alleles into the gene pool.

Gene flow has several significant effects on populations:

• Increases Genetic Diversity: By introducing new alleles, gene flow increases the genetic variation within a population, which can enhance the ability to adapt to changing environments.
• Reduces Genetic Differences Between Populations: Gene flow homogenizes genetic differences, making separate populations more genetically similar.
• Prevents Speciation: By maintaining genetic similarity, gene flow can hinder the process of speciation, where new species arise due to genetic divergence.

An example of gene flow is seen in human populations. Migration between continents has introduced a vast array of genetic diversity, reducing the genetic distinctiveness of isolated populations and enhancing overall genetic variability.

Comparison Table

Summary and Key Takeaways