12.3

Genetic Drift, Natural Selection, and Gene Flow

Introduction to Evolutionary Mechanisms

Evolutionary biology studies the processes that cause changes in allele frequencies within populations over time. The main mechanisms include natural selection, genetic drift, and gene flow. Understanding these mechanisms is essential for explaining patterns of genetic variation and adaptation in nature.

Genetic Drift

Definition and Overview

• Genetic drift refers to random changes in allele frequencies from one generation to the next, especially in small populations.

• Unlike natural selection, genetic drift is not driven by environmental pressures or adaptive value, but by chance events.

Key Features of Genetic Drift

• Significant in small populations, where chance events can have a large impact on allele frequencies.

• Can lead to the loss of genetic variation within populations.

• May cause harmful alleles to become fixed (reach a frequency of 1) or beneficial alleles to be lost.

Examples of Genetic Drift

• Bottleneck Effect: Occurs when a population undergoes a drastic reduction in size due to a sudden environmental change (e.g., flood, fire). The surviving population may have different allele frequencies than the original population, leading to reduced genetic variation.

• Founder Effect: Occurs when a small group of individuals establishes a new population. The new population's gene pool may differ from the source population, often resulting in high frequencies of certain inherited disorders.

Illustrative Example: Bottleneck Effect in Prairie Chickens

• Population of greater prairie chickens in Illinois dropped from millions to fewer than 50 birds in the 1990s due to habitat loss.

• Genetic analysis showed a loss of alleles and reduced hatching success, indicating genetic drift and inbreeding depression.

Summary Table: Effects of Genetic Drift

Effect	Description
Significance in Small Populations	Chance events can alter allele frequencies dramatically
Loss of Genetic Variation	Alleles may be lost from the gene pool
Fixation of Harmful Alleles	Harmful alleles can become fixed by chance
Random Changes	Allele frequencies change unpredictably

Natural Selection

Definition and Mechanism

• Natural selection is the process by which individuals with advantageous heritable traits survive and reproduce more successfully than others, leading to changes in allele frequencies over time.

• It is the only evolutionary mechanism that consistently leads to adaptive evolution.

Key Points

• Acts on phenotypic variation within populations.

• Can increase the frequency of beneficial alleles and decrease the frequency of harmful alleles.

• Requires variation, heritability, and differential reproductive success.

Gene Flow

Definition and Effects

• Gene flow is the transfer of alleles between populations due to the movement of individuals or their gametes (e.g., pollen).

• Can introduce new alleles into a population, increasing genetic variation.

• Can reduce genetic differences between populations, making them more genetically similar.

Example: Lake Erie Water Snake (Nerodia sipedon)

• Mainland and island populations differ in color patterns due to gene flow and local adaptation.

• Gene flow can counteract the effects of natural selection by introducing alleles that are not locally adaptive.

Comparing Evolutionary Mechanisms

Mechanism	Cause	Effect on Genetic Variation	Directionality
Natural Selection	Non-random, adaptive	Can increase or decrease	Consistent, adaptive
Genetic Drift	Random, chance events	Decreases	Unpredictable
Gene Flow	Migration	Increases within, decreases between populations	Variable

Summary

• Evolutionary change in populations is driven by multiple mechanisms, each with distinct effects on genetic variation and adaptation.

• Understanding these processes is essential for interpreting patterns of biodiversity and the evolutionary history of life.