In evolutionary biology, genetic drift becomes a major factor in changing allele frequencies in a population under which of the following conditions?

Introduction / Context:
Evolutionary change in populations can occur through several mechanisms, including natural selection, genetic drift, mutation, migration and non random mating. Genetic drift refers to random changes in allele frequencies from one generation to the next due to chance events. Unlike natural selection, which is directed by differences in fitness, drift is random and is especially important under certain conditions. This question asks when genetic drift becomes a major factor in evolution.

Given Data / Assumptions:

We are focusing on genetic drift, a random process affecting allele frequencies. Options mention male competition, artificial selection, absence of selective pressure, disruptive selection and strong mutation and selection. We assume standard population genetics definitions.

Concept / Approach:
Genetic drift is strongest in small populations where random sampling effects can cause large relative changes in allele frequencies. When there is little or no selection, allele frequency changes are driven mainly by chance, not by differential survival or reproduction. Events such as bottlenecks and founder effects are classic scenarios where drift dominates. In contrast, in large populations under strong selection (natural, sexual or artificial), selection usually overrides drift. Therefore, drift becomes a major factor when population size is small and selective pressures are weak or absent, allowing random fluctuations to shape evolution.

Step-by-Step Solution:
Step 1: Recall that drift refers to random changes, not directed changes due to higher fitness. Step 2: Remember that the effect of random sampling is greater when the sample size (population size) is small. Step 3: Understand that if strong selection is acting, advantageous alleles tend to increase in frequency in a predictable way, reducing the relative importance of drift. Step 4: Look for an option that emphasises small population size and minimal or no selection, which allows chance to dominate. Step 5: Identify option c as the one that mentions little or no selective pressure and implicitly small populations where random sampling effects dominate, fitting the concept of genetic drift.

Verification / Alternative check:
Think about real world examples such as a small isolated island population founded by a few individuals. If there is no strong selection for or against particular alleles, random births and deaths can significantly change allele frequencies over a few generations, sometimes even fixing or eliminating alleles purely by chance. Textbooks highlight bottleneck events, where population size is drastically reduced, and founder effects, where a few individuals colonise a new area, as classic cases where drift is a major evolutionary force. These scenarios match the conditions described in option c.

Why Other Options Are Wrong:
Intense male competition for mates in a large population (option a) is an example of sexual selection, a form of natural selection, rather than drift. Strong artificial selection (option b) imposed by humans, for example in domesticated animals or crops, is a directed process where particular traits are chosen, not random drift. Strong disruptive selection (option d) favours extreme phenotypes and is again a form of selection, not random change. Option e describes strong mutation and selection, which would also make natural selection a dominant force, leaving less room for drift to be the main driver of evolutionary change.

Common Pitfalls:
Students often equate any evolutionary change with selection and overlook the importance of random processes. Another pitfall is to ignore population size, even though drift is much weaker in very large populations. To avoid these mistakes, remember that drift is strongest when populations are small and selection is weak, and it can lead to fixation or loss of alleles regardless of their effect on fitness. Keeping this distinction clear between random drift and directed selection helps you choose the right answer in questions about evolutionary mechanisms.

Final Answer:
Genetic drift becomes a major factor in evolution when there is little or no selective pressure and the population size is small, so random sampling effects dominate.