In evolutionary biology, which statement correctly describes the role of chance events in the process of evolution?

Introduction / Context:
Evolution is driven by a combination of chance events and non random processes such as natural selection. Many exam questions in biology test whether you can distinguish the role of random mutation and genetic drift from the directional action of natural selection. This question asks which statement correctly captures how chance contributes to evolutionary change in populations over time.

Given Data / Assumptions:

• The question presents several statements about alleles, mutations, and evolutionary change.
• We assume a basic understanding of genes, alleles, and DNA.
• We also assume familiarity with the concepts of mutation, genetic drift, and natural selection.

Concept / Approach:
The key concept is that mutations are random changes in DNA sequences, and they are the ultimate source of new genetic variation. Without mutation, no new alleles would arise. Evolution then occurs when allele frequencies change in a population over generations, due to both chance processes such as genetic drift and non random processes like natural selection. To answer the question, we evaluate which option correctly and precisely describes a genuine role of chance in evolution.

Step-by-Step Solution:
Step 1: Read option A. It claims that a beneficial allele that increases fitness can never be lost by chance. This is incorrect, because even beneficial alleles can be lost from small populations due to random sampling effects, especially early on.Step 2: Read option B. It states that the ultimate source of new alleles is mutation, which consists of random changes in DNA sequences. This is a standard textbook statement and accurately describes the role of chance at the genetic level.Step 3: Read option C. It suggests that all evolutionary change is only the result of random changes. This ignores the powerful non random component of natural selection, so it is misleading and incomplete.Step 4: Read option D. It claims that chance has no effect on allele frequencies in small populations, which is the opposite of what genetic drift tells us. Drift is strongest in small populations.Step 5: Based on this analysis, option B is the only statement that is fully accurate and matches accepted evolutionary theory.

Verification / Alternative check:
A quick way to verify the answer is to recall the canonical statement from evolutionary biology that mutation is the ultimate source of genetic variation. Other processes such as genetic recombination reshuffle existing alleles, but only mutation creates brand new alleles. Since the wording in option B matches this idea and correctly mentions random changes in DNA, it fits both the definition of mutation and the role of chance in evolution.

Why Other Options Are Wrong:
Option A is wrong because beneficial alleles can be lost by chance, especially in small populations or in early generations when they are still rare. Option C is wrong because natural selection is a non random process that systematically favors alleles that improve fitness, so evolution is not purely random. Option D is wrong because it denies the existence of genetic drift, which is precisely the chance change in allele frequencies that can be very strong in small populations.

Common Pitfalls:
Students often assume that any mention of chance must refer to genetic drift and may overlook mutation as a random process. Another common mistake is to oversimplify evolution as either completely random or completely non random, instead of recognizing that both components are involved. It is also easy to confuse the origin of variation (mutation) with the sorting of variation (natural selection and drift). Keeping these distinctions clear is essential for correctly answering conceptual evolution questions.

Final Answer:
The ultimate source of new alleles is mutation, random changes in the nucleotide sequences of an organism's DNA.