In evolutionary biology, what is considered the smallest unit that can undergo evolution and show changes in traits over time?

Introduction / Context:
In evolutionary biology and ecology, exams often test whether you know at what biological level evolution really operates. Many students wrongly think that a single organism evolves, but competitive exams expect you to know that evolution is defined in terms of changes in genetic composition over generations. This question checks whether you can correctly identify the smallest unit that can genuinely evolve over time according to the standard scientific definition of evolution.

Given Data / Assumptions:
- Evolution means long term, heritable change in characteristics across generations.
- The question asks for the smallest biological unit that can evolve, not the largest or most inclusive.
- The options mention genes, individuals, populations, and communities, which are all real biological levels of organization.
- We assume the standard textbook definition that evolution involves changes in allele frequencies across generations.

Concept / Approach:
In modern biology, evolution is defined as a change in allele frequencies in a population over time. An allele is a version of a gene, and populations consist of many individuals of the same species that interbreed. While genes carry information and individuals express traits, it is populations that show measurable changes in allele frequencies across generations. Therefore, the smallest unit that can evolve is not a single organism or a single gene in isolation, but the population as a whole. Communities combine many different species and are a larger, more complex unit than a population.

Step-by-Step Solution:
Step 1: Recall that evolution is defined as a change in allele frequencies over generations. Step 2: Note that a single gene can mutate, but one gene alone does not constitute evolution of a biological unit; evolution refers to patterns across many individuals. Step 3: An individual organism can experience changes like growth or learning, but its genetic makeup (apart from rare mutations) does not systematically change during its lifetime, so it does not “evolve” in the strict evolutionary sense. Step 4: A population is a group of interbreeding individuals of the same species in a given area, where allele frequencies can be measured and can change from generation to generation. Step 5: A community includes several populations of different species, and is therefore a higher level than needed to answer “smallest unit”. Step 6: From these definitions, the smallest unit at which we can meaningfully track evolutionary change is the population.

Verification / Alternative check:
A good way to verify this concept is to think about classic examples from population genetics. When we say that a population of insects has become resistant to a pesticide, we mean that the frequency of resistance alleles has increased in that population over generations. We do not say that an individual insect has “evolved” during its lifetime; instead we say it has inherited traits from previous generations. This confirms that the population is the correct level at which evolution is defined and measured in biology.

Why Other Options Are Wrong:
- A single gene within a chromosome: A gene can mutate, but evolution is not defined at the level of a lone gene; it is defined by statistical changes in allele frequencies in a population over time.
- An individual organism: Individuals can adapt physiologically or behaviorally, but these changes are not the long term genetic changes that define evolution; individuals are not considered the unit that evolves.
- An entire biological community: A community includes multiple species, and while communities can change composition, the basic textbooks identify the population, not the community, as the fundamental unit of evolution.

Common Pitfalls:
A very common exam mistake is to choose “individual organism” because we casually talk about an animal “evolving” or “adapting”. In scientific language, this is imprecise. Individuals may acclimatize or show phenotypic plasticity, but they do not evolve in the strict sense. Another error is to focus only on genes and forget that evolution is about how gene variants spread or disappear in a group across generations, not just about the existence of a single mutated gene. Remember also that communities are too broad for this question, because they include many different species and do not represent the smallest unit that evolves.

Final Answer:
The smallest unit that can evolve in evolutionary biology is a population of individuals of the same species.