Which one of the following phenomena most appropriately explains the change in frequency
distributions of species 1 and 2 in the figure given above?
(1) Migration
(2) Character displacement
(3) Colonization
(4) Succession

Character Displacement: Explaining Shifts in Bird Beak Size Distributions Across Habitats

In the study of ecology and evolution, patterns of trait variation among closely related species provide critical insights into how competition shapes biodiversity. One of the most illuminating examples comes from observing how bird species differ in beak size depending on whether they live together or apart. This article explores why character displacement is the most appropriate explanation for changes in beak size frequency distributions among bird species across different habitats—where some species coexist, and others live alone.

The Scenario: Beak Size Distributions Across Habitats

Consider three closely related bird species whose beak sizes are measured in three habitat patches:

• Habitat A: All three species coexist.

• Habitat B: Only species 1 is present.

• Habitat C: Only species 2 is present.

Scientists observe that the frequency distributions of beak sizes for species 1 and 2 are different in habitat A (where they coexist) compared to habitats B and C (where each is found alone). Specifically, beak sizes may be more similar when species are alone, but diverge when they live together.

What Is Character Displacement?

Character displacement is an evolutionary process where closely related species evolve differences in traits—such as beak size—specifically in response to competition when they live together (sympatry). In contrast, when these species live apart (allopatry), their traits are more similar. This pattern is driven by natural selection favoring individuals that can exploit different resources, thereby reducing competition and allowing coexistence125.

Why Character Displacement Explains the Scenario

The observed changes in beak size distributions are best explained by character displacement for several reasons:

• Sympatric Divergence: When species 1 and 2 live together in habitat A, they evolve differences in beak size to reduce competition for food. This results in distinct frequency distributions for each species.

• Allopatric Similarity: When species 1 lives alone in habitat B and species 2 lives alone in habitat C, there is no selective pressure to differentiate beak size. As a result, their beak size distributions are more similar to each other and less distinct than in habitat A.

• Evolutionary Change: The differences in beak size are not just behavioral or temporary; they are the result of evolutionary adaptation driven by competition.

This pattern is classic character displacement, as seen in famous examples like Darwin’s finches, where beak sizes diverge in sympatry but are more similar in allopatry126.

Comparing Character Displacement to Other Phenomena

Let’s consider why the other options do not explain the observed pattern:

• Migration: Migration refers to the movement of individuals between habitats. While migration can affect population size and genetic diversity, it does not explain why beak size distributions would differ between habitats where species coexist versus where they are alone.

• Colonization: Colonization is the process by which a species establishes itself in a new habitat. Colonization may affect which species are present, but it does not explain the evolutionary divergence of traits like beak size.

• Succession: Succession describes how community composition changes over time after a disturbance. While succession can alter which species are present, it does not explain the specific pattern of trait divergence seen in character displacement.

Real-World Examples of Character Displacement

Character displacement is well documented in nature, especially in birds:

• Darwin’s Finches: On the Galápagos Islands, finch species have similar beak sizes when living alone on small islands but evolve distinct beak sizes when living together on large islands. This allows each species to specialize on different seed types and reduce competition125.

• Rock Nuthatches: In Asia, rock nuthatches show differences in beak size and feeding behavior when they coexist, but not when they live apart.

• African Tinkerbirds: These birds exhibit character displacement in both body size and song when living together, which helps reduce competition and reproductive interference8.

These examples demonstrate that character displacement is a widespread and important mechanism for promoting biodiversity and species coexistence.

The Mechanism Behind Character Displacement

Character displacement arises from the following steps:

1. Competition: When two or more similar species share the same habitat, they compete for limited resources (e.g., food, nesting sites).

2. Natural Selection: Individuals with traits that allow them to exploit slightly different resources have a survival advantage.

3. Evolutionary Change: Over time, the species evolve differences in traits (e.g., beak size), reducing competition and allowing coexistence.

This process is rooted in the principle of competitive exclusion, which states that two species cannot indefinitely occupy the same niche unless they differentiate in some way.

Types of Character Displacement

Character displacement can take several forms:

• Ecological Character Displacement: Involves changes in traits related to resource use, such as beak size, body size, or feeding behavior.

• Reproductive Character Displacement: Involves changes in traits related to mating or reproduction, such as song or courtship behavior, to reduce hybridization between species.

Both types help species reduce competition and coexist more harmoniously.

The Importance of Character Displacement in Ecology

Character displacement is a key concept in ecology and evolutionary biology because it:

• Promotes Biodiversity: By allowing more species to coexist in the same habitat, character displacement increases overall biodiversity.

• Drives Speciation: The evolutionary changes that result from character displacement can lead to the formation of new species.

• Structures Communities: Character displacement helps shape the structure of ecological communities by promoting niche differentiation and reducing competition.

Evidence for Character Displacement

Numerous studies provide evidence for character displacement:

• Morphological Differences: Species in sympatry are more distinct in traits related to resource use than those in allopatry.

• Behavioral Differences: Species may evolve different behaviors to reduce competition.

• Physiological Differences: Species may evolve different physiological adaptations to exploit different resources.

These differences are often genetically based and can be passed on to future generations.

The Impact of Character Displacement on Conservation

Understanding character displacement is important for conservation because it:

• Highlights the Importance of Species Interactions: Maintaining the natural interactions between species is crucial for preserving biodiversity.

• Guides Habitat Management: Protecting habitats that allow for natural competition and niche differentiation can help preserve the evolutionary potential of species.

• Informs Restoration Efforts: Restoring ecosystems should consider the role of competition and character displacement in shaping community structure.

Summary Table: Character Displacement in Bird Beak Size

Conclusion

Character displacement is the most appropriate explanation for the observed changes in beak size frequency distributions among closely related bird species across different habitats. When species live together, they evolve differences in beak size to reduce competition, resulting in divergent distributions. When they live alone, their beak sizes are more similar. This process is a fundamental driver of biodiversity and species coexistence, highlighting the importance of competition and evolutionary adaptation in shaping the natural world.