Questions: Character Displacement and Sympatric Evolution

Question 1

Two bird species that evolved in isolation on separate islands each developed similar medium-sized beaks. When one colonizes the other's island, researchers observe their beak sizes diverging over generations — one evolving larger beaks, one smaller. What is the most likely mechanism?

• A. Genetic drift in the smaller island population randomly shifts allele frequencies
• B. Ecological character displacement — individuals whose beaks differ most from the competitor's gain a feeding advantage
• C. Reproductive character displacement — diverging mating signals reduce costly hybridization
• D. Mutation rates increase when species compete, producing faster phenotypic divergence

Answer: (B)

This scenario involves resource competition (food) rather than hybridization costs, making it ecological character displacement. When two species share a resource, individuals with traits that reduce overlap with the competitor face less competition and leave more offspring. Natural selection therefore pushes both species' traits apart — one beak evolves larger to exploit different seeds, one smaller. The key signal is that divergence is in resource-use traits (beak size) driven by feeding competition, not in mating signals driven by hybridization costs.

Question 2

Two closely related species in secondary contact are hybridizing, and their hybrid offspring are significantly less fit than either parental species. Character displacement theory predicts which outcome?

• A. Mating signals will converge so species recognize each other more readily, increasing gene flow
• B. One species will go extinct as the more fit species monopolizes the shared habitat
• C. Mating signals will diverge more rapidly in sympatric zones than in allopatric populations of the same species
• D. Hybridization will continue until the two species merge into a single combined lineage

Answer: (C)

This is reproductive character displacement via reinforcement. When hybrids are less fit, any individual that mates with the wrong species wastes reproductive effort. Selection therefore favors individuals whose mate-recognition signals (songs, color patterns, pheromones) are more distinctive from the other species, because they make fewer costly mating mistakes. The critical prediction is that this divergence is stronger where the species co-occur (sympatry) than in populations of the same species that have never encountered the other (allopatry) — exactly the pattern researchers look for to identify reinforcement.

Question 3

According to data from Darwin's finches, species that co-occur on the same island show more divergent beak sizes than populations of the same species found alone on different islands.

• T. True
• F. False

Answer: True

This is the classic empirical signature of ecological character displacement and was one of the key pieces of evidence used to support the concept. The pattern makes sense: populations living alone face only their own intraspecific competition, so beak size is free to track only local food resources. Populations living with a competitor face interspecific competition and selection pushes them apart. The sympatric populations are therefore more different from each other than their allopatric counterparts, even accounting for time since divergence.

Question 4

Reproductive character displacement and ecological character displacement are driven by the same selective pressure — competition — and differ mainly in which traits they act upon.

• T. True
• F. False

Answer: False

The two mechanisms are driven by different selective pressures, not just different traits. Ecological character displacement is driven by competition for resources — individuals with more distinct resource-use traits face less interspecific competition and leave more offspring. Reproductive character displacement is driven by the cost of hybridization — individuals that avoid mating with the wrong species waste less reproductive effort on unfit offspring. Both operate through natural selection, but the cost being avoided is fundamentally different: wasted food access vs. wasted reproductive investment.

Question 5

What does it mean that character displacement is 'driven by the costs of being too similar,' and what specific costs distinguish ecological from reproductive character displacement?

Model answer: Being too similar imposes costs when species share a habitat. In ecological character displacement, the cost is resource competition: individuals whose traits overlap most with the competitor lose food, habitat, or other resources to them. In reproductive character displacement, the cost is wasted reproduction: individuals who cannot distinguish their own species from the other produce hybrid offspring that are less fit, losing genetic investment. Selection in both cases favors individuals who are most different from the competitor — in resource-use traits for ecological displacement, in mating signals for reproductive displacement.

The concept matters for biodiversity because character displacement provides a mechanism for completing speciation even after secondary contact. Without it, two partially isolated populations that come back into contact might simply merge again. Character displacement sharpens the differences between them — both in traits that prevent hybridization and in traits that reduce competition — allowing stable coexistence rather than fusion or competitive exclusion.