Home / IB DP Biology- A4.1 Evolution and speciation- IB Style Questions For HL Paper 2

IB DP Biology- A4.1 Evolution and speciation- IB Style Questions For HL Paper 2 -FA 2025

IBDP Biology HL Paper 2 -FA 2025- All Chapters

Question

The map illustrates the distribution of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) on opposite sides of the Congo River as it flows towards the Atlantic Ocean in the Democratic Republic of Congo. Bonobos inhabit the dense, humid forest to the south of the Congo River, while chimpanzees occupy woodland and forest habitats to the north of the river. Both species originated from a shared ancestral population.
(a) Name the type of speciation that produced these two species.
(b) Describe how both species could have arisen from their common ancestor.
(c) Bonobos are listed as an endangered species. Explain how ongoing deforestation can lead to a reduction in bonobo population size.

Most-appropriate topic codes (CED):

• TOPIC A4.1: Evolution and speciation — parts (a), (b)
• TOPIC A4.2: Conservation of biodiversity — part (c)
▶️ Answer/Explanation
Detailed solution

(a)
Allopatric speciation.

(b)
The original ancestral population was split by a physical barrier, the Congo River. Once separated, the two groups experienced different environmental conditions and selection pressures in their respective habitats (dense wet forest in the south versus forested regions in the north). Over many generations, random mutations and natural selection caused changes in allele frequencies in each isolated population. This gradual genetic divergence eventually became great enough that individuals from the two groups could no longer interbreed successfully, resulting in two distinct species.

(c)
Deforestation causes extensive loss and fragmentation of habitat for bonobos. As forest is cleared, bonobos lose access to essential food resources, nesting sites, and safe areas for reproduction. Smaller, isolated forest patches can support fewer individuals and increase their vulnerability to predation, hunting, and human conflict. Together, these factors reduce survival and reproductive success, leading to a decline in overall population size.

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