Four taxa share a common ancestor. Trait X (a derived character) is present in taxa B and C but absent in A and D. Under maximum parsimony, which tree minimizes character-state changes for X?
- A
(A, C) sister to (B, D): trait X arises in ancestor of A+C+B+D
- B
All four taxa equally related (polytomy): trait X arises three times
- Ccheck_circle
(A, D) sister to (B, C): trait X arises once in ancestor of B+C
- D
(A, B) sister to (C, D): trait X arises twice independently
Explanation
Grouping B and C as sister taxa requires only ONE evolutionary origin of trait X (in their common ancestor). The other arrangements require multiple independent origins — less parsimonious.