Name the mechanism of evolution that must have operated so that population-two evolved from population-one.

| List I | List II | ||
| A | Down’s syndrome | I | 11th chormosome |
| B | α-Thalassemia | II | ‘X’ chromosome |
| C | β-Thalassemia | III | 21st chromosome |
| D | Klinefelter’s syndrome | IV | 16th chromosome |