Answer
Emergent properties are not obvious until biological organization brings enough parts together, interacting, for the new property to be seen and emerge. Cells change when enough sickle cell hemoglobin is present at low oxygen concentrations because the proteins then form filaments not seen in non-mutants. This leads to sickle-shaped, misshapen cells, even some of those in heterozygotes, which block blood flow in capillaries. At the population level, heterozygotes are selected for because of their malaria resistant phenotype, making the selection directional. This resistance and its effect on populations is also unanticipated because nothing about a filament-forming version of hemoglobin suggests that it will help resist a protist-caused disease.
Work Step by Step
Review the definition of emergent properties in Ch. 1-- it is one of the most common themes in this book, a frequent subject of the essay questions in various chapters. Use your imagination to see how something about mutant hemoglobin could lead to changes in cells and populations-- that is the emergent part. These should be connected back to the mutated protein sequence.
