This question is testing whether you know the actual minimum requirements for natural selection, as opposed to conditions that are common in nature but not logically necessary. Let's check each option one by one against that minimum list.
- There must be variation in the trait: Needed. Selection can only pick among different trait values that already exist in the population; if everyone is identical there is nothing to select for or against.
- The trait must be linked with differential reproductive success: Needed. This is the actual mechanism of selection: individuals with a certain trait value must, on average, leave more surviving offspring than individuals with other values.
- The trait must be either fully or partially heritable: Needed. Even if a trait affects reproduction, the trait composition of the population will not change over generations unless offspring inherit that trait from their parents.
- The population must comprise of sexually reproducing organisms: Not needed. Bacteria, many protists and clonal plants reproduce asexually yet still evolve by natural selection, as long as variation, heritability and fitness differences exist. Sex is a separate biological phenomenon that is not part of the definition of natural selection.
Since variation, heritability, and a link to reproductive success are the three genuine pillars of natural selection, and none of them require sex, the condition that is NOT essential is the one about sexually reproducing organisms, option (D).
Let's summarize:
- Natural selection needs only three things: trait variation, heritability, and a fitness difference tied to the trait.
- Sexual reproduction speeds up the spread of new combinations of alleles through recombination, but it is not a precondition for selection itself.
So the correct choice is (D), since a population can evolve by natural selection whether it reproduces sexually or asexually.