Final answer:
Selection events, such as targeting a single locus, two loci, or specific genotype combinations at two loci can create linkage disequilibrium. In a population already in disequilibrium, selection for one trait, like purple flowers, can influence the evolution of linked traits, such as pollen shape.
Step-by-step explanation:
When a population of pea plants is in linkage equilibrium for two linked loci, flower color (P = purple, p = pink) and pollen shape (L = long, l = round), certain types of selection can create linkage disequilibrium. A selection event at a single locus, such as the death of all pink-flowered plants, can indeed create linkage disequilibrium, especially if the alleles for flower color and pollen shape are not equally distributed among the pink and purple flowers. However, selection at two unlinked loci, like red-flowered and long-pollen plants dying, can also lead to linkage disequilibrium, particularly if these traits are statistically associated in the population.
In the case of selection on a combination of genotypes at two loci, for instance, if only plants that are both pink-flowered and have round pollen grains die, this would have a strong effect in creating linkage disequilibrium. This is because the selection is directly targeting the non-random association of alleles at different loci.
If a population is already in linkage disequilibrium, selection for purple flowers will likely affect the evolution of pollen shape differently compared to a population in linkage equilibrium. This is because in disequilibrium, there is already a non-random association between the alleles for the two traits, which might be disrupted or reinforced by selection, depending on the direction of selection pressure. This would contrast with the scenario where initially, flower color and pollen shape are in linkage equilibrium, and selection would first need to establish disequilibrium before it could exert a coupled effect on the second trait.