Final answer:
The reduction in genetic diversity of maize around 4400 years ago is best explained by selective breeding, where humans choose particular traits to be passed on, leading to less variation in the crop's genetic makeup.
Step-by-step explanation:
The loss of genetic diversity in maize around 4400 years ago can be explained by the process of selective breeding. In agriculture, selective breeding is an intentional process where only those plants with desirable traits are chosen to reproduce. This can result in a reduction of allelic diversity because the gene pool is being limited by human selection rather than natural selection. In the case of maize, early agricultural communities likely selected for traits like larger cob size or sweetness, which would narrow the genetic base of the crop as these selected traits became more prevalent. Other factors like mutation, genetic drift, and the founder effect could contribute to changes in genetic diversity but are typically characterized by more random processes, whereas selective breeding is a directed process.
Genetic drift can occur in natural populations due to chance events that result in only a portion of individuals contributing to the next generation's gene pool. The founder effect is a type of genetic drift that happens when a new population is established by a small number of individuals who carry only a fraction of the total genetic variation found in the original population. Although both genetic drift and the founder effect can lower genetic diversity, the conscious choice of humans to select certain traits in crops like maize is the clearest explanation for the observed loss in genetic diversity 4400 years ago.