Final answer:
The least likely explanation for the G-value paradox, where pufferfish have more genes than gorillas but are less complex, is the assumption that more genes directly cause greater complexity. More plausible explanations might involve non-coding DNA or differences in gene regulation.
Step-by-step explanation:
The G-value paradox refers to the observation that there is not a consistent relationship between the complexity of an organism and the number of genes it has. The observation that a green spotted pufferfish has more genes than a gorilla but is considered less complex is an example of this paradox. The least likely explanation for this observation is that having more genes directly leads to a more complex organism. More plausible explanations might include the presence of non-coding DNA, regulatory sequences, or differences in gene expression that contribute to complexity in ways that are not strictly related to gene count.