I would say energy myself, and this is because energy plays a key role in all of these. With trophic levels energy is being transferred amongst the levels. Food chain's is a map of how the energy is passed along the chain. Food web is a web of how energy is transferred, and this is why I would say energy is a keyword.
Answer:
Crossing over
Explanation:
During prophase I of meiosis, homologous chromosomes pair to form bivalents. The chromatids of bivalents may form intimate contact points called chiasmata. At the chiasmata breakage may occur and exchange of chromatids segments between non sister chromatids may take place. This is crossing over ad results in new gene combinations causing variations.