Answer:
When Organism A has a trait that negatively impacts it, or lacks a trait which would positively impact it, then said organism perishes, and its genes are not passed onto the next generation. On the flip side, when Organism B has a trait that positively impacts it, or lacks a trait that would negatively impact it, then the organism thrives, and its genes are passed onto the next generation.
Therefore, the next generation receives genes from Organism B and does not receive genes from Organism A. So, the next generation has traits that positively impact it and lacks traits that would negatively impact it, thus evolving according to Darwin.
Step-by-step explanation:
What was explained by it? Evolution. But how did it explain evolution? That is in the answer.