Q.How did Darwin explain fitness of organisms?

Step 1. Darwin observed that organisms overproduce offspring relative to available resources, so a struggle for existence (for food, space and mates) inevitably follows.

Step 2. He also observed universal variation — no two individuals are exactly alike — and that some of these variations happen to be useful in coping with the struggle.

Step 3. He proposed that nature acts as a selective force: individuals whose variations make them better suited to their environment are more likely to survive this struggle and successfully reproduce, while less well-suited individuals are less likely to. …

Trace Darwin's logical chain — overproduction, struggle for existence, variation, and selection of the best-adapted variants — …

• Defining fitness only as physical strength or size, rather than as reproductive success relative to the environment. …