Fitness according to Darwin refers to______________.
Number of species in a community
Useful variation in population
Strength of an individual
Reproductive fitness of an organism
The correct option is D Reproductive fitness of an organism
According to Darwin's theory fitness refers to the Reproductive fitness of an organism because it describes the reproductive success of an individual which is measured as the number of offsprings produce by an individual with respect to specific genotype or phenotype. Fitness theory of Darwin doesn't depend on strength of an organism to survive but the ability of an organism to pass on its genetic material to its offsprings.
So, the correct answer is 'Reproductive fitness of an organism'.