The biological fitness of an organism is dependent on its ability to survive and reproduce in a given environment. If different traits or alleles increase the fitness of an organism, those alleles will consequently increase in the gene pool, and that trait will increase in the population. This is how natural selection affects a population.

There is inherent trade-off in biological fitness. A trait that increases ability to survive, but makes an individual sterile, decreases fitness because the organism cannot produce offspring to carry on the trait. Similarly, if a trait increases the ability to reproduce, but makes it harder to the organism to survive, it may die before being able to produce offspring. Both survival and reproduction are essential to defining the fitness of an organism.

Biological fitness in the evolutionary sense is only related to fitness in terms to reproduction. Because the primary goal of all organisms is to reproduce, or to pass their DNA onto offspring, fitness is defined as the ability to reproduce and create viable offspring.

"Favorable" traits, such as intelligence, size, or strength, may increase the ability of an individual to survive and reproduce, thus increasing biological fitness, but cannot be used to directly define the fitness of the individual.

The term "fitness" in evolutionary biology means the ability of an organism to pass on its genetic material to its offspring. Biological or "Darwinian" fitness is being able to live long enough to reproduce and keep the population or species alive. Most students confuse biological fitness with physical fitness because that is the context most often associated with the word.

Biological or Darwinian fitness is defined based on the specimen's ability to reproduce and generate viable offspring. Essentially, the fitness of the individual is based on its ability to pass genetic information on to the next generation, as opposed to any physical characteristic or trait.

Measuring the number of offspring who contribute to the gene pool is the best way to determine how genetically fit the iguana is. No matter how strong, large, old, or free of predation an animal is, if it cannot reproduce, it is not considered fit.

All of the examples given provide an evolutionary advantage. A white rabbit in a snow covered environment has camouflage, which protects it from its predators. The same is true with the black moth living in a  in a soot-covered industrial area. A cheeta that can run fastest has the greatest chance of catching prey and feeding himself/herself and his/her offspring. The same is true for a bird that can crack nuts in an area where nuts are the main source of food. 

The term biological fitness refers to reproductive success and is different than physical fitness. Since the most spotted male fathered the most cubs that survived to adulthood to reproduce themselves, he would be considered the most biologically fit. It is also important to note the inclusion of the "survived to adulthood" aspect since reproductive success is dependent on an organism's offspring being able to reproduce and contribute to the gene pool as well. For example, if the most spotted male had fathered a litter that initially had nine cubs, but only one of them survived to adulthood to have cubs of its own, he would no longer be considered the most biologically fit.