What do you mean by fitness?
Fitness can have an unambiguous mathematical definition in the field of population genetics, but in other areas of biology fitness is hard to pin down semantically. So are words like conflict and cooperation when they are used to describe the behavior of nonhuman organisms—yeast or algae or rabbits. Such words are necessary to communicate what goes on non-mathematically, but they are also sociologically loaded: They imply an ability to make choices. In my lab, we avoid defining fitness using absolute numerical values. We do bookkeeping: We count how many eggs are laid. We count how many eggs are hatched. But we do not suggest that these numbers alone are somehow equivalent to something that we can confidently label fitness.
How has the sociological use of the concept of fitness evolved since the publication of Charles Darwin’s Origin of Species?
Ever since the social Darwinist Herbert Spencer launched the "survival of the fittest" trope 150 years ago, people have equated survival with fitness and fitness with survival. It’s a tautology that causes us trouble, and yet it lingers. For example, the mathematical biologist R. A. Fisher was a founder of population genetics in the 1920s and 1930s, and his work relating fitness to genetic variance was enormously important. But Fisher himself was a eugenicist who believed that upper-class people, because they are upper class, have a genetic predisposition to superior intelligence and “qualities of the moral character.” He feared that since lower-class individuals seemed to have the highest birth rates, the spread of their collectively inferior genes would lead to the downfall of civilization if left unchecked. He advocated that society take certain measures to ensure the proliferation of upper-class genes. This is one reason why scientists need to be crystal-clear that fitness is a statement of fact and not a value judgment.
Then why use these loaded words?
They evolved as a convenient, if inexact, shorthand. In actuality, there may be no words that can replace the mathematical structures without carrying some normative baggage.
What is the meaning of cooperation in experimental biology?
Biological cooperation can occur when two or more individuals—be they single-celled or multicelled units—group together to form a new entity: a new individual or, perhaps, a group of individuals, depending on the perspective of the experimenter. The division of reproductive labor in a multicellular entity requires that cells work together to maximize the reproductive potential of the entity of which they are a part.
To that end, cooperative cells exhibit behaviors that improve the ability of other cells to transmit their genomes, sometimes at the expense of minimizing their own genetic contribution to the next generation. These cells are often called "altruists" in the lab. They may produce or preserve energy resources for the group.
On the other hand, a noncooperative cell—or what is labeled a "defector" in the lab—will move around an ecosystem eating up scarce resources without making any contributions to sustainability. The defector's "selfish" behavior tends to preserve its own genome for intergenerational transmission at the expense of the group.
Sexually reproducing organisms have capitalized on these two conflicting approaches in a way that benefits the entire organism: Somatic cells act as cooperators and germ cells act as defectors. Multicellularity as we know it might not have been possible without having both the defectors and the cooperators—each type of behavior is necessary.