What Is Inclusive Fitness?

Inclusive fitness is the total genetic contribution an individual makes to the next generation, not only through its own offspring (direct fitness) but also by aiding the reproductive success of close relatives who share many of the same genes (indirect fitness). This means that an individual’s evolutionary success is measured by both the children they produce and the additional offspring produced by relatives as a result of the individual’s actions.

The Science Behind Inclusive Fitness

The concept was first formalized by evolutionary biologist W. D. Hamilton in the 1960s. He demonstrated that genes promoting behaviors that help relatives can spread through a population, provided the benefit to the relative (weighted by genetic relatedness) outweighs the cost to the individual. This is summarized by Hamilton’s Rule, which predicts when altruistic behaviors will evolve:

$$rB>C$$

where $r$ is the genetic relatedness between the actor and recipient, $B$ is the benefit to the recipient, and $C$ is the cost to the actor.

Why Inclusive Fitness Matters

• Explains Altruism: Inclusive fitness theory helps explain why animals often help relatives, even at a cost to themselves. For example, worker bees forgoing reproduction to help the queen, or meerkats acting as sentinels to protect their group.

• Kin Selection: Inclusive fitness is the foundation of kin selection, where behaviors that favor the reproductive success of relatives are naturally selected.

• Broader Than Reproductive Fitness: Unlike reproductive fitness, which only counts an individual’s own offspring, inclusive fitness also includes the genetic success achieved by supporting relatives.

Examples in Nature

• Parental care: Parents invest resources in offspring, increasing both their own direct fitness and the survival of their genes.

• Eusocial insects: Worker ants and bees help raise the queen’s offspring, who are their close kin, thereby increasing their inclusive fitness.

• Alarm calls: Animals like ground squirrels give warning calls that may put themselves at risk but protect their relatives.

Key Concepts

Conclusion

The best term for an individual’s ability to pass down their genes through both their own offspring and the offspring of close relatives is inclusive fitness. This concept is fundamental to understanding the evolution of cooperation, altruism, and complex social behaviors in the natural world.

Correct answer: (1) Inclusive fitness