Inclusive Fitness: Definition & Theory

Instructor: Meredith Mikell
The successful passing along of genes is the ultimate driving force behind an organism's fitness. The concept of inclusive fitness goes beyond the individual, also involving the reproductive success of others in a population. Here we will examine the definition of inclusive fitness, explore the theory, and finish with a brief quiz.

Genes At The Controls

In evolutionary biology, it has been suggested that genes are the ultimate controllers of our bodies, our instincts, and even our actions. This concept can get pretty philosophical, even to the extent of viewing ourselves as merely vessels for our genes, executing actions that are only driven toward the passing on of those genes to future generations. Whether or not this is how we might want to view ourselves, it is a useful way of understanding the concept of inclusive fitness.

From a gene's 'point of view,' the survival and reproductive success of the individual organism it inhabits is of foremost importance. But say a gene is found in more than one organism in the population - which it usually is - such as the gene for brown eyes or green eyes? The gene seeks only to be passed along, and if there are other copies of the gene in other individuals, it is beneficial to ensure the success of those other individuals too. This illustrates the concept of inclusive fitness: a theory suggesting that the genetic success of an organism is dependent upon cooperation with other individuals in a population. A gene will be more prevalent in future generations if all individuals who carry that gene are successful at reproducing, rather than just one individual with that gene.

Family First

Since related individuals are those most likely to be carrying common genes, this cooperation is foremost associated with the concept of kin selection, in which related individuals tend to take care of each other, often altruistically, and generally work to ensure each others' successes. Our innate desires to protect those in our families, especially our offspring, can ultimately be attributed to our genes, 'motivating' us to protect those same genes in others!

Organisms tend to care first and foremost for their relatives, especially offspring. This increases inclusive fitness.
birds kin selection

It Takes A Village

In many cases, members of a population do care for non-relatives in the same way as they take care of their kin. Naturally, we can see how it is beneficial for a tribe or group to look after each other, in a way, 'you scratch my back, and I'll scratch your back.' Team work does pay off for individuals in the group. But looking at this from a gene's point of view, it makes sense for a different reason. If we apply the theory of inclusive fitness, a gene is most successful if ALL individuals carrying that gene survive to reproduce. The catch is that it is not easy for an organism to tell (instinctively, not consciously) who in their population also carries a given gene and who does not. Thus, in many species, individuals show care for non-related members of the population, striving to ensure their survival.

In meerkat troops, a single female often babysits all of the young pups, many of whom are not related to her. This can also help increase the inclusive fitness of certain genes in the population.

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