Adaptive Value

How much a trait helps an animal survive in their environment and propagate their genes is called adaptive value. In a non-eusocial species, these traits usually consist of behaviors that increase the animal's ability to reproduce. However, in a eusocial animal (like the ant!) it is sometimes more adaptive for them to sacrifice their own reproductive capabilities for the benefit of the colony, even though this means their own individual genetic material will not be passed on directly via sexual reproduction. Why is this so? Read on!

The adaptive value of eusocial behavior in ants

Kin Selection, Hamilton's Rule

Kin selection refers to the apparent strategies in evolution that favour the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. This model describes the evolutionary advantage of eusocial behavior, despite the fact that it means that most individuals that practice this behavior are sterile and do not pass on their genes. Under this theory, individuals gain fitness in (at least) 2 ways.

  1. Directly, from the offspring they produce. The Queen and Drone ant benefit from this.
  2. Indirectly, from offspring of relatives. The Worker and Soldier ants benefit from this.

Therefore, the total fitness of a eusocial insect = direct fitness + coefficient of relatedness * indirect fitness.

Hamilton's Rule was developed by W. D. Hamilton in 1964, and was the first formal quantitative treatment of kin selection to deal with the evolution of apparently altruistic acts. The formula is as follows:

r * B > C

  • r = the genetic relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same locus is identical by descent
  • B = the additional reproductive benefit gained by the recipient of the altruistic act
  • C = the reproductive cost to the individual performing the act

The coefficient of relatedness (r) in ants can be derived as follows:

Coefficient of Relatedness in Ants

Inclusive Fitness of an organism is the sum of its classical fitness (how many of its own offspring it produces and supports), and the number of equivalents of its own offspring it can add to the population by supporting others. Given what you've read so far, you know that the classical definition for this model does not apply to a eusocial insect. Except for a few members of the colony, eusocial species rely on the success of their close relatives to increase their inclusive fitness.