Kin Selection

The theory of kin selection may explain the evolution of prosocial traits that promote reproductive success among kin even at the expense of individuals. A study published in Science Advances suggests that kin selection enhances reproductive output and encourages cooperation among groups of bulb mite relatives.


Traditional evolutionary wisdom suggests that genes are passed down from parent to offspring to improve the reproductive success of future generations, but other mechanisms through which genes are shared have been suggested. Hamilton’s theory of kin selection offers another explanation. According to the theory, genes that improve the fitness of subsequent generations are shared with other classes of relatives even at the expense of individual fitness. This process allows for the evolution of traits that benefit entire kinship groups and ensure the continuation of their line.

Many scientists believe that the evolution of prosocial traits like altruism may be explained by the theory of kin selection. Prosocial traits could evolve to mitigate threats to females’ reproductive success. Traits like self-restraint may help to ensure reproductive success by regulating competition for resources, such as food supplies, within classes of relatives. Kin selection may also affect the evolution of sexual conflict, which results in harm inflicted on females. While this evolution may occur through reductions in harm inflicted by males, females may also evolve reduced resistance to male harm.

A group of scientists in Poland used experimental evolution under controlled laboratory settings to test the effect of relatedness on female reproductive success. They used evolving populations of bulb mites organized into groups of relatives and nonrelatives to test their hypotheses. The results of their study were published in Science Advances. In general, they hypothesized that related lines of bulb mites would have higher reproductive output (i.e., fecundity) compared to unrelated control lines.

The researchers evolved three “kin selection” lines and three “control” lines of bulb mites. Kin selection lines were full siblings, whereas control lines were unrelated. After adults emerged from the larvae, they were placed in interaction vials where mating occurred; 100 interaction groups were created for each line. During the mating period, adults and larvae in both groups competed for resources and sexual partners. Next, inseminated females from all interaction groups in both lines were combined to lay eggs, and larvae were selected at random to start the next monogamous generation. Monogamy generations were alternated with experimental evolution generations to reduce the possibility of inbreeding in the kin selection lines. The method resulted in 18 generations of experimental evolution.

Three experiments were conducted to test the researchers’ main hypothesis that females from related lines of bulb mites would have higher reproductive output compared to unrelated females from the control lines. In the first experiment, reproductive output in kin selection and control line females was compared under the same conditions in which the two lines evolved in the laboratory. Reproductive output was measured for two days after a five-day interaction period, during which mating occurred among unrelated groups in both lines. The researchers used unrelated groups to control for the direct effects of relatedness versus the effect of relatedness accomplished through experimental evolution. In accordance with their predictions, they found that reproductive output was higher among females in kin selection lines compared to those in the control lines.

In the second experiment, males’ effect on reproductive output under kin selection was assessed. Virgin females from a stock population were paired with virgin males from the kin selection and control lines. After the interaction period, females were separated from the males to lay their eggs. Females paired with males from the kin selection lines showed greater fecundity compared to those paired with males from the control lines.

The third experiment was designed to test the prediction that females evolve decreased resistance to male harm inherent in sexual conflict under kin selection, given that resistance is especially costly to reproductive success. To test this prediction, females from kin-selected and control lines were paired with males from the stock population. Researchers found that females from kin-selected lines were more fecund compared to those from the control lines.

Overall, results supported the major premise of kin selection theory, which suggests that genes are shared among classes of relatives to ensure the continuation of a shared gene pool that promotes survival of the group. Kin selection enhances reproductive output and appears to facilitate cooperation between male and female bulb mites to enhance reproductive success. While researchers controlled factors that may have influenced reproductive output, the experiment occurred in the laboratory, and they could not account for dynamics that would have occurred in a natural environment. Future research should stimulate natural conditions under which the evolution of behaviors and conditions that might contribute to kin selection may occur.


Written By: Suzanne M. Robertson, Ph.D

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