Optimal breeding or just random mating in the wild?

When an individual helps a close relative to produce offspring, the relative will pass on genes to the offspring that are shared in common with the original individual. In this way, some of the original individual’s genes are passed on with out them actually producing their own offspring. The benefit gained from this is known as an increase in inclusive fitness.

An individual can also get this inclusive fitness benefit by mating with a close relative. This occurs if by mating with that relative, the relative is able to produce more offspring than they would otherwise have been able to and therefore passes on more genes shared in common with the original individual. However, any benefit from this increase in inclusive fitness will be balanced against lost fitness due to inbreeding depression in the resulting offspring.

Recent work weighing up these two opposing factors has found that in many situations inclusive fitness benefits are actually expected to outweigh the costs of inbreeding depression. This work suggests that sometimes animals may actually be better off finding an intermediate relative to mate with. This theory is known as optimal inbreeding because it is thought that an intermediate relative might strike the best balance.

This study investigated the level of relatedness that animals were choosing in a natural population under natural conditions. Stephen completed this work on the fruit fly, Drosophila melanogaster. Pairs of flies were collected in the wild while they were actually mating. This meant that the chosen mates were naturally chosen and we knew which individuals were mating. Using genetic techniques we could then work out how closely related the two individuals were. These techniques could also be used to compare how closely related they were to a randomly sampled individual from the population.

Stephen found that males were actually more closely related to the female they were found mating with than they were to a randomly sampled female. This is what would be expected from optimal inbreeding. But not all of the results were clear-cut.

Animals choosing related mates did not explain everything. The most worrying result was that all the animals collected mating in the wild were more closely related to each other than they were to the randomly sampled ones. It wasn’t just the ones that were actually mating that were related to each other.

Was this truly an example of optimal inbreeding or just random mating in the wild?

To read Stephen’s full article click here. Dr Stephen Robinson was an Environmental Advisor/Zoologist with Terrestrial Ecosystems.