“Assortative mating by an obliquely transmitted local cultural trait promotes genetic divergence: a model”
D. Justin Yeh (Jan 2019)
Learned birdsong may promote genetic divergence in contact zones even when birds are not learning from their parents
Many animals copy behaviors from others. For example, passerine birds learn their birdsong, we humans learn languages, and many species across the animal kingdom learn different ways to get food. When these cultural traits are learned accurately, and variation in the behavior leads to different fitness, they can evolve in a way similar to genetic evolution.
Some species learn their mate choice signals, preferences, or both. An interesting consequence is that if two populations learn to not mate with each other, they may become two separate species because they no longer interbreed. However, variation in mating success would not affect a trait’s frequency in the next generation if juveniles learn it from unrelated adults (oblique transmission), because unmated individuals can still teach their unattractive cultural trait to unrelated juveniles. This seems to suggest that oblique transmission, which is often how passerine birds learn their songs, blocks out the effect of sexual selection.
In this paper, Justin Yeh builds a model to show that despite oblique transmission, sexual selection on cultural traits can still contribute to the formation of new species. Specifically, when two genetically distinct populations come into contact, if the two populations each sing their own preferred song (or speak their own language), the song would help them maintain the genetic distinction. This is because foreign birds that sing an unpopular song are less likely to find mates, and even though they can still teach the song to unrelated juveniles, their foreign genes are less successful. This is true even when the cultural traits are changing over time due to random chance and learning errors. In contrast, because hybrid offspring could have learned their song from any adult, selection against them does not help keep the songs distinct. These results help us understand how culture may have affected the evolutionary history of both birds and humans.
The effect of learned culture (e.g., birdsong dialects and human languages) on genetic divergence is unclear. Previous theoretical research suggests that because oblique learning allows phenotype transmission from individuals with no offspring to an unrelated individual in the next generation, the effect of sexual selection on the learned trait is masked. However, I propose that migration and spatially constrained learning can form statistical associations between cultural and genetic traits, which may allow selection on the cultural traits to indirectly affect the genetic traits. Here, I build a population genetic model that allows such statistical associations to form, and find that sexual selection and divergent selection on the cultural trait can indeed help maintain genetic divergence through such statistical associations, while selection against genetic hybrids does not affect cultural trait divergence. Furthermore, I find that even when the cultural trait changes over time due to drift and mutation, it can still help maintain genetic divergence. These results suggest the role of obliquely transmitted traits in evolution may be underrated, and the lack of one-to-one associations between cultural and genetic traits may not be sufficient to disprove the role of culture in genetic divergence.