Birdsong and divergence
What are the causes of song evolution? How are song signals used in social interaction among birds? Would song evolution lead to the formation of premating barriers? The chapter 9 and 10 of T. Price, Speciation in Birds book attempted to answer these questions.
First there are a couple key terms to clarify:
Social selection: “selection on traits that result in advantages in social interactions, usually between member of the same species.”
genetic assimilation: “the process by which a phenotypic trait that is selected by some environmental stimuli, is taken over by genotype, so that is found even in absence of the environmental influence that had at firs been necessary to produce it.”
Song learning and divergence
The three clades of birds that can do vocal learning, learning/improvisation of songs, encompass more than 50% of the total number of bird species, which casts on a hint that song learning might accelerate speciation. Vocal learning is such a cool process in which birds invent orders and compositions of syllables, pitch, among other aspects of the song. Some of the innovations can spread in a population, making the species-recognition signals different than before.
The book considered three ways that could predispose populations to discrete song types : 1) some song variant might produce more stimulus to the receivers than others; 2) new variant can arise through copy errors, and these variants can be established through drift; 3) big scale disruption in song transmission can happen through “withdraw of learning”, when insufficiently tutored birds colonized a new population.
Once the frequency of new song variant is high enough, through drift or selection, there is usually frequency dependent selection, favoring the common song type, speeding up its spread. Moreover genetic assimilation can kick in sometimes following cultural evolution. Notice that genetic assimilation only occur in species that undergo vocal learning as the species that don’t learn their songs starts song evolution with genetic evolution. Repertoire size is likely to evolve genetically in response to selection in the environment (large repertoires are costly, but attractive to the females). Physiological and morphological constraints in the ability to learn certain can also lead to song differences that are underlay by genetic modulation of these constraints. These constrains could differentiate through various level ecological selection pressures, in parapatry, for example.
I can imagine that such song evolution started with vocal learning could lead to allopatric (especially through founder effect), and parametric speciation. After a founder or bottleneck event, the founders might sing insufficiently tutored song, which will be transmitted in the following generations in this population, eventually lead to premating isolation between the founder population and the initial source population. In allopatric populations isolated by certain geographical barriers, the origin and spread of cultural mutations might happen in parallel, leading to divergence in species-recognition signals among isolated populations. At parapatry, ecological selection gradient might result in a gradation of songs (e.g. certain songs might stand out more over certain ecological background) that lead to “regiolects”, termed by Martens (1996), which is a step further than dialect in the process of speciation. These regiolects can be more distinct than dialects among groups, and partially reproductively isolate the groups.
However, song learning might not facilitate sympatry speciation unless there is some other force that reproductively isolate these song variant groups. For example, if some features of female mating bias is linked to the song variants somehow, male song variants can assortatively mate with female bias variants, and cause premating isolation among song clusters. The song variants arise in the population that undergo song learning is insufficient to cause differentiation, because the new variant might either be lost or become common enough to be stabilized with frequency dependent selection, which does not subdivide the population.
Song and social interaction
There are a few interesting facts about how birds use their songs that I should take a note of.
In terms of development of the song, after fledging from the nest, birds started singing plastic songs, in which the order and structure of the syllables are highly variable, and different song types sang by different tutors can be heard in plastic songs. Then at adult song is the crystallized song that deleted many syllable types. Young birds tend to learn the song types that they heard repetitively.
As for response of the songs, birds tend to respond more strongly to the songs that belong to their own dialects, and the intention to retreat at an aggressive encounter is reflected by sing the song type that is not in the opponent’s repertoire; males singing rare songs are more likely to lose their territories. Birds also change their songs among breeding seasons to match their neighbors. There is a relationship between song and population. In larger populations, birds tend to sing songs with greater diversity of syllables.