Image from Wikimedia Commons, Source: Mattias Starkenberg
The history of evolutionary and ecological studies on mimic species is deep and chock-full of familiar names (Bates, Darwin, Muller, Wallace are just a few). There has also been no limit on the number of jaw-droppingly gorgeous species that have been under investigation.
A new paper by Evan Twomey and colleagues presents an integrative view into the process of mimetic speciation in yet another beautiful taxon, a poison dart frog appropriately named Ranitomeya imitator.
This frog species is a mullerian mimic of four different members of Ranitomeya and is interesting due to the multiple, independent instances of mimetic divergence that are in replicate across Peru.
Four species of Ranitomeya dart frog and their associated Peruvian mimic as shown in Figure 1 from Twomey et al (2015)
The authors studied three transition zones in R. imitator by quantifying color phenotypes, genetic variation, and mate choice preferences. Hypothesizing that mimetic divergence is promoting reproductive isolation in these morphs, they predicted that:
- phenotype clines should be narrow
- genetic divergence among mimetic morphs should be apparent
- mimetic morphs should prefer to mate with their own morph
Whether or not these predictions held was dependent on which transition zone was in question, but most were contrary to expectations. While each transition zone varied in the phenotypic width, no transition zone showed any additional genetic differentiation that couldn’t be explained through isolation by distance alone. Additionally, most populations showed no mating preference for their own morph.
The authors used these results to pitch the idea of a speciation continuum among the different morph transition zones of R. imitator:
At the first stage of population divergence, only mimetic color pattern divergence is present (spotted-striped transition zone). In the second stage, clines get narrower, becoming divergent in multiple aspects of overall color pattern, and mating preferences appear among allopatric populations. Lastly, the final stage of the continuum (shown in the striped-veradero transition zone) includes narrow clines that are variable among color pattern components, preferential mating at the transition zone, and restricted gene flow.
The discovery of this continuum is still the beginning of explaining exactly how this differentiation came to be. Selection on single or multiple traits? Geographic barriers? Secondary contact? Differences in divergence time? The authors discuss these options and provide an argument for simultaneous selection on color pattern and body size that has created reproductive isolation during different time frames for each morph.
I just want to know if they need any field help.
Twomey, E., Vestergaard, J. S., Venegas, P. J., & Summers, K. (2016). Mimetic Divergence and the Speciation Continuum in the Mimic Poison Frog Ranitomeya imitator. The American naturalist, 187(2), 205. DOI: 10.1086/684439