Speciation and Population Differentiation
Behavioral and genetic differentiation in canyon treefrogs
Canyon treefrog pair in amplexus
My dissertation research focused on exploring variation in the male advertisement calls and female call preference among populations of the canyon treefrog. As in many anurans, male canyon treefrogs produce calls to attract females during the mating season. These calls are often considered to be species- specific, with different species having distinctly different calls. Thus, within a species, we might expect that genetically more closely related populations will have more similar calls relative to genetically distant related populations.
Because the canyon treefrog has an extensive range, the southwestern US into south-central Mexico, we might expect to see genetically divergent populations and diverging advertisement calls. Furthermore, if females can discriminate and show strong preference for their own population’s call type, this would lead to assortative mating, which would ultimately lead to further genetic isolation and divergence among populations.
Because the canyon treefrog has an extensive range, the southwestern US into south-central Mexico, we might expect to see genetically divergent populations and diverging advertisement calls. Furthermore, if females can discriminate and show strong preference for their own population’s call type, this would lead to assortative mating, which would ultimately lead to further genetic isolation and divergence among populations.
Canyon treefrog (left) Mountain treefrog (right-the green one)
However this pattern was not observed in the southwest US. One canyon treefrog population had calls similar to other populations, but was genetically more closely related to another species, the mountain treefrog.
A previous study used only mitochondrial DNA markers to distinguish within population relationships of the canyon treefrog. We decided to use both mitochondrial and nuclear genes to assess phylogenetic relations. Our study revealed that the genetically distinct population had the nuclear genome of other canyon treefrogs, but the mitochondrial genome of the mountain treefrog (as found in the earlier study). The phenomenon of one species mitochondrial genome being integrated into another species is called mitochondrial introgression. Because the mitochondrial genome is usually inherited through the maternal line, this phenomenon can occur when species hybridize and, through a series of backcrosses with one of the parental species, the mitochondrial genome of one species gets integrated into another species (or population of another species). This has been shown to occur in several different taxa, and can only be determined when both nuclear and mitochondrial markers are analyzed, thus our study emphasizes the importance of using multiple lines of evidence when delimiting species. Furthermore, this introgressed canyon treefrog population is genetically more similar to the surrounding canyon treefrog populations; thus the lack of call differences is not unusual.
I also examined the advertisement calls among US and Mexican populations, and I assessed female choice for these call types. I found biologically meaningful call differences in which females from one US population were able to discriminate against the calls of two Mexican populations of the canyon treefrog, limiting the likelihood of between population mating should these clades come into secondary contact.
To better resolve the within species phylogeny and understand placement of these distinct populations, I used AFLP markers to infer within species relationships. We also found evidence for mitochondrial introgression between the mountain treefrog and two Mexican populations of the canyon treefrog. Furthermore, our results suggest that one of the southern Mexican populations is extremely distinct, and is most likely a biologically distinct species.
A previous study used only mitochondrial DNA markers to distinguish within population relationships of the canyon treefrog. We decided to use both mitochondrial and nuclear genes to assess phylogenetic relations. Our study revealed that the genetically distinct population had the nuclear genome of other canyon treefrogs, but the mitochondrial genome of the mountain treefrog (as found in the earlier study). The phenomenon of one species mitochondrial genome being integrated into another species is called mitochondrial introgression. Because the mitochondrial genome is usually inherited through the maternal line, this phenomenon can occur when species hybridize and, through a series of backcrosses with one of the parental species, the mitochondrial genome of one species gets integrated into another species (or population of another species). This has been shown to occur in several different taxa, and can only be determined when both nuclear and mitochondrial markers are analyzed, thus our study emphasizes the importance of using multiple lines of evidence when delimiting species. Furthermore, this introgressed canyon treefrog population is genetically more similar to the surrounding canyon treefrog populations; thus the lack of call differences is not unusual.
I also examined the advertisement calls among US and Mexican populations, and I assessed female choice for these call types. I found biologically meaningful call differences in which females from one US population were able to discriminate against the calls of two Mexican populations of the canyon treefrog, limiting the likelihood of between population mating should these clades come into secondary contact.
To better resolve the within species phylogeny and understand placement of these distinct populations, I used AFLP markers to infer within species relationships. We also found evidence for mitochondrial introgression between the mountain treefrog and two Mexican populations of the canyon treefrog. Furthermore, our results suggest that one of the southern Mexican populations is extremely distinct, and is most likely a biologically distinct species.