Gray tree frogs (Hyla versicolor) are late spring bloomers to the toads, leopard frogs, and spring peepers. Phenotypically indistinguishable from Cope’s gray tree frog (Hyla chrysoscelis), both species displaying astounding variability in coloration and mottling pattern. Some can be entirely uniformly green or brown, while others have more cryptic markings that blend in to the leaf litter and tree trunks. Of all the ones I’ve seen so far this spring, this individual was by far the most flamboyant with vibrant green bumpy skin, sticking out like a neon piece of lichen. The most reliable character to differentiate H. versicolor & H. chrysoscelis from other tree frogs is the hindleg’s orange flanks. But as for telling them apart purely by observation, the only difference is in their calling pattern: Hyla versicolor trills at a substantially lower rate than H. chrysoscelis. However, trill rate also varies with temperature, which can make it difficult to tell them apart in areas where they are not calling simultaneously.
It turns out that distinguishing between the two species is most easily achieved through genetic differences, and this is actually what is most fascinating about Hyla versicolor— its evolutionary origin. Hyla versicolor is an allotetraploid, meaning it possesses four sets of chromosome (tetraploid) derived from another species (allo-). Tetraploids can arise from genome duplication in which two sets of chromosomes from each diploid progenitor are passed on to their offspring. This occurs due to the fusion of diploid gametes without cell division, an example of nondisjunction. But it is not always so straightforward. In Hyla versicolor, it is hypothesized that the species arose from three hybridization events, between H. chrysoscelis and two extinct ancestors, each pair resulting in allotetraploid hybrids which interbred to yield the modern-day H. versicolor. The roles of hybridization and polyploidy in speciation are illustrated nicely in H. versicolor, but in other taxa the two processes can be very convoluted. Particularly in plants, both processes are rampant throughout evolutionary history, so together with a high reproductive diversity it makes for a rich group to examine these questions.