Friday, June 5, 2009

Ecological Divergence in the Swallowtail

The influence of climate change on the Earth’s ecology can be as conspicuous as a hurricane, or it can be as subtle as a butterfly’s preference in oviposition sites… The interaction between environmental condition and evolutionary trajectory represents a complex and dynamic system in which the slightest deviation can be compounded to produce remarkably substantial outcomes. The idea that slight perturbances in a system’s initial state can result in large effects, is a tenet of Chaos Theory; more appropriately for this post however, this mathematical idea can be referred to as “the butterfly effect.”

Recently, alterations in climate have facilitated a “butterfly effect” in which expansion in the home ranges of two species has resulted in a chain of events that may ultimately lead to designation of a new butterfly species. As chance would have it, one of the two affected species, an Eastern Tiger Swallowtail butterfly, was photographed in my backyard yesterday.

Swallowtails belong to the Papilionidae family of butterflies and are characterized by their robust size, contrasting coloration and antenna-like tails. Their elongate tails mimic the appearance of antenna and redirect the focus of predators away from the head and towards the rear of the butterfly, thereby improving the probability of escape.

Yesterday, this male Eastern Tiger Swallowtail (Papilio glaucus) was photographed in my yard just after emerging from his chrysalis (three photos same specimen):

Common throughout the eastern United States, P. glaucus has a home range that is naturally bounded by ambient temperature (requires warm temperatures to reproduce) and although the pictured specimen will undoubtedly enjoy the opportunities provided by the numerous black cherry trees (Prunus serotina) in my yard – one of the staples of the swallowtail’s diet – and the warm Florida clime, even greater opportunities exist for his kin to the north.

Recently, research conducted by Michigan State University has shown that climate change has allowed the Eastern Tiger Swallowtail to travel further north than was previously tolerable. In taking advantage of its new habitat, the Eastern’s range now overlaps with the territories of the Canadian Tiger Swallowtail (Papilio canadensis) and has resulted in increased rates of hybridization between the two. The hybrids, though not yet reproductively isolated, exhibit some rather distinguishing characteristics that lead the folks at MSU to think that speciation may be at hand.

As with most plant feeding insects, butterfly-to-host plant adaptations are highly specialized towards specific plant species or plant groups. The reason for this is that plants characteristically produce chemicals that either deter or attract insects based on the defensive and mutualistic needs of the plant itself; if butterflies are to take advantage of a plant as a food source they must first adapt tolerances or immunity to the toxic compounds produced by that particular plant as defensive measures. In regards to the swallowtails specifically, although both the Eastern and Canadian feed on a variety of trees (polyphagous), they are very particular about the trees used to oviposition their eggs; the Eastern has a preference for tulip trees and the Canadian is a fan of quaking aspen. One of the distinguishing characteristics exhibited by the hybrid swallowtails is a shift in this ovipositional preference.

Although the northern most range of the Eastern has expanded, its plant of choice, the tulip tree (Liriodendron tulipifera), immigrates a bit slower and is still found only to the south – thus not accessible to the hybrids. In addition, the aspen tree (Populus tremuloides) preference of the Canadian isn’t passed on to the hybrids – i.e. no genetically transmitted tolerance to aspen chemicals. Left with little recourse, the hybrids have begun to oviposit on Fraxinus americana, the white ash tree. White ash isn’t toxic to either hybrid parental species, nor, are they specialized towards it…

This case serves as an excellent example of an ecology-linked evolutionary dynamic in which novel environmental conditions are driving divergent speciation through a compounding process. More to the point, at present a change in climate has resulted in the expansion of the swallowtail’s biogeographic distribution, thus amplifying the frequency of hybridization with similar species, and in turn increasing transmission of a trait-linked behavioral phenotype permitting unique insect-to-host plant combinations. Taken together these changes are pushing the Eastern-Canadian swallowtail hybrids towards reproductive isolation and the advent of a new Papilionidae species.

Sort of like a “butterfly effect”…

Mercader, R., Aardema, M., & Scriber, J. (2008). Hybridization leads to host-use divergence in a polyphagous butterfly sibling species pair Oecologia, 158 (4), 651-662 DOI: 10.1007/s00442-008-1177-9

1 comment:

  1. No competition["shortages"],as one would expect with Darwin's OTHER theory--Divergence[cf letter to Asa Gray]. Too bad Darwin never developed divergence adequately as the ur-ground of natural selection.