The omnivores' solution: Tadpoles independently solve a common problem the same way

One of the key observations in support of evolutionary theory is that similar lifestyles can lead distantly-related living things to evolve strikingly similar traits. Compare an echidna and a hedgehog, distantly related mammals with very similar lifestyles. This kind of convergence can occur on much smaller scales of time and space, too, as a new paper just released online by Proceedings of the Royal Society shows. Its authors demonstrate that populations of spadefoot toads have independently evolved the same response to competition from another toad species [$-a].

Spea multiplicata, the New Mexico spadefoot toad. Photo by J.N. Stuart.

As tadpoles, spadefoot toads (Spea multiplicata) have two feeding strategies: they can be omnivores, feeding on organic debris in the water around them; or carnivores, feeding on aquatic crustaceans and sometimes other tadpoles. The two strategies are linked to a developmental switch -- tadpoles that start eating crustaceans develop larger heads, the better to eat their fellows, presumably. This switch is handy in minimizing competition for food with another, related toad species, S. bombifrons. In ponds where S. multiplicata and S. bombifrons occur together, S. multiplicata tadpoles are much more likely to become omnivores, and S. bombibfrons are more likely to become carnivores, than is the case for either species when they're the only ones in the pond.

This solution to competition might have evolved two ways: it may have turned up once, in a single population of S. multiplicata, which was then able to colonize other ponds containing the competitor; or it may have emerged independently in multiple populations experiencing similar natural selection from competition. The new study's authors show that the second scenario is more likely by comparing the genetic similarity of multiple S. multiplicata populations to the frequency of their competitors, and showing that competition strength, not genetic relatedness, is the better predictor of how likely S. multiplicata tadpoles are to become omnivores.

References

Pfennig, D., & Frankino, W. (1997). Kin-mediated morphogenesis in facultatively cannibalistic tadpoles Evolution, 51 (6) DOI: 10.2307/2411019

Rice, A., Leichty, A., & Pfennig, D. (2009). Parallel evolution and ecological selection: replicated character displacement in spadefoot toads Proc. R. Soc. B DOI: 10.1098/rspb.2009.1337