Galapagos Marine Ecology, ECOL 496O/596O 6.28.06
Sal Celi
Report on the hybridization between the Galapagos Marine and Land Iguanas
Before we can even begin to discuss the degree of hybridization between land and marine iguanas of the Galapagos, it would be important to check for understanding. Hybridization may occur when two populations of closely related but distinct species come into contact with each other (usually through the overlapping of their habitats) where they may mate and reproduce. What seems like a very simple topic is convoluted by the tremendous degree of time windows needed to create speciation—the evolutionary formation of new kinds of organisms.
Now let’s take a look at the land iguana. There are two species: conolophus pallidus (currently found on five of the Galapagos Islands) and conolophus subcristatus (currently found on Santa FeIsland only). As is very typical of cold blooded reptiles, they spend their mornings sprawled out in the sun, their afternoons in the shade and sleep in burrows at night to preserve body heat. With their clawed feet, spiny crests along their back and long heavy tails, they resemble the mythical dragons of our storied past. Their yellowish rough and wrinkled skin (with patches of black, brown or rust) cover a body that weighs up to thirteen kilograms and can stretch over a meter long.
The marine iguana ( amblyrhynchus cristatus) is similar in almost all aspects, with the substantially main difference being the very dark coloration of the skin. Since the ocean waters it gets its food from are very cold and it is a reptile, the dark coloration helps them warm up quicker when they bask in the sunlight.
The two species populations overlap when the marine iguana has the ability to and does come on to the land habitat of the land iguana. Both species mating ritual is very aggressive, with the male chasing down and grabbing the female, who then escapes (after copulation) to lay her eggs in the sand.
I have found several documents on the internet that mention land and marine iguanas mating to create a hybrid, but only one ( Rit.edu/~rhrsbi/GallapagosPages/LandIguana.html) that gives a detailed account on the particulars of the event.
I t reports that male marine iguanas, in a mating frenzy, have been seen attacking and “raping” female land iguanas and that hybrid offspring have occurred. The hybrid’s coloration resembles that o the marine iguana (very dark), but it seems to follow the lifestyle of the land iguana. I could not find any information regarding whether the hybrids are fertile, but they do have evidence (using mitochondrial DNA which occurs in the cytoplasm and comes only from the mother and has been matched in the hybrid) supporting the male marine and female land iguana copulation scenario.
The standard definition of the term “species” includes the presence of a fertility barrier. It has been my experience that the hardest part of understanding how evolution works is trying to wrap your mind around the enormous time windows needed to allow the changes that create the “survival of the fittest” scenario to take place. At this point it is difficult to determine why hybridization occurs so infrequently, except to say that the many necessary conditions to be in place all at the same time is a rare event. Zit would be interesting to examine whether a hybrid can interbreed with a land, marine or another hybrid iguana and determine if that offspring would be able to interbreed as well. It may be that we are witnessing the birth of a new species. Or not.