Welcome to the hybrid zone

by Dr Luis Ortiz-Catedral

Since the early 1980s a number of unusual-looking iguanas has been noticed on South Plaza island, an apparent cross between land and marine iguanas. But why are there hybrids of these species?

Land iguana © Philip Cope
Land iguana (Conolophus subcristatus) © Philip Cope

Among animals, a species can be loosely defined as a collective of individuals that can mate and produce viable offspring, and that is reproductively isolated from other populations. Reproductive isolation is the line in the sand, the boundary that defines a species as a biological unit. The theory suggests that there are morphological and behavioural features that keep individuals from mating with the “wrong” species. After all, mating is expensive, and mating with the wrong individual is likely to affect an individual’s reproductive success and even survival.

Take for instance the elaborate dances of blue-footed boobies: when a pair of these colourful birds engages in courtship dances, both individuals assess the quality of their partner, but also, the accuracy of the moves. If a booby belonging to a different species were to dance in front of a blue-footed individual with the intention of mating, chances are its efforts would go unnoticed or, worse even, the individual might be chased off or injured.

Nevertheless, there are plenty of examples that show the line in the sand is sometimes crossed. In fact, hybrid offspring can end up giving rise to a new subspecies or even species, a lineage that is distinct from either of the parental species. The mockingbirds of Genovesa island (Mimus parvulus bauri) appear to be a case in point, where their genetic makeup strongly suggests there was once interbreeding between the San Cristobal mockingbird (M. melanotis) and another mockingbird species from the northwest of the Archipelago (M. parvulus). In spite of their hybrid ancestry, Genovesa mockingbirds seem happy reproducing amongst themselves and produce healthy, fertile offspring. The same is true for the finches of Daphne Major, where Peter and Rosemary Grant have repeatedly seen hybridisations between finches from different Geospiza species. Over the last 20 years, it has also emerged that many of the giant tortoises, particularly those on Wolf Volcano on Isabela, are hybrids or the descendants of hybrids.

A less well known but no less striking example of inter-species hybridisation has been observed between the land iguanas (Conolophus subcristatus) and marine iguanas (Amblyrhynchus cristatus) on the small uplifted island of South Plaza. These crosses were first noted in 1981 and formerly described in 1997. Since then rangers from the Galapagos National Park and tour guides have reported a handful of other instances of iguana hybridisation. The hybrids have a very distinctive colouration, a blend of dirty yellow and dark grey, and boast dorsal head scales that are intermediate between the two parental species. Genetic analysis of these peculiar individuals confirms that they are indeed hybrids, with a 50:50 mix of land iguana and marine iguana DNA. What is less clear is why these hybrids have only ever been seen on South Plaza, even though there are plenty of other islands where land iguanas and marine iguanas live alongside each other.

Hybrid iguanas have only been found on South Plaza island The only illustrations of hybrid iguanas are those by artist John Bendon based on his own trip to South Plaza island © John Bendon

It may be because this is such a small island – only 200m across at its widest point – that there is just more opportunity for the two iguana species to interact. There are other mysteries too, like whether these hybrid offspring feed exclusively on land vegetation. No hybrids have been seen venturing into the water, which suggests that metabolically, they might be closer to land iguanas. It is also the case that every South Plaza hybrid that’s been studied appears to be the offspring of a male marine iguana and a female land iguana, rather than the other way round.

The explanation for this observation remains unclear, though there are two main hypotheses. In both marine iguanas and land iguanas, males are territorial and display elaborate head-bobbing and posturing to communicate dominance to their fellow males and to attract females. With a hierarchy established, the most dominant males get to control a small group of females and prevent other males from mating with them. In marine iguanas, however, less dominant males may still be able to obtain some paternity by adopting a “satellite” strategy, hanging out at some distance from the main group and sneaking copulation with one of the females should the chance arise. The appearance of hybrid iguanas on South Plaza could be explained if these satellite marine males occasionally direct their efforts at female land iguanas instead. Alternatively, hybridisation could be the result of female choice, with some female land iguanas actively choosing to mate with male marine iguanas with the most exaggerated displays.

© Tui De Roy
Marine iguana (Amblyrhynchus cristatus) © Tui De Roy

Although we don’t yet understand exactly how these hybrids come about, they appear to be infertile, so unlike the Genovesa mockingbirds, the finches of Daphne Major or the giant tortoises on Isabela, the South Plaza hybrids cannot pass on their mixed-up genes to subsequent generations so could never give rise to a new subspecies or species. Nevertheless, they are still of great interest to science, individuals that seem to embody the processes of divergence for which Galapagos has become famous. On your next visit to South Plaza, make sure you ask your tour guide about these one-off iguanas and if you’re lucky, you might even see one.

This article was originally published in the Spring-Summer 2020 issue of Galapagos Matters.

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