Not so Tiny Tortoises

Anyone who has taken an evolutionary biology class knows about Darwin’s famous trip to the Galapagos during which he observed the natural phenomena that influenced the development of his Theory of Evolution. On this trip, one of the most well-known examples of his observations were the giant Galapagos tortoises (Chelonoidis nigra), the largest land turtles, whose shells can grow up to 130 cm and can weigh over 185 kg [1]. In BBC’s Galapagos, David Attenborough talks about how Darwin saw that the tortoises living on the water-rich islands had rounded shells, while the tortoises living on drier islands had peaked shells.

It has been found that Galapagos tortoises originated on these islands two to three million years ago, giving the species ample time to diversify [2]. However, to this day, scientists argue about whether these tortoises from the differing island populations are subspecies of the Chelonoidis genus, or different species all together [1]. Either way, it is clear that the different environments on each island heavily influenced the evolution of each population. A likely method of this divergence was that of allopatric speciation. In allopatric speciation, the formation of a geographic barrier causes a species to diverge according to the differing selective pressures in the new, separated environments. Here, we see turtles separated on different islands being isolated from one another and diverging according to their respective island environments. Tortoises with the rounded shell morphotype live on islands where there is plenty of food and little competition. However, the peaked “saddleback” shells are adaptive on islands on which the food is scarcer and there is more competition, as these shells allow the tortoises to reach up for the higher vegetation [3]. In fact, turtle and tortoise shells have been extensively studied, as they are an interesting example of vertebrate evolution. There is ample opportunity for a shifting in the turtle body plan throughout embryogenesis, during which there is lateral rib growth and caraspace (shell) formation [4]. This means that can be a range of genetically determined physical qualities that can be selected for or against over time, depending on the environments, leading to the varying shells that we see today.

by Sarah Katznelson and Jeffrey Lowenthal

Galapagos with David Attenborough, Episode 3, from 25:04

References

1. McAllister C, Duszynski D, & D Roberts. 2014. A new coccidan (Apicomplexa: eimeriidae) from Galapagos tortoise, Chelonoidis (Testudines: testudinidae), from the Dallas zoo. The Journal of Parasitology 100(1), 128-132.
2. Caccone A, Gentile G, Gibbs. J, et al. 2002. Phylogeography and history of giant Galapagos tortoises. Evolution 56(10): 2052-2066.
3. Fritts T. 1984. Evolutionary divergence of giant tortoises in Galapagos, Biological Journal of the Linnean Society 21(1-2):165–176.
4. Nagashima H, Sugahara F, Takechi M, et. al. 2009. Evolution of the turtle body plan by the folding and creation of new muscle connections. Science 325(5937):193-196.