Thursday, March 4, 2021

Two iguana species evolve on the Galapagos Islands


We have the following problem:

On the Galapagos Islands, several species of iguanas live on land, and some species live only in marine environments. It is possible that both of these species diverged from a common ancestor. Both species lay their eggs in the sand.

The marine iguanas are sustained on a diet of algae in the water, while land iguanas generally eat cactus plants on the islands. And while algae are abundant in the water, cacti are much rarer on land.

Explain how these two species could have developed from a single common ancestor. What do you think could have caused this to happen?

During the course of history, rats and goats have been introduced to the island as nonnative species. The rats usually eat iguana eggs, while goats like to eat the cactus plants found on the island.

How might the introduction of rats and goats have affected both iguana species?

The answers to this question will be found in the comments to this post.

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.


  1. Definition of a species

    In biology, we often consider a “species” to be a set of organisms that can breed within the set (mostly, in nature, they just breed with other individuals of the same species) and produce fertile offspring. If two organisms can breed, but their offspring are infertile, then those two animals belong to two different species.

    This definition was developed by Ernst Mayr (source: de Queiroz K (2005). “Ernst Mayr and the modern concept of species”. Proc. Natl. Acad. Sci. U.S.A. 102 Suppl 1: 6600–7. doi:10.1073/pnas.0502030102), but there are problems when it comes to applying it to all known organisms.

    Darwin himself wrote, “I look at the term ‘species’ as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other … It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake.”

    In other words, scientists don’t really try to define “species,” but rather, they use the term for convenience in describing how things work in the world. Because of the vast level of hybridization that has been observed in nature, it may be true that no single concept of a species applies to all life.

    We could get into a philosophical discussion about how many base pairs have to be different in two organisms’ DNA before they can be considered to belong to two different species, but that would waste a lot of time, when we could be talking about these beautiful iguanas on the Galapagos Islands.

    Practically speaking, though, most biologists rely on Mayr’s description. If you would like more information about what defines a species, read “What is a species?” in Scientific American, June 2008, pp. 72-79.

    In addition to the Scientific American article, there’s also a site at the University of California, Berkeley, that describes the problems inherent in defining a species by its ability to breed and produce fertile offspring. One such question is: What do we do with species that reproduce asexually? Didn’t think about that one, did you? The site is here.

    Definition of a common ancestor

    Certainly the most famous example of a common ancestor would involve Darwin’s finches on these islands (the Gelapagos archepelago).

    Reasons for divergence

    Here is a picture of a land iguana from the Galapagos Islands (stock photo, under copyright):

    And here is a picture of a marine iguana from the Galapagos Islands (stock photo, under copyright):

  2. Charles Darwin’s theory of natural selection has four basic parts to it. In this comment, we will discuss each of these parts in turn.


    Environmental Pressure

    Increased survival


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