Birth of a species

By Sancia van der Meij, Research Fellow

Biologists often refer to the word “species” when they are talking about the animals or plants that they study, but just what exactly is a species? Defining ‘species’ is actually quite tricky…

A basic definition is based on the work of a German biologist called Ernst Mayr, whose simplified description is “a group of interbreeding populations that are reproductively isolated from other groups”. This is a great starting point, but it is difficult to use when studying animals in the field. Biologists therefore use breeding experiments in laboratories and, increasingly, genetics to help determine what a species is.

How and under which circumstances new species evolve remains an important topic in biology. Quite a lot is known about geographical barriers causing the formation of new and distinct species through evolution – a process known as speciation. Mountains, rivers and ocean currents, for example, can divide populations of single species and in the long run – thousands or millions of years – this isolation can cause different populations to evolve in separate, new species.

But a more difficult concept in speciation is how species can evolve in the same geographical area. Together with a colleague, I studied the genetic composition of Opecarcinus hypostegus, a tiny crab species, around 5 mm in size, that only occurs in the Atlantic Ocean. These Gall Crabs are adapted to living in stony corals and often show a clear preference for inhabiting closely related coral species.

We studied over 200 specimens from five different coral species, all collected from the Caribbean island of Curaçao. The results showed that O. hypostegus should be considered a single, valid species. But to our surprise, when we zoomed into the details of the genetic composition of the crab,  we noticed small differences in the DNA of the crabs inhabiting the various coral species. With statistical tests we could prove that the variation in DNA was significantly different between the crabs inhabiting these five different Agaricia corals.

Despite the fact that all the crabs live around the same small Caribbean island, it does appear that we see the very first signs of future speciation in the crab’s DNA. Unfortunately we will not be around to witness the new species as it will likely take several hundreds of thousands of years before the making of these new crab species has neared completion. But how exciting to witness its new beginnings?