Contributed by Heng Cheng
When you go to a zoo, an aquarium, or a botanical garden, you may wonder how nature created countless species. In fact, nature did not “create” species—species arose from a common ancestor through a process called speciation. Speciation can occur through a number of mechanisms. Among these, one mechanism that is widely seen is that populations of the same species diverge from one another because they use different resources in the environment. We called this ecological speciation. We are going to focus on ecological speciation here. Fishes in postglacial lakes provide excellent examples of how the use of different parts of the environment can lead to reproduction isolation then ultimately, speciation.
About 15000 years ago, lakes formed after ice disappeared over the northern part of North America and Eurasia. These lakes were mainly freshwater lakes. Some lakes connected to the ocean, which provided the opportunity for some fishes to go to the ocean. However, fish had different degrees of salt-tolerance–not all the fish were able to live in high salt concentrated ocean water. Only salt-tolerant fish were able to get to the ocean. From this point, fish utilized different habitats. Some used fresh water and some used salted water. This happened in the yummy sockeye salmon. One kind of sockeye salmon, anadromous sockeye salmon, spend their first two years in the lakes then swim to the ocean. In the ocean, they grow into a very big size. Another kind, kokanee salmon, stay in the lakes forever and remain a much smaller size. These two kinds of sockeye salmon are genetically distinct. DNA analyses demonstrate that kokanee salmon evolved from anadromous salmon. This sockeye salmon speciation was an example of sympatric speciation. They lived in the same place but they did not mate with each other.
Another example of ecological speciation can been seen in the European whitefish (Coregonus lavaretus) in Scandinavian lakes. Similar to the sockeye salmon, whitefish utilize different areas of the lakes that were ecologically different. To the extreme, some lakes have up to five ecologically different areas. As a result, whitefishes in these lakes developed into five different species: they have different sizes, consume different resources, and have different spawning sites and times, which lead to reproductive isolation, just like the sockeye salmon.
To read more, see:
Schluter, Dolph. 1996. Ecological Speciation in Postglacial Fishes. Philosophical Transaction of The Royal Society B.Biological Sciences 351(1341): 807-814.
Fahlman, Johan. 2014. Size selective predation of pike on whitefish: The effects on resource polymorphism in Scandinavian whitefish populations. Umeå University. 13p.