Dolph Schluter studies how new species form and become different from one another. His main study organism is the threespine stickleback, which is found in the ocean, but has also colonized freshwater lakes. In some lakes, two distinct forms have evolved. Find out how the flow of ancient genes influences the development of new species.

I'm Dolph Schluter. I'm a professor in the Zoology Department and interim director of the Biodiversity Research Centre. I study how new species form and become different from one another. My main study organism is the three spine stickleback. The threespine stickleback occurs throughout the northern hemisphere in coastal waters and the marine species has been in the sea for a long time, but particularly in this part of the world (British Columbia, Canada) it has colonized fresh water over and over again.

To understand how these freshwater populations form, we have to realize that 13,000 years ago, the land where I am now sitting was covered by ice about a kilometre and a half thick. This ice melted over the next one to two thousand years and the ice was really heavy. With the weight of the ice removed the land rebounded and new lakes were formed. The marine form (stickleback) was able to colonize freshwater at that time.

When we looked at the genes (of these sticklebacks) we discovered two things that we did not expect. The gene variance that benthics had, on different islands where these pairs evolved independently, were more closely related to each other than either is to the variant present in the limnetic species or in the marine species. The second result that we did not expect when we looked at the genes is that the the benthic variant, although it arose from the variant that we see today in the limnetics and in the marine form, the benthic variant is older than the lakes (they live in). Not only is it older than the lakes it's older than the species. It's not just a few years older, it's millions of years older. How's this even possible? No existing freshwater stickleback population is millions of years old.

Our hypothesis is that these old variants are kept in circulation by persistent gene flow between marine and freshwater populations. Here we see the ice retreating, coastal lands rebounding and the marine (sticklebacks) colonizing fresh water. These marines carried genes with them from older populations. Here we see a freshwater population and a marine population meeting where the stream meets the ocean and interbreeding to produce a hybrid. This hybrid individual has both marine variants and freshwater variants. This hybrid joins the marine population and interbreeds with marine individuals generation after generation. After every generation the hybrid offspring range in the number of freshwater variants that they possess, but gradually the freshwater genotypes become scattered. What's produced in the end is a marine population where every individual, or at least most individuals, carry maybe one or two variants that are inherited from an interbreeding event that happened with the freshwater population a long time ago.

The ice recedes, a new lake has formed, the marines colonize and they bring with them some of this variation. Over time within this lake offspring are produced and some of them carry zero, one, two, or three of these genes with the freshwater variants. Those individuals that inherit more copies of freshwater variants do better on average. They survive better and they produce more offspring and this is the process of natural selection. Eventually over a number of generations we produce a freshwater population that is very much like the freshwater population that long ago interbred with the marine form and injected it with these variants.

One of the things we learned from stickleback and the genetic studies of stickleback is that new species can evolve rapidly, and that when they do so they often make use of genes that formed a long time ago. A new species can often be made up, at least in part, by old genes. We are finding that, that seems to be a common story now that we're finding in other systems around the world that other people are working on. Especially when new species evolve very rapidly.


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