Extinction through fornication

Normally, we think of extinction happening because organisms fail to reproduce. In the case of Canadian sticklebacks, some incipient species are going extinct because they are reproducing all too well.

Three-spined stickleback have many populations around the world. The main population lives in marine systems most of their lives. But over and over, small pockets of animals have been caught in freshwater lakes, and started to go off on their own evolutionary trajectory, separating from their marine ancestors.

Within the individual lakes, the sticklebacks can continue to branch off further from each other. Some specialize in lake bottoms; some on the surfaces. In my old stomping grounds of Vancouver Island, this has been going on in Enos Lake. Back in the early 1990s, the bottom- and surface dwellers were said to be separate species, as they were very morphologically distinct and didn’t interbreed with each other. That said, they were never formally described as separate species and given new species names, as far as I can tell.

This may turn out to be a good thing, because when Behm and colleagues sampled the stickleback populations in 2005, the two distinct groups were gone. To be clear, there were still stickleback – but instead of two clearly distinct clusters with no interbreeding, there was one cluster with lots of interbreeding.

Previously, the two different body shapes had different diets, as measured both by direct observation of the prey types in stomach, and by carbon isotopes found in the fish. You’ll forgive me, I hope, if I skip the details of how lake food webs influence the ration of different carbon types the fish ingest. In any case, that difference between body shape and food type had also vanished.

It’s possible that there are a lot of intermediate shaped fish, but maybe they’re not doing so well. Turns out that as of right now, there’s no evidence of that. The intermediates don’t seem to be small, seem to be growing fine, and so on.

All of this raises the question: What the heck has suddenly caused these two distinct groups to start merging back together?

The authors’ surprising suggestion is that it’s because of crayfish. Signal crayfish (Pacifastacus leniusculus) were introduced into Enos Lake around 1990. Behm and company don’t have a clear reason why having crayfish in the lake should have this dramatic impact on the stickleback populations, but the timing of the two events is mighty suspicious. There’s clearly some interesting ecology to track down in this system.

In the last few decades, we’ve really come to appreciate how dynamic and quick evolutionary processes, like speciation, can be. This research shows that speciation, at least in the early stages, can also be a fairly fragile thing, easily falling apart for reasons that are not entirely clear at first glance.

Reference

Behm, J., Ives, A., & Boughman, J. (2009). Breakdown in Postmating Isolation and the Collapse of a Species Pair through Hybridization The American Naturalist DOI: 10.1086/648559

Stickleback picture by user SuperIDR on Flickr, and used under a Creative Commons license.