Photo: Grandchildren at waterfall where we searched for Gambusia montanaensis.
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Okay, during July I posted a few times on Goliad Farm’s Facebook page during a trip starting in Texas to Colorado, Wyoming, Montana, Idaho, Utah, and New Mexico. Susie, my wife, and I caravanned with two of our six children and five of our six grandchildren (those five old enough to go along). When we reached Montana I had a brainstorm. Why not use our trip for educational purposes. To that end I started a tongue-in-cheek mythical expedition to collect rare Gambusia. Before going into that, let’s take care of some natural history first.
Gambusia is New World genus of Poeciliidae livebearers. Some other Poeciliidae livebearers include guppies, mollies, and swordtails. These livebearers have internal fertilization via the male’s modified anal fin, which is called a gonopodium. Females subsequently bear live young that are able to care for themselves immediately. According to the Integrated Taxonomic Information Service (ITIS), there are 43 species in Gambusia, all found in the Atlantic Ocean drainage. Eleven of these are native to the USA. One species, G. affinis, is known as the mosquitofish and has been introduced to all continents but Antarctica for mosquito control. It also has the largest natural range of any Gambusia, from the Virginias across to Kansas and south to the Gulf of Mexico. The other species have much restricted ranges, sometimes single springs, and two are now extinct. Of the eleven species, ten are, or were, found in Texas and New Mexico’s Pecos River drainage. One is from Florida.
Why so many species with highly restricted ranges? The ancestral population once ranged throughout west Texas and eastern New Mexico when the climate was much wetter than now. Starting over 10,000 years ago the climate warmed and got drier, isolating some of these fish in headwater springs as the downstream rivers and creeks dried up. Each spring evolved its own species (as an aside, the same thing happened with pupfishes).
Now back to my brainstorm. I could use a mythical collecting trip as a springboard to a discussion of evolution and geography. See, much speciation seems to occur by geographic isolation of populations. After isolation, a population is free to evolve to meet local conditions without gene flow from other populations. If this goes on long enough, later when the two populations meet up again, they don’t interbreed, or if they do, the resulting hybrids are poorly adapted to either of the niches the two populations now occupy. That leads to strong selection pressure against hybridization.
So, I thought I could pretend (although I’m always looking for new species in unexpected places) to search for made up Gambusia species. How preposterous is this? Not entirely in the case of Montana and New Mexico. It also isn’t for Colorado and Wyoming, but I didn’t dream this up until we were in Montana. I thought about back-dating posts to cover those two states, but that seemed to be cheating. I went with what I had.
First, I made up the name Gambusia montanaensis, which simply means Gambusia from Montana. If I had found such a fish that’s probably what I would have named it, either that or after Susie. Is it feasible there could be in an isolated spring a fish that could be so-named? Highly unlikely. G. affinis, which is likely close the ancestral Gambusia does not naturally range into Montana. It has been introduced into warm springs where it can survive Montana’s cold winters. Like G. affinis, it’s unlikely the ancestral Gambusia ranged into Montana. That’s why my post referred to it as the “very elusive Gambusia montanaensis.”
How about G. idahoensis? By now you know that means Gambusia from Idaho. That is even less likely since all of Idaho is in the Pacific Ocean drainage and Gambusia species are only found naturally in the Atlantic Ocean drainage. The same applies to G. utahensis, it’s wholly in the Pacific drainage.
In respect to G. coloradoensis (we looped back through a corner of Colorado on the return from Montana), the corner we traversed was all in the Pacific drainage, so not likely.
As to G. neomexicana things get more possible. Once we crossed the Continental Divide in central New Mexico into the Atlantic drainage, it could have been possible. Why? Because the ancestral Gambusia ranged into eastern New Mexico and could have been isolated in a spring. But alas, I’ve never found a fish I could name that.
By the way, Google “Gambusia montanaensis.” They pick up topics quickly.