Photo: A young, three to four month old, male Xiphophorus alvarezi.
Xiphophorus alvarezi, the Chiapas Swordtail, is a wild swordtail species from, remarkably enough considering its name, the Mexican state of Chiapas. We acquired this species in the auction at the April 2012 Fort Lauderdale convention of the American Livebearer Association (https://livebearers.org/). Here is link to a very fine discussion of this species:
We presently maintain two breeding colonies of this species. One population has freckles and the other does not. Well not quite; I’m selecting for freckling in one population and no freckling in the other. That’s a work in progress. Both colonies produce the other’s freckling pattern, but as each generation goes by the percentage of the correct type improves. This is natural selection, well…okay, my selection, at work, although part of their “natural” environment is me. They will have to live with that. This, by the way, is an example of what happens to fish under domestication. The breeder’s selection of favorites changes the fish. Not only that, but the fish rapidly genetically adapt to the new conditions, no predators, prepared foods, no seasonality, consistent water conditions, etc. A good example is wild Poecilia latipinna that we collect from nearby streams. The original fish are skittish, hiding from movement. A few generations later they act just like long domesticated commercial mollies, no longer frightened. Why? Because the bolder, less wild fish eat more, mate more, and their genes for calmness increase while genes for fright and fleeing are selected out of the population.
The pictured male at the top is a three to four-month-old from the non-freckled population. He does not show the reddish blush along the back and head of many mature males. His sword will get quite a bit longer. Here are a pair from the freckled population from three years ago when I separated the two populations. Our current freckled breeders have significantly more freckling.